solvatochromic effect

7/30/2019 Solvatochromic effect

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Solvent influence on absorption and fluorescence spectra of

merocyanine dyes: a theoretical and experimental study

I. Baraldi, G. Brancolini1, F. Momicchioli*, G. Ponterini, D. Vanossi

Dipartimento di Chimica, Universitaa di Modena e Reggio Emilia, Via Campi 183, I-41100 Modena, Italy

Received 8 November 2002

Abstract

The solvatonCS INDO model, previously successfully used to describe the solvatochromic properties of merocya-

nines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these

dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively

solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2

was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the ET30 scale. Solvato- andfluorosolvatochromism were described by calculating the S0eq: ! S1FranckCondon and S1eq: ! S0FranckCondon

transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and

ranging from 0 to 1. The absorbing S0eq: and emitting S1eq: units (solute molecule + solvent cage) were approximatedusing the S0 and S1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value

ofke. The calculation results fully confirm that S0 and S1 states of merocyanines can be viewed as a mixture of a neutraland a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral

data (absorption and emission maxima and corresponding Stokes shifts) vs ke are in fairly good agreement with those ofthe experimental data over almost the entire range of the normalized ENT values, thus showing that specific solvent in-

teractions are at least partly simulated within the solvatonCS INDO scheme. The methodological prerequisites for a

correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations.

2003 Published by Elsevier Science B.V.

1. Introduction

We have recently shown [1,2] that solvent effects

on both ground-state properties and absorption

spectra of classic donoracceptor dyes, such as

merocyanines, can be fairly well accounted forwithin the CS INDO scheme [3]. Briefly, the sol-

utesolvent interactions were described by the

simple solvaton model [4] and were incorporated

in the CS INDO Hamiltonian according to previ-

ous basically equivalent all valence electron SCF

approaches [511]. Our procedure, however, is

characterized by a peculiar modelling of the solv-

aton set representing the polarized solvent sur-

rounding the solute. We followed the basic widely

accepted idea that the electronic structure of

Chemical Physics 288 (2003) 309325

www.elsevier.com/locate/chemphys

* Corresponding author. Tel.: +39-59-2055081; fax: +39-59-

373543.

E-mail address: [email protected] (F. Momicch-

ioli).1 Present address: Dipartimento di Chimica G.Ciamician,

Universitaa di Bologna, Via F.Selmi 2, I-40126 Bologna, Italy.

0301-0104/03/$ - see front matter 2003 Published by Elsevier Science B.V.

doi:10.1016/S0301-0104(03)00046-6
http://mail%20to:%[email protected]/http://mail%20to:%[email protected]/


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merocyanines can be described at the p level in

terms of resonance between neutral and charge-

separated forms (Fig. 1) and that the solvato-

chromic behaviour can be traced back to the rel-ative weights of the two structures in the ground

state and their change upon vertical transition to

the electronic excited state [12,13]. Such a scheme

stresses the key role of the p-electron distribution

and suggests that an effective solvent field must be

first of all capable of correctly controlling the drift

of p-electrons from the donor R2N to the ac-ceptor (CO) group. A very elementary way to

simulate a polarized environment is to position

one or more point charges at one or either end of

the chromophore system, as some authors did by

the middle of the 1990s [14,15]. In a more realistic

way, following the solvaton model [4] we associ-

ated with each atom of the conjugated system,

with p net charge Qp, a fictive particle with charge

Qp interacting with all electron and core charges

of the solute according to Borns law. The so-de-

fined solvaton set reflects the composition of the

resonance hybrid (Fig. 1), depending on the nature

of the donor and acceptor groups, and hence may

effectively account for the p-electron redistribution

induced by the solvent polarity. Using such a

solvaton set within the CS INDO CI scheme [1,2],we were able to provide a satisfactory description

of both the positive solvatochromism of two vi-

nylogous streptomerocyanines (Fig. 1, n 2; 4)and the large negative solvatochromism of stil-

bazolium betaine [13] (Fig. 2, M2).

Reasonable structural variations with solvent

polarity, related to variations of the resonance

hybrid composition, were also predicted. The same

twofold problem had previously been addressed by

Albert et al. [16] using the self-consistent reaction

field (SCRF) model within the INDO method, butno choice of the cavity-size parameter had yielded

a reasonable prediction for the two opposite sol-

vatochromic behaviours (for more details see

[1,2]). To our knowledge, the two solvatochromic

trends were qualitatively well reproduced only by

Klamt [17] using the AM1/COSMO method.

Other theoretical studies introducing the solvent

dielectric field through either a set of point charges

[14,15] or the virtual charge model [10] dealt only

with stilbazolium betaine which has attracted great

attention in relation to its uncommonly large

negative solvatochromism and the much-discussed

solvatochromic reversal at low medium polarity

[18]. Independently of the specific (continuum)solvent model, the majority of the cited theoretical

studies [1,2,10,15,17] reproduced qualitatively the

negative solvatochromism of M2. On the other

hand, Morley [14] predicted the opposite trend

combining AM1 structure optimization in the

presence of the solvent and gas phase CNDOVS

calculation of the transition energy for the solvent-

distorted molecular geometry. This result was

interpreted by Morley as evidence that the zwit-

terionic (benzenoid) form, obtained in the polar

medium, absorbs at the red of the neutral (qui-nonoid) form prevailing in non-polar medium.

Such interpretation, however, is questionable

since, no matter what geometry is used, the elec-

tronic structure yielded by an MO calculation

corresponds to a mixture of VB structures. The

zwitterionic form exists only in the presence of a

polar medium and its electronic spectrum can be

calculated only using the solvent-polarized MOs.

As a matter of fact, all calculations complying with

this condition [1,2,10,15,17] predicted solvato-Fig. 1. Neutral and charge-separated mesomeric structures of

simple streptopolymethine merocyanines.

Fig. 2. Investigated compounds. M1: 1,9-di-(N-phenyl-N-me-

thyl)-4,6,dimethylene-nona-1,3,6,8-tetraen-5-one; M2: 40-hy-droxy-1-methylstilbazolium betaine.

310 I. Baraldi et al. / Chemical Physics 288 (2003) 309325


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chromic shifts in qualitative agreement with ex-

periment. 2 Qualitatively equivalent results were

obtained by Benson and Murrell [19] within an

SCF p-electron treatment where the effect of sol-vent was simulated by suitable choice of one-cen-

tre core and electron repulsion integrals for the

nitrogen and oxygen atoms.

From the above brief survey, our solvatonCS

INDO scheme appears to have two main advan-

tages: (i) both ground and excited-state properties

are calculated for the solute molecule embedded in

the solvent, (ii) the use of a solvaton set reflecting

the net p-electron charges enables the solvent in-

teraction to be modelled in keeping with the VB

description of the electronic structure of the solute.

A disadvantage, shared by all continuum models,

is that it formally leaves out specific solvent in-

teractions. In principle, such effects can be ac-

counted for only within semicontinuum type

theories [20,21] or fully discrete type approaches

as, for example, those based on statistical me-

chanics techniques [2224]. However, our very

simple scheme, where the solvent interaction is

introduced essentially through a variation of the

diagonal elements of the Fock matrix corre-

sponding to the AOs of the p system, may im-

plicitly account for specific solvent interactions asfirst argued by Benson and Murrell [19].

In the present work, the solvatonCS INDO

scheme was subjected to further validation by

studying the solvent effects on the fluorescence

spectra (fluorosolvatochromism) 3 which have to

date received relatively little attention from a theo-

retical point of view. As test compounds we chose

two merocyanines, a ketocyanine dye (M1) and

stilbazolium betaine (M2) (Fig. 2), and the solvent-

induced spectral shifts of both absorption and flu-

orescence emission were investigated. Abundantexperimental data concerning the absorption spec-

tra of these dyes in solvents of different polarities are

available ([25], and references cited therein). On the

other hand, to our knowledge solvent effects on the

fluorescence spectra have been reported only for M1[26,27]. Thus, we first of all carried out an experi-

mental exploration of the absorption and emission

solvatochromism of M2. In summary, the experi-

mental data as a whole show that: (i) M1 exhibits

strong positive solvatochromism in both absorption

and emission [2527], (ii) M2, the absorption of

which is characterized by one of the strongest neg-

ative solvatochromisms ever observed, features a

markedly weaker negative fluorosolvatochromism.

The theoretical interpretation of the entire body

of experimental evidence was undertaken by ap-

plying the solvatonCS INDO method within a

usual scheme where the fluorescence emission takes

place from the equilibrium geometry of the lowest

excited singlet state reached very quickly after ver-

tical S0 ! S1 excitation of the equilibrium groundstate. In principle, this should require geometry

optimization of the solvatedsolute molecule in both

the ground state S0 and the emitting S1 (pHpL;

H HOMO, L LUMO) state. In practice, wesimply used S0 and S1 geometries optimized in the

gas-phase approximation and calculated the S0

eq:

! S1Franck

Condon and S1eq: ! S0Franck

Condon transition energies as functions of thesolvent polarity using solvaton sets reflecting the p

net charges of S0eq: and S1eq:, respectively. Thecalculation results will first be thoroughly analysed

by reference to the basic characteristics of the the-

oretical model and will then be subjected to a de-

tailed comparison with experimental observations

in solvents covering the whole scale of solvation

power. It will be shown that both the solvatochro-

mic and fluorosolvatochromic behaviours of M1

and M2 are qualitatively well described within thesolvatonCS INDO scheme.

2. Experimental investigation on dye M2

2.1. Materials, instrumentation and details of

experiments

M2 (4-[(1-methyl-4(1H)-pyridinylidene)ethylid-

ene]-2,5-cyclohexadien-1-one) was purchased from

2 A calculation procedure like the Morley one was applied in

[16] where INDOSCRF optimized geometries were used in gas

phase INDO/S type calculations of the spectra. This may

explain the positive solvatochromism erroneously predicted for

M2 in [16] when using physically reliable values of the cavity

radius.3 Hereafter, the solvent dependence of the position of the

absorption and fluorescence bands will be termed solvatochro-

mism and fluorosolvatochromism, respectively [25].

I. Baraldi et al. / Chemical Physics 288 (2003) 309325 311


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Aldrich and was used as received. All solvents

(Merck and Lab-Scan) were of spectroscopic grade

and were dehydrated with activated molecular

sieves before use. The polar and hygroscopic oneswere treated with solid KOH so as to dissolve the

M2 betaine in the unprotonated form. For the

same reason, measurements in water were carried

out in 102 M NaOH.Absorption spectra were recorded on a Perkin

Elmer k15 spectrophotometer, while a Spex-Jobin

Yvon Fluoromax 2 spectrofluorometer was em-

ployed for the fluorescence measurements. All

experiments were carried out at room temperature

(1821 C). Maximum optical densities were,

typically, between 0.1 and 1 (corresponding to

sample concentrations from 3 106 to 3 105 mol dm3) for absorption measurements,and around 0.10.15 for fluorescence measure-

ments. In the latter, each sample was excited at, at

least, two different wavelengths on the high-energy

side of the absorption band. Emission and exci-

tation spectra were corrected for the instrumental

spectral response. Due to the very weak fluores-

cence emission of M2, especially in low-polarity

solvents, wide monochromator slits were used

(68 nm spectral resolution) to improve the signal-

to-noise ratio. Fluorescence quantum yields UFwere determined in methanol and water with re-

spect to cresyl violet in methanol (UF 0:65 [28])and eosin in methanol (UF 0:60 [29]) accordingto the usual expression: UF UF;rA=Arn2=n2r ODr=OD, where r refers to the reference, A arethe areas of the corrected emission bands, n are the

solvent refractive indexes and the optical-density

(OD) ratios at the excitation wavelengths were

adjusted to unity. Inner filter effects were deemed

negligible on both emission spectra and fluores-

cence quantum yields because of the optical thin-ness of the samples employed and of the low to

very low spectral overlap between absorption and

emission.

2.2. Results and discussion

The choice of test merocyanine dyes suitable to

check thoroughly the capability of the solvaton

CS INDO method of accounting for solvent effects

on both absorption and fluorescence spectra was

not a trivial affair. First, in compliance with the

previous studies limited to effects on the absorp-

tion spectra [1,2], we needed two compounds

characterized by opposite solvatochromisms. In[1,2] simple streptomerocyanines (Fig. 1) and stil-

bazolium betaine (M2 in Fig. 2) were taken as

typical dyes with strong positive and negative

solvatochromism, respectively. In both cases the

intense colour band is due to the lowest p ! p(essentially pH ! pL) transition and the observedsolvent shifts are not affected by the presence of

any forbidden np state at low energies. On theother hand, in order that the study may be ex-

tended to fluorosolvatochromism the test com-

pounds should be characterized by an efficiently

emitting lowest-lying pHpL state. Previous theo-retical and photophysical studies [3032] have

pointed out that in general streptomerocyanine

derivatives have no such characteristics because of

the presence of lowest-lying np states and theoccurrence of fast transcis isomerization follow-

ing direct S0 ! S1pHpL excitation. Thus, in thepresent work we replaced the streptomerocyanines

(Fig. 1) with the ketocyanine M1 (Fig. 2) which is

known to be an efficient fluorophore exhibiting

large red shifts in both absorption and emission

spectra upon increasing the solvent polarity [25].In this case, the theoretical study (Section 3) was

based on the abundant experimental data reported

in the literature [2527].

The suitability of stilbazolium betaine M2 as

test compound for negative solvatochromism in

both absorption and fluorescence required on the

contrary further experimental investigation. As is

well known, dye M2 exhibits a very large negative

solvatochromism in absorption ($6500 cm1 ongoing from CHCl3 to H2O solution) [25]. How-

ever, the absorption spectrum of M2 shows achange in shape on moving from low-polarity

solvents, where a pronounced structure is ob-

served, to highly polar and protic solvents, where

the structure is blurred out. For merocyanines

closely related to M2, the structured spectra ob-

served in different solvents of relatively low po-

larity were attributed to the presence in solution of

two variously identified species ([33], and refer-

ences cited therein). Similar observations had led

other authors to question the reliability of any



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analysis of the solvatochromism of these dyes [34].

In order to check the importance of any extra

contribution to the absorption of M2 in solvents of

low polarity, we carried out a spectrofluorometricanalysis in a 1:1 dichloromethanetoluene mixture.

The emission and excitation spectra showed only a

very modest dependence on, respectively, the ex-

citation and monitoring wavelength. Representa-

tive spectra are shown in Fig. 3. A close

correspondence was observed between the ab-

sorption and excitation spectra, apart from a weak

solvent Raman band (around 560 nm in Fig. 3) ca.

3000 cm1 from the monitoring wavenumber, andsome blurring of the vibronic structure possibly

due to poor instrumental spectral resolution (8

nm). We conclude that the absorption and emis-

sion spectra of M2 in low-polarity solvents are due

to essentially one and the same species. Moreover,

the rather small Stokes shift and the mirror image

relationship between the absorption and emission

peaks suggest that fluorescence emission takes

place from the local minimum of the initially ex-

cited 1pHpL state. This is in keeping with therecently reported femtosecond dynamics of dye

M2 [35]. According to [35], direct S0 ! S1 excita-

tion of unprotonated stilbazolium betaine M2

does not result in transcis isomerization but it is

followed by fast thermally activated relaxation (1.1

ps) to a non-emitting conformational intermediate.Thus, although emission is predicted to occur from

the local minimum of the S1pHpL state, the flu-orescence quantum yield should be very small. As

a matter of fact, we found the fluorescence quan-

tum yield of M2 to be only 2:0 103 in methanol,1:5 103 in water and even lower values weremeasured in solvents of low polarity. From the

foregoing considerations, it ensues that M2, even

though weakly fluorescent, may be used to inves-

tigate solvatochromism in both absorption and

emission. For this purpose, the absorption and

fluorescence emission maxima of M2 were mea-

sured in nine solvents with ET30 solvent po-larity parameter [25] ranging from 39.1

(chloroform) to 63.1 (water), and the results are

reported in Table 1. Both absorption and emission

spectra exhibit a negative solvatochromism, and

shift quite regularly to the blue with increasing the

value of ET30 which represents the overall sol-vation power of the solvents. On the other hand,

the static dielectric constant e, reflecting only

Fig. 3. Absorption (full line), fluorescence excitation (dashed line, kem 660 nm) and emission (dotted line, kexc 580 nm) spectra ofM2 in a 1:1 dichloromethanetoluene mixture.



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non-specific solvent effects, fails to account for

the quite different band shifts induced by aprotic

and protic solvents of comparable polarity. The

maximum solvatochromic shift found for the

absorption, Dmabsmax mchloroform mwater 6260 cm1, is in agreement with data of otherauthors ([25], and references cited therein). Much

smaller hypsochromic shifts were observed foremission, Dmflumax 1340 cm1. We note that aqualitatively similar, yet less pronounced, solvent

dependence of the absorption and fluorescence

spectra has recently been reported for another

negatively solvatochromic merocyanine (MC 540)

[36].

3. Theoretical investigation

3.1. Method and calculation details

Incorporation of the solvaton model into the

CS INDO method has already been described in

detail [1,2]. Here, we only summarize the main

points concerning the calculation of the solvato-

chromic shifts. Briefly, the Fock matrix general

element is expressed as

Fslm F0lm dlmke

XN

B0QB0cAB0 l 2 A; 1

where the first term F0lm is the CS INDO matrixelement in the absence of solvent [3], while the

second term accounts for the electronsolvaton

interaction effects. In particular, ke is a solventpolarity factor related to the static dielectric con-

stant e. In keeping with Costanciel and Tapias

virtual charge model [9], where the polarity factor

is set equal to ffiffiep 1= ffiffiep, we assumed ke torange from 0 (when e is 1) to 1 (for e ! 1). QB0 isthe charge of the solvaton associated with atom B

and cAB0 is taken equal to the electron repulsion

integral cAB. As pointed out in Section 1 (see also

[1,2]), solvatons were only associated with the at-

oms contributing to the p system (C, N, O) and

their charges were taken equal to the negative of

the respective net p-electron charges.

The determination of the solvent effects on the

position of the first absorption bands required the

HartreeFock equations incorporating solutesolvaton interactions to be resolved for the

equilibrium ground-state geometries and the cor-

responding vertical S0 ! S1pHpL transitions tobe calculated at different values of ke. As sug-gested by the experimental observations (Section

2.2) fluorescence emission of both M1 and M2 dyes

takes place from the excited S1pHpL state aftergeometrical relaxation of the solute and reorgani-

zation of the solvent molecules in the solvation

shell. According to this scheme, the fluorescence

Table 1

Absorption and emission maxima of M2 in solvents of different characteristics

Solvent ET30a (kcal/mol) ENT b ec Absorptiond, mmax cm1 Fluorescenced, mmax cm1

Chloroform 39.1 0.259 4.8 16,310 15,850Dichloromethane 40.7 0.309 8.9 16,470 15,820

Acetone 42.2 0.355 20.7 17,040 16,130

DMFe 43.2 0.386 37.0 17,150 16,140

DMSOf 45.1 0.444 46.7 17,390 16,160

Acetonitrile 45.6 0.460 37.5 17,590 16,260

2-Propanol 50.7 0.546 19.9 18,450 16,700

Methanol 55.4 0.762 32.7 20,700 17,120

Water 63.1 1.000 78.4 22,570 17,190

Dmmax cm1 )6260 )1340a Solvent polarity parameter of Dimroth and Reichardt measured at 25 C and 1 bar [25].b Normalized ENT value.c Dielectric constant at room temperature (20 or 25 C) [13].d Typical uncertainties: 30 cm1 on absorption, 60 cm1 on emission.e Dimethylformamide.fDimethyl sulfoxide.



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transitions were calculated at different ke valuesusing geometries and solvaton charges optimized

for the excited state. For the sake of simplicity, in

the present work geometrical distortions inducedby the solvaton field on the solute molecule were

disregarded. In other words, we simply used ge-

ometries optimized in the absence of solvent

ke 0. This simplification appears to be ac-ceptable, since solvatochromic shifts are mainly

determined by polarization and electrostatic effects

that are not very sensitive to small geometry

modifications. However, the solvaton charges cor-

responding to the equilibrium geometries of the

ground and the excited states were adjusted by an

iterative procedure, i.e., by performing a sequence

of SCF (for S0) and SCF CI (for S1) calculations

until convergence of the QB0 values.

To summarize, we carried out the following set

of calculations:

(i) The S0-state equilibrium geometries of both

dyes were calculated by minimizing the total CS

INDO SCF energy of the isolated molecule, as a

function of the internal coordinates of the chro-

mophoric group. With these geometries, SCF cal-

culations were carried out, for different ke values,using solvaton charges corresponding to the S0 net

p-electron charges. The MOs produced at eachke value by the iteratively adjusted solvaton setwere then used for the CI calculation of the ab-

sorption spectrum, thus determining the absorp-

tion spectral shifts.

(ii) An analogous procedure was carried out to

calculate fluorosolvatochromic shifts. First, for

each molecule, the equilibrium geometry of the

S1pHpL state of the isolated molecule was cal-culated by minimizing the excited-state energy

ES1 ES0 DES0 S1 derived from the CIcalculation as a function of the internal coordi-nates of the chromophoric group. The so obtained

geometries were used to generate, at different kevalues, optimized solvaton sets corresponding to

the excited-state net p-electron charges and the

MOs to be used in the CI calculation of the

emission spectra and solvent-induced fluorescence

shifts.

Since the excited state responsible for both the

absorption colour band and the fluorescence

emission, S1pHpL, is fully represented by the

singly excited 1UpHpL configuration, we limitedourselves to standard S-CI calculations. The MO

active spaces included all p and p molecular or-

bitals of the chromophoric groups and, for M1, ofthe benzene rings. In view of the model character

of this study, the solvaton charges suitable for the

equilibrium excited state were simply derived from

the net p-electron charges of the 1UpHpL con-figuration.

The geometry optimizations were carried out

introducing in our CS INDO CI programs an

optimization code where the gradient is calculated

numerically within a FletcherPowell-type opti-

mization algorithm [37]. The way the gradient is

computed makes the program easily adaptable for

calculations of excited-state geometries.

The parameters peculiar to the CS INDO

method [3] were chosen as follows: (i) the screening

constants, klm, where the indexes refer to two hy-

brid atomic orbitals of the CS INDO basis set,

were given the values: krr 1, kpp 0:50,krp 0:65, knp 0:60, knr 0:72, knn 0:68; (ii)the atomic pair parameters, aAB (a.u.) and R

0AB (

AA),

entering the calculation of the core repulsion en-

ergy, were assigned the values aCC 1:35,aCN

1:40, aCO

1:20, R0CC

1:7, R0CO

2:10.

The remaining pair parameters were given theusual values, aAB 1:50 and R0AB 2:00.

Finally, two-electron repulsion integrals were

calculated according to OhnoKlopman [38].

3.2. Results and discussion

3.2.1. Ground- and excited-state equilibrium geom-

etries

After exhaustive test calculations on ground

and excited state geometries of prototypic systems,

not reported here for conciseness [39], the CSINDO based approach described in the Section 3.1

was applied to M1 and M2 in both S0 and S1states. The optimized bond lengths are reported in

Table 2. The structural differences between the two

dyes in the ground state and the geometry changes

induced by the S0 ! S1 excitation can be illus-trated by analysing the extent of bond length al-

ternation (BLA) in the conjugated bridge

connecting the donor (amino) and the acceptor

(carbonyl) groups, which reflects the relative



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weights of the neutral polyenic and zwitterionic

structures in the VB resonance hybrid (Fig. 1). All

such characteristics are well expressed by the value

of the so called BLA parameter (last row of Table

2), defined as the difference between the average

length of the single bonds and that of thedouble bonds [16]. The single and double

bonds were identified by reference to the neutral

VB structures (see Fig. 2), so high positive (nega-

tive) value of the BLA parameter indicates pre-

dominance of the neutral (zwitterionic) mesomeric

structure. A vanishing BLA parameter, on the

other hand, indicates a cyanine-like structure

characterized by similar weights of the two forms.

On this basis, the calculation results of Table 2

indicate that: (i) in the ground state, the charge-

separated form is more important for M2 than forM1 BLA M1;S0 > BLA M2; S0, (ii) in bothmerocyanines bond-length alternation decreases

upon electronic excitation, i.e., the contribution of

the neutral VB structure becomes lower on moving

from S0 to S1 BLA M1; S0 > BLA M1;S1;BLA M2;S0 > BLA M2;S1. The differences

between the two dyes become more evident if theBLA parameters of M2 are calculated in terms of

the bonds (d, e, f) of the central polymethinic

fragment. As a matter of fact, in this case BLA is

already rather small in the ground state (0.017)

and changes sign in S1, thus reflecting an opposite

bond alternation with respect to S0. The goodness

of our theoretical predictions should now be veri-

fied by comparing the calculated ground-state ge-

ometries of Table 2 with the experimentally

determined structures as well as the results of

previous theoretical studies. However, as far as we

know, structure determinations and calculations

have been reported only for dye M2. From the

crystal structure determination of M2 trihydrate

[40] the molecule was found to be almost planar

and to have a zwitterionic (benzenoid) structure

with an essentially double CC central bond (1.346AA) connected to the six-membered rings by bonds

of $1.44 AA, and a CO bond (1.304 AA) ratherlonger than a double C@O bond (1.215 AA). As is

evident, such structure is at variance with the CS

INDO optimized geometry of the isolated mole-

cule (Table 2) which corresponds to a mixture ofthe two VB forms, with a slight prevalence of the

neutral (quinonoid) one (Fig. 4). This is not sur-

prising since in the crystal the zwitterionic form is

imposed by the formation of hydrogen bonds be-

tween the carbonyl end of the chromophore and

the crystal water. Thus, the crystal structure is

assimilable to that of M2 in aqueous solution.

The CS INDO optimized geometry of M2 is in

qualitative agreement with previous theoretical

studies [10,14,16,19] in that all calculations predict

the quinonoid form to be (more or less) prevailingin the isolated molecule. However, with reference

to the central polymethine fragment (e, d, f CC

bonds), our geometry exhibits the smallest BLA

Fig. 4. Neutral (quinonoid) and zwitterionic (benzenoid) forms of dye M2.

Table 2

Bond lengths and bond length alternation (BLA) parameters

for the ground state S0 and the first pp excited state S1 ofdyes M1 and M2

Bonds M1 M2

S0 S1 S0 S1

a 1.374 1.368 1.365 1.362

b 1.398 1.411 1.396 1.402

c 1.446 1.430 1.447 1.444

d 1.398 1.415 1.419 1.427

e 1.458 1.450 1.436 1.421

f 1.245 1.253 1.418 1.430

g 1.455 1.447

h 1.401 1.406

i 1.450 1.447

l 1.250 1.254

BLA 0.055a

0.027a

0.038b

0.024b

0.017c )0.008c

For labelling of the bonds, see Fig. 2.a BLA M1 c e b d=2.b BLA M2 c e g i b d f h=4.c BLA M2 e d f=2.



9/17

value (present work: 0.017; [19]: 0.030; [16]: 0.038;

[14]: 0.058; [10]: 0.077). In other words, our de-

scription reflects a resonance hybrid with similar

weights of the two limiting structures, while theother theoretical predictions favour more decid-

edly the quinonoid structure. We do not deny that

our calculation may have underestimated the

contribution of the quinonoid form, but we can

say that this result is consistent with the observed

solvatochromic behaviour of M2, i.e., an initial

small red shift followed by a large blue shift of the

first absorption band on going from low polar to

highly polar solvents [1,2,13,18].

3.2.2. Absorption and fluorescence solvatochromism

Using the S0 and S1 optimized geometries, the

vertical S0eq: ! S1 and S1eq: ! S0 transitionenergies of M1 and M2 were calculated in a me-

dium of changing polarity (ke 00.9) by the CSINDO CI procedure described in Section 3.1. The

results DE are collected in Tables 36, togetherwith the oscillator strengths of the transitions (f),

the electric dipole moments (l), and the net char-

ges on the nitrogen and oxygen atoms in the

S0 QN;O and S1 QN;O states.First of all, let us point out some essential

characteristics of the theoretical description.Tables 3 and 4 show that on going from ke 0(gas phase) to ke 0:9 (highly polar medium),both the absorption and fluorescence maxima of

M1 are predicted to undergo marked bathochro-

mic shifts (DmS0 ! S1 3550 cm1 andDmS1 ! S0 5000 cm1) 4, while the oscillatorstrengths are found to be both quite high (>2) andnot very sensitive to solvent polarity. The batho-

chromic shifts are related to the increase of the

solute dipole moment on the S0 ! S1 transition,resulting in a solvent-induced stabilization of theexcited state relative to the ground state, which

increases with increasing solvent polarity. The

polarization effects induced by the solvaton field

on the electronic structure of the solute result in a

quite regular growing of both lS0 and lS1 on going

from ke 0 to 0.9. Such growing is little higherwhen using the equilibrium geometry of the excited

state (Table 4), but Dl lS1 lS0 is predicted to

be almost the same in absorption (Table 3) and

emission (Table 4) at all values ofke. However, itis worth noting that the permanent dipole moment

of M1 is directed along the twofold axis containingthe C@O bond (see Fig. 2), while the S0S1 tran-

sition is polarized perpendicularly to the twofold

axis 1B, i.e., is associated to charge transfer alongthe longitudinal molecular axis. Thus, the dipole-

moment change occurring on the S0S1 transition

is not as effective a parameter of the solvatochro-

mic effect as in the case of the parent streptopen-

tamethinemerocyanine [1,2] where both

permanent and transition dipole moments are

aligned with the chromophoric chain. As a matter

of fact, the S0 and S1 dipole moments of the ke-

tocyanine M1 (and their increase with increasing

ke) turn out to be decidedly smaller than those ofits pentamethinemerocyanine moiety, while the

contrary occurs for oscillator strength and solvent

shift of the S0 ! S1 transition (see [2]). For a moreprecise comparison, we carried out CS INDO SCI

test calculations for pentamethinemerocyanine

(Fig. 1; n 2, R Me) and its N-phenyl derivativein the absence of solvent. The calculation results

(Table 7) indicate that the phenyl substitution

produces a small red shift and intensity increase of

the transition, while lS0 and lS1 remain almostunchanged and substantially higher than those of

the ketocyanine (Table 3, first row). The absorp-

tion data in dichloromethane, also reported in

Table 7, corroborate the theoretical prediction

even if transition energies and spectral shift are a

little overestimated because of the adopted sim-

plifications (essentially, singly excited CI and co-

planarity of the phenyl ring). A more correct

correlation between the solvatochromic behaviour

of ketocyanine M1 and that of its parent chro-

mophore emerges by changing from the dipolemoments (i.e., global electrical properties) to the

atomic net charges reflecting the local charge dis-

tributions. As a matter of fact, Table 3 shows that

the net charges on the nitrogen and oxygen atoms

in the ground state, as well as their changes due to

an increase of ke and/or S0 ! S1 excitation,closely resemble those previously found for strep-

topentamethinemerocyanine [2]. In particular, in

S0 an increase ofke results in a drift of electronsfrom the two nitrogen atoms (donors) to the4 Dm mnon-polar solvent mpolar solvent.



10/17

Table 4

Calculated S1 ! S0 emission properties of M1 as functions of the solvent polarity factor ke (see caption of Table 3 for the meaning ofsymbols)

ke DE (eV) f lS0 (D) lS1 (D) QN QO QN QO0.0 3.118 2.411 5.67 8.10 )0.12 )0.73 )0.03 )0.78

0.2 2.968 2.384 6.44 8.99 )0.08 )0.83 +0.01 )0.87

0.4 2.823 2.362 7.17 9.81 )0.04 )0.91 +0.05 )0.95

0.6 2.686 2.345 7.86 10.58 0.00 )0.98 +0.10 )1.01

0.8 2.558 2.331 8.53 11.30 +0.05 )1.05 +0.14 )1.07

0.9 2.498 2.326 8.86 11.64 +0.07 )1.08 +0.16 )1.10

Table 5

Calculated S0 ! S1 absorption properties of M2 as functions of the solvent polarity factor ke (see caption of Table 3 for the meaningof symbols)

ke DE (eV) f lS0 (D) lS1 (D) QN QO QN QO0.0 2.782 2.075 17.92 18.36 )0.04 )0.73 +0.01 )0.71

0.2 2.766 1.904 22.09 21.30 +0.02 )0.84 +0.05 )0.81

0.4 2.790 1.702 26.88 24.73 +0.09 )0.94 +0.11 )0.91

0.6 2.944 1.425 33.80 30.58 +0.21 )1.04 +0.22 )1.02

0.8 3.184 1.234 39.91 37.54 +0.35 )1.13 +0.37 )1.11

0.9 3.291 1.163 43.12 41.81 +0.45 )1.16 +0.47 )1.15

Table 3

Calculated S0 ! S1 absorption properties of M1 as functions of the solvent polarity factor ke: vertical transition energy DE, os-cillator strength (f), ground and excited-state dipole moments lS0, lS1 and net charges on nitrogen QN;QN and oxygen QO;QOatoms

ke DE (eV) f lS0 D lS1 D QN QO QN QO0.0 3.258 2.378 5.59 8.03 )0.13 )0.72 )0.03 )0.77

0.2 3.163 2.367 6.11 8.65 )0.10 )0.81 0.00 )0.86

0.4 3.060 2.353 6.64 9.27 )0.06 )0.91 +0.04 )0.94

0.6 2.952 2.321 7.10 9.87 )0.01 )1.01 +0.09 )1.03

0.8 2.855 2.263 7.55 10.47 +0.07 )1.09 +0.16 )1.10

0.9 2.818 2.223 7.79 10.78 +0.12 )1.13 +0.21 )1.14

Table 6

Calculated S1 ! S0 emission properties of M2 as functions of the solvent polarity factor ke (see caption of Table 3 for the meaning ofsymbols)

ke DE (eV) f lS0 (D) lS1 (D) QN QO QN QO0.0 2.732 2.227 19.37 17.56 )0.02 )0.74 +0.01 )0.70

0.2 2.725 2.126 21.67 19.51 +0.02 )0.83 +0.05 )0.79

0.4 2.724 2.007 24.49 21.79 +0.08 )0.92 +0.10 )0.89

0.6 2.732 1.884 27.56 24.27 +0.15 )1.02 +0.16 )1.00

0.8 2.750 1.757 30.91 26.98 +0.23 )1.11 +0.23 )1.09

0.9 2.763 1.699 32.53 28.33 +0.28 )1.15 +0.28 )1.13



11/17

oxygen atom (acceptor), traceable to an increased

weight of the two equivalent zwitterionic forms

giving rise to a nonamethinecyanine-like elec-

tronic structure relative to the neutral form (M1

in Fig. 1). Moreover, the comparison between QN,QO and QN, QO provides an estimate of the realcharge transfer occurring during the S0 ! S1transition at each ke. Table 4 shows that, in-verting the direction of the charge transfer, similar

considerations apply to the transition from the

relaxed excited state to the FranckCondonground state, S1eq: ! S0. In conclusion, the ke-tocyanine M1 provides an example of a system

where the solvatochromic effects may be described

within the solvaton model better than by using

SCRF models adopting the Onsager dipolar ap-

proximation [16].

Merocyanine M2 is predicted to have a very

different solvatochromic behaviour both in ab-

sorption and in emission (Tables 5 and 6). In going

from ke 0 to 0.9, the absorption maximum isfound to undergo a small bathochromic shift (untilke 0:2) followed by a large hypsochromic shiftleading to a global strongly negative solvatochro-

mism Dm ffi 4100 cm1 (Table 5). This behav-iour follows that found in our previous theoretical

study [1,2], even if the transition energies and the

solvatochromic shift turn out to be somewhat re-

duced because of some differences in the calcula-

tion procedure concerning the parametrization

(kpp 0:5 instead of 0.55) and the use of fixedgeometries optimized at ke 0. The blue-shift of

the absorption maximum of M2 with increasing

ke, is accompanied by a substantial decrease of

the oscillator strength attributable to the increas-

ing importance of the zwitterionic form (Fig. 4). In

this case, where permanent and transition dipole

moments are both parallel to the longitudinal

molecular axis, the S0 and S1 dipole moments

should be fully representative of the solvatochro-

mic behaviour. Briefly, Table 5 shows that lS0 is

quite high at k

e

0, due to a substantial con-

tribution of the zwitterionic VB structure, andgrows rapidly with increasing ke owing togrowing solvent-induced polarization. At ke 0,lS1 is slightly higher than lS0 but, from ke 0:2on, the FranckCondon S1 state is 23 D less di-

polar than S0. Thus, both the small red shift be-

tween ke 0 and 0.2, and the successive strongblue shift can be rationalized in terms of net sta-

bilization of S1 or S0 induced simply by dipolar

solutesolvent interactions, as expected for a polar

solute in a polar solvent [20,25]. Similar consider-

ations can be made by analysing the net atomiccharges. Table 5 shows, in particular, that in both

S0 and S1 an increase in solvent polarity gives rise

to an increase in electron population on the oxy-

gen atom, while the vertical S0 ! S1 transitionresults in a reduction of such electron-charge ac-

cumulation. The latter phenomenon, which is the

opposite of that found with M1 (see Table 3),

suggests that S0 ! S1 excitation produces an en-richment of the resonance hybrid in the quinonoid

form. The same set of calculations performed for

Table 7

Calculated and experimental properties of the S0 ! S1 transition of the streptopentamethinemerocyanine and its N-phenyl derivativeMolecule Calculateda f lS0 (D) lS1 (D) Experimental

b 103emax

DE (eV) DE (eV)

Me2NCH@CH2CHO 4.087 1.211 8.64 13.43 3.430 51PhNMeCH@CH2CHOc 3.797 1.360 8.49 13.17 3.337 65a At ke 0.b In dichloromethane, [41].c The phenyl ring was assumed to be coplanar with the chomophore chain and to have the benzene structure.



12/17

the vertical S1eq: ! S0 transition, i.e., using ge-ometry and solvaton charges of the relaxed excited

state, led to similar trends of the dipole moments

and net charges (Table 6). Two points, however,have to be noted: (i) lS1 lS0 is negative over thewhole range ofke, (ii) the increase oflS1 and lS0with increasing ke is less marked than that foundfor the vertical S0eq: ! S1 transition (Table 5).Such trend, which is contrary to that exhibited by

M1 (Tables 3 and 4) (where lS1 > lS0), is related tothe fact that the solvaton field acting on the

equilibrium excited state is weak compared with

that acting on the equilibrium ground state. As a

consequence, the S1eq: ! S0 transition energyturns out to be little affected by solvent polarity,

with a global negative solvatochromism of just

250 cm1 ()0.031 eV) (Table 6).The above discussion has shown how the

adopted theoretical scheme is reflected in the cal-

culation results. Now, these results have to be

compared with the observed solvato- and fluoro-

solvatochromic behaviours of M1 and M2. In or-

der to make such comparison as concise and

comprehensive as possible, we have plotted in a

same diagram the calculated and experimental

band frequencies (in cm1) using, respectively, the

ke and ENT [25] scales, both ranging from 0.0 to1.0. This way, we obtained four diagrams corre-

sponding to the absorption and fluorescence

properties of M1 (Figs. 5 and 6) and M2 (Figs. 7

and 8). Let us comment first on Figs. 5 and 6.

As is evident, the calculated mmax values are

overestimated with respect to the experimental

values. However, the deviation is nearly the same

for absorption (Fig. 5) and emission (Fig. 6), and

keeps almost constant over the entire range of sol-

vent polarity ($0.5 eV at ke, ENT 0:1 and $0.45eV at ke, E

N

T 0:9). Thus, apart from the sys-tematic blue-shift of the calculated S0eq: ! S1and S1eq: ! S0 transitions (essentially due to theadopted CI truncation), our simple solvatonCS

INDO scheme describes fairly well the absorption

and fluorescence properties of ketocyanine M1

as well as their dependence on the solvent charac-

teristics. As a matter of fact, the calculated Dmmaxvalues mk 0 mk 0:9 for absorption3550 cm1 and emission 5000 cm1 are ingood qualitative agreement with the experimental

ones mENT 0:1 mENT 1:0 ([26]: 4010,3980 cm1; [27]: 3600, 3880 cm1). Interest-ingly, the agreement becomes even better if the

comparison is made over the same interval (0.10.9)

of solvent-polarity parameter. In this case, the sol-

vatochromic shifts, derived by simple linear inter-

polations, were found to be: Dmabs=cm1 ffi 3170calcd:, 3260 [26], 2800 [27], Dmfluo ffi 4350

Fig. 5. Wavenumber of the calculated and experimental ab-

sorption maximum of M1 vs, respectively, the ke and nor-malized ENT values of solvent polarity. The experimental mmaxvalues in eight solvents (toluene, THF, acetone, DMF, 2-pro-

panol, ethanol, methanol and water) with ENT ranging from

0.090 to 1.000, are reported from [26,27].

Fig. 6. Wavenumber of the calculated and experimental emis-

sion maximum of M1 vs, respectively, the ke and normalizedENT values of solvent polarity. The experimental mmax values are

reported from [26,27] (see caption of Fig. 5 for the solvent

characteristics).



13/17

calcd:, 3810 [26], 3710 [27]. Such estimatesindicate that excepting solvents with very high ENT ,

where specific solutesolvent interactions become

very important, in M1 the fluorescence is more

sensitive than the absorption to solvent polarity.

This behaviour, originating in the marked orien-

tational stabilization of the emitting state S1 [20]

is evidenced by the fact that, leaving out solvents

with ENT > 0:7, the remaining experimental points

are aligned almost parallely to the theoretical plots

(Figs. 5 and 6). The same thing emerges from the

plots of the Stokes shifts (mabsmax mfluomax Dmabsfluo) vs the normalized solvent-polarity parame-ters (Fig. 9). In spite of the large fluctuations of the

experimental data, it is evident that calculated and

observed Stokes shifts of M1 exhibit similar plots

until ENT ffi 0:7 . Beyond this value, the theoreticalplot goes on increasing, while the experimental ones

first bend and then undergo a dramatic drop down

in water (ENT 1:0). We can conclude that: (i) due tothe water peculiarity, any discussion considering

only the overall spectral shifts between water

(ENT 1:0) and toluene (ENT ffi 0:1) may lead towrong interpretations, (ii) except for EN

T

P 0:7, inM1 solvato- and fluorosolvatochromic shifts seem

to be to a great extent determined by non-specifc

solutesolvent interactions.

Now, let us consider the stilbazolium betaine

M2 (Figs. 7 and 8). In low polarity solvents, the

absorption and emission maxima of M2 are red-

shifted with respect to those of M1 of more or less

0.5 eV. This is correctly predicted by calculations

(e.g., at ke 0 the calculated red shifts are 0.48and 0.38 eV, respectively; see Tables 3 and 5 and

Tables 4 and 6) even if the individual transition

energies are rather overestimated ($0.7 eV at ke,ENT 0:3) due to the adopted CI limitations. Fig. 7shows that both the initial small red shift and the

Fig. 7. Wavenumber of the calculated and experimental ab-

sorption maximum of M2 vs, respectively, the ke and ENTvalues of solvent polarity. The experimental data in 25 solvents

covering the entire range of ENT were taken from [18]. The mmaxvalues of Table 1 are also reported.

Fig. 8. Wavenumber of the calculated and experimental emis-

sion maximum of M2 vs, respectively, the ke and normalizedENT values of solvent polarity. The experimental mmax values are

those of Table 1.

Fig. 9. Calculated and experimental Stokes shift of M1 vs, re-

spectively, the ke and normalized ENT values of solvent po-larity. Experimental values were derived from [26,27] (see

caption of Fig. 5 for the solvent characteristics).



14/17

successive strong blue shift of the absorption band

of M2 observed on increasing the ENT value are

qualitatively well reproduced by the calculations.

Quantitatively speaking, the large negative solva-tochromism of M2 (mabschloroform mabswater 6260cm1 (Table 1), 6490 cm1 [18,25]) is somewhatunderestimated by the solvatonCS INDO calcu-

lations (mabske0:2 mabske0:9 ffi 4230 cm1, see Table5 and Fig. 7). According to the previous discussion

on the values and solvent-induced changes of di-

pole moments, in this case lS1 < lS0 fluorosol-vatochromism should be expected to be weaker

than solvatochromism, contrary to what occurs in

the case of M1 lS1 > lS0. This is fully confirmedby our fluorescence study on M2 in various sol-

vents showing that mfluochloroform mfluowater 1340 cm1(Table 1 and Fig. 8), i.e., about a fifth of the blue-

shift observed in absorption. Calculations under-

estimate fluorosolvatochromic effects to the same

extent as the solvatochromic ones, so mfluoke0:2mfluoke0:9 reduces to $ 300 cm1 (Table 6, Fig. 8).In other words, the calculation results agree with

experiment as regards the direction of the spectral

shifts of absorption and fluorescence maxima, but

in both cases the effects are predicted to be weaker

than the experimental ones. This allows for a much

better agreement between calculated and observedStokes shifts (Dmabs fluo), as clearly shown byFig. 10. As is evident, the theoretical and experi-

mental plots of Dmabs fluo are in very goodagreement over the experimentally investigated

range of ENT values, and indicate that the Stokes

shift of M2 increases rapidly on increasing thesolvent polarity. Interestingly, contrary to what is

found for M1 (Fig. 9), in this case the experimental

plot displays no inversion 5 and remains parallel to

the theoretical one until ENT 1:0.Figs. 9 and 10 lend themselves to the following

concluding remarks. Although dyes M1 and M2

exhibit opposite solvatochromic and fluorosolva-

tochromic behaviours, in both systems the Stokes

shift increases considerably (with the only excep-

tion of M1 in water) with increasing the solvation

power of the solvent (as expressed by the EN

T

va-

lue). This occurs for opposite reasons in the two

dyes: in M1 the Stokes shift raises because the

bathochromic shift of the fluorescence is greater

than that of the absorption; on the contrary, in M2

the same phenomenon is due to the hypsochromic

shift of the absorption being greater than that of

the fluorescence. Both phenomena are well de-

scribed within the solvatonCS INDO scheme and

can be rationalized in terms of the different free-

molecule electronic structures (i.e., the different

relative weights of the neutral and zwitterionic VB

structures) and of their evolution with increasingthe solvent polarity. Moreover, it should be noted

(see also Figs. 58) that the solvatonCS INDO

approach, formally including only electrostatic

solutesolvent interactions, actually accounts

fairly well for the effects of media characterized by

quite high ENT values where specific (H-bond) in-

teractions are expected to be as important as di-

polar interactions. The reason for this somewhat

surprising result lies in the way the solutesolvaton

interactions were included in the molecular Ham-

Fig. 10. Calculated and experimental Stokes shift of M2 vs,

respectively, the ke and normalized ENT values of solvent po-larity. The experimental values derive from the absorption and

emission data of Table 1.

5 Fig. 10 shows that the solvation power of hydrogen-bond

donor solvents towards M2 (in both the ground and the excited

states) is well described by the ET30 ENT parameter. On thecontrary (Fig. 9), the solvation power of such solvents

(primarily water) towards M1 is poorly described within the

ET30 ENT ) solvent scale. This is likely due to the fact that theprobe used to develop this scale (pyridinium N-phenolate

betaine) [25] is chemically more similar to M2 than to M1. A

better general description could be achieved with proper

multiparameter approaches [25,36], but such correlation anal-

ysis does not fall within the purposes of this work.



15/17

iltonian. As reminded in Sections 1 and 3.1, our

procedure acts prevalently on the p-electron sys-

tem by correction terms linearly related to the

solvation parameter ke (Eq. (1)). Within thisscheme, the modifications of the Fs

ppelements at

high ke values may well account for the effects ofH-bond interactions occurring in the molecular

plane (e.g., those involving the sp2-like oxygen

lone pairs). This interpretation is plausible since it

has been shown [19] that the effects of protic sol-

vents on merocyanine spectra may be described

within p-electron theory using heteroatom pa-

rameters corresponding to the molecule proton-

ated on the oxygen.

A complete validation of the calculated ground

and excited-state dipole moments was not possible

for lack of experimental data. However, using the

solvatochromic comparison method, Banerjee et

al. [27] evaluated the ratio lS1=lS0 to be equal to1.4 for dye M1. Table 5 shows that for M1 the

theoretical prediction is in very good agreement

with experiment (at ke 0, lS1=lS0 1:44).

4. Summary and conclusions

The main purpose of the present paper was toappraise the capability of the solvatonCS INDO

model to arrange both solvatochromism and flu-

orosolvatochromism of merocyanine dyes. In or-

der to carry out an exhaustive test, we searched for

two sample merocyanines characterized by oppo-

site solvent effects. As a prototypic dye exhibiting

large positive solvatochromism on both absorp-

tion and emission, we chose the ketocyanine M1

which has been the subject of extensive experi-

mental work. The stilbazolium betaine M2 was

chosen for the large negative solvatochromism ofits absorption band, which has been widely studied

both experimentally and theoretically. Since, to

our knowledge, the emission properties of M2 had

never been reported before, we carried out an ex-

perimental investigation on the matter. We found

that M2 is rather weakly fluorescent and exhibits

negative fluorosolvatochromism, even if the emis-

sion is much less sensitive to a change of solvent

polarity than the absorption. Moreover, the anal-

ysis of the fluorescence emission and excitation

spectra led us to conclude that, as it happens in the

case of the strongly fluorescent M1, the emission

of M2 originates from the local minimum of the

solvated S1pHpL state.According to this body of experimental evidence,

the theoretical descriptions of solvatochromism and

fluorosolvatochromism were undertaken within

the same conventional scheme, i.e., calculating

the shifts induced by a solvent-polarity change

on the vertical S0eq: ! S1FranckCondon andS1eq: ! S0FranckCondon transitions. Such ascheme was directly applied within the solvatonCS

INDO method. The equilibrium absorbing and

emitting units (molecule + solvaton pattern) and the

corresponding S0!

S1 and S1!

S0 vertical tran-

sitions were studied for M1 and M2 as a function of

a polarity factor formally related to the dielectric

constant, ke ffiffiep 1= ffiffiep. For the sake of sim-plicity, the molecular geometries in the equilibrium

S0 and S1 states were approximated by those opti-

mized in the absence of solvent, and the respective

solvaton patterns, setup using the subset of the net

p-electron charges, were adjusted iteratively start-

ing from those corresponding to the unsolvated

molecules.

The solvent-dependent composition of the res-

onance hybrid between the covalent and the zwit-terionic VB structures was taken as the key to

interpret the calculated ground and excited-state

properties of the two merocyanines at the various

ke values. The analysis of the gas-phase opti-mized geometries in terms of BLA parameter

showed that in the ground state M1 has a markedly

covalent character while in M2 the covalent char-

acter is nearly balanced by the zwitterionic one. On

moving from S0 to S1, the contribution of the

charge-separated structure increases in both dyes,

even if the phenomenon is less marked in M2 thanin M1. Introduction of a polar solvent stabilizes the

zwitterionic forms but, due to the difference in the

starting electronic structures, it results in opposite

solvatochromic behaviours in both absorption and

emission. The use of solvaton sets reflecting the net

p-electron charges proved to be most effective to

rule the evolution of the resonance hybrid com-

position of S0 and S1 as the solvent polarity in-

creases. The description of the solutesolvent

interaction in terms of local charge distributions



16/17

makes the solvaton model free from symmetry

imposed restraints, this being at variance with all

solvaton models (e.g., SCRF) where the solute is

treated as a point dipole. Thus, the fact that M1 hasmoderate S0 and S1 dipole moments perpendicular

to the conjugated chain (the longitudinal compo-

nents being equal to zero by symmetry) while M2

has large dipole moments aligned with the long

molecular axis leads to quite different reaction

fields (and solutesolvent interaction energies)

within the dipolar approximation in spite of the

basic similarities of the two systems. On the con-

trary, the solvaton model provides fully consistent

descriptions for the M1 and M2 solvatochromisms.

A thorough comparison with the experimental

data was made possible by the construction of di-

agrams where calculated and observed frequencies

of the absorption and emission maxima were

plotted against the ke and ENT solvent polarityparameters, both ranging from 0 to 1. Apart from a

systematic overestimation (0.50.7 eV) of the

transition energies the solvatonCS INDO calcu-

lations provided good descriptions of the opposite

solvatochromic behaviours of M1 and M2 in both

absorption and emission. Quantitatively speaking,

the solvent shifts of the absorption and fluores-

cence spectra were correlated better for the posi-tively solvatochromic ketocyanine than for the

negatively solvatochromic stilbazolium betaine,

where the solvatochromic ranges (mnon-polarsolvent mhighly polar solvent) of absorptionand emission were both somewhat underestimated.

However, in very good agreement with experiment

the Stokes shifts (Dmabs fluo) of both M1 andM2 were predicted to substantially increase (espe-

cially in the case of M2) on increasing the solvent

polarity. We notice that, except for the peculiar

behaviour of M1 in water, the solvatonCS INDOmodel is capable of accounting for the solvation

effects on merocyanine spectra over almost the

entire range of the normalized ENT values. This

happens since the polarization of the p-electron

system induced by specific (H-bond) solvent inter-

actions taking place in the molecular plane is im-

plicitly accounted for within our formulation of the

solvaton model.

To sum up, the solvatonCS INDO method

was applied to the study of solvent effects on both

absorption and fluorescence spectra of positively

and negatively solvatochromic merocyanines.

Such a severe test has been passed rather well, at

least as far as the main qualitative aspects areconcerned (but also the sizes of the solvent shifts

were in general well reproduced). An elementary

condition underlying these results is that the elec-

tronic transitions of the solute were generated in

the presence of the polarized solvent. Quite sur-

prisingly, the controversy that arose some years

ago in the interpretation of the solvatochromism

of M2 (see Section 1) came from theoretical

treatments devoid of this prerequisite.

Acknowledgements

This research was jointly supported by the

MURST (Roma) and the University of Modena

and Reggio Emilia within the Programmi di

Ricerca di Interesse Nazionale. We are indebted

to Dr. J.P. Flament for providing us with his ge-

ometry optimization program and to Prof. G.

Berthier for valuable discussions.

References

[1] I. Baraldi, F. Momicchioli, G. Ponterini, D. Vanossi,

Chem. Phys. 238 (1998) 353.

[2] I. Baraldi, F. Momicchioli, G. Ponterini, D. Vanossi, Adv.

Quantum Chem. 36 (1999) 121.

[3] F. Momicchioli, I. Baraldi, M.C. Bruni, Chem. Phys. 82

(1983) 229.

[4] G. Klopman, Chem. Phys. Lett. 1 (1967) 200.

[5] H.A. Germer, Theor. Chim. Acta 34 (1974) 145.

[6] H.A. Germer, Theor. Chim. Acta 35 (1974) 273.

[7] S. Miertus, O. Kysel, Chem. Phys. 21 (1977) 27.

[8] S. Miertus, O. Kysel, Chem. Phys. Lett. 65 (1979) 395.

[9] R. Costanciel, O. Tapia, Theor. Chim. Acta 48 (1978)383.

[10] A. Botrel, A. Le Beuze, P. Jacques, H. Straub, J. Chem.

Soc. Faraday Trans. II 80 (1984) 1235.

[11] A. Botrel, B. Aboad, F. Corre, F. Tonnard, Chem. Phys.

194 (1995) 101.

[12] J. Griffiths, Colour and Constitution of Organic Molecules,

Academic Press, London, 1976.

[13] C. Reichardt, Solvent Effects in Organic Chemistry, Verlag

Chemie, Weinheim, 1979.

[14] J.O. Morley, J. Mol. Struct. (Theochem) 304 (1994) 191.

[15] L. da Silva, C. Machado, M.C. Rezende, J. Chem. Soc.

Perkin Trans. II (1995) 483.



17/17

[16] I.D.A. Albert, T.J. Marks, M.A. Ratner, J. Phys. Chem.

100 (1996) 9714.

[17] A. Klamt, J. Phys. Chem. 100 (1996) 3349.

[18] P. Jacques, J. Phys. Chem. 90 (1986) 5535.

[19] H.G. Benson, J.N. Murrell, J. Chem. Soc. Faraday Trans.II 68 (1972) 137.

[20] L. Salem, Electrons in Chemical Reactions: First Princi-

ples, Wiley, New York, 1982 (Chapter 8).

[21] J. Tomasi, M. Persico, Chem. Rev. 94 (1994) 3027.

[22] J.T. Blair, K. Krogh-Jespersen, R.M. Levy, J. Am. Chem.

Soc. 111 (1989) 6948.

[23] K. Coutinho, S. Canuto, M.C. Zerner, J. Chem. Phys. 112

(2000) 9874.

[24] K.J. de Almeida, K. Coutinho, W.B. de Almeida, W.R.

Rocha, S. Canuto, Phys. Chem. Chem. Phys. 3 (2001)

1583.

[25] C. Reichardt, Chem. Rev. 94 (1994) 2319.

[26] M.A. Kessler, O.S. Wolfbeis, Spectrochim. Acta 47A

(1991) 187.

[27] D. Banerjee, A. Kumar Laha, S. Bagchi, J. Photochem.

Photobiol. A: Chem. 85 (1995) 153.

[28] S.J. Isak, E.M. Eyring, J. Phys. Chem. 96 (1992) 1738.

[29] G.R. Fleming, A.W.E. Knight, J.M. Morris, R.J.S. Mor-

rison, G.W. Robinson, J. Am. Chem. Soc. 99 (1977) 4306.

[30] I. Baraldi, S. Ghelli, Z.A. Krasnaya, F. Momicchioli, A.S.

Tatikolov, D. Vanossi, G. Ponterini, J. Photochem. Pho-

tobiol. A: Chem. 105 (1997) 297.

[31] P. Milliee, F. Momicchioli, D. Vanossi, J. Phys. Chem. B

104 (2000) 9621.[32] I. Baraldi, F. Momicchioli, G. Ponterini, A.S. Tatikolov,

D. Vanossi, Phys. Chem. Chem. Phys. 5 (2003).

[33] J.O. Morley, R.M. Morley, A.L. Fitton, J. Am. Chem. Soc.

120 (1998) 11479.

[34] J. Catalaan, E. Mena, W. Meutermans, J. Elguero, J. Phys.

Chem. 96 (1992) 3615.

[35] C. Burda, M.H. Abdel-Kader, S. Link, M.A. El-Sayed, J.

Am. Chem. Soc. 122 (2000) 6720.

[36] B. CCunderlkovaa, L. SSikurovaa, Chem. Phys. 263 (2001)

415.

[37] R. Fletcher, M.J. Powell, Comput. J. 6 (1963) 163.

[38] K. Ohno, Theor. Chim. Acta 2 (1964) 219;

G. Klopman, J. Am. Chem. Soc. 86 (1964) 4550.

[39] G. Brancolini, Doctorate thesis, University of Modena and

Reggio Emilia, 2002.

[40] D.J.A. De Ridder, D. Heijdenrijk, H. Schenk, R.A.

Dommisse, G.L. Lemieere, J.A. Lepoivre, F.A. Alder-

weireldt, Acta Crystallogr. C 46 (1990) 2197.

[41] S.S. Malhotra, M.C. Whiting, J. Chem. Soc. (1960) 3812.


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