Relaxation Pathways of Excited N-(Triphenylmethyl)salicylidenimine

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Relaxation Pathways of Excited N-(Triphenylmethyl)salicylidenimine in Solutions Renata Karpicz,*,† Vidmantas Gulbinas,†,‡ Aleksandra Lewanowicz,§ Mindaugas Macernis,|| Juozas Sulskus,|| and Leonas Valkunas†,|| †

Institute of Physics, Center for Physical Sciences and Technology, Savanoriu Ave 231, LT-02300 Vilnius, Lithuania Faculty of Physics, Department of General Physics and Spectroscopy, Vilnius University, Sauletekio Ave 9, LT-10222 Vilnius, Lithuania § Institute of Physical and Theoretical Chemistry, Wroczaw University of Technology, Wybrze_ze Wyspianskiego 27, 50-370 Wroczaw, Poland Faculty of Physics, Department of Theoretical Physics, Vilnius University, Sauletekio Ave 9, LT-10222 Vilnius, Lithuania

)

‡

ABSTRACT: Excited state relaxation of N-(triphenylmethyl)salicylidenimine (MS1) in protic and aprotic solvents has been investigated by means of absorption pump-probe spectroscopy with femtosecond time resolution and fluorescence spectroscopy with picosecond time resolution. Short-lived excited states and long-lived photoproducts have been identified from the differential absorption spectra. Excited states and photoproducts were different under excitation of enol-closed and cisketo tautomers. As a result, the commonly accepted excited state relaxation model of aromatic anils, which assumes an ultrafast transformation of excited enol-closed tautomers into cis-keto tautomers, has been modified. Performed quantum chemical calculations suggest that hydrogen-bonded ethanol molecules facilitate formation of cis-keto tautomers and are responsible for their different relaxation pathways in comparison with relaxation of excited enol-closed tauromers. Fluorescence decay on a nanosecond time scale was attributed to aggregated MS1 molecules.

’ INTRODUCTION Functional groups, such as -OH or -NH2, in combination with -CO, -NdN-, or dNH groups, are known for their ability in constructing pathways of inter- and intramolecular proton transfer. This type of reaction plays an important role in chemical physics and biophysics. Bacteriorhodopsin is a good example demonstrating such proton transfer initiated by electronic excitation of the retinal chromophore linked to the protein via protonated Schiff base. The origin of the active proton driving force in bacteriorhodopsin remains still to be disclosed.1-4 For this purpose the proton transfer reaction is widely considered in various model systems constituting intramolecular hydrogen bonds. Imine is a typical moiety present in molecular compounds containing the Schiff base as a functional group. Aromatic anils corresponding to the imine type molecules derived from aniline with phenyl or substituted phenyl moieties are well-known for their photo- and thermochromic properties.5-33 Salicylideneaniline (SA) is the most widely studied anil.5,6,9,11-15,18,21,23,24,26,30 Intramolecular proton transfer in anils is associated with the enol-keto tautomerization. The tautomerization reaction makes anils interesting for their possible applications in photochemical energy conversion and photoprotection.34,35 Depending on the molecule structure and environmental conditions the anil type molecules may exist in the enol and r 2011 American Chemical Society

keto tautomeric forms. The enol tautomers are usually more stable and, thus, dominate in solutions. Proton-donating properties of solvents are of major importance in determining tautomeric equilibrium. The long wavelength absorption band, commonly associated with the cis-keto tautomer, usually appears only in protic solvents. However, this is not a strict rule; some anil type molecules may exist in a cis-keto tautomeric form in aprotic solvents.22 Tautomeric forms of anil molecules may also vary under their excitation causing their photochromic properties, which have been widely studied.6,9,12,13,16,18-20,23,24,26,29-31,36 After photoexcitation of the dominating enol tautomers, fluorescent cis-keto tautomers are formed by a very fast proton transfer in the excited state.12,13,18,19,23,24,30-33,36 The excited cis-keto tautomers, relax to the ground state very rapidly, on the picosecond time scale, either re-forming the enol tautomeric form or forming the photoproduct, trans-keto tautomers. This photoproduct usually has a long lifetime of more than several nanoseconds12,13,18 or even of several microseconds.19,24,31,33 Such a tautomerization reaction in the excited state has been observed in a wide variety of anil molecules; however, it still Received: October 1, 2010 Revised: December 20, 2010 Published: February 22, 2011 1861

dx.doi.org/10.1021/jp109426a | J. Phys. Chem. A 2011, 115, 1861–1868

The Journal of Physical Chemistry A

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Figure 1. Chemical structures of different tautomeric forms of MS1 compound.

remains unclear how this reaction depends on the molecular structure and environment. Here we present an experimental and theoretical study of the excited state dynamics of N-(triphenylmethyl)salicylidenimine (MS1) of different tautomeric forms (see Figure 1) in protic and aprotic solvents. The MS1 molecules have an extended threephenyl ring system attached to the carbon atom of the methyl group. This bulky attachement is expected to significantly hamper conformational motions of molecules during the tautomerization reaction and, thus, change the excited state dynamics. We demonstrate that relaxation of enol MS1 tautomers occurs in a very similar way, as is the case in other anil type molecules, while excitation of cis-keto tautomers leads to formation of different short-lived transients and long-lived photoproducts. We attribute this difference to the influence of ethanol molecules attached by hydrogen bonds to the cis-keto MS1 tautomers, rather than to the influence of the three-phenyl ring. The obtained experimental results were verified by the quantum chemical calculations used to define geometrical structures and energies of different tautomeric forms of MS1 molecules and their complexes with ethanol molecules.

’ MATERIALS AND METHODS The MS1 molecules were synthesized at the Wroczaw University of Technology (Poland).37 Figure 1 shows chemical structures of different tautomeric forms of MS1 molecules. Reagent grade solvents, ethanol and cyclohexane, were used for the preparation of MS1 solutions. A standard ground state density functional theory (DFT)38 with three-parameter Becke3-Lee-Yang-Parr (B3LYP)39 exchange-correlation functional and the 6-311þþG(2d,p) basis set was used for the geometry optimization of the MS1 molecules in the ground electronic state. The time dependent DFT (TDDFT)40-42 with the same exchange-correlation functional and basis set was used for the evaluation of singlet excitation energies. The Gaussian 03 quantum chemistry package43 was used for calculations of electronic structures of different tautomers and their complexes with ethanol molecules, and the GaussView-3.09 package44 was invoked for analysis and visualization of the results. The influence of nuclear vibrations through zero point energies was taken into account by considering the relative stability of the structures under consideration in some particular cases. Steady state absorption spectra of MS1 solutions (using 10-mm quartz cell and solute concentration of about 10-5 mol/L) were recorded with a Perkin-Elmer Lambda 950 spectrometer. Fluorescence spectra and relaxation kinetics were measured with the Edinburg Instruments time correlated single photon counting fluorescence spectrometer F900. The picosecond pulsed diode laser EPL-375 emitting 56 ps pulses at 375 nm or the picosecond pulsed light emitting diode EPLED-300 emitting

850 ps pulses at 300 nm was used for the sample excitation. The time resolution of the setup accounting for the deconvolution procedure was about 50 ps in the case of the diode laser application. Fluorescence spectra were corrected for the instrument sensitivity. All experiments were carried out at room temperature. Transient absorption investigations were performed by using the time-resolved pump-probe technique with a femtosecond time resolution. The spectrometer was based on the amplified femtosecond Ti:sapphire laser Quantronix Integra-C generating 130 fs duration pulses at a 1 kHz repetition rate. Radiation of the parametric generator TOPAS-C and the second harmonics (402 nm) of the fundamental laser radiation (805 nm) were used for the sample excitation. White light continuum, generated by focusing fraction of fundamental radiation onto a thin sapphire plate, was used as the probe light. The time resolution of the setup was about 150 fs.

’ EXPERIMENTAL RESULTS AND QUATUM CHEMICAL CALCULATIONS Steady State Absorption. Steady state absorption spectra of MS1 in cyclohexane and ethanol are presented in Figure 2. The spectra contain two strong UV absorption bands at about 260 and 320 nm, and a weak absorption band at about 400 nm noticeable only in ethanol solution. Similar spectra have been also observed in other Schiff base containing compounds, SA,13,18,23 salicylidene-1-naphthylamine (SN)26 and N-salicylidene-pmethylaniline.17 In aprotic solvents, acetonitrile or cyclohexane, these compounds have absorption bands in the UV region attributed to the enol tautomeric form.13,17,18,23,24,26 The long wavelength absorption band with the maximum at about 340 nm for SA,13,18,23,26,45 at 350 nm for SN,26 and at 360 nm for salicylaldehyde azine (SAA)24,29 has been assigned to the lowest energy π-π* transition of enol tautomers. By analogy, we also attribute absorption band of MS1 at 320 nm to the enol-closed tautomeric form (Figure 1). A weak visible absorption band located in the 400-470 nm region has been also observed for SA and similar Schiff bases in polar protic solvents such as methanol or ethanol.8,9,13,17,25 Absorption spectra of some anils in solid states (crystalline or polycrystalline) also exhibit bands at about 450-470 nm assigned to the cis-keto tautomeric form.10,21 Since polar protic solvents shift the tautomeric equilibrium toward the keto form,22,25 the long wavelength absorption band in solutions has been usually attributed to the keto form. On the other hand, a weak long-wavelength absorption tail of N,N0 -bis(salicylidene)p-phenylenediamine (BSP),19 SA, and SAA15,24 in acetonitrile has been assigned to the n-π* transitions of the enol tautomeric form. On the basis of these assignments and by taking into account that the weak visible absorption band of MS1 molecules 1862

dx.doi.org/10.1021/jp109426a |J. Phys. Chem. A 2011, 115, 1861–1868


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Figure 2. Stationary absorption and fluorescence spectra of MS1 in ethanol and cyclohexane solutions obtained with different wavelength excitation. The inset shows a magnified long wavelength part of the absorption spectrum.

appears only in protic solvent ethanol, we attribute this band to the cis-keto tautomers, which are evidently present in ethanol solution in addition to the dominating enol-closed tautomers. Fluorescence. Fluorescence spectra of MS1 in ethanol and cyclohexane are presented in Figure 2. Two weak fluorescence bands were observed in both solutions under excitation at 300 nm, while only the long wavelength fluorescence band at 505 nm was observed in ethanol solution under excitation at 375 nm. Fluorescence of MS1 in cyclohexane under excitation at 375 nm was too weak to be measured. Two fluorescence bands of MS1 molecules in polar protic solvents were also observed in ref 25 and attributed to different tautomeric forms. A very weak short-wavelength fluorescence band at about 420 nm has been also observed for some other anils and assigned to the enol form.19,24,25,36 Large Stokes shift of the long wavelength fluorescence band of MS1 and its strong dependence on the solvent parameters indicate that the intramolecular proton transfer takes place in MS1 molecules in the excited state, which is in agreement with the relaxation model developed for similar anil molecules.9,13,18,19,24 Fluorescence of MS1 in ethanol and cyclohexane shows nonmonoexponential decay (not shown). Kinetics is well-fitted with three-exponentials characterized by lifetimes of less than 50 ps (limited by the instrument resolution), 1.8 ( 0.1 ns, and several nanoseconds. The fluorescence decay was slightly faster in the long-wavelength region. This fluorescence relaxation is much slower than reported for BSP molecules,19 SA, SAA,24 or the main relaxation component of 2-hydroxy-4-(dodecyloxy)-40 carboxysalicylidineaniline (DCSA),13 where fluorescence relaxation during several or several tens of picoseconds has been observed. However, a similar decay on the nanosecond time scale was observed for salicylidine-3,4,7-methylamine (SMA) in water.46 The long-time component observed in DCSA has been attributed to the aggregated structure.13 Since the fluorescence kinetics of our samples is much slower than the transient absorption kinetics described below, we also attribute it to the aggregated structures present at low concentration in solutions, which evidently have much higher fluorescence yield than the monomer molecules.

Figure 3. Transient differential absorption spectra at various delay times (a) and transient absorption kinetics at different probe wavelengths (b) of MS1 in ethanol obtained with excitation at 340 nm.

Femtosecond Transient Absorption. Transient absorption spectra of the MS1 molecules in ethanol obtained with excitation at 340 nm are presented in Figure 3a. The spectra contain a strong induced absorption band in the blue spectral region and the negative signal in the red region at 550-700 nm. Since the steady state absorption is absent in this spectral region, the negative signal should be unambiguously attributed to the gain due to the stimulated emission. Quite similar transient absorption spectra have been observed for SA12,13,18,46 and N-(3methylsalicylidene)-3-methylaniline (Me2SA).30 Similar stimulated emission of SA molecules has been attributed to excited cis-keto form.12,13,19,24,30,32 Transient absorption relaxation kinetics in the induced absorption and stimulated emission regions are shown in Figure 3b. The gain band decays with the time constant of about 12 ps. It is worthwhile to note that the gain band is shifted to the long wavelength side in comparison with the fluorescence band and decays much faster than the fluorescence. It supports the attribution of the fluorescence to the minor aggregate species, which only negligibly contribute to the gain. The induced absorption contains a slow relaxation component, decaying much longer than our investigation time domain. Transient absorption spectra and kinetics of MS1 in cyclohexane solution were obtained to be identical within the experimental accuracy to those of MS1 in ethanol. Figure 4a shows the transient absorption spectra of MS1 in ethanol under excitation at 402 nm. The transient absorption spectrum has several evident differences in comparison with that 1863



Figure 4. Transient differential absorption spectra at various delay times (a) and transient absorption kinetics at different probe wavelengths (b) of MS1 in ethanol obtained with excitation at 402 nm.

obtained under 340 nm excitation: (i) an additional induced absorption band at about 560 nm is well distinguished, (ii) no gain was observed in the long wavelength region, (iii) the negative signal in the short wavelength region (below 440 nm) is evident in the zero delay time spectrum, and (iv) the long-lived induced absorption spectrum is shifted to the short wavelength side. The induced absorption spectrum obtained under 402 nm excitation changes significantly on a time scale of several picoseconds. The 560 nm band relaxes in about 4 ps and the 470 nm induced absorption band shifts by about 10 nm to the blue spectral region in about 50 ps. The transient absorption kinetics of MS1 in ethanol solution excited at 402 nm is presented in Figure 4b. The strong shortlived negative signal in the short wavelength region reveals some very fast relaxation process. It is plausible to attribute it to the stimulated emission because (i) the steady state absorbance in this spectral region is very weak, and thus, only weak absorption bleaching is expected, and (ii) the absorption bleaching would last longer, at least not shorter than the excited state lifetime. The stimulated emission evidently corresponds to the FranckCondon state of excited cis-keto tautomers. The lifetime of this state is comparable or shorter than the time resolution of our setup. The transient absorption decay measured in the long wavelength induced absorption band region evidently reflects the excited state relaxation, which takes place during several tens of picoseconds. The long-lived photoproduct characterized by the absorption band below 500 nm appears during this time. It shoud

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be also noted that the relaxation kinetics obtained at different probe wavelengths cannot be fitted with biexpentnaial relaxation functions with some fixed time constants. This is because spectra shifts related to the environment relaxation and conformational changes of molecules take place simultaneously with the transitions between excited states. No transient absorption was observed for the MS1 in cyclohexane under 402 nm excitation because MS1 molecules of the enol-closed tautomeric form do not absorb at this excitation wavelength. Hence, the transient absorption of the ethanol solution excited at 402 nm should be attributed to the excited cis-keto tautomers. Quantum Chemical Calculations. Experimental results are not sufficient to make decisive conclusions concerning the excited state dynamics of the MS1 molecules; therefore, quantum chemical calculations were performed to determine available molecular structures, their electronic spectrum and the possible role of the H-bond formation with solvent (ethanol) molecules. Geometries of molecules were defined by means of quantum chemical computations by (i) neglecting any external influences from the solvent and by (ii) modeling the influence of the solvent explicitly positioning solvent molecules in the vicinity of the molecule under investigation. Calculations of isolated MS1 molecules reveal four major tautomeric forms (Figure 1): cis-keto and trans-keto, as well as enol-closed and enol-open. The cis-keto and trans-keto tautomers contain protonated Schiff base, while the hydrogen atom bonds to the oxygen atom in the enol-closed and enol-open tautomers (Figure 1). The geometry optimization has shown that the three-phenyl ring system has two stable orientations with slightly different energies for every of the above-described tautomers. The enol-open tautomer is the only exception, which has only one stable orientation. Energy differences between the two orientations of the same tautomer are below 0.02 eV for the cis-keto and enol-closed tautomers and about 0.1 eV for the trans-keto tautomer. Total energies and dihedral angles of phenyl ring positions of the most stable forms of different tautomers are presented in Table 1. The enol-closed tautomer has the lowest ground state energy. Figure 5 summarizes these results and also presents energies of three lowest singlet excited states and oscillator strengths of corresponding electronic transitions. The excited state energies were obtained by adding excitation energies calculated by the TD-DFT method to the total ground state energies of tautomers calculated as described above. Only the electronic transition to the S1 state, which is of π-π* character, has sizable oscillator strength. The other two transitions are optically forbidden. Different hydrogen bonded complexes are expected to be formed in protic solvents since MS1 tautomers may form H-bonds with one or several molecules from the protic solvent. The most stable structures of enol-closed and cis-keto MS1 tautomers with two ethanol molecules added in the vicinity are shown in Figure 6. The characteristic energies corresponding to formation of different possible complexes are presented in Table 2. Formation energies were determined as differences between total energies of investigated complexes and total energies of separate molecules defined from calculation based on the B3LYP/ 6-311þþG(2d,p) method. The zero point energies were also taken into account. Almost all investigated H-bond complexes are formed by H-bonds of similar type (H-bonds of hydrogen atoms of ethanol with oxygen atoms of MS1 or ethanol) and corresponding formation energies may be regarded as energies of 1864



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Table 1. Most Stable Tautomers of the N-(Triphenylmethyl)salicylidenimine Molecule in the Ground Electronic State and Their Relative Energies without Addition of Solvent Moleculesa structure cis-keto trans-keto enol-closed enol-open

dihedral angle of ring 3,b deg

ΔE (S0), eV



-131

-26

109

0.18

33 -9

147 121

-105 -117

0.72 0.0

-117

18

124

0.39

a

The zero point energy was not taken into account. b The dihedral angle is defined by four atoms: C-N-C atoms from the Schiff base and the second C atom from the selected phenyl ring (the numbering of C atoms in phenyl rings starts from the C atom connected to the Schiff base; see Figures 1 and 6).

H-bond between two ethanol molecules. Thus, the total energy associated with the H-bond formation between MS1 and ethanol equals ΔEhb ¼ -Ehb ðMS1-ethanolÞ þ Ehb ðethanol-ethanolÞ

Figure 5. Calculated ground state and three lowest energy excited state energies, and corresponding oscillator strengths of MS1 in different tautomeric forms.

one or two H-bonds. The H-bond energies range from 0.145 eV for the enol-closed tautomer complex with 1 ethanol molecule to 0.314 eV for H-bond formation of the enol-open tautomer with the second ethanol molecule. The latter H-bond is the strongest one and is of a different origin compared with all other investigated H-bonds. It is formed between the oxygen atom from the ethanol molecule and the hydrogen atom from the OH group of the enol-open form of MS1 molecule. It should be noted that much higher H-bond energies were obtained for the cis-keto tautomer than for the enol-closed tautomer, thus making the cisketo tautomer complex with two H-bonded ethanol molecules energetically most favorable. Stabilization of the keto tautomeric form by H-bonded solvent molecules was also demonstrated for 2-(N-methyl-R-iminoethyl)phenol molecules.27 The H-bond formation is expected to convert the enol-closed tautomer, which is the most energetically favorable in case of isolated molecules, to the cis-keto tautomer in the complex of MS1 with two ethanol molecules.

’ DISCUSSION Formation of all possible H-bonds including H-bonds between solvent molecules should be accounted for the evaluation of the stability of different tautomeric forms in solution. This is because the H-bond formation between MS1 and solution molecules is in competition with the formation of H-bonds between two solvent molecules. According to our estimations the energy of a single H-bond between two ethanol molecules equals 0.167 eV. Thus, it is reasonable to assume that the H-bond formation between the MS1 and ethanol molecule breaks the

It is evident that this energy is negative if the H-bond energy between MS1 and ethanol is larger than the H-bond energy between two ethanol molecules and is positive in the opposite case. The total tautomer energies in the ground state presented in Figure 7 were calculated as a sum of energies of isolated MS1 molecules for structures given in Table 1 and the H-bond formation energies. The vibrational zero point energy was taken into account during these calculations. As we can see, the enolclosed tautomer without H-bonded ethanol molecules has the lowest energy. This tautomer with one or two H-bonded ethanol molecules has a higher energy by 0.22 and 0.42 eV, respectively. The cis-keto tautomer with one or two H-bonded ethanol molecules has almost equal energies, which are higher by 0.49 eV than the energy of the enol-closed tautomer without H-bonded ethanol molecules. Simple estimation based on the Boltzmann distribution implies that about 14% of MS1 molecules should be in the cis-keto form with one or two H-bonded ethanol molecules. While remaining molecules should be in the enol-closed form, most of them (about 62%) are expected to be not H-bonded with ethanol molecules. This is in qualitative agreement with the experimentally defined absorption spectrum, which shows only a weak cis-keto absorption band in ethanol solution, suggesting that only about 3% of MS1 molecules are in cis-keto form. On the basis of our calculations, we suppose that even in protic solvents the enol-closed tautomers do not form H-bonds. This explains the very close similarity of the steady state and transient absorption properties of the enol-closed tautomers of the MS1 molecules in ethanol and cyclohexane. The calculations also support the conclusion that the MS1 molecules in aprotic solvent are in an enol-closed form. The close similarity of absorption spectra of the MS1 and SA molecules12,13,18,23,24 indicates that the three-phenyl ring attachment has a weak impact on the molecule properties in the ground state. Transient absorption dynamics of the excited enol-closed form molecules is also in agreement with the excited state relaxation mechanism proposed in ref 18. According to this mechanism, an ultrafast intramolecular proton transfer takes place in the excited enol-closed MS1 tautomers transferring them to the excited cis-keto tautomeric form. This process is supported by our calculations, which show that the excited cisketo tautomers have lower energy in comparison with the excited enol-closed tautomers. Subsequent relaxation of the molecules proceeds via two channels: directly to the ground state of cis-keto 1865



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Figure 6. Calculated structures of enol-closed and cis-keto MS1 tautomers with two hydrogen bonded ethanol molecules.

Table 2. Relative Ground State Energies of Investigated Complexesa H-bond

a

bond energy, eV

ethanol-ethanol

0.167

cis-keto-ethanol

0.282

cis-keto with ethanol-ethanol

0.175

enol-closed-ethanol enol-closed with ethanol-ethanol

0.145 0.147

trans-keto-ethanol

0.295

trans-keto with ethanol-ethanol

0.285

enol-open-ethanol

0.155

enol-open with ethanol-ethanol

0.314

cis-keto-enol-closed

0.172

The zero point energy was taken into account.

tautromers or to the ground state of the trans-keto tautomeric form via conformational transformation. Since the cis-trans transformation involves a large scale conformational motion, one may expect that it may be strongly influenced by the presence of bulky three-phenyl substituent. Indeed, our data show that the excited state relaxation time of the MS1 molecules is about 10 ps in comparison with the 6 ps time as indicated in ref 18 for SA molecules possessing no any substituent. Judging from the intensity of the long-lived induced absorption, trans-keto tautomer formation yield for MS1 is quite similar to that for other anil molecules.19,30 The influence of the three-phenyl substituent is surprisingly weak. The molecule has several possible degrees of freedom during the isomerization reaction, and hampering of one of the motion coordinates evidently only weakly influences the reaction dynamics. Excitation of the cis-keto tautomers creates a short-lived excited state transient and the photoproduct with different spectrum in comparison with the spectrum of the photoproduct created by excitation of the enol-closed tautomers. According to the model developed in ref 18, excitation of the cis-keto tautomers should produce the same excited cis-keto transient directly, which is produced in about 50 fs after excitation of the enolclosed tautomers. Hence, the differences in the excited state relaxation and properties are expected only on a femtosecond time scale. Different transient absorption spectra obtained with excitation of the enol-closed and cis-keto tautomers show that

Figure 7. Calculated ground state and the π-π* excited state energies of different MS1 tautomers with different numbers of hidrogen bonded ethanol molecules.

this model should be reconsidered. Similar transient absorption spectra possessing two induced absorption bands were also observed for BSP in a very strongly proton-donating solvent under excitation of keto form molecules31 and even for N-(3,5ditert- butylsalicylidene)-2-aminopyridine (2P) and N-(3,5-ditert- butylsalicylidene)-4-aminopyridine (4P) in aprotic solvents,32 containing only enol form molecules. However, the origin of the long wavelength induced absorption band has not been discussed, a priori attributing it to the intrinsic properties of molecules under investigation. Our results unambiguously show that the transient species of MS1 with the long wavelength induced absorption band may be created only by direct excitation of the cis-keto form molecules. We suggest that the difference between the cis-keto form obtained by direct excitation and by electron transfer from the excited enol-closed form is related to H-bonded ethanol molecules. Direct excitation prepares excited cis-keto species with one or two H-bonded ethanol molecules in contrast to the excited cisketo form prepared from the excited enol-closed form, which has no H-bonded ethanol molecules. The H-bonded solvent molecules influence electronic properties of excited states and/or change their relaxation pathways. 1866



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Figure 8. Relaxation model of excited enol-closed (left part) and cis-keto (right part) MS1 tautomers.

The absence of the gain band is an another important feature of directly excited cis-keto molecules. The gain is observed only at a zero delay time at short probe wavelength (Figure 4a). At longer delays, when the gain band shifts to the long wavelength side, it is evidently masked by the strong excited state absorption. Another possible explanation is that the transition dipole moment decreases simultaneously with the shift of the spectrum during the excited state stabilization. Such a decrease may be related to the conformational motion of molecules. Rotation of some molecular bonds may lead to formation of “dark” states with the weak transition dipole moment.47 This assumption is supported by the results presented in ref 32, where only a weak gain band of 2P and 4P molecules was observed in solution, while the gain band was much stronger in a solid state, where conformational changes of molecules are hindered. Formation of photoproducts has been observed in numerous investigations of anil class molecules.12,13,18,19,23,24,29-33,36 The photoproduct has been typically attributed to the trans-keto tautomer formed during the excited state relaxation. The shape of the long-lived induced absorption spectrum of the MS1 molecules observed under excitation of the enol-closed tautomers is very similar to that typically observed for other anil molecules and thus should be also attributed to the trans-keto tautomers. Our calculations show that the trans-keto tautomers of MS1 have a narrow energy gap and, thus, may indeed be responsible for the observed long-lived induced absorption. However, the photoproduct formed under direct excitation of the cis-keto tautomers is different. We call it photoproduct 2 (Figure 8). The absorption spectrum of photoproduct 2 is less red-shifted than that formed by excitation of the enol-closed tautomers. The two photoproducts evidently differ by their conformation. The absorption spectrum of photoproduct 2 is only about 20-30 nm red-shifted relative to the steady state absorption spectrum of cis-keto tautomers. Therefore, it is more plausible to attribute it to conformationaly modified cis-keto tautomers as revealed by our quantum chemical calculations, rather than to the trans-keto tautomers. However, this interpretation meets with the difficulty to explain

why conformationally modified cis-keto tautomers are not formed under excitation of enol-open tautomers. The most plausible explanation is that H-bonded ethanol molecules significantly impede conformational motions of the MS1 molecules. The MS1 molecule contains large three-phenyl groups on one side of the molecule (Figure 6). Attached ethanol molecules are connected to the other part of the molecule and impede motion of one part relative to another during the tautomerization reaction. Our relaxation model is presented in Figure 8. The left part describing relaxation of the excited enol-open tautomers is identical to that developed in ref 18, while the right part describes our relaxation model of excited cis-keto tautomers.

’ CONCLUSIONS Subnanosecond fluorescence and absorption pump-probe spectroscopy with femtosecond time resolution, together with quantum chemical calculations have been used for the investigation of relaxation of the photoexcited MS1 molecules. Fluorescence of MS1 molecules is very weak and is completely prevailed by the fluorescence with several nanosecond lifetime of some minor species present in solutions, probably MS1 aggregates. The absorption pump-probe spectroscopy revealed different spectral properties and relaxation mechanisms of the MS1 excited state when enol-closed or cis-keto form molecules have been excited. Relaxation of the excited enol-closed form molecules occurs in a very similar way as reported for other anil molecules. The attached three-phenyl substitutient does not influence the relaxation mechanism substantially. Photoexcited cis-keto form molecules have an additional long wavelength excited state absorption band and produce photoproduct with the absorption spectrum less shifted to the long wavelength side in comparison with that produced under excitation of enolclosed form molecules. We attribute it to the conformationaly distorted cis-keto form. On the basis of the quantum chemistry calculations we suggest that the difference in the excited state properties of the excited enol-closed and cis-keto molecules is related to the hydrogen bonded ethanol molecules. 1867



’ AUTHOR INFORMATION Corresponding Author

*Fax: þ370 5 2602317. E-mail: renata@ar.fi.lt.

’ ACKNOWLEDGMENT This research was supported by the global grant of the Lithuanian Scientific Council according to the operational program for the human resources development and by BalticTaivan collaborative grant. We are much indebted to Professor Miroszaw Soroka (Department of Chemistry, Wroczaw University of Technology) for his gift of MS1 sample. ’ REFERENCES (1) Mathies, R. A.; Lin, S. W.; Ames, J. B.; Pollard, W. T. Annu. Rev. Biophys. Biophys. Chem. 1991, 20, 491–518. (2) Krebs, M. P.; Khorana, H. J. J. Bacteriol. 1993, 175, 1555–1560. (3) Kietis, P.; Saudargas, P.; Valkunas, L. Lith. J. Phys. 2005, 45, 397– 409. (4) Kietis, P. B.; Saudargas, P.; Varo, G.; Valkunas, L. Eur.Biophys. J. 2007, 36, 199–211. (5) Cohen, M. D.; Schmidt, G. M. J. J.Chem. Soc. 1964, 1969. (6) Becker, R. S.; Richey, W. F. J. Am. Chem. Soc. 1967, 89, 1298– 1302. (7) Richey, W. F.; Becker, R. S. J. Chem. Phys. 1968, 49, 2092. (8) Potashnik, R.; Ottolenghi, M. J. Chem. Phys. 1969, 51, 3671– 3681. (9) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J. Am. Chem. Soc. 1980, 102, 2786–2791. (10) Lewis, J. W.; Sandorfy, C. Can. J. Chem. 1982, 60, 1738–1746. (11) Yuzawa, T.; Takahashi, H.; Hamaguchi, H. Chem. Phys. Lett. 1993, 202, 221–226. (12) Mitra, S.; Tamai, N. Chem. Phys. Lett. 1998, 282, 391–397. (13) Mitra, S.; Tamai, N. Chem. Phys. 1999, 246, 463–475. (14) Shen, M. Y.; Zhao, L. Z.; Goto, T.; Mordzinski, A. J. Lumin. 2000, 87-89, 667–669. (15) Zgierski, M.; Grabowska, A. J. Chem. Phys. 2000, 112, 6329– 6337. (16) Ogawa, K.; Harada, J.; Fujiwara, T.; Yoshida, S. J. Phys. Chem. A 2001, 105, 3425–3427. (17) Vargas, V.; Amigo, L. J. Chem. Soc., Perkin Trans. 2001, 2, 1124– 1129. (18) Mitra, S.; Tamai, N. Phys. Chem. Chem. Phys. 2003, 5, 4647– 4652. (19) Ziolek, M.; Kubicki, J.; Maciejewski, A.; Naskrecki, R.; Grabowska, A. Chem. Phys. Lett. 2003, 369, 80–89. (20) Rospenk, M.; Krol-Starzomska, I.; Filarowski, A.; Koll, A. Chem. Phys. 2003, 287, 113–124. (21) Ogawa, K.; Harada, J. J. Mol. Struct. 2003, 647, 211–216. (22) Fabian, W. M. F.; Antonov, L.; Nedeltcheva, D.; Kamounah, F. S.; Taylor, P. J. J. Phys. Chem. 2004, 108, 7603–7612. (23) Okabe, C.; Nakabayashi, T.; Inokuchi, Y.; Nishi, N.; Sekiya, H. J. Chem. Phys. 2004, 121, 9436–9442. (24) Ziolek, M.; Kubicki, J.; Maciejewski, A.; Naskrecki, R.; Grabowska, A. Phys. Chem. Chem. Phys. 2004, 6, 4682–4689. (25) Lewanowicz, A.; Olszowski, A.; Dziekonski, P.; Leszczynski, J. J. Mol. Model 2005, 11, 398–406. (26) Rodriguez-Cordoba, W.; Zugazagoitia, J. S.; Collado-Fregoso, E.; Peon, J. J. Phys. Chem. A 2007, 111, 6241–6247. (27) Macernis, M.; Kietis, B. P.; Sulskus, J.; Lin, S. H.; Hayashi, M.; Valkunas, L. Chem. Phys. Lett. 2008, 466, 223–226. (28) Ortiz-Sanchez, J. M.; Gelabert, R.; Moreno, M.; Lluch, J. M. J. Chem. Phys. 2008, 129, 214308. (29) Ziolek, M.; Filipczak, K.; Maciejewski, A. Chem. Phys. Lett. 2008, 464, 181–186.

ARTICLE

(30) Ruckebusch, C.; Sliwa, M.; Rehault, J.; Naumov, P.; Huvenne, J. P.; Buntinx, G. Anal. Chim. Acta 2009, 642, 228–234. (31) Ziolek, M.; Burdzinski, G.; Karolczak, J. J. Phys. Chem. A 2009, 113, 2854–2864. (32) Sliwa, M.; Mouton, N.; Ruckebusch, C.; Aloise, S.; Poizat, O.; Buntinx, G.; Metivier, R.; Nakatani, K.; Masuhara, H.; Asahi, T. J. Phys. Chem. C 2009, 113, 11959–11968. (33) Sliwa, M.; Mouton, N.; Ruckebusch, C.; Poisson, L.; Idrissi, A.; Aloise, S.; Potier, L.; Dubois, J.; Poizat, O.; Buntinx, G. Photochem. Photobiol. Sci. 2010, 9, 661–669. (34) Hadjoudis, E.; Mavridis, I. M. Chem. Soc. Rev. 2004, 33, 579– 588. (35) Sliwa, M.; Letard, S.; Malfant, I.; Nierlich, M.; Lacroix, P. G.; Asahi, T.; Masuhara, H.; Yu, P.; Nakatani, K. Chem. Mater. 2005, 17, 4727–4735. (36) Ziolek, M.; Kubicki, J.; Maciejewski, A.; Naskrecki, R.; Luniewski, W.; Grabowska, A. J. Photochem. Photobiol. A 2006, 180, 101–108. (37) Lewanowicz, A. Photoactivity of organic systems. Photochromism and photolumescence of 1,4-dihydropyridines and Schiff bases; Scientific Papers of the Faculty of Chemistry; Wroclaw University of Technology: Wroclaw, 2007; No. 4. (38) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules: Oxford University Press: Oxford, U.K., 1989. (39) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (40) Dreuw, A.; Head-Gordon, M. Chem. Rev. 2005, 105, 4007. (41) Casida, M. E. Recent Advances in Density Functional Methods; World Scientific: Singapore, 1995, 155. (42) Marques, M. A. L.; Ullrich, C. A.; Nogueira, F.; Rubio, A.; Burke, K.; Gross, E. K. U. Time-Dependent Density Functional Theory, Lecture Notes in Physics: Springer: Berlin, 2006. (43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M., Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (44) Frisch, A.; Nielsen, A. B.; Holder, A. J. Gaussview Users Manual; Gaussian Inc.: Pittsburgh, PA, 2000. (45) Otsubo, N.; Okabe, C.; Mori, H.; Sakota, K.; Amimoto, K.; Kawato, T.; Sekiya, H. J. Photochem. Photobiol. A 2002, 154, 33–39. (46) Mukhopadhyay, M.; Banerjee, D.; Koll, A.; Filarowski, A.; Mukherjee, S. Chem. Phys. Lett. 2006, 420, 316–320. (47) Gulbinas, V.; Kodis, G.; Jursenas, S.; Valkunas, L.; Gruodis, A.; Mialocq, J.-C.; Pommeret, S.; Gustavsson, T. J. Phys. Chem. A 1999, 103, 3969–3980.

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