Femtosecond spectroscopic study on photochromic salicylideneaniline

23 January 1998

Chemical Physics Letters 282 Ž1998. 391–397

Femtosecond spectroscopic study on photochromic salicylideneaniline Sivaprasad Mitra, Naoto Tamai School of Science, Kwansei Gakuin UniÕersity, 1-1-155 Uegahara, Nishinomiya 662, Japan Received 14 August 1997; in final form 20 October 1997

Abstract Photochromic reactions of salicylideneaniline ŽSA. have been studied in solution using femtosecond transient absorption spectroscopy and picosecond fluorescence measurements. In the excited state, SA undergoes intramolecular enol–keto tautomerism and subsequent isomerization to give a colored photoproduct. Both the proton transfer and photochromic product formation were found to occur within few hundreds of femtosecond. The mechanism of these two ultrafast processes and their solvent dependence are discussed. q 1998 Elsevier Science B.V.

1. Introduction Recently there has been a considerable interest to study excited-state intramolecular proton transfer ŽESIPT. reactions due to its wide applications in both the fundamental and technological point of view w1–3x. The literature shows the common mechanism that, in particular cases, where the functional groups with opposite p K a tendencies Že.g., OH and C5O groups. occupy the adjacent positions of a molecule, often undergoes keto–enol tautomerization in the excited state due to the translocation of the acidic proton to the basic moiety. The process occurs at the ultrafast time scale and in most cases the initially excited state is nonfluorescent. Another phenomenon that has also peaked up in present-day research is to study molecular photochromism based on cis–trans isomerization w4,5x. The most important

application of this process is to develop memory storage devices. The photo- and thermo-chromic behaviors of aromatic anils have been the subject of immense interest for more than three decades w6–8x. N-Ž2-hydroxy benzylidene. aniline ŽI, Fig. 1. or most commonly known as salicylideneaniline ŽSA. is the simplest compound of aromatic anils that is known to possess both the above properties. It is generally agreed that, SA undergoes ultrafast proton transfer upon photoexcitation to give the keto tautomer ŽPT, II. which undergoes subsequent isomerization to the colored photochromic product ŽIII.. There has been a lot of controversy over the structure of the photoproduct and now it is believed to have at least two different structures, one is trans quinoid ŽQ T . and the other is cis-zwitterionic ŽQ C . w4x. In a recent report Yuzawa et al. w9x proposed a hybrid structure for the transient

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 2 7 9 - 7

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2. Experimental

Fig. 1. Structural representation of salicylideneaniline ŽSA. and its photoproducts.

species of the photochromic salicylideneaniline. It was also suggested that the relative contribution of Q T and Q C to the hybrid structure of the transient species depends on the solvent polarity. The longlived photochromic product reverts back to the anil by a thermal or photochemical back reaction w10,11x. Most of the previous studies on the photochromism of SA were devoted to elucidate the structure of the photoproduct and the mechanism associated with this process w11–13x, but studies on the dynamics of this process are relatively scarce. Barbara et al. w14x studied picosecond fluorescence kinetics of the proton-transferred form of SA in various environments. They proposed that ESIPT from the excited enol form occurs within 5 ps and photochromic product is formed from the vibrationally hot keto tautomer. However, ESIPT time could not be resolved due to the limitations in instrument response and kinetics of the photochromic product formation was not probed. In this Letter, we report our studies on the dynamics of excited-state intramolecular proton transfer and photochromic product formation processes of salicylideneaniline in cyclohexane and ethanol using femtosecond transient absorption and picosecond fluorescence spectroscopy.

Salicylideneaniline ŽSA. was purchased from Tokyo Kasei, which was recrystallized from methanol. The dried sample was checked to be optically pure with reproducibility of the literature spectral data w14x. The solvents cyclohexane and ethanol were spectroscopic grade and used without further purification. The solution concentration was ; 8 = 10y4 M. All experiments were carried out at 294 " 1 K. The experimental set-up is essentially same as reported elsewhere w15x. Briefly, the laser system consisted of a hybridly mode-locked, dispersion compensated femtosecond dye laser ŽCoherent, Satori 774. and a dye amplifier ŽContinuum, RGA 60-10 and PTA 60.. The dye laser Žgain dye Pyridine 2 and saturable absorber DDI. was pumped with a cw mode-locked Nd:YAG laser ŽCoherent, Antares 76S.. The sample was excited by second harmonic Ž360 nm. of the fundamental Žcenter wavelength 720 nm, pulse width ; 200 fs FWHM. at a repetition rate of 10 Hz. Residual part of the fundamental output is focused in a 1-cm H 2 O cell to generate a femtosecond supercontinuum probe pulse. A computer-controlled translational stage was used to change the time delay between the pump and the probe pulse. Transient spectra were obtained by averaging over 200 pulses and analyzed by an intensified multichannel detector ŽPrinceton Instruments ICCD-576. as a function of probe delay time. The spectra were corrected for the intensity variation and time dispersion of the supercontinuum. Rise and decay curves at a fixed wavelength were measured with a photodiode–monochromator ŽJapan Spectroscopic, CT-10. combination. The fluorescence decay curves were measured with the same laser system as above. The repetition rate of the excitation pulse was reduced with an external pulse picker ŽConoptics, model 360-80, 25D and 305.. The detection system for fluorescence was composed of a monochromator ŽJapan Spectroscopic, CT-10 ., a microchannel-plate photomultiplier ŽHamamatsu MCP R2809U., a constant fraction discriminator ŽTennelec TC454. and a time-to-amplitude convertor ŽTennelec TC864.. The instrument response function was ; 30 ps fwhm. The fluorescence decay and one-wavelength rise

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and decay curves of transient absorption were analyzed by a non-linear least-squares iterative convolution method based on the Marquardt algorithm w16x.

3. Results 3.1. Transient absorption spectra Fig. 2 shows the transient absorption spectra of SA in cyclohexane in different time delay. The spectral features are same in both the solvents studied here. The characteristic features of the spectra can be summarized as below: Ž1. A broad band covering the wavelength region 400–500 nm with a maximum at ; 440 nm is observed just after the excitation. The spectral shape and intensity of this band change very rapidly with time. Ž2. After that a highly intense, sharp band centering the wavelength of ; 420 nm appears very rapidly. The maximum of this band is shifted blue

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than the band maximum appeared just after the excitation. The intensity and the spectral shape of this band change very rapidly Žsee below.. It is also interesting to note the appearance of negative absorption in transient absorption spectrum around 620 nm during this time which is due to the stimulated emission of the proton-transferred form. Ž3. A shoulder appears at around 485 nm remaining all other properties Že.g., spectral shape. almost same. After this the intensity of the 420 nm band decreases with time. Ž4. After a few tens of picoseconds again there remains a very weak and broad band covering the wavelength region 425–550 nm which does not change its shape and intensity even up to several nanoseconds. In order to estimate the time constant of the excited-state photophysics of SA, we have analyzed the time profiles of transient absorption at some selected wavelengths, e.g. 420, 485 and 620 nm in both the solvents. The time profile of 420 nm, where the most intense transient absorption was found in Fig. 2, consists of a ultrafast rise of ; 210 fs in cyclohexane ŽFig. 3. and ; 380 fs in ethanol. The decay consists of two parts, the major component with a few picoseconds Ž4 ps in cyclohexane, 12 ps in ethanol. and relatively smaller part of the excited species relaxes with a very long Ž) 3 ns. decay time in both the cases. The time dependence of the 485 nm transient absorption was also analyzed. It is seen that, here also, the spectrum is associated with a solvent-dependent ultrafast rise time of about a few hundred of femtoseconds and a fast decay time of about a few picoseconds. The tail of the decay curve having a long decay component is also observed in this case. The rise part of the 620 nm stimulated emission could not be identified with our present apparatus due to the very weak absorption intensity of this band. However, the decay part shows a solvent-dependent long decay time of ; 4 ps in cyclohexane and ; 11 ps in ethanol. 3.2. Time-resolÕed fluorescence spectra

Fig. 2. Transient absorption spectra of salicylideneaniline in cyclohexane solution at different time delays.

To analyze the transient absorption data more precisely, we performed fluorescence decay analysis

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Fig. 3. Rise and decay curve of salicylideneaniline transient absorption in cyclohexane at 420 nm. The solid line is the simulation curve of the experimental points Ždots. by the convolution of the laser pulse and a sum of three exponential functions. One rise component of 210 fs and two decay components of 4 ps and ) 3 ns were obtained. The upper part shows the distribution of weighted residuals obtained from best fitting the experimental points.

of SA in both the solvents. Time-resolved fluorescence data were taken at the reported emission maximum of SA Ž; 540 nm.. Time resolution of our present system Ž30 ps fwhm. was found to be incapable to analyze the very fast rise component of the fluorescence or the wavelength dependence of the emission spectrum. The decay of fluorescence data was analyzed with both one and a sum of two exponential functions. The statistical factors like reduced chi-square Ž xr2 . or Durbin–Watson ŽDW. parameter gave a reasonable fitting with two exponential functions but an interestingly pre-exponential factor associated with the long decay was found to be - 0.1% always. So the decay is mostly associated with only one exponential function. The fluores-

cence decay times are found to be ; 4 ps in cyclohexane and ; 11 ps in ethanol. It is seen that these data have very close resemblance with the decay of 620 nm stimulated emission and the fast decay of the 420 nm transient absorption. 4. Discussion To start the discussion of the spectral results obtained from the femtosecond pump–probe experiment, we first consider the broad absorption band just after the excitation with a peak maximum at ; 440 nm. As the excitation with 360 nm laser pulse is at the red edge of the absorption maximum of SA Ž345 nm., we can consider the excitation to the S 1

S. Mitra, N. Tamair Chemical Physics Letters 282 (1998) 391–397

state of the normal keto form of SA. The transient observed just after excitation at 440 nm can be safely considered as due to the S n - yS1 absorption of the normal enol form of SA. Although, as our knowledge, there is no such report of the transient absorption of the initially excited enol form of SA, we have identified this species due to the ultrashort laser pulses with femtosecond temporal resolution. Now we consider the most important part of our experimental result, i.e. the spectra obtained within a few picoseconds after the excitation. The sharp band centering around 420 nm is assigned as due to the proton-transferred keto form of excited SA. The spectral shape changes very rapidly with the formation of a shoulder at ; 485 nm and negative absorption at 620 nm. The time-resolved analysis of the 420 nm absorption and 620 nm stimulated emission gives a decay time of ; 4 ps in cyclohexane. These data correlated well with the fluorescence decay analysis of the SA emission using time-correlated single-photon counting technique. Moreover, the change in the spectral evolution, i.e. increase in the transient absorption intensity and the gain in the stimulated emission with time, indicates that both of

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these appear from the same species. All these observations suggest that the 420 nm absorption is due to the proton-transferred keto form of SA. The rise time of this band gives the proton transfer time in excited SA which is found to be ; 210 fs in cyclohexane and ; 380 fs in ethanol. Another important observation which concludes that the 420 nm absorption is due to the proton-transferred tautomeric form is the transient gain recorded at the spectral region of phototautomeric fluorescence. Although the fluorescence maximum of the tautomeric emission of SA appears at ; 540 nm, the stimulated emission shifts to ; 620 nm due to the mutual overlap of this band with the transient absorption of the proton-transferred keto tautomer. The phenomenon of spectral gain occurring in the phototautomeric fluorescence region is a common feature in intramolecularly hydrogen bonded Schiff bases w17x. The decay of this stimulated emission is almost the same as the decay of the transient absorption ŽFig. 4.. It is noted that the 420 nm absorption band shifts blue and the spectral shape narrows with time as illustrated in Fig. 5. The time constant for this

Fig. 4. Rise and decay curve of salicylideneaniline transient absorption in ethanol, excited with 360 nm laser pulse and monitored at Ža. 420 nm and Žb. 620 nm.

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Fig. 5. Normalized transient absorption spectra of salicylideneaniline in ethanol at the initial time region showing spectral narrowing and blue shift of the band maximum.

spectral shift and narrowing of the 420 nm absorption are estimated to be similar within the experimental error, which is estimated to be ; 400 fs in cyclohexane and ; 600 fs in ethanol. It is well documented that in the case of large molecules in solution, transfer of excess vibrational energy to the surrounding solvent medium occurs within a few to few tens of picoseconds whereas intramolecular vibrational redistribution ŽIVR. or vibrational dephasing within a few hundreds of femtoseconds is very common w18–20x. So we can consider the time-dependent change in 420 nm transient absorption of SA is due to the IVR process. It is also known that the formation of photochromic product from the excited proton-transferred keto form of SA is excitation wavelength-dependent w14,21x. Picosecond studies at low temperature by Barbara et al. w14x showed the presence of bimodal fluorescence kinetics of SA. The short-lived component of the fluorescence, blue-shifted from the long-lived component, was assigned to the vibrationally excited state. It was also proposed that the vibrationally hot state of the proton-transferred form is a common precursor toward the formation of photochromic product and the vibrationally cold fluorescing species. Therefore, the time-dependent spectral shift and narrowing of the 420 nm band in our study can be considered as due to the relaxation of vibrationally hot molecules, leading to the intrinsic reaction process for the formation of photochromic products from the proton-transferred keto tautomer.

The proton transfer rate is strongly modulated by the solvent environment, e.g. Ž210 fs.y1 in cyclohexane and Ž380 fs.y1 in ethanol. It is known that intramolecular proton transfer rate in hydrogen bonding solvents is decreased due to the competition with the possible formation of ‘intermolecular’ hydrogen bonds with the solvent. The photochromic product formation time was also found to be solvent-dependent. The time constant for this process is ; 400 fs in cyclohexane and ; 600 fs in ethanol. As the photochromic product formation is due to the intrinsic reaction process Žleading to relaxation. of the vibrationally hot proton-transferred state, it is difficult to explain at this stage the role of solvent on this process. There may occur some perturbation on the IVR process by the intermolecular hydrogen bond formation of SA with ethanol. Even vibrational energy transfer to the solvent molecules may contribute to some extent in the relaxation process. The broad and very weak transient absorption at longer time delay may be considered as due to the long-lived photochromic product in the ground state which persists over few nanoseconds. The spectral position is similar to that observed by Kownacki et al. w13x from their nanosecond transient absorption spectroscopy. The long decay Ž) 3 ns. associated with the 420 nm transient absorption is due to the presence of this species in the similar spectral region. In conclusion, the dynamics of salicylideneaniline photochromism has been analyzed by means of femtosecond transient absorption and picosecond timeresolved fluorescence spectroscopy. Both the rate of proton transfer and photochromic product formation are found to be solvent-dependent. Further investigation using various solvents is now in progress. The results w22x will show the role of solvent on the dynamics of the photochromic reaction. Isotopic replacement of phenolic hydrogen can suggest the mechanism of the proton transfer Žwhether tunneling or internal vibrational relaxation. and the principal mode associated with vibrational relaxation in the S1 state of proton-transferred keto tautomer of salicylideneaniline, leading to the photochromic products. Acknowledgements SM thanks the Japan Society for the Promotion of Science for a post-doctoral fellowship. A part of this

S. Mitra, N. Tamair Chemical Physics Letters 282 (1998) 391–397

work was supported by a Grant-in-Aid ŽNo. 08218259. for Scientific Research on Priority Area ŽPhotoreaction Dynamics..

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