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Molecular Physics, Vol. 104, Nos. 5–7, 10 March–10 April 2006, 943–955

Excited state intramolecular proton transfer

in 1-(trifluoroacetylamino)naphthaquinone:a CASPT2//CASSCF computational studyy

ALESSANDRO CEMBRAN and JIALI GAO*

University of Minnesota, USA

(Received 24 April 2005; in final form 9 August 2005)

The excited state intramolecular proton transfer (ESIPT) in 1-(trifluoroacetylamino)-

naphthaquinone (TFNQ) has been investigated using the CASSCF and CASPT2 computa-

tional approaches with the 6-31G(d) basis set. The structures and relative energies of critical

points along the proton transfer reaction coordinate were optimized and the associated

spectroscopic and electrostatic properties obtained. Combined quantum mechanical and

molecular mechanical (QM/MM) Monte Carlo simulations were performed to elucidate

solvent effects on the vertical excitation S0!S1. It was found that the ESIPT reaction is a

barrierless process that takes place on a very flat potential energy surface (PES) and the

tautomeric structure of the reaction product is the only minimum on the excited state surface.

The PES for both the ground and excited state from accurate electronic structure calcualtions

will be used to parameterize empirical force fields in subsequent molecular dynamics

simulations of the reaction in solution.

1. Introduction

Excited state intramolecular proton transfer (ESIPT)

reactions were observed by Weller [1] in methyl salicy-

late and later by Kasha [2] in 3-hydroxyflavone. These

reactions have been a subject of great interest both

experimentally [3–9] and theoretically [10–13] thanks to

the potential applications in lasers [14], energy and

data storage devices, optical switches [14–20], Raman

filters [21], polymers photostabilizers [22], triplet

quenchers [23, 24] and LED materials [24]. In many

cases, the donor atom (to which the hydrogen is initially

bonded) is an oxygen atom and the reaction is driven

by the redistribution of the charge density to the

proton acceptor. In 1-aminoanthraquinones, however,

the donor group consists of a nitrogen atom, forming

an intramolecular hydrogen bond with the acceptor

quinonoid oxygen [3]. The compound is forced to be

planar by the rigidity of the anthraquinone framework.Smith et al. [3] studied the excited state properties

of the four 1-(acylamino)anthraquinones derivatives

shown in figure 1 by static fluorescence, time-resolved

emission and transient absorption experiments. After

photoexcitation to the spectroscopic �!�* excited

state, two fluorescence bands are observed, providingevidence for the existence of two different emitting

tautomeric structures. The short wavelength emission

(SWE), centred around 510 nm (235 kJmol�1), hasbeen attributed to the ‘normal’ structure N, in which

the hydrogen is bonded to the nitrogen. The long

wavelength emission (LWE), centered around 630 nm

(190 kJmol�1), is from the tautomeric form T, wherethe proton is transferred to the oxygen. The relative

intensities of these two bands depend on the sub-

stituents, in particular HPAQ displays mostly SWE,while only LWE is observed for TFAQ. This means that

electron-withdrawing groups favour the ESIPT by

shifting the equilibrium towards the T structure.CAAQ and DCAQ show both bands and their relative

intensity ratio is influenced by the solvent. In particular,

polar solvents yield strong SWE bands with respect tothat of the LWE, by greater stabilization of the N

structure than the T configuration. The roles played

by substituents on the two structures are reflectedin the ESIPT reaction rate. While for TFAQ, the

ESIPT is nearly instantaneous, a rate constant of

110 fs was observed for compounds with a lesselectron-withdrawing substituent like CAAQ [25].

*Corresponding author. Email: [email protected]

yThis paper is dedicated to Professor Michael Robb on the occasion

of his 60th birthday.

Molecular PhysicsISSN 0026–8976 print/ISSN 1362–3028 online � 2006 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/00268970500417556

In the other extreme, HPAQ does not undergo fastproton transfer (PT). The lifetime of these moleculesranges from 80 to 700 ps. In connection with the rateconstant of intersystem crossing kisc for 1-acetylaminoanthraquinone [26], which is 1.43�109 s�1,of similar time scale to the slow ESIPT reactions, itsuggests that the decay to the ground state takes placeby intersystem crossing.Using Raman spectrscopic techniques, Blank

et al. [27] investigated the ESIPT reaction by probingthe solvent (acetonitrile) response directly. Their resultsconfirmed that HPAQ does not undergo ESIPT, whileTFAQ proceeds with a ‘ballistic’ and complete ESIPT.Interestingly, they found that initially CAAQ undergoesfast ESIPT, but the reaction returns to the N structurein about 1 ps, suggesting that the potential energysurface of the excited state evolves as the solventreorganizes in response to the charge distributionof the excited state species.In association with this work, we seek to understand

the mechanism of the ESIPT process in 1-(acylamino)anthraquinones, TFAQ is the most suitable derivativeto be examined thanks to its fast and complete PTmechanism. In the following, we first summarize thecomputational details employed in the present study.Due to the computational cost of the CASPT2//CASSCF approach [28, 29], we will show in the thirdsection that the smaller model compound 1, lacking oneof the aromatic rings (see figure 2) is a reasonablesimplification of the anthracene system. Then, wepresent the computational results and discussionin the third section. We conclude this paper witha summary of the major findings of this work.

2. Computational details

2.1. Electronic structure calculations

The ground state S0 and first excited state S1 potential

energy surfaces (PESs) for the ESIPT process have beendetermined by full optimizations at the CASSCF/6-31G(d) level, which is followed by single-pointCASPT2 computations on the optimized structures.

All the optimizations have been performed with singlestate calculations, with the exception of MT –S1

(see figures 2 and 3) for which a state averagewavefunction with equal weights for S0 and S1 has

been used. The 6-31G(d) basis set is used throughout.All computations have been carried out using theGaussian 98 [30] and the MOLCAS-5.2 [31] quantum

chemistry programs.In order to apply the CASPT2//CASSCF approach

using an adequate number of orbitals, it is necessary toreduce the dimension of the system. The HOMO andLUMO orbitals involved in the �!�* excitation are

located, respectively, in the amide-bearing ring andin the quinone ring and no important orbitals are foundin the third aromatic ring for the reaction studied here.Therefore, it is a reasonable approximation to remove

this fragment and use the reduced model compound 1,1-(trifluoroacetylamino)-naphthaquinone (TFNQ).

The selection of the active space is a crucial stepin CASSCF calculations. In this system, there are 16�electrons in 15� orbitals that can be potentially included

into the active space. In addition, it might be necessaryto include the � orbitals for the N–H bond and the lonepair on the oxygen to which the proton is transferred.In all, the target active space could be as large as

20 electrons in 17 orbitals, which is too large to befeasible computationally. By examining the occupanciesof different orbitals in various tests, we found

that a balanced, smaller active space that includes

RHPAQ n-C6H13CAAQ CH2ClDCAQ CHCl2TFAQ CF3

N T

N

O

OH

O

R

N

O

OH

O

R

Figure 1. Normal (N) and tautormer (T) structures forthe four different 1-(acylamino)anthraquinones of reference [3]1-(heptanoylamino)anthraquinone (HPAQ), 1-(chloro-acetylamino)anthraquinone (CAAQ), 1-(dichloroacety-lamino)anthraquinone (DCAQ) and 1-(trifluoroacetylamino)anthraquinone (TFAQ).

N

O

OH

O

CF3

1

12

3445

6

78 8a

1

2

Figure 2. 1-(Trifluoroacetylamino)naphthaquinone (TFNA),1, used in all the calculations as a reduced model of TFAQ.

944 A. Cembran and J. Gao

10 electrons and 10 orbitals (10e/10o) is adequate forthe proton transfer process in CASSCF optimizations.The optimized geometries are further refined by singlepoint (12e/12o) state averaged (equal weight for S0and S1) CASSCF calculations, followed by perturbationcorrections at the CASPT2 level. Interestingly, the two �orbitals are not essential in these calculations for thereaction studied here since even in the transitionstate region their occupation numbers are very closeto two.For each optimized structure, the dipole moment

and partial atomic charges are determined using theelectrostatic potential (ESP) fitting scheme at theCASSCF(12e/12o) and the CASSCF(10e/10o) level.To assess the efficiency and probability of radiativetransition between S0 and S1, the RASSI [32, 33]approach is used to compute the oscillator strength f

of the vertical transitions for the optimized structures.

2.2. Combined QM/MM Monte Carlo simulations

In order to evaluate solvent effects on the S0!S1transition energy, statistical Monte Carlo simulations

are carried out using a combined quantum mechanical

and molecular mechanical (QM/MM) potential for 1

in tetragonal boxes of different solvents, including

CH2Cl2 [34], CH3CN [35] and TIP4P [36] H2O. The

boxes dimensions are approximately 30� 30� 46 A3

for CH2Cl2, 29� 29� 43 A3 for CH3CN and 20� 20�

30 A3 for H2O, containing a total of 380 solvent

molecules in each system. The solute molecule electronic

structure in the ground and first excited states is

described by a configuration-interaction with single

excitations (CIS) within an active space of six doubly

occupied and six unoccupied molecular orbital obtained

from the semiempirical Austin Model 1 (AM1) wave-

function [37, 38]. All Monte Carlo simulations are

executed in an isothermal-isobaric (NPT) ensemble at

25�C and 1 atm. A spherical cutoff distance of 12 A is

used for both solvent–solvent and solute–solvent inter-

actions based on solute heavy atom and water oxygen

distances. The van der Waals parameters used for the

solute molecule are taken from reference [39]. In all

simulations, the molecular geometry of 1 is kept fixed

at the configuration optimized at the HF/6-31G* level

in the gas phase.

S0

Reaction coordinate (RNH–ROH) (Å)

−1 −0.5 0 0.5 1

Ene

rgy

(kJ

mol

−1)

MN-S0

TS–S1

MT–S0

MT–S1

MN–S1

TS–S0

0

100

200

300

400

323.50.080

268.40.115

135.20.141

187.20.176

S1

O N ONH

N T

MN–S0

0

100

200

300

400

323.50.080

268.40.115

135.20.141

187.20.176

H

Figure 3. S0 and S1 PES for the proton transfer reaction in 1 along the reaction coordinate defined as the difference between theN–H and O–H bond distances. All the structures are optimized at the CASSCF[10e,10o]/6-31G(d). The CASSCF[12e,12o]/6-31G(d)energy profiles are reported with dashed lines while the CASPT2 corrected profiles are shown with solid lines. In dashedarrows the absorption from the S0 minimum and the fluorescence energies from S1 are reported together with the oscillatorstrength value in italic.

Excited state intramolecular proton transfer in 1-(trifluoroacetylamino)anthraquinone 945

In each case, 106 configurations of Monte Carlosimulations were performed to equilibrate the system,which was followed by additional 106 configurationsfor data averaging. New configurations were generatedby randomly translating a randomly selected moleculein the three Cartesian directions and rotating it arounda randomly chosen axis. To facilitate convergenceof solvent near the solute molecular, Owicki–Scheraga[40, 41] preferential sampling technique was usedsuch that solvent moves are made proportional to1/(R2

þW), where W¼ 250 A2 for CH2Cl2 andW¼ 200 A2 for CH3CN and H2O. Volume moveswere attempted on every 2375 configurations with amaximum allowed variation of �585 A3, and the solutemolecule was moved in every 90 configurations.Different ranges for the rotation and translations wereused for the different solvents and for the solute: forthe solvent molecules the ranges are from �0.15to �0.25 A and from � 15� to �23�, while for thesolute they span from �0.10 to �0.13 A and from �7�

to �10�. These options yielded an acceptance rateof about 45% on all attempted moves. All simulationswere performed using the MCQUM [42] programinterfaced with the MOPAC [43] package.

3. Results and discussion

3.1. Validation of the model compound

In order to check if compound 1 is a good model forthe larger system, both molecules were optimized at theHF/6-31G(d) level in the ground state and verticalexcitations energies were computed at the semiempiricalZINDO and CIS/6-31G(d) levels.The transition energies for 1 at the ZINDO

and CIS level are, respectively, 356.9 kJmol�1 and478.6 kJmol�1, in reasonable agreement with valuesof 369.4 kJmol�1 and 484.9 kJmol�1 obtained at thesame levels for TFAQ. These results confirm thatthe third aromatic ring has little participation in the�!�* transition and that the reduced model 1, TFNQ,can be used to approximate the electronic structuresof TFAQ used in experiments.

3.2. CASPT2//CASSCF study

We have determined the S0 and S1 energy profiles alongthe proton transfer reaction coordinate at the CASSCFand CASPT2 levels of theory using the 6-31G(d) basisset, which are shown in figure 3. The optimizedstructures are depicted in figure 4 along with keygeometrical parameters, and table 1 summarizes theabsolute and relative energies computed using thesemethods.

At the CASSCF level, following excitation fromthe ground state minimum (MN –S0) to the ��*state S1, the system relaxes to a minimum (MN –S1) inwhich the proton is still bonded to the nitrogen atom.A transition state (TS –S1) connecting this minimumto the tautomeric structure (MT –S1) was located, whichhas a barrier height of 4.5 kJmol�1. The tautomerminimum lies about 41 kJmol�1 below the Franck–Condon state. Accompanying the proton transferreaction, geometrical changes are found in a numberof bond distances, particularly those involving atomsdirectly associated with the proton transfer reaction.While in the ground state, the proton is bonded to theamide nitrogen with a bond distance of 0.998 A, it formsa covalent bond with the oxygen atom at a distanceof 0.977 A in the excited state tautomer structureMT –S1. In the transition state structure, by slightlyreducing the bending angles, the nitrogen and oxygenatoms are closer than that in the equilibrium statesto minimize the bond distances with the proton. Thiscooperative process assists the PT reaction by reducingits barrier with donor–acceptor geometrical distortions.

In going from the ground state MN –S0 geometry tothe excited state product form MT –S1, the N–C bondshortens from 1.40 to 1.33 A and the O–C bondlengthens from 1.20 to 1.30 A, indicating alterationin single and double bond characters. Although the�-electrons are fully delocalized, there are no detectablechanges in single and double bonding characters overthe aromatic rings.

We also constructed the ground state adiabaticpotential energy profile for the proton transfer from 1

(MN –S0) to the tautomer structure MT –S0, which is alocal minimum at the CASSCF level. The barrier at thetransition state, TS –S0, separating these two structuresis 89.4 kJmol�1 from MN –S0 to MT –S0.

When dynamic correlation corrections are includedat the CASPT2 level, both the ground and excitedstate potential energy surfaces change dramatically.First, figure 3 shows that the entire S1 PES is shifteddownward by 80 kJmol�1, relative to the globalminimum of the ground state, MN –S0. Furthermore,electron correlation effects affect the Frank–Condonstate MN –S1 and the transition state TS –S1 more thanthe charge (proton) transfer state MT –S1. The S1 PEShas flattened out, resulting in a potential surface thatis barrierless for the proton transfer from theFrank–Condon region to the final product. Thus, boththe MN –S1 and TS –S1 structures are no longerstationary points at the CASPT2 level of theory, andthe single minimum structure is the proton transferproduct state, MT –S1, just 2.5 kJmol�1 below theenergy at the fixed MN –S1 configuration. The groundstate PES has also undergone major changes. The main


TS–S1

TS–S0

0.998

1.860

1.4041.2011.495

1.415

1.3591.199

1.532

1.306

1.322

1.405

1.389

1.392

1.390

1.415

1.498

1.1941.482

1.341

1.486

1.754

1.0121.364

1.374

1.197

1.531

1.319

1.306

1.2321.448

1.461

1.426

1.409

1.390

1.449

1.413

1.481

1.2041.462

1.352

1.465

1.272

1.1941.331

1.454

1.481

1.275

1.3741.200

1.529

1.308

1.319

1.421

1.424

1.366

1.462

1.370

1.507

1.200

1.455

1.373

1.418

0.977

1.7281.331

1.3551.208

1.530

1.312

1.324

1.447

1.414

1.436

1.357

1.453

1.381

1.4751.303

1.421

1.363

1.4641.194

1.513

0.965

1.7941.296

1.3791.201

1.529

1.310

1.321

1.469

1.3771.314

1.464

1.356

1.4441.357

1.463

1.467

1.345

1.478

1.500

1.196

1.021

1.554

1.355

1.205

1.342

1.531

1.311

1.324

1.425

1.4151.266

1.442

1.369

1.423

1.367

1.441

1.492

1.193

1.488

1.344

1.468

MN–S1MN–S0

MT–S1 MT–S0

Figure 4. Structures for 1-(trifluoroacetylamino)naphthaquinone, 1, optimized at the CASSCF[10e,10o]/6-31G(d). Bond distancesare reported in angstroms.

Table 1. Computed absolute and relative energies for 1-(trifluoroacetylamino)naphthaquinone, 1, at theCASSCF[10e,10o]/6-31G(d) and CASPT2[12e,12o]/6-31G(d) levels of theory. State-averaged optimizations (equal weightsfor S0 and S1) have been used in the CASPT2 calculations and the weight ! for the zeroth-order CASSCF wavefunctionis reported to show the reliability of the perturbation correction. The computed oscillator strengths f for the radiative

transition between S0 and S1 are also given for selected points.

Structure State CASSCF (au) CASPT2 (au) ! F �E (kJmol�1)

MN –S0 S0 �1035.41752 �1037.94696 0.57 0.080 0.0S1 �1035.26839 �1037.82372 0.54 323.5

TS –S0 S0 �1035.38258 �1037.91292 0.57 89.4S1 �1035.27655 �1037.83614 0.56 290.9

MT –S0 S0 �1035.39036 �1037.90528 0.57 109.4

S1 �1035.26264 �1037.81916 0.56 335.5MN –S1 S0 �1035.40575 �1037.94460 0.57 0.115 6.2

S1 �1035.28109 �1037.84238 0.55 274.6

TS –S1 S0 �1035.36373 �1037.91771 0.56 0.176 76.8S1 �1035.27937 �1037.84640 0.55 264.0

MT –S1 S0 �1035.36436 �1037.89782 0.56 0.141 129.0

S1 �1035.29674 �1037.84633 0.56 264.2


effect is that the minimum on the tautomer side

disappears, resulting in an energy profile that directly

leads to the ground state minimum at MN –S0 without

transition state.Our best estimate of the S0!S1 transition energy

is 323.5 kJmol�1 (3.35 eV) at the CASPT2(12,12)/

6-31G(d)//CASCF(10,10)/6-31G(d) level. This is in

good agreement with the experimental value of

310.0 kJmol�1 (3.21 eV) for TFAQ in cyclohexane,

since studies [44–46] have shown that there is essentially

no spectral shift in going from the gas phase to non-

polar organic solvents [3]. The computed oscillator

strength f is 0.080, which is consistent with a molar

extinction coefficient of 6070Lmol�1 cm�1 determined

experimentally in hexaney.The CASPT2 potential energy surfaces are

consistent with the experimental finding of fast

proton transfer following photoexcitation to the S1Franck–Condon state, which is about 60 kJmol�1

higher than the tautomeric structure. Further, there

is no barrier separating the Franck–Condon region

and the proton transfer product. For comparison, the

excited state intramolecular proton transfer reaction

of TFAQ is an ultrafast process, with an upper

(instrumentation) limit of 80 fs.The experimental fluorescence maximum is centered

at 188.4 kJmol�1 in cyclohexane [3]. Since the PES

in the TS –S1/MT –S1 region is flat, it is not possible

to attribute the decay to a specific stable structure

on the excited state surface. It is interesting to notice

that the vertical emission energy from the TS –S1

structure to the ground state is in perfect agreement

with the experimental value; however, the fluorescence

transition can originate from any location along the

proton transfer coordinate between TS –S1 andMT –S1.

The smallest energy gap between the S1 and S0 states

is 135 kJmol�1, which basically rules out the possibility

of finding a low-energy conical intersection between

these two states that could provide a mechanism

for fast decay. This finding is in agreement with the

experimental data in that the observed lifetime of TFAQ

is over 100 ps and the decay takes place by intersystemcrossing [26].

To gain insights into the charge transfer process,we monitored the variation of atomic partial chargesdetermined by fitting the quantum mechanical electro-static potentials (ESP) (see supporting information),along the reaction coordinate. Although the quantitativeresults depend on the specific method used in derivingthese charges, the qualitative trends are reasonable usingthe ESP charges. It can be seen that the S0!S1transition involves a gradual charge migration fromthe aromatic ring to which the amide group is attachedto the quinonoid moiety. The net charge transferis �0.35 e from the ESP procedure. Since the dominantfactor in the �!�* transition is charge transfer fromthe HOMO into the LUMO of the compound, thisobservation is consistent with the fact that the formerorbital is primarily localized on the aromatic ring,whereas the LUMO is largely on the quinone ring.The negative charge continues flowing to the quinonering along the reaction coordinate, so that at the TS –S1

structure the quinone moiety gains about �0.60 e.Concomitant to the electronic charge transfer, theproton is also transferred from the amide nitrogenon to the quinone oxygen. These two compensatingresult in a net charge transfer of �0.27 e to the quinonesystem in the MT –S1 structure.

The charge shift upon photoexcitation is reflected bya change in the dipole moment both in magnitude andin its directionality as shown in table 2. The S0!S1

Table 2. The CASSCF/6-31G* (12e/12o) dipole momentsvectors and their modules (Debye units) are reported forboth S0 and S1. The structures to which they refer can befound in the supporting information. The XY plane is the

mirror plane in the CS symmetry group to which thismolecule belongs, therefore the dipole moment componentalong Z is always zero. Since the orientation of the moleculedoes not change along the energy profile, it is meaningfulto report the angle of twisting � of the dipole moment with

respect the ground state of the MN –S0 structure.

Structure State X Y Total �

MN –S0 S0 2.61 �0.41 2.64 0.0

S1 0.14 1.62 1.63 94.1TS –S0 S0 2.82 �0.05 2.82 8.1

S1 2.91 0.04 2.91 9.8MT –S0 S0 1.34 0.48 1.42 28.8

S1 3.48 �0.03 3.48 8.6MN –S1 S0 2.51 �0.47 2.56 1.6

S1 �0.71 2.48 2.58 115.0

TS –S1 S0 3.87 �1.27 4.08 �9.1S1 0.01 2.81 2.81 98.9

MT –S1 S0 2.59 �0.06 2.59 7.6

S1 4.50 �0.58 4.54 1.7

yFrom reference [47], for a broadband excitation, the oscillator

strength f can be related to the molar extinction coefficient " by the

formula:

f ¼ 4:33� 10�9

Z"d ~v

in which the integral runs over the band shape and ~v is the frequency in

cm�1. From reference [3] we can estimate an absorption band width of

8000 cm�1. By describing the band as a Gaussian function

" ~�ð Þ ¼ 6070 expð�� ~�2Þ with an � value of 3� 10�7 (which makes the

function decay to zero around �4000 cm�1) and integrating, it turns

out that f should be around 0.085.


excitation leads to a 90� rotation in the direction ofthe molecular dipole moment, which lies in the planeof the conjugated ring system. The magnitude of thedipole moment is slightly reduced in the Franck–Condon region. It is interesting to note that afterthe proton transfer is complete in the MT –S1 structure,the excited state (S1) dipole is again aligned in thesame direction as on the ground state minimumMN –S0. However, the magnitude of the dipole isgreatly increased, a reflection of the charge transfereffect noted above.It is interesting to examine solvation effects on

the excited state proton transfer reaction. Since thetimescale required for solvent reorganization is muchgreater than the ESIPT process (a few picoseconds vs.less than 80 fs), the Franck–Condon state will not besignificantly stabilized by the slow solvent response,which is further exacerbated by a tilt in dipole orien-tation. Remarkably, the equilibrium solvent configura-tion that stabilizes the ground state species is perfectlysuited for stabilizing the proton transfer productin the excited state both because of an enhanced dipolemoment of the chromophore molecule and the restora-tion of the dipole orientation to the initial groundstate direction. This suggests that in solution theMT –S1 structure is immediately (in less than 80 fs)stabilized by solvent configuration that is in equilibriumwith the donor state in the ground state electronicsurface. Furthermore, even over a longer timescale,the excited state MT –S1 structure will be the moststabilized configuration by solvation, rather than othergeometries, along the S1 proton transfer coordinatebecause the magnitude of the dipole moment is thelargest. Therefore, it can be inferred that in the solvatedsystem, the S1 PES would show a more pronouncedwell in the MT –S1 region than the system in gas phase.

3.3. Solvatochromic shifts

The computed solvatochromic spectral shifts for 1

in various solvents are listed in table 1. The S0!S1

(�!�*) energy difference is computed to be343.0 kJmol�1 from a configuration interaction calcu-lation that includes single excitations only using thesemiempirical Austin Model 1 (AM1) model. Thesecalculations utilized the HF/6-31G(d) optimized struc-ture and six electrons and six active orbitals are includedin the CIS calculation. Despite the rough approxima-tion, the AM1 value is in good accord with that(356.9 kJmol�1) obtained using the ZINDO method,which is also a CIS procedure. For comparison, theCASPT2 vertical excitation energy is 323.5 kJmol�1

using the structure optimized at the CASSCF level.Thus, it is suggested that it is reasonable to use thesemiempical CIS method, coupled with statisticalQM/MM Monte Carlo simulations, to access solventeffects on the excitation energies. The computedtransition energy is in good agreement also with theexperimental data of 310.0 kJmol�1 in cyclohexanefor TFAQ [3].

Experimentally, Barbara et al. [3] observed a verysmall blue shift of the absorption band for TFAQin going from non-polar to polar aprotic solvents(6.6 kJmol�1 from cyclohexane to acetonitrile). Thecomputed spectral shift in going from the gas phaseto water from QM-CIS/MM simulations is less than2 kJmol�1, in reasonable accord with experiment.The results presented here confirm that solvent effectson the absorption maximum for TFAQ is almostnegligible. We attribute the small spectral shift to thesmall change in dipole moment (less than 1 Debye).Although the change in dipole orientation in the excitedstate is about 90�, it is not sufficient to cause significantdestabilization to the excited state solvation relativeto that in the ground state.

We also recorded the radial distribution functionbetween the transferred proton with the oxygen atomof water (see supporting information). There are nopeaks that can be identified as hydrogen-bond interac-tions, indicating that hydrogen atom is fully utilizedin forming an intramolecular hydrogen bond. Thisobservation supports the picture that the protontransfer is primarily an intramolecular process, inwhich the solvent does not actively participate andconfirms that the reaction path obtained in the gasphase is a reasonable model for the proton transferprocess in solution.

4. Conclusions

The excited state intramolecular proton transfer reactionof a model compound for TFAQ on the spectroscopic��* state S1 has been studied in gas phase at theCASPT2//CASSCF level. The results show that the

Table 3. Solvatochromic shifts for TFNQ, 1. The S0–S1(�!�*) transition energies are given in kJmol�1 together

with statistical errors. Experimental values are listedwhere available.

Solvent QM/MM Exp.

Gas Phase 343.0 310.0a

CH2Cl2 344.5� 0.7 312.4CH3CN 343.8� 0.5 316.6

H2O 344.6� 0.4 n.a.

a Solvent cyclohexane.


reaction is a barrierless process in the excited state,

consistent with the experimental results that the proton

transfer takes place in less than 80 fs. While an extended

energy plateau in the S1 state has been found inthe gas phase along the proton transfer coordinate,

supporting the view that the proton is weakly bound

to neither the acceptor nor the donor atoms and can

undergo large amplitude oscillations, solvent effects can

stabilize the proton-transfer tautomer due to its larger

dipole moment along the excited state potentialenergy surface.The computed excitation energy is in good agreement

with the experimental data. Furthermore, the large

energy gap between the S1 and S0 state along the

entire reaction coordinate excludes the possibility of

a fast decay through low-energy conical intersections.A comparison of the gas phase computed S1/S0 energy

gaps with the experimental fluorescence maximum

suggests that the radiative decay could take place from

a structure in which the O–H bond is stretched and

the hydrogen is about halfway between the nitrogen

and oxygen atoms.Statistical Monte Carlo QM/MM (AM1-CIS) simu-

lations have been performed to investigate the effect

of various solvents on the absorption maximum. It is

found that even in a polar protic solvent like water the

spectral shift is still very small (less than 2 kJmol�1).

This is consistent with the experimental observationof little solvatochromic spectral shifts of TFAQ from

non-polar to polar solvents. In all, the present quantum

chemical analysis of the ESIPT reaction provided

a framework to construct a molecular mechanics

force field that can be used in dynamics simulations to

investigate the mutual effects of solute proton transferand solvent reorganization dynamics in the excited state.

Acknowledgements

The authors thank the National Institutes of Health

for partial support of this research.

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Supporting Information:

Excited State Intramolecular Proton Transfer in

1-(trifluoroacetylamino)anthraquinone: a CASPT2//CASSCF

Computational Study

ALESSANDRO CEMBRAN and JIALI GAO

Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA

The geometries for the optimized structures are reportedtogether with the ESP charges for S0 and S1 for each structure.

Geometries (Cartesian coordinates in angstroms):

MN –S0

1 H1 6.203831 5.590033 0.0000002 H2 1.946793 7.594354 0.0000003 H3 �1.853402 4.972858 0.000000

4 H4 �1.694711 �1.936280 0.0000005 H5 5.622437 �5.558546 0.0000006 H6 9.446509 �2.791984 0.000000

7 C1 0.133661 1.444091 0.0000008 C2 2.547508 0.293964 0.0000009 C3 4.723073 1.848855 0.00000010 C4 4.518034 4.467536 0.000000

11 C5 2.130499 5.572617 0.00000012 C6 �0.037911 4.093917 0.00000013 C7 2.891388 �2.509929 0.000000

14 C8 7.330234 0.743600 0.00000015 C9 �6.302200 �1.620836 0.00000016 C10 5.505952 �3.533064 0.000000

17 C11 7.559170 �2.047532 0.00000018 C12 �4.500113 0.645487 0.00000019 N1 �2.037780 �0.081413 0.00000020 O1 1.163463 �3.981928 0.000000

21 O2 9.172993 2.045015 0.00000022 O3 �5.344617 2.747284 0.00000023 F1 �5.912238 �3.046311 2.014261

24 F1 �5.912238 �3.046311 �2.01426125 F2 �8.657550 �0.883663 0.000000

TS –S0

1 H1 6.469052 5.087260 0.000000

2 H2 2.375711 7.419652 0.0000003 H3 �1.637933 5.178167 0.0000004 H4 �1.063921 �2.746972 0.000000

5 H5 4.697889 �5.952609 0.0000006 H6 8.831859 �3.675031 0.0000007 C1 0.000000 1.424560 0.0000008 C2 2.359761 0.126534 0.000000

9 C3 4.715087 1.494004 0.00000010 C4 4.712980 4.076619 0.00000011 C5 2.365334 5.388779 0.000000

12 C6 0.096028 4.147327 0.00000013 C7 2.429887 �2.546253 0.000000

14 C8 7.174229 0.112854 0.00000015 C9 �6.421657 �1.227111 0.000000

16 C10 4.833770 �3.929239 0.00000017 C11 7.055137 �2.696858 0.00000018 C12 �4.488029 0.924804 0.00000019 N1 �2.099291 0.001604 0.000000

20 O1 0.469860 �3.918307 0.00000021 O2 9.168265 1.166569 0.00000022 O3 �5.248827 3.071759 0.000000

23 F1 �6.131129 �2.683417 2.01427224 F1 �6.131129 �2.683417 �2.01427225 F2 �8.747426 �0.372941 0.000000

MT –S0

1 H1 6.679868 4.962066 0.0000002 H2 2.619699 7.375935 0.0000003 H3 �1.428516 5.235363 0.000000

4 H4 �0.891572 �3.117352 0.0000005 H5 4.877336 �5.980888 0.0000006 H6 9.021301 �3.751672 0.0000007 C1 0.114922 1.406975 0.000000

8 C2 2.512793 0.009437 0.0000009 C3 4.901055 1.402966 0.00000010 C4 4.915025 3.967398 0.000000

11 C5 2.559105 5.346189 0.00000012 C6 0.282788 4.169386 0.00000013 C7 2.600375 �2.590952 0.000000

14 C8 7.375482 0.033130 0.00000015 C9 �6.334051 �1.018029 0.00000016 C10 5.012888 �3.956992 0.00000017 C11 7.253785 �2.757531 0.000000

18 C12 �4.365153 1.096987 0.00000019 N1 �1.956098 0.101175 0.00000020 O1 0.636898 �4.112517 0.000000

21 O2 9.352829 1.126785 0.00000022 O3 �5.063453 3.256352 0.00000023 F1 �6.060714 �2.466052 2.016086

24 F1 �6.060714 �2.466052 �2.01608625 F2 �8.637875 �0.111125 0.000000

MN –S1

1 H1 6.215668 5.575026 0.000000

2 H2 1.986762 7.690806 0.000000

(continued)


Geometries (Cartesian coordinates in angstroms):

3 H3 �1.884014 5.041029 0.000000

4 H4 �1.555507 �1.893154 0.0000005 H5 5.540042 �5.536904 0.0000006 H6 9.370571 �2.822610 0.000000

7 C1 0.107148 1.487178 0.0000008 C2 2.556866 0.211961 0.0000009 C3 4.719046 1.778595 0.00000010 C4 4.496577 4.507769 0.000000

11 C5 2.140974 5.670158 0.00000012 C6 �0.062986 4.176639 0.00000013 C7 2.828607 �2.511504 0.000000

14 C8 7.309307 0.716504 0.00000015 C9 �6.236522 �1.661545 0.00000016 C10 5.409817 �3.511896 0.000000

17 C11 7.498875 �2.040282 0.00000018 C12 �4.487177 0.641822 0.00000019 N1 �1.979029 �0.027601 0.00000020 O1 1.020246 �3.978010 0.000000

21 O2 9.128078 2.085077 0.00000022 O3 �5.347667 2.733479 0.00000023 F1 �5.811925 �3.066008 2.015285

24 F1 �5.811925 �3.066008 �2.01528525 F2 �8.602453 �0.961209 0.000000

TS –S1

1 H1 6.333672 5.297023 0.000000

2 H2 2.222280 7.602362 0.0000003 H3 �1.784599 5.176727 0.0000004 H4 �1.279488 �2.117597 0.000000

5 H5 4.869754 �5.837779 0.0000006 H6 8.910786 �3.400681 0.0000007 C1 �0.009212 1.515972 0.000000

8 C2 2.395313 0.184968 0.0000009 C3 4.582837 1.568155 0.00000010 C4 4.555908 4.331732 0.000000

11 C5 2.294528 5.575508 0.00000012 C6 �0.018187 4.201033 0.00000013 C7 2.477854 �2.611752 0.000000

14 C8 7.116799 0.268010 0.00000015 C9 �6.320999 �1.504424 0.00000016 C10 4.874658 �3.808731 0.000000

17 C11 7.104480 �2.481140 0.00000018 C12 �4.515003 0.749964 0.00000019 N1 �2.025979 0.012363 0.000000

20 O1 0.402676 �3.835902 0.00000021 O2 8.999710 1.532689 0.00000022 O3 �5.326104 2.867684 0.00000023 F1 �5.937007 �2.918096 2.016120

24 F1 �5.937007 �2.918096 �2.01612025 F2 �8.674281 �0.748042 0.000000

MT –S1

1 H1 6.721370 5.003915 0.0000002 H2 2.698763 7.462094 0.0000003 H3 �1.390638 5.209048 0.000000

4 H4 �0.882624 �3.007279 0.0000005 H5 4.874778 �6.033710 0.0000006 H6 9.022620 �3.725843 0.000000

7 C1 0.154532 1.470068 0.0000008 C2 2.511512 0.082485 0.0000009 C3 4.778051 1.377707 0.000000

10 C4 4.902661 4.120671 0.00000011 C5 2.702425 5.432901 0.00000012 C6 0.317478 4.137198 0.00000013 C7 2.630342 �2.701876 0.000000

14 C8 7.282810 �0.000668 0.00000015 C9 �6.299785 �1.038208 0.00000016 C10 4.976582 �4.008199 0.000000

17 C11 7.238269 �2.766488 0.00000018 C12 �4.310413 1.058798 0.00000019 N1 �1.943399 0.081212 0.000000

20 O1 0.605229 �4.101527 0.00000021 O2 9.200285 1.190134 0.00000022 O3 �5.011375 3.231801 0.000000

23 F1 �6.050148 �2.499167 2.01488224 F1 �6.050148 �2.499167 �2.01488225 F2 �8.600960 �0.116908 0.000000


MN –S0 TS –S0 MT –S0 MN –S1 TS –S1 MT –S1

ESP charges:S01 H 0.167511 0.158320 0.156043 0.178809 0.187828 0.1759582 H 0.158224 0.142611 0.145159 0.164945 0.154536 0.1494413 H 0.230088 0.239035 0.245932 0.231392 0.235399 0.241965

4 H 0.371946 0.457685 0.445797 0.372356 0.452239 0.4522815 H 0.185219 0.200038 0.196429 0.179196 0.183931 0.2044826 H 0.169832 0.179706 0.181640 0.168287 0.173586 0.1870947 C 0.567965 0.962558 1.021366 0.537045 0.718590 0.946600

8 C �0.461321 �0.657902 �0.626332 �0.461959 �0.585516 �0.6186979 C �0.002650 0.018135 0.003465 0.021670 0.080588 0.06110010 C �0.170269 �0.167234 �0.151753 �0.211536 �0.261468 �0.188315

11 C �0.108656 �0.009027 �0.017167 �0.096513 �0.038686 �0.03388412 C �0.306797 �0.465232 �0.499360 �0.312588 �0.397392 �0.45921913 C 0.837721 0.721835 0.609620 0.833557 0.835639 0.649128

14 C 0.656594 0.667137 0.670355 0.651804 0.643964 0.64293115 C 0.599355 0.581605 0.619205 0.610489 0.607978 0.63633616 C �0.315490 �0.260205 �0.220337 �0.294708 �0.270486 �0.258965

17 C �0.206607 �0.247482 �0.279523 �0.205656 �0.194096 �0.26039218 C 0.643909 0.734982 0.729535 0.621312 0.654985 0.70048119 N �0.766447 �0.949812 �0.910032 �0.733162 �0.865846 �0.88455420 O �0.583653 �0.615687 �0.625730 �0.600788 �0.644360 �0.618614

21 O �0.536797 �0.542235 �0.547413 �0.538824 �0.536149 �0.54434122 O �0.501652 �0.503393 �0.480576 �0.488203 �0.499539 �0.49482723 F �0.207796 �0.210439 �0.215189 �0.205883 �0.206307 �0.223639

24 F �0.207796 �0.210439 �0.215189 �0.205883 �0.206307 �0.22363925 F �0.212434 �0.224559 �0.235945 �0.215160 �0.223110 �0.238708

S11 H 0.145515 0.169474 0.178637 0.154531 0.172584 0.196231

2 H 0.156967 0.152643 0.148335 0.167170 0.162501 0.1516253 H 0.223582 0.226766 0.232390 0.221825 0.213311 0.2308284 H 0.362883 0.454911 0.471408 0.357886 0.420022 0.471639

5 H 0.194813 0.219493 0.221377 0.192885 0.208052 0.2224956 H 0.183073 0.187905 0.183499 0.185750 0.196511 0.1875237 C 0.587663 0.584153 0.690769 0.550908 0.543646 0.6688508 C �0.370958 �0.434303 �0.552553 �0.353873 �0.367836 �0.539274

9 C 0.090937 0.159335 0.250903 0.108164 0.200473 0.30131410 C �0.074512 �0.240129 �0.324413 �0.091004 �0.182269 �0.36223011 C �0.174332 �0.086518 �0.024804 �0.180737 �0.146196 �0.033183

12 C �0.288163 �0.325636 �0.346992 �0.261594 �0.245458 �0.34338013 C 0.644850 0.687138 0.748611 0.595541 0.525201 0.72844214 C 0.576931 0.629615 0.619897 0.580477 0.589934 0.601198

15 C 0.617633 0.620413 0.659685 0.629674 0.642547 0.66697416 C �0.301436 �0.341254 �0.361951 �0.282855 �0.302734 �0.35675217 C �0.264440 �0.273058 �0.255342 �0.295657 �0.336795 �0.24764218 C 0.628924 0.650946 0.645912 0.603217 0.598388 0.637807

19 N �0.654004 �0.725338 �0.821388 �0.575524 �0.553833 �0.81711420 O �0.621932 �0.627356 �0.653088 �0.656258 �0.686588 �0.64213621 O �0.549038 �0.519432 �0.511100 �0.554411 �0.536701 �0.503994

22 O �0.475580 �0.489566 �0.498025 �0.456757 �0.452979 �0.50716223 F �0.211800 �0.226241 �0.231718 �0.210419 �0.217117 �0.23577324 F �0.211800 �0.226241 �0.231718 �0.210419 �0.217117 �0.235773

25 F �0.215775 �0.227718 �0.238334 �0.218520 �0.227544 �0.240514


0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12

R (Å)

g (R

)

Figure S1. Radial distribution function for the transferred proton and the TIP4P water oxygen.


excited state intramolecular proton transfer in 1 ... · pdf fileexcited state intramolecular...

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