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Femtosecond transient absorption spectroscopy of non-substituted photochromic spirocompounds
8 December 2000
Chemical Physics Letters 331 (2000) 378±386
www.elsevier.nl/locate/cplett
Femtosecond transient absorption spectroscopy of non-substituted photochromic spirocompounds S.A. Antipin *, A.N. Petrukhin, F.E. Gostev, V.S. Marevtsev, A.A. Titov, V.A. Barachevsky, Yu.P. Strokach, O.M. Sarkisov N.N. Semenov Institute of Chemical Physics, O.M. Sarkisov Laboratory, Russian Academy of Sciences, Kosygin str. 4, 117977 Moscow, Russian Federation Received 8 September 2000
Abstract Primary processes in spironaphtopyran (SNP), spironaphtoaxazine (SNO) and spirophenantrooxazine (SPO) after excitation by the femtosecond light pulse with the carrier wavelength 308 nm were studied by the absorption `pumpprobe' method with femtosecond time resolution. Probing was performed by means of the super-continuum light pulse with the carrier wavelength in the range 420±580 nm. Registration of the photoinduced absorption spectra dynamics allowed to observe time evolution of all intermediates. It appeared that photoinduced transformation of all three compounds has the same mechanism. Kinetic scheme is suggested and all rate constants entering this scheme are determined. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Under irradiation, photochromic compounds can suer reversible intramolecular transformations, which are accompanied by variation of the absorption spectrum [1]. This property of the photochromic compounds may be used in variety of applications. One of a most promising directions is the creation of the 3D optical memory devices [2]. In the photochromic spirocompounds, two polycyclic parts of the molecule connected by one carbon spiroatom are mutually orthogonal and interact but weakly [3]. Absorption spectrum of the spirocompounds lies in the UV region. Cleav-
*
Corresponding author. E-mail address: [email protected] (S.A. Antipin).
age of the bond between the spiroatom and oxygen atom takes place under absorption of the light quantum [4]. The molecule gets the possibility of free rotation about the bonds of the open ring. This leads to the formation of the so-called form B which has a structure resembling that of merocyanine dyes. Absorption spectrum of the form B lies in the visible region. Structure formulas of spirocompounds of interest and scheme of the photochromic transformation are given in Fig. 1. It is well known that photochemical reaction in unsubstituted spirocompounds proceeds only via singlet excited state [5]. Characteristic times of the corresponding primary processes are very small (of the order of 1 ps). Therefore, spectral methods with the ultrahigh time resolution are needed to study these processes. Several works [6±10] were published, in which photochromic reactions of non-substituted
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 2 1 3 - 6
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
379
Fig. 1. Structure formulas of initial (A) and colored (B) forms of SNO and initial forms of SNP and SNO.
spirocompounds were studied using picosecond or sub-picosecond spectroscopy. First of all there are the works of Ernsting [7,8] on spironaphtopyran (SNP) and Tamai and Masuhara [10] on spironaphtoaxazine (SNO). In [8], studies of SNP and spirobenzopyran were performed with the time resolution of 0.42 ps. It was shown that increase of the form B absorption takes place with the characteristic time 1.4 ps and already up to 10 ps steady distribution of isomers is reached. Kinetic curves presented in [8] have vaguely pronounced minimum at times smaller than 1 ps. But because of the insucient time resolution, this fact was not paid attention except the correction of spectra, the necessity of which is conditioned by the dispersion (`chirp') of the super-continuum light pulse, methods of such corrections were described in the later work by Ernsting [11], and also in [12]. In [10], photoisomeration of SNO was studied in the wide enough spectral range (420±700 nm) with the time resolution of 170 fs. Here the appearance of absorption at small times near 490 nm was ascribed to the absorption from the ®rst excited singlet state. After that increase of the form B, absorption was observed. However, use of zinc tetraphenylporphine solution for determination of the delay time zero and correction of spectra seems to be impossible in view of vibrational relaxation in this substance [12].
Thus in the cited works, it was shown that photochemical reaction in both SNP and SNO takes place during less than 10 ps after excitation. The goal of the present Letter was the study of the primary photochromism processes in structurally similar unsubstituted spirocompounds: SNP, SNO and spirophenantrooxazine (SPO), using the methods of registration and correction of the photoinduced absorption spectra at ultrashort times developed in our Letter [12]. It was needed to pick out absorption of possibly all intermediates in the molecule photoisomeration process, identify all these intermediates and determine the rate constants of their transformations. Results obtained for dierent compounds were to be compared in order to reveals eect of the unsubstituted spirocompounds structure on their photochromism. 2. Experimental The femtosecond setup, which includes the initial generator of the femtosecond light pulses, double-cascade laser ampli®er, pumped by pulses of the second harmonics of the ND:YAG ± laser, prism compressor and optical delay line, was used in experiments. Generator of the femtosecond light pulses is a dye laser pumped by a continuous argon laser. A complete description of the setup is given
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S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
in [13]. At the exit of the setup, we got a sequence of the light pulses with the duration shorter than 50 fs, energy 300 lJ and the pulse frequency 25 Hz. In order to perform the pump-probe experiments with the detection of the photoinduced absorption in liquid phase, systems for formation of the pump and probe light pulses and absorption spectra recording were built up. Second harmonics (308 nm) of the pulses produced by the femtosecond setup, generated in non-linear KDP crystal, was used as a pump pulse. The so-called supercontinuum was used as a probe pulse. Supercontinuum pulses, which have large spectral widths, were generated as focusing pulses, given the femtosecond setup, on a quartz cuvette with heavy water. Pump and probe pulses were brought together inside a quartz cuvette through which the solution of the substance under study in hexane was pumped. Absorption spectra were recorded using polychromator and two CCD arrays. A detailed description of the pulse formation and spectra recording can be found in [12]. Super-continuum pulses are strongly chirped and as a result their dierent spectral components reach the sample at dierent moments of time. Therefore, the recorded photoabsorption spectra are not variable spectra for one de®nite moment of time. Corresponding correction of the spectra was performed using the method developed in our work [12]. Pure toluene was used as a reference substance. 3. Results Dynamics of the photoinduced absorption spectra for SNP is presented in Fig. 2. Corrected for the chirp photoinduced absorption spectra for SNO are shown in Figs. 3a±c and for SPO in Figs. 3d±f. Recording of 10 spectra was performed every 7.38 fs. These results were averaged over every ®ve spectra giving the spectra the time interval of about 37 fs. The spectra were recorded and shown in ®gures in the time range of about 4 ps. For SNO and SPO, the photoinduced absorption spectra were recorded and were shown also at large time (with
Fig. 2. Dynamics of the photoinduced absorption spectra for SNP.
the pumping of the solution turned o). Time evolution of all the three compounds spectra may be conditionally divided into three stages. At the ®rst stage (see Fig. 3d), at times up to 250 fs, absorption with the maximum near 490 nm is observed. At the second stage (see Fig. 3e), at times from 250 to 800 fs, decrease of this absorption accompanied by a shift of the maximum to the short-wave region takes place. At the third stage (see Fig. 3f), from 800 fs and further, the decrease of the short-wave absorption with a simultaneous increase of the long-wave one is seen. 4. Discussion Absorption of the quantum of light with the wavelength 308 nm moves the molecule up into the second excited electronic state S2 [14] A S0 hm ! A S2 :
1
The fact that at zero delay between pump and probe pulses photoinduced absorption is not recorded (see Fig. 3a,d) witnesses that compound in the state S2 does not absorb in the spectral range 420±580 nm. Increase of the signal with the maximum at about 490 nm during ®rst 100s of fs (see Fig. 3a,d) corresponds to the population of the
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
381
Fig. 3. Dynamics of the photoinduced absorption spectra for SNO (a, b, c) and for SPO (d, e, f); a ± time delay is: )110 fs (1), 0 fs (2), 73 fs (3), 185 fs (4); b ± time delay is: 665 fs (1), 740 fs (2), 810 fs (3), 885 fs (4), 960 fs (5), 1035 fs (6); c ± time delay is: 1225 fs (1), 3600 fs (2), colored form spectrum shape (3); d ± time delay is: )115 fs (1), )40 fs (2), 35 fs (3), 110 fs (4), 255 fs (5); e ± time delay is: 255 fs (1), 475 fs (2), 550 fs (3), 625 fs (4), 775 fs (5); f ± time delay is: 775 fs (1), 1225 fs (2), 2155 fs (3), colored form spectrum shape (4).
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®rst excited singlet electronic state S1 due to the radiationless transition from S2 . A S2 ! A S1 :
2
The signal observed at this is due to the absorption of probe pulse light quanta leading to transitions from the state S1 to higher singlet states Sn . It is to be noted that the maximum of this adsorption band lies near 490 nm for all three compounds studied. The position of this maximum coincides with those obtained for SNO and SNP in [9,10]. To interpret the observed dynamics of the photoinduced absorption spectra, it is suitable to consider the photochromic reaction model suggested in [15] and presented in Fig. 4. Within this model, the potential surface of the state S1 has two minima. The left one corresponds to the equilibrium con®guration of the initial form A in the electronic state S1 . The right one is lower and originates from the crossing of the ground electronic states, S0 potential surfaces of the forms A and B.
Fig. 4. Photochromic reaction model.
As mentioned above, during the increase and subsequent decrease of photoinduced absorption from the electronic state S1 a smooth shift of this absorption maximum to the short-wave region takes place. It is seen for SPO especially clearly (see Fig. 3d,e). We explain this as follows. Radiationless transition from S2 produces S1 predominantly in the excited vibrational states. During subsequent vibrational relaxation in the state S1 populations of the higher vibrational levels decrease and of the lower ones increase. As a result of this relaxation, the system can with some probability ®nd itself either in the left or in the right potential well of the state S1 (see Fig. 4). From a physical point of view, it means the following. Location of the system in the left potential well corresponds to the ®rst excited singlet state of the initial form A, i.e., a form in which the C±O bond is not broken. When the system is in the right well, then the distance between C and O atoms is much larger. This means that C±O is broken, but the system remains in the excited electronic state. Earlier in the works on photochromism of spirocompounds, there was evidence that the coloring reaction proceeds via some intermediate X, which corresponds to the molecular electronic state with the practically initial structure, but broken C±O bond. First, indication of this sort was made in [16] for SNP. However, due to a very short lifetime of this state, especially for non-substituted spirocompounds, experimental data on their spectroscopic characteristics are missing in the literature. Within the model assumed here, intermediate X will be identi®ed with the system located in the right well of the electronic state S1 . Probabilities of the system to be found in the left or in the right well of state S1 can be estimated based on the results of [14,17]. In [17], it was shown that for SNO and several of its derivatives at 77 K the C±O bond photocleavage at 365 nm (it corresponds to the excitation of the state S1 ), which in the present model corresponds to the formation of X, has quantum yield equal to unity. This conclusion was made based on the absence of ¯uorescence in this Letter. We assume that the situation is analogous for SPO, which is similar to that of SNO structure. In [14], it was shown that
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
for SNP at low temperatures relative quantum yield of ¯uorescence under excitation by light with wavelength of 310 nm is smaller than 0.2. This means that more than 80% of the excited molecules suer C±O bond cleavage and are converted to the intermediate X. Thus, we consider that during photochemical reaction all three compounds under study undergo vibrational relaxation and as a result found ®nally themselves in the right potential well of the state S1 , i.e., in the intermediate X: A S1 ! X:
3
Intermediate X transforms to the ground electronic state S0 . According to the accepted model, the potential well corresponding to X is approximately above the barrier on the potential energy surface of the ground electronic state S0 . The potential well of this surface to the left of the barrier corresponds to the initial form A and that to the right ± to the ®nal photoproduct B. After radiationless transition from S1 to S0 , the molecular system can `roll down' either to the initial form A (closure of the C±O bond) or to the ®nal merocyanine form B. In the latter case, turn of the molecule around the bonds of the open ring takes place. X ! B S0 ;
4
X ! A S0 : Turn now to the spectra of photoinduced absorption obtained here (see Fig. 3). We assign the absorption at wavelengths shorter than 460 nm to the absorption of the intermediate X. The maximum of this absorption lies at wavelengths shorter than 420 nm, i.e., out of the wavelength range studied here. As time grows, a decrease of this absorption is observed with a simultaneous increase of the absorption in the longer-wave region. The latter absorption has maxima at 510 nm for SNP, 540 nm for SNO and 550 nm for SPO. This time variation of absorption spectrum re¯ects nonadiabatic transition from X to S0 with the subsequent turn of the molecule leading to the formation of the form B. Especially visually this successive transition is observed for SNO, where the so-called isosbestic point is present (see
383
Fig. 3b). Long-wave absorption is the colored form B absorption. This conclusion is supported by the identity of this absorption spectrum form with that of the photoinduced absorption spectra recorded at in®nitely large time (with the pumping of the solution turned o) for SNO and SPO (see Fig. 3c,f) and after 1 ns (when the coloring reaction is completed) for SNP. One more peculiarity of the spectra time evolution is to be noted. It appeared that the maximum of the absorption from the state S1 is not always displacing uniformly in time. In particular, in the case of SPO at times 400±700 fs (see Fig. 3e), the maximum retains its position near 400 nm for a relatively long time. This fact can be explained assuming the presence of two potential wells on the S1 potential energy surface. Then absorption in SPO with the maximum at 450 nm can be assigned to the absorption from the potential well of S1 close to the left minimum. The form B formation occurs as a result of the rotation around the bonds of the open ring during the time of the order of ps. At the same time, a little narrowing of form B absorption spectrum takes place during a time lesser than 10 ps (see Fig. 3c,f). Similar narrowing was observed in [8], where it was assumed that absorption at the spectrum wings is connected to the high internal temperature of the molecule. Vibrational±translational energy transfer to the surrounding molecules with characteristic time of the order of 10 ps leads to the narrowing of the spectrum. This narrowing can be interpreted in dierent ways assuming that during the time of the order of 10 ps equilibrium concentrations of the colored form B isomers are established. Thus, within the model assumed here mechanism of the photochemical transformation of the three compounds studied includes the following stages: 1. excitation to the electronic state S2 ; 2. radiationless transition to the excited vibrational states of the electronic state S1 ; 3. vibrational relaxation to the intermediate X, corresponding to the potential well in the S1 , which is located just above the barrier on the ground electronic state S0 (overcoming this barrier leads to the cleavage of the C±O bond);
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4. radiationless transition from X to the ground electronic state leading to the formation of the colored form B (rotation of the molecule about the bonds of the open ring. The above mechanism is presented in the Scheme 1. By the channel characterized by the rate constant k5 , we understand the return of the system back to the initial form A without the C±O bond cleavage, i.e., via the left potential well in the state S1 . As it was mentioned above, this channel can be neglected for all three compounds studied. It is to be noted that the above kinetic scheme is highly approximate because the fast photoreaction process includes relaxation stages and is, generally speaking, dynamical. Therefore, a simple kinetic description seems to be not quite accurate. Nevertheless, we have used Scheme 1 to determine time characteristics of the transformation. The kinetic scheme was described by a system of the ®rst-order dierential equations. It was assumed that the state S2 is populated by the transform-limited light pulse with the duration of 50 fs. The rate constant k4 was expressed via k3 and the quantum yield / of the total photocoloring reaction. If k5 0, then / k3 = k3 k4 . As result a system of dierential equations with three unknown rate constants arises
This system was solved numerically and the values of unknown rate constants were selected in order to ®t the experimental kinetic curves of photoabsorption at dierent wavelengths. The wavelength values for which the kinetic curves were plotted were chosen based on the analysis of the spectra recorded. Kinetic curves recorded for SPO at 510 and 420 nm are shown in Fig. 5. Only state S1 and form B absorb at 510 nm. Therefore, corresponding kinetic curve was modeled by the curve Y1 t e1 y1 t e3 y3 t; where the coecients e1 and e2 are proportional to the extinctions of A S1 and B at 510 nm. These coecients were determined assuming that at
dy0 t=dt p t ÿ k1 y0 t; dy1 t=dt k1 y0 t ÿ k2 y1 t; dy2 t=dt k2 y1 t ÿ k3 y2 t=/; dy3 t=dt k3 y2 t: Here p t is a time envelope of the transformlimited pump light pulse, y0 t A S2 t; y1 t A S1 t; y2 t X t; y3 t B S0 t.
Scheme 1.
Fig. 5. Experimental photoinduced absorption kinetic curves for SPO (daggers) and calculation ®tting (lines): 420 nm (a) and 510 nm (b).
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
small times (shorter than 300 fs), e1 y1 t e3 y3 t and therefore, Y1 t e1 y1 t, and at large times (longer than 5 ps), e1 y1 t e3 y3 t and Y1 t e3 y3 t. Thus, e1 and e3 are equal to the ratios of the measured and calculated values of the absorption at small (in the point of the ®rst maximum) and large (on the plateau) times. Subsequent calculations con®rmed this assumption. Rate constants determined in such a way were used after those in modeling kinetic curve, recorded at 420 nm. Here, already three particles are absorbing: A S1 , X and B and Y2 t e1 y1 t e2 y2 t e3 y3 t
5
was calculated. Fitting only three coecients e1 ; e2 and e3 without variation of the rate constants, it appeared possible to get a good agreement with the experimental curve. Thus, the kinetic scheme suggested allowed to describe the experimental data in the whole spectral range studied and estimate rate constants of the processes taking place. It is to be noted that accuracy of the rate constants' estimations depends on the accuracy of the total reaction quantum yield determination. Values of the quantum yield of the reaction at room temperature available in the literature have large scatter, Therefore average values were used. The rate constants obtained are presented in Table 1. Their accuracy is not worse than 15%. It is seen from Table 1 that all three compounds have practically the same rate constants k1 and k2 . Dierence in the values of k3 is more signi®cant. This dierence may be due to the dierence in the molecular structure. As it was already mentioned, the rate of the form B formation is determined by the rate of the rotation about C±C and C±N bonds. For two almost the same molecules SNO and SNP, the formation of the form B of SNO is two times faster than that of SNP. The reason for Table 1 Calculated rate constants for photoisomerization processes
SNP SNO SPO
k1ÿ1 (fs)
k2ÿ1 (fs)
k3ÿ1 (fs)
100 80 100
250 230 300
1800 1000 2000
385
this can be the easier rotation about C±N bonds than about C±C bonds [17]. 5. Conclusion The technique allowing recording and correcting the photoinduced absorption spectra in the wavelength range 420±590 nm with the time resolution 120 fs was developed. The technique was used to study the processes taking place in SNP, SNO and SPO from the moment of the absorption of the light quantum with the wavelength 308 nm up to the formation of the ®nal photoproduct. It is shown that in all three compounds, the photoreaction has the same mechanism, and the dierences are connected only with the dierent rates of intermediate stages. Time evolution of all intermediates is recorded and kinetic model of the reaction is suggested. According to this model, cleavage of the C±O bond takes place in the excited electronic state. Corresponding to this state, the intermediate absorbs at wavelengths shorter than 460 nm. Rate constants of all the processes entering the kinetic scheme are determined.
Acknowledgements Authors wish to thank S.Ya. Umanskii for the discussions and D.V. Malahov for the help in carrying out experiments. The work was supported by the Russian Foundation of Basic Research (grant No. 99-03-33500) and Ministry of Science and Technology of Russian Federation (grant No. 08.02.33).
References [1] Y. Hirshberg, Compt. Rend. 231 (1950) 903. [2] D.A. Partnenopoulos, P.M. Rentzepis, Science 245 (1989) 843. [3] N.W. Tyer, R.S. Becker, J. Am. Chem. Soc. 92 (1970) 1289. [4] E. Fisher, Y. Hirshberg, J. Chem. Soc. (1952) 4522. [5] C. Lenoble, R.S. Becker, J. Photochem. 34 (1986) 83. [6] S. Sneider, F. Mindl, G. El®nger, M. Melzig, Ber. Bunsenges Phys. Chem. 91 (1987) 1222. [7] N.P. Ernsting, Chem. Phys. Lett. 159 (1989) 526.
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[8] N. Ernsting, T. Arthen-Engeland, J. Phys. Chem. 95 (1991) 502. [9] S. Aramaki, G.H. Atkinson, J. Am. Chem. Soc. 114 (1992) 438. [10] N. Tamai, H. Masuhara, Chem. Phys. Lett. 191 (1992) 189. [11] S.A. Kovalenko, A.L. Dobryakov, J. Ruthman, N.P. Ernsting, Phys. Rev. A 59 (1999) 1. [12] A.N. Petrukhin, S.A. Antipin, F.E. Gostev, V.S. Marevtsev, A.A. Titov, D.G. Tovbin, V.A. Barachevsky, Yu.P. Strokach, O.M. Sarkisov, Chem. Phys. Rep. 19 (2000) 90. [13] F.E. Gostev, A.A. Kachanov, S.A. Kovalenko, V.V. Lozovoy, S.I. Panov, O.M. Sarkisov, E.E. Sviridenkov,
[14] [15]
[16] [17]
A.A. Titov, D.G. Tovbin, Instrum. Exp. Tech. 39 (1996) 567. R.S. Becker, E. Dolan, D.E. Balke, J. Chem. Phys. 50 (1969) 239. D. Phillips, J. Lemaire, C.S. Burton, W.A. Noyes Jr., in: W.A. Noyes Jr., G.S. Hammond, J.N. Pits Jr. (Eds.), Advances in Photochemistry, vol. 5, Interscience, New York, 1968, p. 329. R. Heiligman-Rim, Y. Hirshberg, E. Fisher, J. Phys. Chem. 66 (1962) 2470. V.S. Marevtsev, N.L. Zaichenko, J. Photochem. Photobiol. A 104 (1997) 197.
Chemical Physics Letters 331 (2000) 378±386
www.elsevier.nl/locate/cplett
Femtosecond transient absorption spectroscopy of non-substituted photochromic spirocompounds S.A. Antipin *, A.N. Petrukhin, F.E. Gostev, V.S. Marevtsev, A.A. Titov, V.A. Barachevsky, Yu.P. Strokach, O.M. Sarkisov N.N. Semenov Institute of Chemical Physics, O.M. Sarkisov Laboratory, Russian Academy of Sciences, Kosygin str. 4, 117977 Moscow, Russian Federation Received 8 September 2000
Abstract Primary processes in spironaphtopyran (SNP), spironaphtoaxazine (SNO) and spirophenantrooxazine (SPO) after excitation by the femtosecond light pulse with the carrier wavelength 308 nm were studied by the absorption `pumpprobe' method with femtosecond time resolution. Probing was performed by means of the super-continuum light pulse with the carrier wavelength in the range 420±580 nm. Registration of the photoinduced absorption spectra dynamics allowed to observe time evolution of all intermediates. It appeared that photoinduced transformation of all three compounds has the same mechanism. Kinetic scheme is suggested and all rate constants entering this scheme are determined. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Under irradiation, photochromic compounds can suer reversible intramolecular transformations, which are accompanied by variation of the absorption spectrum [1]. This property of the photochromic compounds may be used in variety of applications. One of a most promising directions is the creation of the 3D optical memory devices [2]. In the photochromic spirocompounds, two polycyclic parts of the molecule connected by one carbon spiroatom are mutually orthogonal and interact but weakly [3]. Absorption spectrum of the spirocompounds lies in the UV region. Cleav-
*
Corresponding author. E-mail address: [email protected] (S.A. Antipin).
age of the bond between the spiroatom and oxygen atom takes place under absorption of the light quantum [4]. The molecule gets the possibility of free rotation about the bonds of the open ring. This leads to the formation of the so-called form B which has a structure resembling that of merocyanine dyes. Absorption spectrum of the form B lies in the visible region. Structure formulas of spirocompounds of interest and scheme of the photochromic transformation are given in Fig. 1. It is well known that photochemical reaction in unsubstituted spirocompounds proceeds only via singlet excited state [5]. Characteristic times of the corresponding primary processes are very small (of the order of 1 ps). Therefore, spectral methods with the ultrahigh time resolution are needed to study these processes. Several works [6±10] were published, in which photochromic reactions of non-substituted
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 2 1 3 - 6
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
379
Fig. 1. Structure formulas of initial (A) and colored (B) forms of SNO and initial forms of SNP and SNO.
spirocompounds were studied using picosecond or sub-picosecond spectroscopy. First of all there are the works of Ernsting [7,8] on spironaphtopyran (SNP) and Tamai and Masuhara [10] on spironaphtoaxazine (SNO). In [8], studies of SNP and spirobenzopyran were performed with the time resolution of 0.42 ps. It was shown that increase of the form B absorption takes place with the characteristic time 1.4 ps and already up to 10 ps steady distribution of isomers is reached. Kinetic curves presented in [8] have vaguely pronounced minimum at times smaller than 1 ps. But because of the insucient time resolution, this fact was not paid attention except the correction of spectra, the necessity of which is conditioned by the dispersion (`chirp') of the super-continuum light pulse, methods of such corrections were described in the later work by Ernsting [11], and also in [12]. In [10], photoisomeration of SNO was studied in the wide enough spectral range (420±700 nm) with the time resolution of 170 fs. Here the appearance of absorption at small times near 490 nm was ascribed to the absorption from the ®rst excited singlet state. After that increase of the form B, absorption was observed. However, use of zinc tetraphenylporphine solution for determination of the delay time zero and correction of spectra seems to be impossible in view of vibrational relaxation in this substance [12].
Thus in the cited works, it was shown that photochemical reaction in both SNP and SNO takes place during less than 10 ps after excitation. The goal of the present Letter was the study of the primary photochromism processes in structurally similar unsubstituted spirocompounds: SNP, SNO and spirophenantrooxazine (SPO), using the methods of registration and correction of the photoinduced absorption spectra at ultrashort times developed in our Letter [12]. It was needed to pick out absorption of possibly all intermediates in the molecule photoisomeration process, identify all these intermediates and determine the rate constants of their transformations. Results obtained for dierent compounds were to be compared in order to reveals eect of the unsubstituted spirocompounds structure on their photochromism. 2. Experimental The femtosecond setup, which includes the initial generator of the femtosecond light pulses, double-cascade laser ampli®er, pumped by pulses of the second harmonics of the ND:YAG ± laser, prism compressor and optical delay line, was used in experiments. Generator of the femtosecond light pulses is a dye laser pumped by a continuous argon laser. A complete description of the setup is given
380
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
in [13]. At the exit of the setup, we got a sequence of the light pulses with the duration shorter than 50 fs, energy 300 lJ and the pulse frequency 25 Hz. In order to perform the pump-probe experiments with the detection of the photoinduced absorption in liquid phase, systems for formation of the pump and probe light pulses and absorption spectra recording were built up. Second harmonics (308 nm) of the pulses produced by the femtosecond setup, generated in non-linear KDP crystal, was used as a pump pulse. The so-called supercontinuum was used as a probe pulse. Supercontinuum pulses, which have large spectral widths, were generated as focusing pulses, given the femtosecond setup, on a quartz cuvette with heavy water. Pump and probe pulses were brought together inside a quartz cuvette through which the solution of the substance under study in hexane was pumped. Absorption spectra were recorded using polychromator and two CCD arrays. A detailed description of the pulse formation and spectra recording can be found in [12]. Super-continuum pulses are strongly chirped and as a result their dierent spectral components reach the sample at dierent moments of time. Therefore, the recorded photoabsorption spectra are not variable spectra for one de®nite moment of time. Corresponding correction of the spectra was performed using the method developed in our work [12]. Pure toluene was used as a reference substance. 3. Results Dynamics of the photoinduced absorption spectra for SNP is presented in Fig. 2. Corrected for the chirp photoinduced absorption spectra for SNO are shown in Figs. 3a±c and for SPO in Figs. 3d±f. Recording of 10 spectra was performed every 7.38 fs. These results were averaged over every ®ve spectra giving the spectra the time interval of about 37 fs. The spectra were recorded and shown in ®gures in the time range of about 4 ps. For SNO and SPO, the photoinduced absorption spectra were recorded and were shown also at large time (with
Fig. 2. Dynamics of the photoinduced absorption spectra for SNP.
the pumping of the solution turned o). Time evolution of all the three compounds spectra may be conditionally divided into three stages. At the ®rst stage (see Fig. 3d), at times up to 250 fs, absorption with the maximum near 490 nm is observed. At the second stage (see Fig. 3e), at times from 250 to 800 fs, decrease of this absorption accompanied by a shift of the maximum to the short-wave region takes place. At the third stage (see Fig. 3f), from 800 fs and further, the decrease of the short-wave absorption with a simultaneous increase of the long-wave one is seen. 4. Discussion Absorption of the quantum of light with the wavelength 308 nm moves the molecule up into the second excited electronic state S2 [14] A S0 hm ! A S2 :
1
The fact that at zero delay between pump and probe pulses photoinduced absorption is not recorded (see Fig. 3a,d) witnesses that compound in the state S2 does not absorb in the spectral range 420±580 nm. Increase of the signal with the maximum at about 490 nm during ®rst 100s of fs (see Fig. 3a,d) corresponds to the population of the
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381
Fig. 3. Dynamics of the photoinduced absorption spectra for SNO (a, b, c) and for SPO (d, e, f); a ± time delay is: )110 fs (1), 0 fs (2), 73 fs (3), 185 fs (4); b ± time delay is: 665 fs (1), 740 fs (2), 810 fs (3), 885 fs (4), 960 fs (5), 1035 fs (6); c ± time delay is: 1225 fs (1), 3600 fs (2), colored form spectrum shape (3); d ± time delay is: )115 fs (1), )40 fs (2), 35 fs (3), 110 fs (4), 255 fs (5); e ± time delay is: 255 fs (1), 475 fs (2), 550 fs (3), 625 fs (4), 775 fs (5); f ± time delay is: 775 fs (1), 1225 fs (2), 2155 fs (3), colored form spectrum shape (4).
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®rst excited singlet electronic state S1 due to the radiationless transition from S2 . A S2 ! A S1 :
2
The signal observed at this is due to the absorption of probe pulse light quanta leading to transitions from the state S1 to higher singlet states Sn . It is to be noted that the maximum of this adsorption band lies near 490 nm for all three compounds studied. The position of this maximum coincides with those obtained for SNO and SNP in [9,10]. To interpret the observed dynamics of the photoinduced absorption spectra, it is suitable to consider the photochromic reaction model suggested in [15] and presented in Fig. 4. Within this model, the potential surface of the state S1 has two minima. The left one corresponds to the equilibrium con®guration of the initial form A in the electronic state S1 . The right one is lower and originates from the crossing of the ground electronic states, S0 potential surfaces of the forms A and B.
Fig. 4. Photochromic reaction model.
As mentioned above, during the increase and subsequent decrease of photoinduced absorption from the electronic state S1 a smooth shift of this absorption maximum to the short-wave region takes place. It is seen for SPO especially clearly (see Fig. 3d,e). We explain this as follows. Radiationless transition from S2 produces S1 predominantly in the excited vibrational states. During subsequent vibrational relaxation in the state S1 populations of the higher vibrational levels decrease and of the lower ones increase. As a result of this relaxation, the system can with some probability ®nd itself either in the left or in the right potential well of the state S1 (see Fig. 4). From a physical point of view, it means the following. Location of the system in the left potential well corresponds to the ®rst excited singlet state of the initial form A, i.e., a form in which the C±O bond is not broken. When the system is in the right well, then the distance between C and O atoms is much larger. This means that C±O is broken, but the system remains in the excited electronic state. Earlier in the works on photochromism of spirocompounds, there was evidence that the coloring reaction proceeds via some intermediate X, which corresponds to the molecular electronic state with the practically initial structure, but broken C±O bond. First, indication of this sort was made in [16] for SNP. However, due to a very short lifetime of this state, especially for non-substituted spirocompounds, experimental data on their spectroscopic characteristics are missing in the literature. Within the model assumed here, intermediate X will be identi®ed with the system located in the right well of the electronic state S1 . Probabilities of the system to be found in the left or in the right well of state S1 can be estimated based on the results of [14,17]. In [17], it was shown that for SNO and several of its derivatives at 77 K the C±O bond photocleavage at 365 nm (it corresponds to the excitation of the state S1 ), which in the present model corresponds to the formation of X, has quantum yield equal to unity. This conclusion was made based on the absence of ¯uorescence in this Letter. We assume that the situation is analogous for SPO, which is similar to that of SNO structure. In [14], it was shown that
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
for SNP at low temperatures relative quantum yield of ¯uorescence under excitation by light with wavelength of 310 nm is smaller than 0.2. This means that more than 80% of the excited molecules suer C±O bond cleavage and are converted to the intermediate X. Thus, we consider that during photochemical reaction all three compounds under study undergo vibrational relaxation and as a result found ®nally themselves in the right potential well of the state S1 , i.e., in the intermediate X: A S1 ! X:
3
Intermediate X transforms to the ground electronic state S0 . According to the accepted model, the potential well corresponding to X is approximately above the barrier on the potential energy surface of the ground electronic state S0 . The potential well of this surface to the left of the barrier corresponds to the initial form A and that to the right ± to the ®nal photoproduct B. After radiationless transition from S1 to S0 , the molecular system can `roll down' either to the initial form A (closure of the C±O bond) or to the ®nal merocyanine form B. In the latter case, turn of the molecule around the bonds of the open ring takes place. X ! B S0 ;
4
X ! A S0 : Turn now to the spectra of photoinduced absorption obtained here (see Fig. 3). We assign the absorption at wavelengths shorter than 460 nm to the absorption of the intermediate X. The maximum of this absorption lies at wavelengths shorter than 420 nm, i.e., out of the wavelength range studied here. As time grows, a decrease of this absorption is observed with a simultaneous increase of the absorption in the longer-wave region. The latter absorption has maxima at 510 nm for SNP, 540 nm for SNO and 550 nm for SPO. This time variation of absorption spectrum re¯ects nonadiabatic transition from X to S0 with the subsequent turn of the molecule leading to the formation of the form B. Especially visually this successive transition is observed for SNO, where the so-called isosbestic point is present (see
383
Fig. 3b). Long-wave absorption is the colored form B absorption. This conclusion is supported by the identity of this absorption spectrum form with that of the photoinduced absorption spectra recorded at in®nitely large time (with the pumping of the solution turned o) for SNO and SPO (see Fig. 3c,f) and after 1 ns (when the coloring reaction is completed) for SNP. One more peculiarity of the spectra time evolution is to be noted. It appeared that the maximum of the absorption from the state S1 is not always displacing uniformly in time. In particular, in the case of SPO at times 400±700 fs (see Fig. 3e), the maximum retains its position near 400 nm for a relatively long time. This fact can be explained assuming the presence of two potential wells on the S1 potential energy surface. Then absorption in SPO with the maximum at 450 nm can be assigned to the absorption from the potential well of S1 close to the left minimum. The form B formation occurs as a result of the rotation around the bonds of the open ring during the time of the order of ps. At the same time, a little narrowing of form B absorption spectrum takes place during a time lesser than 10 ps (see Fig. 3c,f). Similar narrowing was observed in [8], where it was assumed that absorption at the spectrum wings is connected to the high internal temperature of the molecule. Vibrational±translational energy transfer to the surrounding molecules with characteristic time of the order of 10 ps leads to the narrowing of the spectrum. This narrowing can be interpreted in dierent ways assuming that during the time of the order of 10 ps equilibrium concentrations of the colored form B isomers are established. Thus, within the model assumed here mechanism of the photochemical transformation of the three compounds studied includes the following stages: 1. excitation to the electronic state S2 ; 2. radiationless transition to the excited vibrational states of the electronic state S1 ; 3. vibrational relaxation to the intermediate X, corresponding to the potential well in the S1 , which is located just above the barrier on the ground electronic state S0 (overcoming this barrier leads to the cleavage of the C±O bond);
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4. radiationless transition from X to the ground electronic state leading to the formation of the colored form B (rotation of the molecule about the bonds of the open ring. The above mechanism is presented in the Scheme 1. By the channel characterized by the rate constant k5 , we understand the return of the system back to the initial form A without the C±O bond cleavage, i.e., via the left potential well in the state S1 . As it was mentioned above, this channel can be neglected for all three compounds studied. It is to be noted that the above kinetic scheme is highly approximate because the fast photoreaction process includes relaxation stages and is, generally speaking, dynamical. Therefore, a simple kinetic description seems to be not quite accurate. Nevertheless, we have used Scheme 1 to determine time characteristics of the transformation. The kinetic scheme was described by a system of the ®rst-order dierential equations. It was assumed that the state S2 is populated by the transform-limited light pulse with the duration of 50 fs. The rate constant k4 was expressed via k3 and the quantum yield / of the total photocoloring reaction. If k5 0, then / k3 = k3 k4 . As result a system of dierential equations with three unknown rate constants arises
This system was solved numerically and the values of unknown rate constants were selected in order to ®t the experimental kinetic curves of photoabsorption at dierent wavelengths. The wavelength values for which the kinetic curves were plotted were chosen based on the analysis of the spectra recorded. Kinetic curves recorded for SPO at 510 and 420 nm are shown in Fig. 5. Only state S1 and form B absorb at 510 nm. Therefore, corresponding kinetic curve was modeled by the curve Y1 t e1 y1 t e3 y3 t; where the coecients e1 and e2 are proportional to the extinctions of A S1 and B at 510 nm. These coecients were determined assuming that at
dy0 t=dt p t ÿ k1 y0 t; dy1 t=dt k1 y0 t ÿ k2 y1 t; dy2 t=dt k2 y1 t ÿ k3 y2 t=/; dy3 t=dt k3 y2 t: Here p t is a time envelope of the transformlimited pump light pulse, y0 t A S2 t; y1 t A S1 t; y2 t X t; y3 t B S0 t.
Scheme 1.
Fig. 5. Experimental photoinduced absorption kinetic curves for SPO (daggers) and calculation ®tting (lines): 420 nm (a) and 510 nm (b).
S.A. Antipin et al. / Chemical Physics Letters 331 (2000) 378±386
small times (shorter than 300 fs), e1 y1 t e3 y3 t and therefore, Y1 t e1 y1 t, and at large times (longer than 5 ps), e1 y1 t e3 y3 t and Y1 t e3 y3 t. Thus, e1 and e3 are equal to the ratios of the measured and calculated values of the absorption at small (in the point of the ®rst maximum) and large (on the plateau) times. Subsequent calculations con®rmed this assumption. Rate constants determined in such a way were used after those in modeling kinetic curve, recorded at 420 nm. Here, already three particles are absorbing: A S1 , X and B and Y2 t e1 y1 t e2 y2 t e3 y3 t
5
was calculated. Fitting only three coecients e1 ; e2 and e3 without variation of the rate constants, it appeared possible to get a good agreement with the experimental curve. Thus, the kinetic scheme suggested allowed to describe the experimental data in the whole spectral range studied and estimate rate constants of the processes taking place. It is to be noted that accuracy of the rate constants' estimations depends on the accuracy of the total reaction quantum yield determination. Values of the quantum yield of the reaction at room temperature available in the literature have large scatter, Therefore average values were used. The rate constants obtained are presented in Table 1. Their accuracy is not worse than 15%. It is seen from Table 1 that all three compounds have practically the same rate constants k1 and k2 . Dierence in the values of k3 is more signi®cant. This dierence may be due to the dierence in the molecular structure. As it was already mentioned, the rate of the form B formation is determined by the rate of the rotation about C±C and C±N bonds. For two almost the same molecules SNO and SNP, the formation of the form B of SNO is two times faster than that of SNP. The reason for Table 1 Calculated rate constants for photoisomerization processes
SNP SNO SPO
k1ÿ1 (fs)
k2ÿ1 (fs)
k3ÿ1 (fs)
100 80 100
250 230 300
1800 1000 2000
385
this can be the easier rotation about C±N bonds than about C±C bonds [17]. 5. Conclusion The technique allowing recording and correcting the photoinduced absorption spectra in the wavelength range 420±590 nm with the time resolution 120 fs was developed. The technique was used to study the processes taking place in SNP, SNO and SPO from the moment of the absorption of the light quantum with the wavelength 308 nm up to the formation of the ®nal photoproduct. It is shown that in all three compounds, the photoreaction has the same mechanism, and the dierences are connected only with the dierent rates of intermediate stages. Time evolution of all intermediates is recorded and kinetic model of the reaction is suggested. According to this model, cleavage of the C±O bond takes place in the excited electronic state. Corresponding to this state, the intermediate absorbs at wavelengths shorter than 460 nm. Rate constants of all the processes entering the kinetic scheme are determined.
Acknowledgements Authors wish to thank S.Ya. Umanskii for the discussions and D.V. Malahov for the help in carrying out experiments. The work was supported by the Russian Foundation of Basic Research (grant No. 99-03-33500) and Ministry of Science and Technology of Russian Federation (grant No. 08.02.33).
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