research article a new approach for studying bond rupture...

Research ArticleA New Approach for Studying Bond RuptureClosure ofa Spiro Benzopyran Photochromic Material ReactivityDescriptors Derived from Frontier Orbitals and DFT ComputedElectrostatic Potential Energy Surface Maps

M S A Abdel-Mottaleb and Sarah N Ali

Nano-Photochemistry and Solarchemistry Labs Department of Chemistry Faculty of Science Ain Shams UniversityAbbassia Cairo 11566 Egypt

Correspondence should be addressed to M S A Abdel-Mottaleb phochem08photoenergyorg

Received 1 November 2015 Revised 27 January 2016 Accepted 11 February 2016

Academic Editor Mark van Der Auweraer

Copyright copy 2016 M S A Abdel-Mottaleb and S N Ali This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

This paper focuses on computations technique within the framework of the TD-DFT theory for studying the relationship betweenstructure-properties of reversible conversion of photochromicmaterials Specifically we report on 1101584031015840-dihydro-8-methoxy-110158403101584031015840-trimethyl-6-nitrospiro[2H-1-benzopyran-221015840-(2H)-indole] (SP) and its isomers TD-DFT calculated UV-Vis electronic spectra ofthe closed and open isomers of this photochromic material are in excellent agreement with the experimental results Moreover thispaper reports on the results of theoretical investigations of reactivity indices that may govern the conversion between spiropyransand its isomers In addition the solvent and rigidity of themedium significantly control the thermal bleaching of the photogeneratedcolored isomers and hence the switch ability pattern of the photochromic material The effect of molecular structure computedby DFT in gas-phase and solvents on Cspiro-O bond length has been shown to correlate with photochromic properties For thiscompound DFT optimized geometry could be used to predict photochromism Furthermore in an attempt to predict the drivingforce forMCrarr SP this work explores for the first time profitable exploitation of the calculated and visualizedmapped electrostaticpotential energy surfaces (ESP map) Interestingly it seems that the electrostatic potential forces over the molecular fragmentsgovern spirobond ruptureclosure reactions Thermodynamically all-trans-colored isomer (CTT) is the most stable merocyanine-like form

1 Introduction

Recently there has been an exponential growth of interestin photochromism for profitable applications in importanttechnologies [1ndash13] Spiropyrans are one of the major classesof organic photochromes [2 3] Its behavior is usuallyenhanced if a molecule possesses strong electron acceptorsubstituents such as a nitro group in the 6 andor 8 positionsof its 2H-chromene fragment Spiropyrans have been exten-sively studied due to their potential exploitations in severalimportant areas including optical switching high-densityoptical storage image processing and display

The majority of spiropyrans exist in the dark as a neutralclosed form that absorbs light only in the ultraviolet regionof electromagnetic spectrum When exposed to UV light

the spiropyran undergoes a molecular rearrangement toproduce an open form The open form of spiropyrans (alsocalled merocyanine (MC)) is colored and shows solva-tochromic behavior even in the solid state [7 12 13] Theopen form of spiropyrans can revert back to the closedform thermally or photochemically by visible light Thusthe molecules may be switched from the closed to theopen form (coloration) with UV light (or thermally in thedark in presence of transition metal ions in polar solvents)and from the open form to the closed form (decoloration)with visible light or thermally with different time scalesrsquoranges from microseconds to minutes (see Scheme 1) Thephotochromism of the spiropyrantrans-merocyanine pairhas been reviewed previously [3] The rate and mechanismof the reaction of some spiropyrans and their analogues in

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2016 Article ID 6765805 9 pageshttpdxdoiorg10115520166765805

2 International Journal of Photoenergy

Δor

VIS

UVA

OO

O

O

O

O

OO

O

OO O

O O

O OO

O

O

O O

O

O

O

O

O

O

O

N

N

NN

N

NN

NN

N

N

NN

(Z)

(a) (b)

(E)(E)

(E)

(E)

Δ or VISUVA

N

Scheme 1 Closed and open forms of spiropyran as well as resonance between the open Z and E isomers (a) (b) The conversion between themost probable E isomers Methyl groups are hidden for clarity

different media have been examined with different spec-troscopic techniques [4ndash14] Spiropyran photoisomerizationwas studied using ion mobility-mass spectrometry and threemajor conformers were identified Assignment of conformersis based on DFT-B3LYP energy minimized structures andcollision cross sections for data collected on light-inducedchanges in samples The three conformers were assignedto the spiropyran (SP) a cisoid merocyanine (CCC) anda transoid (TTT) merocyanine This is the first experi-mental evidence for a cisoid merocyanine intermediate inspiropyran photoisomerization on a millisecond timescale[15]

Evidence for the TTC form of 6-nitro-BIPS comes fromlaser desorptionelectron diffraction and excited state dy-namics [16 17]

Based on conceptual density functional theory [18] moststudies have been conducted on simple model compoundsFewer theoretical studies of the excited state species have beenreported due to computational constrains in the prediction ofthe excited state properties

It should be mentioned that the gas-phase dynamics oftwo classes of photochromic molecules three spiropyransand one spirooxazine have been investigated using bothtime-resolved mass spectrometry and photoelectron spec-troscopy approaches [19] The comparison of the dynamicsof the studied four molecules has been used to proposea sequential photoisomerization mechanism involving fourstESP occurring in the first 100 psThe authors have been ableto characterize two of these although questions still remainunanswered for the others

Previous study [20ndash22] pointed to the fact that the struc-ture of the photochemically produced colored species thenature of any reaction intermediates and the detailed mech-anistic processes involved in SPMC reversible pathwaysare still unclear (see Scheme 1) Here we try to present theresults of a more detailed study about reactivity descriptorsand electrostatic potential energy surface maps (ESP maps)

that may provide a mechanistic image of the photochemi-calthermochemical conversion which enhances the ratio-nal design of new photochromic materials with improvedperformance In particular in this paper the results of TD-DFT investigations of the conversion between a spiropy-ran namely 1101584031015840-dihydro-8-methoxy-110158403101584031015840-trimethyl-6-nitrospiro[2H-1-benzopyran-221015840-(2H)-indole] (SP) and itsmerocyanines-like isomers (see Scheme 1) on the groundstate and the lowest excited state are reported Potentialenergy surfaces and reactivity indices (such as chemicalpotential hardness and softness parameters) developed fromHOMO-LUMO frontier orbitals are also presented anddiscussed The data computed for electronic singlet statesprovide a detailed view of the electronic spectroscopy of theisomers of spiropyrans

2 Materials and Methods

21 Materials The spiropyran (SP) (1101584031015840-dihydro-8-me-thoxy-110158403101584031015840-trimethyl-6-nitrospiro[2H-1-benzopyran-221015840-(2H)-indole]) (Sigma-Aldrich 97) was used as receivedPure grade (Aldrich) solvents were used

22 Instruments UV-Vis absorption spectra were measuredin the range of 250ndash800 nm using diode array ocean opticsspectrometer with spectra suite operating software Thesource of UV-Vis irradiation is a homemade photoirradiationapparatus with an 8 Watt UVA lamp

23 General Procedure for the SP Photochemical ReactionsThe color developing reaction in a quartz cell is carried outusing 8 Watt UVA lamp and monitored spectrophotomet-rically The color fading up reaction monitored in the darkusing the repeat scan mode of Spectra Suite Ocean Opticssoftware All the measurements were carried out at roomtemperature asymp 22∘C

International Journal of Photoenergy 3

(a) (b)

Figure 1 (a) The optimized geometry of the colorless SP structure (H atoms are hidden for clarity) and (b) the optimized geometry of theTTT isomer (see Supplementary Materials (Tables 1 and 2) available online at httpdxdoiorg10115520166765805 for detailed optimizedgeometry parameters of the SPMC isomers in gas-phase and in water ethanol and toluene)

24 Theoretical Computations The theoretical calculationsare carried out using Gaussian 09 version D (64-Bit Linda)Gaussian Inc (USA) and Spartanrsquo14 parallel 64-bit ver-sion (Wavefunction Inc USA) quantum chemical packageswithin the framework of DFT and TD-DFT (with 6 MOstaken into account in the CI) and visualized by Gaussview05 program (in case of Gaussian 09 computations) 6-and 12-core pro-MAC computer were used to perform thecomputationsThe geometry optimization of closed and openforms of the photochromic dye was carried out in vacuum asimplemented in the Gaussian 09 package [17] The geometryof the dye was optimized using B3LYP [23] functional withthe 6-21G-basis set which is a good compromise betweenaccuracy and efficiencyThe further expansion of the basis setsuch as using 6-31+G(d) has less impact on the accuracy of themolecular parameters Attempts with other basis sets resultin no noticeable changes However using 6-311Glowast basis setimproves the UV-Vis absorption spectral parameters (120582 andoscillator strengths) Potential energy surfaces are calculatedusing B3LYP with 6-311Glowast basis set First excited state (S

1)

calculations are obtained at Configuration Interaction Singles(CIS) level with similar basis sets (6-311Glowast)

DataGraph 32 (Visual Data Tools Inc USA) softwarewas used for the graphical representation of the TD-DFTUV-Vis electronic spectra

The DFT-B3LYP method has been demonstrated to pre-dict excellent geometries and energies In addition this levelof computation was found to take into account different geo-metrical orientations of merocyanine-like isomers Vibrationfrequency calculations were performed to ensure geometryoptimization with minimum energy structure The UV-Visspectrum was also studied in gas-phase as well as in tworepresentative solvents (polar protic ethanol and nonpolartoluene) The useful reaction field model used for solva-tion is the conductor polarized continuum model (CPCM)[24]

3 Results and Discussion

31 The SPMC Conversion Irradiation in the UV regionleads to the cleavage of theCspiro-O bond resulting in differentisomers or conformers Figure 1 shows the optimized geom-etry of the SP closed form and one of its colored MC isomer

It is suggested that irradiation results in the cis-MC(Zusammen Z isomer) which dominantly exist in the morestable TTT form (see Scheme 1(a)) The rotation about the

central C-C bonds in cis-MC yields trans-MC (Scheme 1(a))The MC product is a hybrid of different CCC CCT TTCand TTT forms However it seems plausible to consider thedirect ring opening through Cspiro-O bond rupture to yieldthe energetically apparent stable Z isomer as represented inScheme 1(b)

The MC rarr SP reverse isomerization usually occursspontaneously and can be accelerated thermally or by visiblelight [2 3 19 20] Spiropyrans are also capable of isomerizingin the presence of metal ions even under dark conditionssince the merocyanine form is stabilized by coordination tothe metal ions [3 11 22] The main problem of spiropyransis their low stability and for this reason several attemptshave been performed in order to incorporate or dispersethem into a polymer matrix [25] This results in delayedcolor fading of the MC-like isomers which accompanies thedark conversion to the closed isomer (see Scheme 1) Specificinteractions between the polymer functional groups and theMC-like forms as well as polymer free volume available forthe isomerization are the obvious reasons for the noticeddelay of MC-like isomers to revert back to SP form Thedark fading is also retarded in presence of transition metalions such as samarium ions due to complexation with thechelating groups of the MC-like isomers Consequently thenumber of switching cycles should be decreased significantlySubsequently inclusion of closed SP in PMMA thin film orthe presence of a lanthanide ion significantly stabilizes thephotogenerated TTT isomer [26 27]

To explore the relationship between photochromism andstructural parameters such as particular bond lengths [20 28]for thismaterial within the framework of theDFTwe studiedthe molecular geometry of the closed and open forms inthe gas-phase Upon the spiropyran-merocyanine transfor-mation the C

7atom (Cspiro see Figure 1(a)) changed from sp3

hybridized to sp2 hybridized and the two aromatic moietiesof the molecule changed from being perpendicular andbecame coplanar substantially Generally when compared togas-phase calculations the solvent environment alters thecharge distribution of molecules This is reflected in bondlength change Bond length Cspiro-O increases noticeably inthe order gas-phase lt toluene lt ethanol lt water reflectingeasier bond breaking in polar solventMoreover the localizedpositivenegative Mulliken [29] charge on the C120575+spiro-O

120575minus

atoms decreases slightly in the reverse order leading to arelatively less electrostatic attraction between Cspiro-O atoms


Table 1 HOMOLUMO energies chemical potential hardness and softness of SPMC isomers

Reactivity indices calculated from HOMOLUMOenergies (au)

SP closed form Merocyanine-like isomersGas-phase Toluene EtOH CCC TTC TTT

120598LUMO minus007782 minus008822 minus008812 minus010667 minus010023 minus010994120598HOMO minus019082 minus01925 minus019290 minus019487 minus019139 minus019217120583 minus01343 minus01404 minus01405 minus015077 minus014581 minus015101120578 +01130 +01044 +01047 +00882 +009116 +008223119878 (au)minus1 88496 95785 95511 113379 109697 121610

This synergistically with the bond length favors easier bondbreaking in more polar medium (Online Resources seeTable 1 C

7and O

13) Moreover based on the larger Mulliken

negative charge values of the substituted benzo ketonemoietyit could be easily predicted that the ketonic O as well as themethoxy O atoms has larger abilities to coordinate with atransition or a lanthanide metal positive ion in both SPMCforms

See Supplementary Tables 1 and 2 (Online Resources)Supplementary Table 1 shows parameters of the optimizedgeometries and Mulliken charges of SP in different media(refer to Figure 1(a) for atom tags bond connectivity andangles) Supplementary Table 2 shows parameters of theoptimized geometries and Mulliken charges of the differentMC isomers (refer to Scheme 1(a) and Figure 1(b))

It is noteworthy to mention that in the application ofquantum mechanical calculations to molecular systems thecalculation of effective atomic charge plays an important rolein such cases Mulliken atomic charges are calculated bydetermining the electron population of each atom as definedby the basis function

32 DFT Local Indices for Reactivity and Liability of ElectronDensity Local properties are highly desirable in establishinga reactivity-oriented description of molecular systems [30ndash34]

Global reactivity indices were estimated according tothe equations recommended by Parr and Yang [30 31] Inparticular the electronic chemical potentials (120583) chemicalhardness (120578) and softness (119878 = 1120578) of the SPMC isomersstudied were evaluated in terms of the one-electron energiesof the frontier molecular orbital HOMO and LUMO usingthe following equations

120583 = minus

1

2

(119868 + 119860) asymp

1

2

(120598LUMO + 120598HOMO)

120578 = (119868 minus 119860) asymp (120598LUMO minus 120598HOMO)

(1)

where 120598HOMO and 120598LUMO are the energies of the highestoccupied and the lowest unoccupied molecular orbitalsHOMO and LUMO respectively The energies of Kohn-Sham frontier molecular orbital have been used to calculatereactivity descriptors coming from conceptual DFT [30ndash33]

The chemical potential 120583measures the escaping tendencyof electron from equilibrium and the global hardness 120578 canbe seen as the resistance to charge transfer (or the band gap)while softness 119878 gives indication of how large is electron

transfer tofrom the molecule when the chemical potentialchanges

The knowledge of reactivity on a molecule is of a crucialinterest because it allows understanding interactions thatare operating during a reaction mechanism In particularelectrostatic interactions have been successfully explainedby the use of the molecular electrostatic potential [18 34ndash36] The calculated values are tabulated in Table 1 Escapingtendency of electronic charge 120583-value increases by increasingthe polarity of the solvent andor by Cspiro-O bond cleavageto form MC isomers Also resistance to charge transfermeasured by 120578 decreases and thus softness of moleculesincreases It indicates larger charge transfer fromto the MCrelative to SP following the order

SP (gas-phase) lt SP (EtOH) le SP (Toluene) lt TTC

lt MCCT lt TTT(2)

(see Table 1) The calculated reactivity descriptors listed inTable 1 favor largest charge transfer liability in case of TTTisomer This is in agreement with findings of a recent studyof photoisomerization among ring-open merocyanines byreaction dynamics and wave packet oscillations induced bytunable femtosecond pulses [35]

33 UV Irradiation and Thermal Relaxation Scheme 1(a)describes collectively the spectral observations of liquid-phase experiments depicted in Figure 2 (bathochromic shiftof SP peak forming isosbestic point as the time of irradiationincreases leading to large enhancement of the color of theMC-like isomers generated)

Our spectra calculations (Table 2) show a reaction path-way represented by the following scheme SP rarr CCC rarrTTC rarr TTT (see Figure 2(a)) This is in agreement withthe results obtained above which shows that the calculatedreactivity descriptors listed in Table 1 favor largest chargetransfer liability in case of TTT isomer This is reflected inits red shifted longest wavelength absorption peak relative toother MC isomers

Some representative molecular orbitals involved in theelectronic transitions are shown in Figure 3 In case of SPclosed form it is obvious that electron transfer transitionbetween HOMO and LUMO reflects the localized nature ofthe donor (indole) and acceptor (nitro benzene derivative)parts of the two perpendicular moieties of the moleculessee Figure 3 Some representative molecular orbitals (MOs)involved in the electronic transitions listed in Table 2 for


Table 2 Computed spectroscopic characteristics of the SPMC isomers and MOs involved in the CI transitions

Excited stateClosed form (SP) Open form (CCC) Open form (TTC) Open form (TTT)120582 eV nm (119891) 120582 eV nm (119891) 120582 eV nm (119891) 120582 eV nm (119891)MOs involved MOs involved MOs involved MOs involved

12672 4640 (0011) 1737 7139 (0075) 1716 7224 (0029) 1805 6869 (0039)

93rarr 94 93rarr 94 92rarr 94 92rarr 9493rarr 95 93rarr 94 93rarr 94

2

3483 3560 (0025) 2252 5505 (0031) 2096 5915 (0234) 2015 6152 (0245)93rarr 95 91rarr 94 92rarr 94 92rarr 94

92rarr 94 93rarr 94 93rarr 9493rarr 94 93rarr 95 93rarr 9693rarr 95

3


93rarr 94 91rarr 94 91rarr 9493rarr 95

4

3691 3359 (0194) 2778 4463 (0017) 2806 4417 (0152) 2716 4564 (0117)91rarr 94 91rarr 94 90rarr 94 90rarr 9492rarr 94 92rarr 94 92rarr 95 93rarr 94

92rarr 95 92rarr 96 93rarr 9593rarr 94 93rarr 9693rarr 95

5

3850 3219 (0036) 3056 4057 (0011) 2810 4412 (0004) 2782 4456 (0034)90rarr 94 90rarr 94 90rarr 94 90rarr 9491rarr 94 90rarr 95 91rarr 94 91rarr 9492rarr 94 91rarr 94 92rarr 94 92rarr 9692rarr 95 92rarr 95 92rarr 95 93rarr 96

TTCTTT

CCC

SP

200 250 300 350 400 450 500 550 600 650 700 750

(nm)

0

5000

10000

15000

20000

25000

30000

Epslo

n

(a)

400 500 600 700Wavelength (nm)

0

0

90

90

00

04

08

Abso

rban

ce

(b)

Figure 2 (a) Theoretically computed electronic spectra of the SP (red)MC [CCC (cyan) TTC (green) and TTT(purple)] isomers (b)Experimentally determined photo-induced transformation (120582irradiation = 365 nm on the absorption spectrum of 5 times 10minus5M of SP in toluene)Time of irradiation from the bottom = 0 10 20 30 40 60 90 secThe sharp isosbestic point in (b) reveals the isomerization reaction betweena closed SP and an open merocyanine form


minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

HOMO-1 HOMO-1 HOMO-1 HOMO-1

LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO

SP closed form CCC opened all cisoid form TTC opened form TTT opened form

Figure 3 Orbital energies and some MOs involved in the electronic transitions of different isomers

SP formESP map is of PE ranges from minus169 to+116 kJmolminus1 difference = 43kJmolminus1

Mulliken BO of Cspiro-O = 090

S1 excited state SPlowast

ESP map of PE ranges from minus193to +126 the difference = 67 kJmolminus1

Cspiro-O BOlowast = 083

h997888rarr

Scheme 2

SPTTT isomer and some are depicted graphically inFigure 3

It should be pointed out that previous quantum chemicalcalculations have shown that TTC is the most stable andTTT is the second most stable [20 37ndash41] However severalcomputational studies used a reduced atom set to facilitatecomputations making comparison to the parent systemmoredifficult

34 Analysis ofMolecular Electrostatic Surface Potential (ESP)The electrostatic surface potential (ESP) provides a visualmethod to understand the relative polarity of the com-pounds [34 36]Mapped electrostatic surfaces potential (ESPmap) are shown in Figure 4 which illustrates the 3D chargedistributions of the molecule The ESP map is a plot ofelectrostatic potential mapped onto the constant (HOMO)electron density or total electron density surface One ofthe main purposes of finding the electrostatic potential isto find the reactive site of a molecule The importance ofwhich lies in the fact that it simultaneously displaysmolecularsize shape and positive negative and neutral electrostaticpotential energy regions in terms of color grading (Figure 4)and is very useful in molecular structuremdashphysiochemicalproperty relationship [22 42 43] Knowledge of the chargedistributions can be used to determine how molecules inter-act with one another In molecular ESP maps the negativeelectrostatic potentials are shown in red the intensity ofwhich is proportional to the absolute value of the potential

energy and positive electrostatic potentials are shown in bluewhile green indicates surface areas where the potentials areclose to zero These surfaces are computed at the 00004 auisodensity surface Potential increases are in the followingorder red lt orange lt yellow lt green lt blue (Figure 4)The figure provides a visual representation of the chemicallyactive sites and comparative reactivity of atoms In SPMCthe blue indicates the strongest attraction and red indicatesthe strongest repulsion Regions of negative value are usuallyassociated with the lone pair of electronegative atoms Ascan be seen from the ESP map of the studied molecules(Figure 4) while regions having the negative potential areover the indole part of the SP the regions having the zeroor small positive potential are over the aromatic moiety thathas methoxy and nitro group especially on the Cspiro-O Itis interesting to note that negative and positive potentialsaccording to the ESP map are uniformly distributed overthe whole TTT molecule Referring to Figure 4 it could beseen that upon light excitation the potential energy rises asScheme 2

Consequently Cspiro-O bond rupture occurs easily dueto the increase in PE and the decrease in Cspiro-O bondorder within femtosecond time domain [19] generating theCCC form which is characterized by ESPmap with potentialenergy values range between minus207 and +141 kJmolminus1 of66 kJmolminus1 difference Isomerization to TTC (of ESP mapminus200 to +176 kJmolminus1 difference = 34 kJmolminus1) occurs withsubsequent fast conversion around the bonds of the bridge


TTT ESP map (potential ranges from minus196 to +186 kJmolminus1)Difference = 10kJmolminus1

TTC ESP map (potential ranges from minus200 to +176kJmolminus1)Difference = 34kJmolminus1

CCC ESP map (potential ranges from minus207 to +141kJmolminus1)Difference = 66kJmolminus1

SP ESP map (potential ranges from minus169 to +116 kJmolminus1)Difference = 43kJmolminus1

116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196

and below is SP in state PE rangesminus193 to +126 and the difference = 67kJmolminus1from

S1

Figure 4 The molecular electrostatic potential mapped onto totalSCF electron density for the different forms of closed SP andmerocyanine-like isomer (isovalue for the isosurfaces) with differentcolor codes from minusve value (red) to +ve value (blue) expressed inkJmolminus1

to the most stable TTT (minus196 to +186 kJmolminus1 difference =10 kJmolminus1) which has among other forms the smallestdifference between the maximum and minimum potentialenergy value of 10 kJmolminus1

ESP maps clearly distinguish the electrostatic potentialsover the different molecular isomers and could be used toexplain the driving force for ring closer of TTT and otheropen form isomers ESPmaps point to the spreading of lowestattractiverepulsive energy over the SP form suggesting avisualizing way to explain driving force for the reverse darkreaction CCCharr TTCharr TTTrarr SP

4 Conclusions

The results reveal that the photo-induced ring openingprocess of the SP closed form generates three possible planarmerocyanine-like (MC) colored isomers (CCC TTC andTTT) This paper tackled the issue of the photo-inducedisomers of SP in a new way using new theoretical approachThis is done via considering results of reactivity descriptorsand ESP map

The calculated reactivity descriptors such as chemicalpotential hardness and softness of the photochromic mate-rial and its open forms isomers favor the largest chargetransfer liability in case of TTT isomer This finding alsocorrelates withMulliken charges and TD-DFT calculated andexperimentally measured electronic spectra

The effect of molecular structure generated by DFT ingas-phase and solvents on calculated Cspiro-O bond lengthhas been shown to correlate with photochromic propertiesFor this compound DFT optimized geometry could beused to predict the absorption wavelength of the coloredphotochromicmaterialThe larger the Cspiro-O bond distanceis the longer the absorption wavelength is

Favorable SP ESP map of lowest energy limits relative toother open forms encourages bleaching of the photogener-ated open colored MC forms suggesting a visualizing way toexplain driving force for the reverse pathway CCCharr TTCharrTTTrarr SP

The detailed theoretically obtained knowledge (such asreactivity descriptors derived from HOMO-LUMO energiesand ESP maps) should result in considering a set of newparameters that are important guidance for predicting newphotochromic materials with improved performance

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] R C Bertelson ldquoPhotochromic processes involving heterolyticcleavagerdquo in Techniques of Chemistry G H Brown Ed vol 3pp 45ndash433 Wiley-Interscience New York NY USA 1971

[2] H Duerr andH Bouas-Laurent PhotochromismMolecules andSystems Elsevier New York NY USA 1990

[3] G Berkovic V Krongauz and V Weiss ldquoSpiropyrans andspirooxazines for memories and Switchesrdquo Chemical Reviewsvol 100 no 5 pp 1741ndash1753 2000

[4] R Guglielmetti ldquoChapter 8mdash4n+2 systems spiropyransrdquo inPhotochromism Molecules and Systems H Durr and H Bouas-Laurent Eds pp 314ndash466 Elsevier Amsterdam Netherlands1990

[5] R Guglielmetti ldquoChapter 23mdashspiropyrans and related com-poundsrdquo in Photochromism Molecules and Systems H Durrand H Bouas-Laurent Eds pp 855ndash878 Elsevier AmsterdamNetherlands 1990

[6] M S Attia M H Khalil M S A Abdel-Mottaleb M BLukyanova YuAAlekseenko andB Lukyanov ldquoEffect of com-plexation with lanthanide metal ions on the photochromism


of (133-trimethyl-51015840-hydroxy-61015840-formyl- indoline-spiro221015840-[2h]chromene) in different mediardquo International Journal ofPhotoenergy vol 2006 Article ID 42846 9 pages 2006

[7] B S Lukyanov A V Metelitsa N A Voloshin et al ldquoSolidstate photochromism of spiropyransrdquo International Journal ofPhotoenergy vol 7 no 1 pp 17ndash22 2005

[8] J Berthet S Delbaere V Lokshin A Samat J C Micheauand G Vermeersch ldquoNMR studies of the polyphotochromicbehaviour of biphotochromic compoundsrdquo International Jour-nal of Photoenergy vol 6 no 4 pp 215ndash220 2004

[9] S Delbaere J CMicheau J Berthet andG Vermeersch ldquoCon-tribution of NMR spectroscopy to the mechanistic understand-ing of photochromismrdquo International Journal of Photoenergyvol 6 no 4 pp 151ndash158 2004

[10] A O Bulanov L D Popov I N Shcherbakov et al ldquoSynthesisIR UVvis- 1HNMR andDFT study of chelatophore function-alized 13-benzoxazinone spiropyransrdquo Spectrochimica ActamdashPart A Molecular and Biomolecular Spectroscopy vol 71 no 3pp 1146ndash1152 2008

[11] E Bakeir G M Attia M Lukyanova B Lukyanov and M SA Abdel-Mottaleb ldquoThe effect of Tb and Sm ions on thephotochromic behavior of two spiropyrans of benzoxazineseries in solutionrdquo Research Letters in Physical Chemistry vol2008 Article ID 314898 4 pages 2008

[12] K Kinashi S Nakamura Y Ono K Ishida and Y Ueda ldquoRe-verse photochromism of spiropyran in silicardquo Journal of Photo-chemistry and Photobiology A Chemistry vol 213 no 2-3 pp136ndash140 2010

[13] F Zhang X Zou W Feng et al ldquoMicrowave-assisted crystal-lization inclusion of spiropyran molecules in indium trimesatefilms with antidromic reversible photochromismrdquo Journal ofMaterials Chemistry vol 22 no 48 pp 25019ndash25026 2012

[14] M Campredon R Guglielmetti A Samat and A AlbertildquoESR studies on some spiropyrans spironaphthropyrans andspirooxazinesrdquo The Journal of Chemical Physics vol 91 no 11-12 pp 1830ndash1836 1830

[15] R A Rogers A R Rodier J A StanleyNADouglas X Li andW J Brittain ldquoA study of the spiropyran-merocyanine systemusing ion mobility-mass spectrometry experimental supportfor the cisoid conformationrdquo Chemical Communications vol50 no 26 pp 3424ndash3426 2014

[16] A Gahlmann I-R Lee and A H Zewail ldquoDirect structuraldetermination of conformations of photoswitchable moleculesby laser desorption-electron diffractionrdquoAngewandte ChemiemdashInternational Edition vol 49 no 37 pp 6524ndash6527 2010

[17] C J Wohl and D Kuciauskas ldquoExcited-state dynamics ofspiropyran-derived merocyanine isomersrdquoThe Journal of Phys-ical Chemistry B vol 109 no 47 pp 22186ndash22191 2005

[18] P Geerlings F de Proft and W Langenaeker ldquoConceptualdensity functional theoryrdquo Chemical Reviews vol 103 no 5 pp1793ndash1873 2003

[19] L Poisson K D Raffael B Soep J-M Mestdagh and GBuntinx ldquoGas-phase dynamics of spiropyran and spirooxazinemoleculesrdquo Journal of the American Chemical Society vol 128no 10 pp 3169ndash3178 2006

[20] Y Sheng J Leszczynski A A Garcia R Rosario D Gust andJ Springer ldquoComprehensive theoretical study of the conversionreactions of spiropyrans substituent and solvent effectsrdquo TheJournal of Physical Chemistry B vol 108 no 41 pp 16233ndash162432004

[21] A-K Holm O F Mohammed M Rini E Mukhtar E TJ Nibbering and H Fidder ldquoSequential merocyanine prod-uct isomerization following femtosecond UV excitation of aspiropyranrdquo Journal of Physical Chemistry A vol 109 no 40pp 8962ndash8968 2005

[22] V I Minkin A V Metelitsa I V Dorogan B S Lukyanov SO Besugliy and J-C Micheau ldquoSpectroscopic and theoreticalevidence for the elusive intermediate of the photoinitiated andthermal rearrangements of photochromic spiropyransrdquo Journalof Physical Chemistry A vol 109 no 42 pp 9605ndash9616 2005

[23] M J Frisch G W Trucks H B Schlegel et al GAUSSIAN 09Revision D01 Gaussian Inc Wallingford Conn USA 2013

[24] J Tomasi B Mennucci and R Cammi ldquoQuantum mechanicalcontinuum solvation modelsrdquo Chemical Reviews vol 105 no 8pp 2999ndash3093 2005

[25] R Klajn ldquoSpiropyran-based dynamic materialsrdquo Royal Societyof Chemistry vol 43 no 17 pp 1ndash488 2014

[26] A Samoladas D Bikiaris T Zorba K M Paraskevopoulosand A Jannakoudakis ldquoPhotochromic behavior of spiropyranin polystyrene and polycaprolactone thin filmsmdasheffect of UVabsorber and antioxidant compoundrdquo Dyes and Pigments vol76 no 2 pp 386ndash393 2008

[27] S N Ali Photochromism and spectroscopic studies of somespiropyran complexes of some transition metal ions [MS thesis]Faculty of Science Ain Shams University Cairo Egypt 2015

[28] S Kumar K Velasco and A McCurdy ldquoX-ray kinetics andDFT studies of photochromic substituted benzothiazolinicspiropyransrdquo Journal of Molecular Structure vol 968 no 1ndash3pp 13ndash18 2010

[29] R S Mulliken ldquoA new electroaffinity scale Together with dataon valence states and on valence ionization potentials andelectron affinitiesrdquo The Journal of Chemical Physics vol 2 no11 pp 782ndash793 1934

[30] J Hobley and V Malatesta ldquoEnergy barrier to TTCndashTTT iso-merisation for the merocyanine of a photochromic spiropyranrdquoPhysical Chemistry Chemical Physics vol 2 no 1 pp 57ndash591989

[31] R G Parr and W Yang ldquoDensity functional approach to thefrontier-electron theory of chemical reactivityrdquo Journal of theAmerican Chemical Society vol 106 no 14 pp 4049ndash40501984

[32] PW Ayers J S M Anderson and L J Bartolotti ldquoPerturbativeperspectives on the chemical reaction prediction problemrdquoInternational Journal of Quantum Chemistry vol 101 no 5 pp520ndash534 2005

[33] F Zielinski V Tognetti and L Joubert ldquoCondensed descriptorsfor reactivity amethodological studyrdquoChemical Physics Lettersvol 527 pp 67ndash72 2012

[34] A Savin C J Umrigar and X Gonze ldquoRelationship of Kohn-Sham eigenvalues to excitation energiesrdquo Chemical PhysicsLetters vol 288 no 2ndash4 pp 391ndash395 1998

[35] P Politzer and J SMurray ldquoThe fundamental nature and role ofthe electrostatic potential in atoms and moleculesrdquo TheoreticalChemistry Accounts vol 108 no 3 pp 134ndash142 2002

[36] J S Murray and P Politzer ldquoThe electrostatic potential anoverviewrdquo Wiley Interdisciplinary Reviews Computational Mo-lecular Science vol 1 no 2 pp 153ndash163 2011

[37] Y Futami M Lim L S Chin S Kudoh M TakayanagiandMNakata ldquoConformations of nitro-substituted spiropyranand merocyanine studied by low-temperature matrix-isolation


infrared spectroscopy and density-functional-theory calcula-tionrdquo Chemical Physics Letters vol 370 no 3-4 pp 460ndash4682003

[38] JHobleyU Pfeifer-FukumuraM Bletz TAsahiHMasuharaand H Fukumura ldquoUltrafast photo-dynamics of a reversiblephotochromic spiropyranrdquoThe Journal of Physical Chemistry Avol 106 pp 2265ndash2270 2002

[39] J Hobley andVMalatestaPhysical Chemistry Chemical Physics2000

[40] J Hobley V Malatesta R Millini L Montanari and W ONeil Parker Jr ldquoProton exchange and isomerisation reactionsof photochromic and reverse photochromic spiro-pyrans andtheir merocyanine formsrdquo Physical Chemistry Chemical Physicsvol 1 no 14 pp 3259ndash3267 1999

[41] J Hobley V Malatesta W Giroldini and W Stringo ldquo120587-Cloudand non-bonding or H-bond connectivities in photochromicspiropyrans and their merocyanines sensed by 13C deuteriumisotope shiftsrdquo Physical Chemistry Chemical Physics vol 2 no1 pp 53ndash56 2000

[42] G Cottone R Noto and G La Manna ldquoTheoretical studyof spiropyran-merocyanine thermal isomerizationrdquo ChemicalPhysics Letters vol 388 no 1ndash3 pp 218ndash222 2004

[43] E Scrocco and J Tomasi ldquoElectronic molecular structurereactivity and intermolecular forces an euristic interpretationby means of electrostatic molecular potentialsrdquo in Advancesin Quantum Chemistry P Lowdin Ed pp 115ndash193 AcademicPress New York NY USA 1978

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


Δor

VIS

UVA

OO

O

O

O

O

OO

O

OO O

O O

O OO

O

O

O O

O

O

O

O

O

O

O

N

N

NN

N

NN

NN

N

N

NN

(Z)

(a) (b)

(E)(E)

(E)

(E)

Δ or VISUVA

N

Scheme 1 Closed and open forms of spiropyran as well as resonance between the open Z and E isomers (a) (b) The conversion between themost probable E isomers Methyl groups are hidden for clarity

different media have been examined with different spec-troscopic techniques [4ndash14] Spiropyran photoisomerizationwas studied using ion mobility-mass spectrometry and threemajor conformers were identified Assignment of conformersis based on DFT-B3LYP energy minimized structures andcollision cross sections for data collected on light-inducedchanges in samples The three conformers were assignedto the spiropyran (SP) a cisoid merocyanine (CCC) anda transoid (TTT) merocyanine This is the first experi-mental evidence for a cisoid merocyanine intermediate inspiropyran photoisomerization on a millisecond timescale[15]

Evidence for the TTC form of 6-nitro-BIPS comes fromlaser desorptionelectron diffraction and excited state dy-namics [16 17]

Based on conceptual density functional theory [18] moststudies have been conducted on simple model compoundsFewer theoretical studies of the excited state species have beenreported due to computational constrains in the prediction ofthe excited state properties

It should be mentioned that the gas-phase dynamics oftwo classes of photochromic molecules three spiropyransand one spirooxazine have been investigated using bothtime-resolved mass spectrometry and photoelectron spec-troscopy approaches [19] The comparison of the dynamicsof the studied four molecules has been used to proposea sequential photoisomerization mechanism involving fourstESP occurring in the first 100 psThe authors have been ableto characterize two of these although questions still remainunanswered for the others

Previous study [20ndash22] pointed to the fact that the struc-ture of the photochemically produced colored species thenature of any reaction intermediates and the detailed mech-anistic processes involved in SPMC reversible pathwaysare still unclear (see Scheme 1) Here we try to present theresults of a more detailed study about reactivity descriptorsand electrostatic potential energy surface maps (ESP maps)

that may provide a mechanistic image of the photochemi-calthermochemical conversion which enhances the ratio-nal design of new photochromic materials with improvedperformance In particular in this paper the results of TD-DFT investigations of the conversion between a spiropy-ran namely 1101584031015840-dihydro-8-methoxy-110158403101584031015840-trimethyl-6-nitrospiro[2H-1-benzopyran-221015840-(2H)-indole] (SP) and itsmerocyanines-like isomers (see Scheme 1) on the groundstate and the lowest excited state are reported Potentialenergy surfaces and reactivity indices (such as chemicalpotential hardness and softness parameters) developed fromHOMO-LUMO frontier orbitals are also presented anddiscussed The data computed for electronic singlet statesprovide a detailed view of the electronic spectroscopy of theisomers of spiropyrans

2 Materials and Methods

21 Materials The spiropyran (SP) (1101584031015840-dihydro-8-me-thoxy-110158403101584031015840-trimethyl-6-nitrospiro[2H-1-benzopyran-221015840-(2H)-indole]) (Sigma-Aldrich 97) was used as receivedPure grade (Aldrich) solvents were used

22 Instruments UV-Vis absorption spectra were measuredin the range of 250ndash800 nm using diode array ocean opticsspectrometer with spectra suite operating software Thesource of UV-Vis irradiation is a homemade photoirradiationapparatus with an 8 Watt UVA lamp

23 General Procedure for the SP Photochemical ReactionsThe color developing reaction in a quartz cell is carried outusing 8 Watt UVA lamp and monitored spectrophotomet-rically The color fading up reaction monitored in the darkusing the repeat scan mode of Spectra Suite Ocean Opticssoftware All the measurements were carried out at roomtemperature asymp 22∘C


(a) (b)



1)












120575minus








7and O







120583 = minus

1

2

(119868 + 119860) asymp

1

2

(120598LUMO + 120598HOMO)


(1)






lt MCCT lt TTT(2)








12672 4640 (0011) 1737 7139 (0075) 1716 7224 (0029) 1805 6869 (0039)


2



3



4



5


TTCTTT

CCC

SP

200 250 300 350 400 450 500 550 600 650 700 750

(nm)

0

5000

10000

15000

20000

25000

30000

Epslo

n

(a)


0

0

90

90

00

04

08

Abso

rban

ce

(b)



minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)


LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO








h997888rarr

Scheme 2











116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


(a) (b)



1)












120575minus








7and O







120583 = minus

1

2

(119868 + 119860) asymp

1

2

(120598LUMO + 120598HOMO)


(1)






lt MCCT lt TTT(2)








12672 4640 (0011) 1737 7139 (0075) 1716 7224 (0029) 1805 6869 (0039)


2



3



4



5


TTCTTT

CCC

SP

200 250 300 350 400 450 500 550 600 650 700 750

(nm)

0

5000

10000

15000

20000

25000

30000

Epslo

n

(a)


0

0

90

90

00

04

08

Abso

rban

ce

(b)



minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)


LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO








h997888rarr

Scheme 2











116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of







7and O







120583 = minus

1

2

(119868 + 119860) asymp

1

2

(120598LUMO + 120598HOMO)


(1)






lt MCCT lt TTT(2)








12672 4640 (0011) 1737 7139 (0075) 1716 7224 (0029) 1805 6869 (0039)


2



3



4



5


TTCTTT

CCC

SP

200 250 300 350 400 450 500 550 600 650 700 750

(nm)

0

5000

10000

15000

20000

25000

30000

Epslo

n

(a)


0

0

90

90

00

04

08

Abso

rban

ce

(b)



minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)


LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO








h997888rarr

Scheme 2











116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




12672 4640 (0011) 1737 7139 (0075) 1716 7224 (0029) 1805 6869 (0039)


2



3



4



5


TTCTTT

CCC

SP

200 250 300 350 400 450 500 550 600 650 700 750

(nm)

0

5000

10000

15000

20000

25000

30000

Epslo

n

(a)


0

0

90

90

00

04

08

Abso

rban

ce

(b)



minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)


LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO








h997888rarr

Scheme 2











116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)

minus9

minus8

minus7

minus6

minus5

minus4

minus3

minus2

minus1

Orb

ital e

nerg

y (e

V)


LUMOLUMO

LUMO LUMO

HOMO HOMO HOMO HOMO








h997888rarr

Scheme 2











116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of






116

minus169

126

minus193

141

minus207

17604

minus200

186

minus196


S1




4 Conclusions






Competing Interests


References

























































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



















































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

research article a new approach for studying bond rupture...

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