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Transient behavior of 2-(2′,4′-dinitrobenzyl)pyridine photochromism studied by ultrafast laser spectroscopy
Chemical Physics 306 (2004) 185–189 www.elsevier.com/locate/chemphys
Transient behavior of 2-(2 0,4 0 -dinitrobenzyl)pyridine photochromism studied by ultrafast laser spectroscopy S. Mitra
a,*
, H. Ito b, N. Tamai
b
a
b
Department of Chemistry, North-Eastern Hill University, Permanent Campus, Umshing, Shillong 793 022, India Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gaku-en, Sanda, Hyogo 669 1337, Japan Received 22 April 2004; accepted 26 July 2004 Available online 20 August 2004
Abstract Ultrafast transient behavior of photochromic 2-(2 0 ,4 0 -dinitrobenzyl)pyridine has been studied using femtosecond transient absorption spectroscopy. Three different transient signals could be found in the experiment at different time delays. The dynamics of transient behavior in the short time scale reveal that very fast relaxation (400 ± 100 fs) from the initial excited state leads to the formation of a relatively stable intermediate which is the precursor of proton transfer. The intermediate relaxes to the ground state of proton transferred aci-nitro structure in nanosecond time scale. The relatively slow proton transfer in DNBP is due to the necessity of extensive structural rearrangement before proton transfer. Solvent hydrogen bonding in ethanol seems to play an important role in the deactivation of the excited state giving entirely different photophysical behavior. 2004 Elsevier B.V. All rights reserved.
1. Introduction Molecular systems showing thermal bistability are promising candidates in terms of optical data processing and memory storage devices [1–4]. Often, in these systems, both the forms can be interconverted (ÔswitchingÕ in the forward direction and ÔfadingÕ in the reverse direction) with some external stimulation such as light, temperature, etc. However, the use of these systems in practical application depends on the rate of conversion and stability of the system under repeated cycle. 2(2 0 ,4 0 -Dinitrobenzyl)pyridine (DNBP) is a potential candidate in this regard. For many years, it is known that the pale yellow crystals of DNBP, having the methylene bridge between the dinitro benzene and pyridine moiety (CH form), changes to dark blue upon UV irradiation [4–7]. The dark blue form is due to the transfer of the
*
Corresponding author. Tel.: +91 364 272 2634; fax: +91 364 255 0486. E-mail address: [email protected] (S. Mitra). 0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.07.031
hydrogen from methylene group to the pyridine nitrogen (NH form). The reverse reaction to the original CH form occurs within a time scale of ls–ms. It is reported that the stability of the final form can be increased substantially with judicious substitution in the pyridine ring [8,9]. The mechanism of DNBP photochromism, as known so far, can be depicted as in Fig. 1. The conversion of initial CH form (I) to the final NH form (III) may proceed either directly or through an intermediate geometry in which the methylene hydrogen resides on oxygen of the nitro group in the ortho position (aci-nitro or OH form, II). This intermediate undergoes towards the formation of final product in a relatively slower time scale (100 ls in benzene [10]). All the forward and reverse processes may be activated either by photochemically or thermally. Although the photochromism of DNBP is reported as early as in 1925 [11], the dynamics of this process, complete mechanism of formation and the nature of the transients are not clear so far. Recent flash photolysis experiments in nanosecond time scale on DNBP
186
S. Mitra et al. / Chemical Physics 306 (2004) 185–189 NO2 N h ∆
O
H
C H NO
NH-form (III)
+
N O– N
C H2
N
hν
+
O
O
–
∆
NO2
CH-form (I) N
C H NOOH OH-form (II)
Fig. 1. Representation of different photochromic and thermochromic routes of DNBP.
photochromism have shown the presence of two competitive proton transfer routes exist for the CH ! OH conversion [10]. The first one assigned as a direct temperature independent excited state process and the other one, a much slower, thermally activated multistep proton transfer in the ground state via a colorless precursor (P). It may be interesting to study this proton transfer process with the use of ultrafast laser spectroscopy. Femtosecond pump–probe experiments at selected wavelengths reported earlier indicates that proton transfer from the methylene bridge to the ortho nitro group occurs within a time scale of 320–500 fs [12]. However, considering the complex nature of the transients involved in the DNBP photochromic pathway, observation of dynamic behavior at selected wavelengths may not be sufficient to get a complete picture. It is also important to study the time evolution of the transient spectra after excitation with ultrashort laser pulse with an emphasis to observe the dynamic behavior for the formation of the precursor state, P, proposed by Corval et al. [10] during the conversion of the initially excited CH form (I) towards the formation of the ground state OH structure (II).
urements to avoid the formation of final product. All measurements were performed at 295 ± 2 K. Steady state absorption measurements were performed using Hitachi U-3210 spectrophotometer. The details of laser and transient absorption detection system can be found elsewhere [13,14]. In brief, the system contains a hybridly mode-locked, dispersion-compensated femtosecond dye laser (Coherent Satori 774). This dye laser was pumped by a cw mode-locked Nd:YAG laser (Coherent Antares 76S). The output of the dye laser was amplified upto 400 lJ at a center wavelength of 720 nm by a regenerative amplifier system (Continuum RGA60 and PTA60) with a repetition rate of 10 Hz. The pump pulse at 360 nm was obtained by passing the amplified light through a 1-mm BBO crystal. The residual part of the fundamental light was focused in 1 cm water cell to generate white light supercontinuum which was used as probe pulse after passing through computer controlled delay stage (Sigma Koki, STM – 500X). The system response function in the pump–probe method was estimated to be 200 fs FWHM. Transient signals were obtained by averaging over 200 accumulations in a microcomputer controlled intensified multichannel detector (Princeton instruments, ICCD 576-G). A temporal dispersion of the white-light continuum was corrected for the transient absorption spectra. The analysis of the transient signal at a particular wavelength was done with a non-linear least-square deconvolution program based on Marquardt algorithm [15,16].
3. Results Fig. 2 shows the steady state absorption spectra of DNBP in ethanol at room temperature. The UV absorption band at 250 nm is considered as due to the colorless CH form (I) of DNBP [17]. Irradiation of DNBP with UV light causes the photochromic reaction to occur and the color changes to dark blue having an absorption maximum at 550 nm consisting of NH form. A close
2. Experimental DNBP was purchased from Tokyo Kasei and recrystallized from absolute alcohol before use. Chloroform and ethanol were of spectroscopic grade and used without any further purification. Dilute solutions (105 mol dm3) of DNBP was used for steady state experiments whereas for transient absorption spectra, 5 · 102 mol dm3 solutions were used to get sufficient concentration at the excitation wavelength (360 nm). The sample solutions were allowed to flow through a 2-mm flow cell using a magnetic gyre pump (Micropump, 040-332) during the transient absorption meas-
Fig. 2. Steady state absorption spectra of DNBP solution in ethanol.
S. Mitra et al. / Chemical Physics 306 (2004) 185–189
187
Fig. 3. Transient absorption spectra DNBP solution in chloroform at different time delays (mentioned in the figure) after excitation.
look in Fig. 2 shows the presence of a very weak shoulder at 350 nm. However, we could not get any detectable fluorescence by exciting the sample at this absorption. In Fig. 3, the transient absorption spectra of DNBP in chloroform pumped by a 200-fs pulse at 360 nm and probed by white light supercontinuum are shown. Excitation by 360 nm laser in pump–probe experiment corresponds to the shoulder of the absorption spectra. The time delays of probing after the excitation in the transient absorption spectra are also indicated in Fig. 3. It is seen that at initial time region there exist two broad bands, one at 400–500 nm and the other at 600– 700 nm (with the maxima at 640 nm) region. With the progress of time, the intensity of the second band increases. It is also noted that the peak at 640 nm shifts to the blue side of the spectrum very fast (within 1 ps) and finally the band maxima appears at 600 nm. Around hundreds of picosecond time delay between the pump and probe pulses, the band intensity at 600 nm starts decreasing with a simultaneous appearance of another broad band again at 400–500 nm region of the spectrum. The time dependence of the 600 nm transient signal was analyzed with 200 fs Gaussian laser pulse (Fig. 4). The simulation result gives about 400 fs rise and a slow decay component in nanosecond time scale. The observation of the transient behavior at 420 nm indicates (Fig. 5) that the absorption intensity at this wavelength consist of very fast decay followed by a slow rise in the measured time window.
Fig. 4. Simulation of 600 nm transient absorption intensity variation with 200 fs Gaussian laser pulse.
be explained by Sn S1 absorption of initially excited CH form. Recently, we made similar observation in femtosecond pump–probe spectra for other photochromic
4. Discussion The time evolution of the transient signal for DNBP in chloroform can be explained as below. The broad band around 400–500 nm region at the early time can
Fig. 5. Variation of transient absorption intensity at 420 nm at different delay time.
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S. Mitra et al. / Chemical Physics 306 (2004) 185–189
compounds also [18]. These photoexicited molecules undergo rapid relaxation in the excited state potential energy surface to form relatively stable conformers. The time evolution of transient spectra around 600 nm is most important and needs careful attention. The intensity of this transient absorption increases gradually upto 1 ps and then starts decaying with a concomitant grow in transient signal corresponding to OH form in 400–500 nm region. Previous reports suggest the presence of a precursor state (P) in the slow ground state conversion of CH form to OH form [19,20] in DNBP photochromism. This precursor was proposed to have a structure intermediate to that of CH and OH forms having an absorption peak at 335 nm. This additional ground state conformer was detected kinetically, however, any spectroscopic characterization was lacking due to the instrumental limitation. The initial broad band in the 600–700 nm region in femtosecond transient spectra (Fig. 3) is believed to be originated from this species when pumped with 360 nm laser pulse. As the intensity of this transient absorption increases with the delay time of the probe pulse, we can assume that ground state population of P is also fed from the initially excited CH species. The time constant of this process can be obtained from the rise time of 600 nm band (Figs. 3 and 4), which is found to be 400 fs. The time constant for the very fast decay of the 420 nm transient intensity could not be fitted due to very poor signal to noise ratio, however, visual inspection of Fig. 5 indicates a similar time constant as observed for the growing in of the 600 nm band. This very fast time constant indicates a barrierless (or with very low barrier) relaxation process, i.e., the excited molecules slide down in the potential energy surface towards the formation of the ground state of intermediate, P. This prediction is further supported from the observed blue shift of transient band at 640 nm. In femtosecond pump–probe experimetns, the transient absorption spectral shift occurs mainly due to vibrational cooling [21,22]. The transient absorption of this intermediate at 640 nm shift to the blue side due to very fast intramolecular vibrational relaxation as shown in Scheme 1. The variation of 420 nm transient absorption intensity given in Fig. 5 also supports the discussion made above. Very fast decay of this signal indicates the conversion of initially excited CH form (I) to the metastable intermediate, P and the slow growing in nanosecond time scale is a signature of the formation of OH structure (II). There is no indication of any stimulated emission in the entire spectral region. The stable intermediate undergoes deactivation towards the formation of ground state aci-nitro structure (OH form, II). Transient absorption at 400–500 nm regions at long time delay can be considered as originated from S1 S0 absorption of this species. The spectral position of this band is similar to
TrA1
CH* ~ 400 fs
TrA2
TrA3
hν
P OH (II) CH (I) Scheme 1. Schematic diagram for the photophysical properties of DNBP photochromism in ultrafast time scale. TrAs indicate the transient absorptions at different time delays.
that obtained in nanosecond pump–probe experiments reported earlier [6,17]. Continuous irradiation of DNBP solution by UV light showed that the final photochromic product (NH form) gives an absorption maxima at 550 nm [11]. Nanosecond laser flash photolysis experiments in DNBP showed that the formation of NH form (having absorption peak at 550 nm) from the thermal reaction of OH form (having characteristic absorption at 420 nm) occurs at about 1 ls time scale [6]. So, we do not observe the absorption of the final product within the time limit of our experiment (3 ns). The transient behavior of ethanol differs considerably from that in chloroform. In this case the broad absorption at 400–500 nm region does not disappear in the whole time window. However, the time evolution of the spectra at 600 nm is almost similar in both ethanol and chloroform except the lower absorption intensity in the first case. So, it may be possible that in ethanol, intermolecular hydrogen bonding plays an important role in the photo(thermo)chromic pathway. The early formation of OH species having absorption peak at 400–450 nm indicates the possibility of solvent assisted proton transfer. Solvent assisted proton transfer reactions for many intramolecular proton transfer systems are found to occur in ultrafast time scale [23–25]. This phenomenon becomes much more important in those cases where the initial geometry for proton transfer is not favorable, e.g., when the groups involving the proton transfer are relatively far. However, in presence of alcoholic solvents, a suitable geometry may be formed with bridged hydrogen bond that favors the transfer of the proton more easily. A similar prediction can be made for DNBP too. Solvent assisted proton transfer of excited CH form in ethanol leads to formation of OH structure relatively easily. The lower absorption intensity at 600 nm, corresponding to the precursor state P, indicates that direct formation of OH structure from CH form is more favored pathway in this solvent.
S. Mitra et al. / Chemical Physics 306 (2004) 185–189
5. Conclusion Ultrafast photochromic transients of DNBP are studied in chloroform and ethanol. It was observed for the first time that a stable ground state intermediate, known as a precursor state P as mentioned by Corval et al. [10], is formed within 400 fs during the proton transfer from the excited CH state to the ground state of acinitro structure (OH form) in non-hydrogen bonding solvents. Although the presence of the precursor state before the formation of OH structure has been substantiated unambiguously from ultrafast transient absorption measurements, the structure of this species demands an extensive high level theoretical calculation. In hydrogen bonding solvents like ethanol, the more favored pathway for the photochromic process is the direct conversion of CH form to the OH structure. Acknowledgements S. Mitra thanks JSPS for a fellowship (L-03554) to visit Kwansei Gakuin University. This work is supported in part by Grant-in-Aid (No. 10450326) for Scientific Research (B) from Japanese Ministry of Education, Science and Culture. The authors gratefully acknowledge the comments by one of the reviewers in improving the content of this manuscript.
References [1] U.P. Wild, A. Renn, in: H. Du¨rr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Studies in Organic Chemistry, vol. 40, Elsevier, Amsterdam, 1990. [2] J.-M. Lehn, Angew. Chem., Int. Ed. Engl. 29 (1990) 1304. [3] C. Brau¨chle, N. Hampp, D. Osterhelt, Adv. Mater. 3 (1991) 420.
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[4] R. Ao, S. Jahn, L. Ku¨mmerl, R. Weiner, D. Haarer, Jpn. J. Appl. Phys. 31 (1992) 693. [5] H.S. Mosher, E.R. Hardwick, D. Ben Hurr, J. Chem. Phys. 37 (1962) 904. [6] E. Klemn, D. Klemn, A. Graness, J. Kleinschmidt, Chem. Phys. Lett. 55 (1978) 113. [7] H. Sixl, R. Warta, Chem. Phys. 94 (1985) 147. [8] M. Scherl, D. Haarer, J. Fischer, A. DeCian, J.-M. Lehn, Y. Eichen, J. Phys. Chem. 100 (1996) i16175. [9] Y. Eichen, J.-M. Lehn, M. Scherl, D. Haarer, R. Casalegno, A. Corval, K. Kuldova, H.P. Trommsdorff, J. Chem. Soc., Chem. Commun. (1995) 713. [10] A. Corval, R. Casalegno, O. Ziane, H.D. Burrows, J. Phys. Chem. 106 (2002) 4272. [11] A.E. Chichibabin, B.M. Kunidzhi, S.W. Benewolenskaja, Ber. Dtsch. Chem. Ges. 28 (1925) 1580. [12] J. Takeda, D.D. Chung, J. Zhou, K.A. Nelson, Chem. Phys. Lett. 290 (1998) 341. [13] S. Mitra, N. Tamai, Chem. Phys. Lett. 282 (1998) 391. [14] S. Mitra, N. Tamai, Phys. Chem. Chem. Phys. 5 (2003) 4647. [15] M. Van den Zegel, N. Boens, D. Daems, F.C. DeSchryver, Chem. Phys. 101 (1986) 311. [16] P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969. [17] E. Hadjoudis, A. Toska, G. Wettermark, J. Photochem. 8 (1978) 233. [18] S. Mitra, N. Tamai, Chem. Phys. 246 (1999) 463. [19] O. Ziane, R. Casalegno, A. Corval, Chem. Phys. 250 (1999) 199. [20] K. Kuldova´, A. Corval, H.P. Trommsdorff, J.M. Lehn, J. Phys. Chem. A 101 (1997) 6850. [21] H. Miyasaka, M. Hagihara, T. Okada, N. Mataga, Chem. Phys. Lett. 188 (1992) 259. [22] A. Seilmeier, W. Kaiser, in: W. Kaiser (Ed.), Ultrashort Laser Pulses and Applications, Topics in Applied Physics, vol. 60, Springer, Berlin, 1988 (Chapter 7). [23] P.F. Barbara, H.P. Trommsdorff (Eds.), Special Issue on Spectroscopy and Dynamics of Elementary Proton Transfer in Polyatomic Systems, Chem. Phys. 136 (1989) 153. [24] D. McMorrow, M. Kasha, J. Am. Chem. Soc. 105 (1983) 5133; Proc. Natl. Acad. Sci. USA 81 (1984) 3375. [25] B.J. Schwartz, L.A. Peteanu, C.B. Harris, J. Phys. Chem. 96 (1992) 3591.
Transient behavior of 2-(2 0,4 0 -dinitrobenzyl)pyridine photochromism studied by ultrafast laser spectroscopy S. Mitra
a,*
, H. Ito b, N. Tamai
b
a
b
Department of Chemistry, North-Eastern Hill University, Permanent Campus, Umshing, Shillong 793 022, India Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gaku-en, Sanda, Hyogo 669 1337, Japan Received 22 April 2004; accepted 26 July 2004 Available online 20 August 2004
Abstract Ultrafast transient behavior of photochromic 2-(2 0 ,4 0 -dinitrobenzyl)pyridine has been studied using femtosecond transient absorption spectroscopy. Three different transient signals could be found in the experiment at different time delays. The dynamics of transient behavior in the short time scale reveal that very fast relaxation (400 ± 100 fs) from the initial excited state leads to the formation of a relatively stable intermediate which is the precursor of proton transfer. The intermediate relaxes to the ground state of proton transferred aci-nitro structure in nanosecond time scale. The relatively slow proton transfer in DNBP is due to the necessity of extensive structural rearrangement before proton transfer. Solvent hydrogen bonding in ethanol seems to play an important role in the deactivation of the excited state giving entirely different photophysical behavior. 2004 Elsevier B.V. All rights reserved.
1. Introduction Molecular systems showing thermal bistability are promising candidates in terms of optical data processing and memory storage devices [1–4]. Often, in these systems, both the forms can be interconverted (ÔswitchingÕ in the forward direction and ÔfadingÕ in the reverse direction) with some external stimulation such as light, temperature, etc. However, the use of these systems in practical application depends on the rate of conversion and stability of the system under repeated cycle. 2(2 0 ,4 0 -Dinitrobenzyl)pyridine (DNBP) is a potential candidate in this regard. For many years, it is known that the pale yellow crystals of DNBP, having the methylene bridge between the dinitro benzene and pyridine moiety (CH form), changes to dark blue upon UV irradiation [4–7]. The dark blue form is due to the transfer of the
*
Corresponding author. Tel.: +91 364 272 2634; fax: +91 364 255 0486. E-mail address: [email protected] (S. Mitra). 0301-0104/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2004.07.031
hydrogen from methylene group to the pyridine nitrogen (NH form). The reverse reaction to the original CH form occurs within a time scale of ls–ms. It is reported that the stability of the final form can be increased substantially with judicious substitution in the pyridine ring [8,9]. The mechanism of DNBP photochromism, as known so far, can be depicted as in Fig. 1. The conversion of initial CH form (I) to the final NH form (III) may proceed either directly or through an intermediate geometry in which the methylene hydrogen resides on oxygen of the nitro group in the ortho position (aci-nitro or OH form, II). This intermediate undergoes towards the formation of final product in a relatively slower time scale (100 ls in benzene [10]). All the forward and reverse processes may be activated either by photochemically or thermally. Although the photochromism of DNBP is reported as early as in 1925 [11], the dynamics of this process, complete mechanism of formation and the nature of the transients are not clear so far. Recent flash photolysis experiments in nanosecond time scale on DNBP
186
S. Mitra et al. / Chemical Physics 306 (2004) 185–189 NO2 N h ∆
O
H
C H NO
NH-form (III)
+
N O– N
C H2
N
hν
+
O
O
–
∆
NO2
CH-form (I) N
C H NOOH OH-form (II)
Fig. 1. Representation of different photochromic and thermochromic routes of DNBP.
photochromism have shown the presence of two competitive proton transfer routes exist for the CH ! OH conversion [10]. The first one assigned as a direct temperature independent excited state process and the other one, a much slower, thermally activated multistep proton transfer in the ground state via a colorless precursor (P). It may be interesting to study this proton transfer process with the use of ultrafast laser spectroscopy. Femtosecond pump–probe experiments at selected wavelengths reported earlier indicates that proton transfer from the methylene bridge to the ortho nitro group occurs within a time scale of 320–500 fs [12]. However, considering the complex nature of the transients involved in the DNBP photochromic pathway, observation of dynamic behavior at selected wavelengths may not be sufficient to get a complete picture. It is also important to study the time evolution of the transient spectra after excitation with ultrashort laser pulse with an emphasis to observe the dynamic behavior for the formation of the precursor state, P, proposed by Corval et al. [10] during the conversion of the initially excited CH form (I) towards the formation of the ground state OH structure (II).
urements to avoid the formation of final product. All measurements were performed at 295 ± 2 K. Steady state absorption measurements were performed using Hitachi U-3210 spectrophotometer. The details of laser and transient absorption detection system can be found elsewhere [13,14]. In brief, the system contains a hybridly mode-locked, dispersion-compensated femtosecond dye laser (Coherent Satori 774). This dye laser was pumped by a cw mode-locked Nd:YAG laser (Coherent Antares 76S). The output of the dye laser was amplified upto 400 lJ at a center wavelength of 720 nm by a regenerative amplifier system (Continuum RGA60 and PTA60) with a repetition rate of 10 Hz. The pump pulse at 360 nm was obtained by passing the amplified light through a 1-mm BBO crystal. The residual part of the fundamental light was focused in 1 cm water cell to generate white light supercontinuum which was used as probe pulse after passing through computer controlled delay stage (Sigma Koki, STM – 500X). The system response function in the pump–probe method was estimated to be 200 fs FWHM. Transient signals were obtained by averaging over 200 accumulations in a microcomputer controlled intensified multichannel detector (Princeton instruments, ICCD 576-G). A temporal dispersion of the white-light continuum was corrected for the transient absorption spectra. The analysis of the transient signal at a particular wavelength was done with a non-linear least-square deconvolution program based on Marquardt algorithm [15,16].
3. Results Fig. 2 shows the steady state absorption spectra of DNBP in ethanol at room temperature. The UV absorption band at 250 nm is considered as due to the colorless CH form (I) of DNBP [17]. Irradiation of DNBP with UV light causes the photochromic reaction to occur and the color changes to dark blue having an absorption maximum at 550 nm consisting of NH form. A close
2. Experimental DNBP was purchased from Tokyo Kasei and recrystallized from absolute alcohol before use. Chloroform and ethanol were of spectroscopic grade and used without any further purification. Dilute solutions (105 mol dm3) of DNBP was used for steady state experiments whereas for transient absorption spectra, 5 · 102 mol dm3 solutions were used to get sufficient concentration at the excitation wavelength (360 nm). The sample solutions were allowed to flow through a 2-mm flow cell using a magnetic gyre pump (Micropump, 040-332) during the transient absorption meas-
Fig. 2. Steady state absorption spectra of DNBP solution in ethanol.
S. Mitra et al. / Chemical Physics 306 (2004) 185–189
187
Fig. 3. Transient absorption spectra DNBP solution in chloroform at different time delays (mentioned in the figure) after excitation.
look in Fig. 2 shows the presence of a very weak shoulder at 350 nm. However, we could not get any detectable fluorescence by exciting the sample at this absorption. In Fig. 3, the transient absorption spectra of DNBP in chloroform pumped by a 200-fs pulse at 360 nm and probed by white light supercontinuum are shown. Excitation by 360 nm laser in pump–probe experiment corresponds to the shoulder of the absorption spectra. The time delays of probing after the excitation in the transient absorption spectra are also indicated in Fig. 3. It is seen that at initial time region there exist two broad bands, one at 400–500 nm and the other at 600– 700 nm (with the maxima at 640 nm) region. With the progress of time, the intensity of the second band increases. It is also noted that the peak at 640 nm shifts to the blue side of the spectrum very fast (within 1 ps) and finally the band maxima appears at 600 nm. Around hundreds of picosecond time delay between the pump and probe pulses, the band intensity at 600 nm starts decreasing with a simultaneous appearance of another broad band again at 400–500 nm region of the spectrum. The time dependence of the 600 nm transient signal was analyzed with 200 fs Gaussian laser pulse (Fig. 4). The simulation result gives about 400 fs rise and a slow decay component in nanosecond time scale. The observation of the transient behavior at 420 nm indicates (Fig. 5) that the absorption intensity at this wavelength consist of very fast decay followed by a slow rise in the measured time window.
Fig. 4. Simulation of 600 nm transient absorption intensity variation with 200 fs Gaussian laser pulse.
be explained by Sn S1 absorption of initially excited CH form. Recently, we made similar observation in femtosecond pump–probe spectra for other photochromic
4. Discussion The time evolution of the transient signal for DNBP in chloroform can be explained as below. The broad band around 400–500 nm region at the early time can
Fig. 5. Variation of transient absorption intensity at 420 nm at different delay time.
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S. Mitra et al. / Chemical Physics 306 (2004) 185–189
compounds also [18]. These photoexicited molecules undergo rapid relaxation in the excited state potential energy surface to form relatively stable conformers. The time evolution of transient spectra around 600 nm is most important and needs careful attention. The intensity of this transient absorption increases gradually upto 1 ps and then starts decaying with a concomitant grow in transient signal corresponding to OH form in 400–500 nm region. Previous reports suggest the presence of a precursor state (P) in the slow ground state conversion of CH form to OH form [19,20] in DNBP photochromism. This precursor was proposed to have a structure intermediate to that of CH and OH forms having an absorption peak at 335 nm. This additional ground state conformer was detected kinetically, however, any spectroscopic characterization was lacking due to the instrumental limitation. The initial broad band in the 600–700 nm region in femtosecond transient spectra (Fig. 3) is believed to be originated from this species when pumped with 360 nm laser pulse. As the intensity of this transient absorption increases with the delay time of the probe pulse, we can assume that ground state population of P is also fed from the initially excited CH species. The time constant of this process can be obtained from the rise time of 600 nm band (Figs. 3 and 4), which is found to be 400 fs. The time constant for the very fast decay of the 420 nm transient intensity could not be fitted due to very poor signal to noise ratio, however, visual inspection of Fig. 5 indicates a similar time constant as observed for the growing in of the 600 nm band. This very fast time constant indicates a barrierless (or with very low barrier) relaxation process, i.e., the excited molecules slide down in the potential energy surface towards the formation of the ground state of intermediate, P. This prediction is further supported from the observed blue shift of transient band at 640 nm. In femtosecond pump–probe experimetns, the transient absorption spectral shift occurs mainly due to vibrational cooling [21,22]. The transient absorption of this intermediate at 640 nm shift to the blue side due to very fast intramolecular vibrational relaxation as shown in Scheme 1. The variation of 420 nm transient absorption intensity given in Fig. 5 also supports the discussion made above. Very fast decay of this signal indicates the conversion of initially excited CH form (I) to the metastable intermediate, P and the slow growing in nanosecond time scale is a signature of the formation of OH structure (II). There is no indication of any stimulated emission in the entire spectral region. The stable intermediate undergoes deactivation towards the formation of ground state aci-nitro structure (OH form, II). Transient absorption at 400–500 nm regions at long time delay can be considered as originated from S1 S0 absorption of this species. The spectral position of this band is similar to
TrA1
CH* ~ 400 fs
TrA2
TrA3
hν
P OH (II) CH (I) Scheme 1. Schematic diagram for the photophysical properties of DNBP photochromism in ultrafast time scale. TrAs indicate the transient absorptions at different time delays.
that obtained in nanosecond pump–probe experiments reported earlier [6,17]. Continuous irradiation of DNBP solution by UV light showed that the final photochromic product (NH form) gives an absorption maxima at 550 nm [11]. Nanosecond laser flash photolysis experiments in DNBP showed that the formation of NH form (having absorption peak at 550 nm) from the thermal reaction of OH form (having characteristic absorption at 420 nm) occurs at about 1 ls time scale [6]. So, we do not observe the absorption of the final product within the time limit of our experiment (3 ns). The transient behavior of ethanol differs considerably from that in chloroform. In this case the broad absorption at 400–500 nm region does not disappear in the whole time window. However, the time evolution of the spectra at 600 nm is almost similar in both ethanol and chloroform except the lower absorption intensity in the first case. So, it may be possible that in ethanol, intermolecular hydrogen bonding plays an important role in the photo(thermo)chromic pathway. The early formation of OH species having absorption peak at 400–450 nm indicates the possibility of solvent assisted proton transfer. Solvent assisted proton transfer reactions for many intramolecular proton transfer systems are found to occur in ultrafast time scale [23–25]. This phenomenon becomes much more important in those cases where the initial geometry for proton transfer is not favorable, e.g., when the groups involving the proton transfer are relatively far. However, in presence of alcoholic solvents, a suitable geometry may be formed with bridged hydrogen bond that favors the transfer of the proton more easily. A similar prediction can be made for DNBP too. Solvent assisted proton transfer of excited CH form in ethanol leads to formation of OH structure relatively easily. The lower absorption intensity at 600 nm, corresponding to the precursor state P, indicates that direct formation of OH structure from CH form is more favored pathway in this solvent.
S. Mitra et al. / Chemical Physics 306 (2004) 185–189
5. Conclusion Ultrafast photochromic transients of DNBP are studied in chloroform and ethanol. It was observed for the first time that a stable ground state intermediate, known as a precursor state P as mentioned by Corval et al. [10], is formed within 400 fs during the proton transfer from the excited CH state to the ground state of acinitro structure (OH form) in non-hydrogen bonding solvents. Although the presence of the precursor state before the formation of OH structure has been substantiated unambiguously from ultrafast transient absorption measurements, the structure of this species demands an extensive high level theoretical calculation. In hydrogen bonding solvents like ethanol, the more favored pathway for the photochromic process is the direct conversion of CH form to the OH structure. Acknowledgements S. Mitra thanks JSPS for a fellowship (L-03554) to visit Kwansei Gakuin University. This work is supported in part by Grant-in-Aid (No. 10450326) for Scientific Research (B) from Japanese Ministry of Education, Science and Culture. The authors gratefully acknowledge the comments by one of the reviewers in improving the content of this manuscript.
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