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Picosecond vibrational relaxation in the excited-state proton-transfer of 2-(3′-hydroxy-2′-naphthyl)benzimidazole
Volume 217, number 5,6
28 January 1994
CHEMICAL PHYSICS LETTERS
Picosecond vibrational relaxation in the excited-state proton-transfer of 2- ( 3’-hydroxy-2’-naphthyl)benzimidazole A. Douhal a,b, F. Amat-Guerri ‘, A.U. Acufia b and K. Yoshihara a ’ Institutefor MolecularScience, Myodaiji,Okuzaki444, Japan b Institutede Q&mica Fisica Rocasolano,CSIC, Sewano 119.28006 Madrid, Spain ’ Institutede Quimica Orghica. CSIC, Juan de la Cierva 3.28006 Madrid, Spain Received 30 August 1993; in final form 4 November 1993
The fluorescenceof 2-(3’-hydroxy-2’-naphthyl) bcnzimidazole in solution shows two bands at ~400 and 600 MI, respectively assigned to the open-enol form bound to the solvent, and to the phototautomer produced from the closedenol form. Both enol species coexist in the ground state. A fluorescence risetime of 16 ps has been attributed to the vibrational cooling-process of the excited phototautomer.
1. Introduction
Excited-state intramolecular proton or hydrogenatom transfer (ESIPT ) in organic molecules has been widely investigated as an example of a simple and fast reaction (for recent reviews, see ref. [ 1 ] ) . Compounds showing this phenomenon are also of practical interest with uses such as photostabilizers of polymers [ 2 1, in tunable lasing media [ 3- 111 and, possibly, as devices for storing information [ 12 1. Recent technical advances in laser spectroscopy have allowed the determination of ESIPT reaction rates as fast as 1012-10” s-‘, suggesting a low, or a lack of, energy barrier for the process [ 1,13- 191. The driving force for the reaction is a femtosecond electrondensity redistribution upon electronic excitation, giving rise to a simultaneous increase in both the acidity of the proton (or hydrogen-atom) donor and the basicity of the acceptor of the two conjugated aromatic groups. Due to this charge redistribution, the solute-solvent orientations may be different after the ESIPT reaction. The phototautomer is formed with a large excess energy, in the range 3000-6000 cm-‘, which is eventually converted into vibrational and rotational excitation. The intramolecular redistribution of the vibrational energy may be detected experimentally by using picosecond spectroscopic techniques. Recently, using time-resolved absorp-
tion spectroscopy, a cooling time of 30 ps has been determined for the highly excited enol molecules generated by the fast back proton transfer in the ground state of 2-(2’-hydroxy-S-methylphenyl)benzotriazole in solution [ 151. Similarly, we have found a value of 14 ps for the time constant of the relaxation process in 2-(2’-hydroxyphenyl)imidazo [ 1,2-a ] pyridine [ 19 1. In this work we report a direct observation of the cooling process during the vibrational relaxation in the electronically excited-state of the phototautomer formed by the ESIPT reaction in 2- (3’-hydroxy-2’naphthyl)benzimidazole (HNB). 4
H4
..
The picosecond time-resolved emission of HNB in solution shows that the fluorescence of the tautomer generated by ESIPT shows a risetime of 16 ps. We present evidence indicating that this risetime is due to the energy-redistribution process. We also discuss the dual fluorescence shown by this molecule in solution.
0009-2614/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ OOOOS-2614(93)El426-H
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2. Experimental 2- ( 3’-hydroxy-2’naphthyl) benzimidazole was obtained by condensation of o-phenylenediamine with an equimolar amount of 3-hydroxy-2-naphthoic acid (Aldrich ) , in polyphosphoric acid at 160- 180 oC under argon atmosphere, as described for related 2-( 2’hydroxyphenyl)benzimidazoles [ 9 1. The final product was purified by crystallization from ethanolwater. Mp 268-272°C (decomp.); mass spectrum (electronic impact), m/z (O/o), 260 (M+, loo), 232 (43), 231 (33), 140 (7), 116 (21); ‘H NMR (300 MHz, 3O”C, DMSO-de, tetramethylsilane as internal reference), 6 7.32 (broad m, 2 H, H-5 and H-6), 7.38 (m,J=l.O, 6.8 and8.1 Hz, 1 H, H-7’), 7.42 (s, 1 H, H-4’), 7.51 (m, J=l.O, 6.8 and 8.1 Hz, 1 H, H-6’), 7.72 (broad m, 2 H, H-4 and H-7), 7.79 (dd, J= 1.0 and 8.1 Hz, 1 H, H-S), 7.90 (dd, J= 1.O and 8.1 Hz, 1 H, H-8’), 8.71 (s, 1 H, H-l’), 10.0 (broads, 2 H, NH and OH). Steady-state absorption and fluorescence spectra were recorded on a Shimadzu XIV-3 100 spectrophotometer and on a SLM 8000 fluorometer. Spectra are presented in photon units and have been corrected for instrumental factors. The apparatus for picosecond time-resolved fluorescence measurements has been described in detail elsewhere [ 20 1. In brief, a Coherent Antares 76-S mode-locked Nd:YAG laser pumped a Spectra Physics 375-00 dye laser; a Spectra Physics 344 cavity dumper was used to produce a train (4 MHz) of 10 ps pulses at 650 nm. The output of the dye laser was doubled with an angle-tuned KTP crystal to produce radiation at 325 nm. This beam, filtered through an UV filter (BG3) to eliminate red radiation, was used to excite the sample. The fluorescence, at a 90” angle to the excitation beam, passed through a Nikon P-250 monochromator, and was detected by a Hamamatsu R 2809U.01 microchannel-plate photomultiplier and conventional photon counting electronics. A 340 nm cutoff filter was placed between the sample and the emission monochromator to remove scattered light. The instrumental response function (typically 60 ps ) was determined by measuring the intensity profile of radiation at 325 nm scattered from phosphate buffer solution containing a drop of milk. Fluorescence decay data were analyzed using the ‘Global’ nonlinear least-squares package [ 2 11. Exponential decay func620
28 January 1994
CHEMICAL PHYSICS LETTERS
tions were convoluted with the instrument response function and fitted to the experimental decays. The quality of the fits were characterized in terms of the reduced x2 values, the distribution of residuals, and the residuals autocorrelation function. Using these procedures, decay components as short as 10 ps could be recovered. All solvents were of spectroscopic grade (Merck). Acetonitrile was distilled over P20s and methanol and ethylene glycol were used as received.
3. Results and discussion 3.1. Steady-state absorption and fluorescence spectra The steady-state absorption and fluorescence spectra of HNB in acetonitrile are shown in fig. 1. The absorption spectrum shows five maxima, at 290,320, 330, 357 and 370 nm, and is similar to that of the parent compound 2-( 2’-hydroxyphenyl)benzimidazole (HPB) [ 5,9,22,23]. The value of the molar absorption coefficient at 330 nm (24500 M-l cm-‘) is indicative of a (x, n*) transition [9]. The absorption spectrum of HNB in acetonitrile did not show any change in the concentration range from 1O-6 to 1O-4 M. The room-temperature fluorescence spectrum of HNB in acetonitrile depends on the excitation wavelength (fig. 1). When the sample is excited at the ab‘h
waveleng th/nm
Fig. 1. Absorption (A) and fluorescence (F) spectra of 2-( 3’hydroxy-2’-naphthyl)benzimidazole (HNB) 5 X 1Oe6 M in acetonitrile at 295 K. Excitation wavelengths were 320 nm (p) and 380 nm (- - -).
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sorption maximum (320-330 nm) the emission shows two bands, with maxima at 390 and 620 nm, the first one with some vibrational structure and normal Stokes shift, and the second one without structure and largely Stokes shifted. This dual fluorescence is observed in other solvents (fig. 2). The ratio of the fluorescence intensity of the red/blue bands decreases in solvents with higher hydrogenbonding ability: acetonitrile > methanol > ethylene glycol. This is a common feature of molecules with an intramolecular hydrogen bond (IHB) and dual fluorescence in solution [ 5,9,22-271. In the following, the steady-state and time-resolved emission of HNB will be discussed for the acetonitrile solution. The excitation spectra of the two fluorescence bands are different in acetonitrile (fig. 3 ) , indicating that the two emissions are due to different groundstate molecules. The excitation spectrum of the 400 nm emission presents a maximum at 3 18 nm, with shoulders at 350 and 358 nm. With the exception of the red-edge tail, this spectrum is similar to that of the so-called open-en01 form of HPB in hydrogenbonding solvents, where the maximum peaks at 3 13 nm [ 9 1. We assign the species emitting at z 400 nm to an open conformation where the hydroxy group is not hydrogen bonded with the imidazole nitrogen, but with the solvent (conformer I in scheme 1) and therefore this conformer is identified as the open-en01 form, as in the previous work. As expected, this form was in larger concentration when methanol or ethylene glycol was used as solvent. The intensity ratio of the absorption bands at 320 and 370 nm is slightly
400
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500 wavelength/nm
600
700
0
Fig. 2. Absorption (A) and fluorescence (F) spectra of HNEll 0e6 M in methanol (-) and ethylene glycol (- - -) (&=330 nm).
,
0 250
350
300 waveleng
“\A_ 400
4
th/nm
Fig. 3. Fluorescence excitation spectra of a 5 x 10m6M solution of HNB in acetonitrile observed at 400 nm (- - -) and 590 nm (-).
higher in methanol and ethylene glycol than in acetonitrile (figs. 1 and 2). The second fluorescence band (at 620 nm in acetonitrile, at 590 nm in methanol or ethylene glycol) presents a large Stokes shift ( 11000 cm- ’ ). The corresponding fluorescence excitation spectrum has three maxima, at 3 16,330, and 360 (broad) nm. The HNB conformation in weakly hydrogen-bonding solvents is most likely the closed-enol form (conformer II in scheme 1). Electronic excitation of this conformer leads to the ESIPT process, with formation of a tautomer emitting with the large Stokes shift. Interestingly, the excitation spectrum of the red fluorescence is almost identical to the absorption one, indicating that the main ground-state of HNB species is the closed-enol form, with a relative concentration of 85%-90O%1in acetonitrile [ 91. Based on the above similarities and on the well-documented ESIPT process, the red emission band is assigned to the fluorescence of the phototautomer generated by the ESIPT reaction, namely the keto tautomer. The driving force of the ESIPT is a fast electronic charge redistribution leading to a favorable change in the acidity and basicity of the naphthol and benzimidazole moieties, respectively. The excited-state pK: values of 2-naphthol and that of the imino group in benzimidazole derivatives are ~2.5 [2,28-311 621
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(q&&q-&e*
A
A
thermal relavatbn 320 nm
390 nm
330 nm
fl”o~o++J
4.88 ns
6
*z&ns
HNB(I)
HNB(III)
HNB(II)
Scheme 1. The photophysical processes of 2-( 3’-hydroxy-2’-naphthyl)benzimidazole cence parameters taken from A acetonitrile s&u&n at i95 K. _
and 7 [ 9,221, respectively, and the change would facilitate the shift of an hydrogen atom from the OH in the naphthol moiety to the benzimidazole imino nitrogen. The structure of the phototautomer in scheme 1 (HNB(II1) ) shows a quinonoid configuration, rather than a zwitterionic one. We favor the former assignment by analogy with what has been observed in a similar derivative, 2-( 2’-hydroxyphenyl)benzothiazole, by time-resolved IR spectroscopy [ 13 1. The quinonoid structure resulting from the transfer step requires extensive electronic density rearrangements, reflected in the large Stokes shift characteristic of this process. 3.2. Fluorescence dynamics The decay of the fluorescence of HNB in acetonitrile solution, measured at 400 and 640 nm using the picosecond technique described above, is shown in fig. 4, together with the instrument response function to indicate the time resolution of the experiment. The fluorescence kinetics of the phototautomer (640 nm) shows a single exponential decay with 1.68 ns and a risetime of 16 ps (pre-exponential factors of 0.015 and -0.014, respectively, x2= 1.11). At 400 nm, the fluorescence kinetics follows a simpler behavior, with a mono-exponential decay time of 4.88 ns. No short-lived component was found at this wavelength. This blue emission is due to a small 622
(HNB) in solution, with absorption and fluores-
fraction of open forms in the solution, as commented above, and is not of main interest here. The different fluorescence lifetime values of HNB at 400 nm (4.88 ns) and at 640 nm ( 1.64 ns) indicates that the two emitting species are not in equilibrium in the excited state. On the other hand, since the lifetime of the open-en01 form is in the nanosecond range and the keto form shows a picosecond risetime, this phototautomer cannot be generated from the open-en01 configuration in the excited-state. In methanol solution, the proton-transfer fluorescence at 640 nm presents the same dynamics as in acetonitrile: a risetime of 17 ps and a decay constant of 2.4 1 ns. In ethylene glycol, the corresponding values are 15 ps and 3.11 ns. It should be noted that a similar risetime value ( 15 ps) has been determined by measuring the formation of the phototautomer by picosecond transient absorption spectroscopy [ 32 1. As noted above, the red fluorescence originates from the keto tautomer formed by the ESIPT reaction. The risetime of this emission might be due to: (a) an activation barrier in the ESIPT process; (b) a specific solvation of the excited keto tautomer; or (c) a vibrational relaxation process from the initially produced hot phototautomer to the nanosecond emitting form. The observed spectra give little information on the nuclear configuration of the electronically excited HNB molecule. The broad absorption band at 360
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0
2.5
5
7.5 time/ns
12.5
10
1.80
0.80
timeps 3.21 2.57 In E ! 5
1.93
12 0% -1 UI 5
1.28 a.3 = -0.014
0.84 lll,l,
0.00
r;= a.=
16 ps 0.015
‘I= X=
::;:
I
“s
1
~.
480
960
I,,,,!
1440 1,1,,1,1,11 time/w
1920
2400
Fig. 4. Time-resolved fluorescence decay of HNB in acetonitrile: (a) ,I,=400 nm; (b) I,=640 nm; and (c) the rise and decay of the fluorescence recorded at 640 nm (A.,= 325 nm).
nm might be assigned to a charge-transfer transition, from the naphthol ring to the benzimidazole moiety.
In that case, a quasi-planar conformation of the ground and excited states brought about by the intramolecular hydrogen bond would facilitate this process. On the other hand, the maximum and bandwidth of the red fluorescence band are not affected by the viscosity of the solvent (methanol, ethylene
28 January 1994
glycol, fig. 2), and the fluorescence of HNB in ethylene glycol shows a risetime of 15 ps, a value similar to that found in methanol ( 17 ps). All these observations suggest that there are no large solvent-dependent geometrical rearrangements in the form initially excited and in the tautomer, and that only a single emission occurs under this band. As noted above, it is likely that the electronic configuration of the phototautomer approaches a quinonoid structure with a fairly rigid conformation. Hence, its fluorescence would not be affected by the viscosity of the solvent, as is actually observed in methanol and ethylene glycol. Therefore, it is unlikely that the risetime of the keto tautomer fluorescence is due to a kinetic barrier in the ESIPT process [ 1,13- 19,27 1. If specific interactions between the keto tautomer and solvent molecules were the origin of the observed risetime, one would expect different values of this kinetic component in acetonitrile (weak protonaccepting solvent) and methanol (strong proton-donating and proton-accepting solvent) solutions, as is actually observed in the nanosecond decay times. The similar values recorded in the three solvents used here indicate that the 16 ps risetime is not due to specific interactions between the keto tautomer and the solvent. The other alternative is that the risetime is due to the vibrational relaxation of the excess energy of the phototautomer. Taking into account the Stokes shift ( 11000 cm- * ), the energy of the exciting photon (30770 cm-‘), and of the O-O absorption band (227000 cm-‘), this excess energy can be estimated as z 9000 cm-l and, therefore, a high vibrational temperature should be expected. Energy redistribution in hot molecules of large aromatic compounds by collisions with solvent molecules takes place in the picosecond time scale (5-50 ps) (for a recent review, see ref. [ 33 ] ) . In addition, a cooling time of 14 ps has been determined [ 19 ] for the vibrationally excited tautomers formed in the ESIPT reaction of 2- ( 2’-hydroxyphenyl )imidazo [ 1,2-a ] pyridine in cyclohexane. For this compound, it was observed that the intensity of the red tail of the transient absorption spectra decreases with time, while the position of the maximum remained constant [ 19 1. Recently, Frey and Elsaesser reported [ 15 ] a cooling time of 30 ps for the vibrationally excited closed-enol conformer of 2- (2’-hydroxy-5’-methyl623
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CHEMICAL PHYSICS LETTERS
phenyl)-benzotriazole, generated by the back proton-transfer in the ground state. Accordingly, it is likely that the risetime of the fluorescence of HNB determined here in acetonitrile ( 16 ps), methanol ( 17 ps) and ethylen glycol ( 15 ps) is due to the kinetics of the energy redistribution of the vibrationally excited (hot) tautomer formed in the ESIPT reaction. This is also consistent with the observed transient spectra and kinetic parameters of the absorption and stimulated emission of HNB in acetonitrile at different wavelengths, recorded by picosecond techniques [ 321. In scheme 1, the main results of the present work and some photophysical data of HNB in acetonitrile are summarized. In this scheme, the reverse ground-state intramolecular proton-transfer (GSIPT ) reaction regenerates the closedenol form. Although there is not yet information concerning this process, the time constant of the GSIPT reaction in compounds showing ESIPT has been found to be in the nanosecond-subpicosecond range, depending on the molecule [ 13-l 5,19,34]. The risetime value ( 16 ps) of fluorescence in acetonitrile is independent on the temperature in the 273-334 K range. However, the fluorescence lifetime at x 600 nm decreases significantly in the same range (1.87, 1.68, 1.51 and 1.38 ns, at 273, 293, 313 and 333 K, respectively), with an apparent activation energy of 1 kcal/mol. This, and the low emission quantum yield ( x 10V4) indicate that most of the relaxed phototautomers return to the ground state by radiationless channels. Acknowledgement This work was supported, in part, by Projects PB90-0102 and MAT93-0369 from the Spanish Direccion General de Investigation Cientifica y TCcnica. AD thanks the Japanese Society for the Promotion of Science for a fellowship.
References [ 1] P.F. Barbara and H.P. Trommsdorff eds., Spectroscopy and dynamics of elementary proton transfer in polyatomic systems, Chem. Phys. 136 (1989) (special issue); Papers presented at the Conference on Photoinduced Proton Transfer in Chemistry, Biology and Physics, in honour of M. Kasha, J. Phys. Chem. 95 ( 199 1).
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[2] W. Kliipfer, Advan. Photochem. 10 (1977) 311.
[ 31 P.T. Chou, D. McMorrow, T.J. Aartsma and M. Kasha, J. Phys. Chem. 88 (1984) 5652. [4] A.U. Acufia, A. Costela and J.M. Muiioz, J. Phys. Chem. 90 (1986) 2807. [ 5] A.U. Acuila, F. Amat, J. Catalan, A. Costela, J.M. Figuera and J.M. Mutioz, Chem. Phys. Letters 132 (1986) 567. [6] A. Costela, F. Amat, J. Catalan, A. Douhal, J.M. Figuera, J.M. Munoz and A.U. Acuiia, Opt. Commun. 64 (1987) 457. [ 71 A. Costela, J.M. Muiioz, A. Douhal, J.M. Figuera and A.U. Acutia, Appl. Phys. B 49 (1989) 545. [ 8 ] A.U. Acutia, F. Amat-Guerri, A. Costela, A. Douhal, J.M. Figuera, F. Florida and R. Sastre, Chem. Phys. Letters 187 (1991) 98. [9] A. Douhal, F. Amat-Guerri, M.P. Lillo and A.U. Act&q J. Photochem. Photobiol. A, in press. [ 10 ] J. Sepiol, H. Bulska and A. Grabowska, Chem. Phys. Letters 140 (1987) 607. [ 111 P.T. Chou, M.L. Martinez and J.H. Clements, Chem. Phys. Letters 204 (1993) 395. [ 121 D. Haarrer, Japan J. Appl. Phys. 26 (1986) 3362. [ 131 T. Elsaesser and W. Kaiser, Chem. Phys. Letters 128 (1986) 231; T. Elsaesser, W. Kaiser and W. Ltittke, J. Phys. Chem. 90 (1986) 2901. [ 141 W. Wiechmann, H. Port, F. Laermer, W. Frey and T. Elsaesser, Chem. Phys. Letters 165 (1990) 28. [ 15 ] W. Frey and T. Elsaesser, Chem. Phys. Letters 189 ( 1992) 565. [ 161 B.J. Schwartz, L.A. Peteanu and C.B. Harris, J. Phys. Chem. 96 (1992) 3591. [ 171 T. Arthen-Engeland, T. Bultmann, N.P. Emsting, M.A. Rodriguez and W. Thiel, Chem. Phys. 163 ( 1992) 43. [ 18 ] J.L. Herek, S. Pedersen, L. Bafiares and A.H. Zewail, J. Phys. Chem. 97 ( 1992) 9046. [ 191 A. Douhal, H. Kandori, K. Yoshihara and F. Amat-Guetri, to be published. [ 201 T. Yamazaki, N. Tamai, H. Kume, H. Tsuchiya and K. Oba, Rev. Sci. Instrum. 56 (1985) 1187. [21] J.M. Beechem, E. Gratton and W. Mantulin, Globals Unlimited, University of Illinois, Champain-Urbana, IL, 1990. [22] H.K. Sinha and S.K. Dogra, Chem. Phys. 102 (1986) 337. [23] K. Das, N. Sarkar, D. Majumdar and K. Bhattacharyya, Chem. Phys. Letters 198 ( 1992) 443. [24] P.F. Barabara, P.K. WalshandL.E. Brus, J. Phys. Chem. 93 (1989) 29. [25] T.C. Swinney and D.F. Kelly, J. Phys. Chem. 95 (1991) 10369. [26] G.A. Bruker, T.C. Swinney and D.F. Kelly, J. Phys. Chem. 95 (1991) 3190. [27]T.P. Smith, K.A. Zaklika, K. Thakur, G.C. Walker, K. Tominaga and P.F. Barbara, J. Phys. Chem. 95 ( 1991) 10465. [28] W.R. Laws and L. Brand, J. Phys. Chem. 83 (1979) 795.
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[29] CM. Harris and B.K. Selinger, J. Phys. Chem. 84 (1980) 891. [ 301 G.J. Woolfe and P.J. Thistlethwaite, J. Am. Chem. Sot. 103 (1981) 3849. [31] G.W. Robinson, P.J. Thistlethwaite and J. Lee, J. Phys. Chem. 90 (1986) 4224.
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[ 32 ] H. Kandori, A. Douhal, F. Amat-Guerri, A.U. Act&a and K. Yoshihara, to be published.
[ 331 T. Elsaesser and W. Kaiser, Ann. Rev. Phys. Chem. 42 (1991) 83.
[ 341 F. Laermer, T. Elsaesser and W. Kaiser, Chem. Phys. Letters 148 (1988) 119.
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Picosecond vibrational relaxation in the excited-state proton-transfer of 2- ( 3’-hydroxy-2’-naphthyl)benzimidazole A. Douhal a,b, F. Amat-Guerri ‘, A.U. Acufia b and K. Yoshihara a ’ Institutefor MolecularScience, Myodaiji,Okuzaki444, Japan b Institutede Q&mica Fisica Rocasolano,CSIC, Sewano 119.28006 Madrid, Spain ’ Institutede Quimica Orghica. CSIC, Juan de la Cierva 3.28006 Madrid, Spain Received 30 August 1993; in final form 4 November 1993
The fluorescenceof 2-(3’-hydroxy-2’-naphthyl) bcnzimidazole in solution shows two bands at ~400 and 600 MI, respectively assigned to the open-enol form bound to the solvent, and to the phototautomer produced from the closedenol form. Both enol species coexist in the ground state. A fluorescence risetime of 16 ps has been attributed to the vibrational cooling-process of the excited phototautomer.
1. Introduction
Excited-state intramolecular proton or hydrogenatom transfer (ESIPT ) in organic molecules has been widely investigated as an example of a simple and fast reaction (for recent reviews, see ref. [ 1 ] ) . Compounds showing this phenomenon are also of practical interest with uses such as photostabilizers of polymers [ 2 1, in tunable lasing media [ 3- 111 and, possibly, as devices for storing information [ 12 1. Recent technical advances in laser spectroscopy have allowed the determination of ESIPT reaction rates as fast as 1012-10” s-‘, suggesting a low, or a lack of, energy barrier for the process [ 1,13- 191. The driving force for the reaction is a femtosecond electrondensity redistribution upon electronic excitation, giving rise to a simultaneous increase in both the acidity of the proton (or hydrogen-atom) donor and the basicity of the acceptor of the two conjugated aromatic groups. Due to this charge redistribution, the solute-solvent orientations may be different after the ESIPT reaction. The phototautomer is formed with a large excess energy, in the range 3000-6000 cm-‘, which is eventually converted into vibrational and rotational excitation. The intramolecular redistribution of the vibrational energy may be detected experimentally by using picosecond spectroscopic techniques. Recently, using time-resolved absorp-
tion spectroscopy, a cooling time of 30 ps has been determined for the highly excited enol molecules generated by the fast back proton transfer in the ground state of 2-(2’-hydroxy-S-methylphenyl)benzotriazole in solution [ 151. Similarly, we have found a value of 14 ps for the time constant of the relaxation process in 2-(2’-hydroxyphenyl)imidazo [ 1,2-a ] pyridine [ 19 1. In this work we report a direct observation of the cooling process during the vibrational relaxation in the electronically excited-state of the phototautomer formed by the ESIPT reaction in 2- (3’-hydroxy-2’naphthyl)benzimidazole (HNB). 4
H4
..
The picosecond time-resolved emission of HNB in solution shows that the fluorescence of the tautomer generated by ESIPT shows a risetime of 16 ps. We present evidence indicating that this risetime is due to the energy-redistribution process. We also discuss the dual fluorescence shown by this molecule in solution.
0009-2614/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ OOOOS-2614(93)El426-H
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Volume 217, number 5,6
2. Experimental 2- ( 3’-hydroxy-2’naphthyl) benzimidazole was obtained by condensation of o-phenylenediamine with an equimolar amount of 3-hydroxy-2-naphthoic acid (Aldrich ) , in polyphosphoric acid at 160- 180 oC under argon atmosphere, as described for related 2-( 2’hydroxyphenyl)benzimidazoles [ 9 1. The final product was purified by crystallization from ethanolwater. Mp 268-272°C (decomp.); mass spectrum (electronic impact), m/z (O/o), 260 (M+, loo), 232 (43), 231 (33), 140 (7), 116 (21); ‘H NMR (300 MHz, 3O”C, DMSO-de, tetramethylsilane as internal reference), 6 7.32 (broad m, 2 H, H-5 and H-6), 7.38 (m,J=l.O, 6.8 and8.1 Hz, 1 H, H-7’), 7.42 (s, 1 H, H-4’), 7.51 (m, J=l.O, 6.8 and 8.1 Hz, 1 H, H-6’), 7.72 (broad m, 2 H, H-4 and H-7), 7.79 (dd, J= 1.0 and 8.1 Hz, 1 H, H-S), 7.90 (dd, J= 1.O and 8.1 Hz, 1 H, H-8’), 8.71 (s, 1 H, H-l’), 10.0 (broads, 2 H, NH and OH). Steady-state absorption and fluorescence spectra were recorded on a Shimadzu XIV-3 100 spectrophotometer and on a SLM 8000 fluorometer. Spectra are presented in photon units and have been corrected for instrumental factors. The apparatus for picosecond time-resolved fluorescence measurements has been described in detail elsewhere [ 20 1. In brief, a Coherent Antares 76-S mode-locked Nd:YAG laser pumped a Spectra Physics 375-00 dye laser; a Spectra Physics 344 cavity dumper was used to produce a train (4 MHz) of 10 ps pulses at 650 nm. The output of the dye laser was doubled with an angle-tuned KTP crystal to produce radiation at 325 nm. This beam, filtered through an UV filter (BG3) to eliminate red radiation, was used to excite the sample. The fluorescence, at a 90” angle to the excitation beam, passed through a Nikon P-250 monochromator, and was detected by a Hamamatsu R 2809U.01 microchannel-plate photomultiplier and conventional photon counting electronics. A 340 nm cutoff filter was placed between the sample and the emission monochromator to remove scattered light. The instrumental response function (typically 60 ps ) was determined by measuring the intensity profile of radiation at 325 nm scattered from phosphate buffer solution containing a drop of milk. Fluorescence decay data were analyzed using the ‘Global’ nonlinear least-squares package [ 2 11. Exponential decay func620
28 January 1994
CHEMICAL PHYSICS LETTERS
tions were convoluted with the instrument response function and fitted to the experimental decays. The quality of the fits were characterized in terms of the reduced x2 values, the distribution of residuals, and the residuals autocorrelation function. Using these procedures, decay components as short as 10 ps could be recovered. All solvents were of spectroscopic grade (Merck). Acetonitrile was distilled over P20s and methanol and ethylene glycol were used as received.
3. Results and discussion 3.1. Steady-state absorption and fluorescence spectra The steady-state absorption and fluorescence spectra of HNB in acetonitrile are shown in fig. 1. The absorption spectrum shows five maxima, at 290,320, 330, 357 and 370 nm, and is similar to that of the parent compound 2-( 2’-hydroxyphenyl)benzimidazole (HPB) [ 5,9,22,23]. The value of the molar absorption coefficient at 330 nm (24500 M-l cm-‘) is indicative of a (x, n*) transition [9]. The absorption spectrum of HNB in acetonitrile did not show any change in the concentration range from 1O-6 to 1O-4 M. The room-temperature fluorescence spectrum of HNB in acetonitrile depends on the excitation wavelength (fig. 1). When the sample is excited at the ab‘h
waveleng th/nm
Fig. 1. Absorption (A) and fluorescence (F) spectra of 2-( 3’hydroxy-2’-naphthyl)benzimidazole (HNB) 5 X 1Oe6 M in acetonitrile at 295 K. Excitation wavelengths were 320 nm (p) and 380 nm (- - -).
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sorption maximum (320-330 nm) the emission shows two bands, with maxima at 390 and 620 nm, the first one with some vibrational structure and normal Stokes shift, and the second one without structure and largely Stokes shifted. This dual fluorescence is observed in other solvents (fig. 2). The ratio of the fluorescence intensity of the red/blue bands decreases in solvents with higher hydrogenbonding ability: acetonitrile > methanol > ethylene glycol. This is a common feature of molecules with an intramolecular hydrogen bond (IHB) and dual fluorescence in solution [ 5,9,22-271. In the following, the steady-state and time-resolved emission of HNB will be discussed for the acetonitrile solution. The excitation spectra of the two fluorescence bands are different in acetonitrile (fig. 3 ) , indicating that the two emissions are due to different groundstate molecules. The excitation spectrum of the 400 nm emission presents a maximum at 3 18 nm, with shoulders at 350 and 358 nm. With the exception of the red-edge tail, this spectrum is similar to that of the so-called open-en01 form of HPB in hydrogenbonding solvents, where the maximum peaks at 3 13 nm [ 9 1. We assign the species emitting at z 400 nm to an open conformation where the hydroxy group is not hydrogen bonded with the imidazole nitrogen, but with the solvent (conformer I in scheme 1) and therefore this conformer is identified as the open-en01 form, as in the previous work. As expected, this form was in larger concentration when methanol or ethylene glycol was used as solvent. The intensity ratio of the absorption bands at 320 and 370 nm is slightly
400
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500 wavelength/nm
600
700
0
Fig. 2. Absorption (A) and fluorescence (F) spectra of HNEll 0e6 M in methanol (-) and ethylene glycol (- - -) (&=330 nm).
,
0 250
350
300 waveleng
“\A_ 400
4
th/nm
Fig. 3. Fluorescence excitation spectra of a 5 x 10m6M solution of HNB in acetonitrile observed at 400 nm (- - -) and 590 nm (-).
higher in methanol and ethylene glycol than in acetonitrile (figs. 1 and 2). The second fluorescence band (at 620 nm in acetonitrile, at 590 nm in methanol or ethylene glycol) presents a large Stokes shift ( 11000 cm- ’ ). The corresponding fluorescence excitation spectrum has three maxima, at 3 16,330, and 360 (broad) nm. The HNB conformation in weakly hydrogen-bonding solvents is most likely the closed-enol form (conformer II in scheme 1). Electronic excitation of this conformer leads to the ESIPT process, with formation of a tautomer emitting with the large Stokes shift. Interestingly, the excitation spectrum of the red fluorescence is almost identical to the absorption one, indicating that the main ground-state of HNB species is the closed-enol form, with a relative concentration of 85%-90O%1in acetonitrile [ 91. Based on the above similarities and on the well-documented ESIPT process, the red emission band is assigned to the fluorescence of the phototautomer generated by the ESIPT reaction, namely the keto tautomer. The driving force of the ESIPT is a fast electronic charge redistribution leading to a favorable change in the acidity and basicity of the naphthol and benzimidazole moieties, respectively. The excited-state pK: values of 2-naphthol and that of the imino group in benzimidazole derivatives are ~2.5 [2,28-311 621
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(q&&q-&e*
A
A
thermal relavatbn 320 nm
390 nm
330 nm
fl”o~o++J
4.88 ns
6
*z&ns
HNB(I)
HNB(III)
HNB(II)
Scheme 1. The photophysical processes of 2-( 3’-hydroxy-2’-naphthyl)benzimidazole cence parameters taken from A acetonitrile s&u&n at i95 K. _
and 7 [ 9,221, respectively, and the change would facilitate the shift of an hydrogen atom from the OH in the naphthol moiety to the benzimidazole imino nitrogen. The structure of the phototautomer in scheme 1 (HNB(II1) ) shows a quinonoid configuration, rather than a zwitterionic one. We favor the former assignment by analogy with what has been observed in a similar derivative, 2-( 2’-hydroxyphenyl)benzothiazole, by time-resolved IR spectroscopy [ 13 1. The quinonoid structure resulting from the transfer step requires extensive electronic density rearrangements, reflected in the large Stokes shift characteristic of this process. 3.2. Fluorescence dynamics The decay of the fluorescence of HNB in acetonitrile solution, measured at 400 and 640 nm using the picosecond technique described above, is shown in fig. 4, together with the instrument response function to indicate the time resolution of the experiment. The fluorescence kinetics of the phototautomer (640 nm) shows a single exponential decay with 1.68 ns and a risetime of 16 ps (pre-exponential factors of 0.015 and -0.014, respectively, x2= 1.11). At 400 nm, the fluorescence kinetics follows a simpler behavior, with a mono-exponential decay time of 4.88 ns. No short-lived component was found at this wavelength. This blue emission is due to a small 622
(HNB) in solution, with absorption and fluores-
fraction of open forms in the solution, as commented above, and is not of main interest here. The different fluorescence lifetime values of HNB at 400 nm (4.88 ns) and at 640 nm ( 1.64 ns) indicates that the two emitting species are not in equilibrium in the excited state. On the other hand, since the lifetime of the open-en01 form is in the nanosecond range and the keto form shows a picosecond risetime, this phototautomer cannot be generated from the open-en01 configuration in the excited-state. In methanol solution, the proton-transfer fluorescence at 640 nm presents the same dynamics as in acetonitrile: a risetime of 17 ps and a decay constant of 2.4 1 ns. In ethylene glycol, the corresponding values are 15 ps and 3.11 ns. It should be noted that a similar risetime value ( 15 ps) has been determined by measuring the formation of the phototautomer by picosecond transient absorption spectroscopy [ 32 1. As noted above, the red fluorescence originates from the keto tautomer formed by the ESIPT reaction. The risetime of this emission might be due to: (a) an activation barrier in the ESIPT process; (b) a specific solvation of the excited keto tautomer; or (c) a vibrational relaxation process from the initially produced hot phototautomer to the nanosecond emitting form. The observed spectra give little information on the nuclear configuration of the electronically excited HNB molecule. The broad absorption band at 360
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0
2.5
5
7.5 time/ns
12.5
10
1.80
0.80
timeps 3.21 2.57 In E ! 5
1.93
12 0% -1 UI 5
1.28 a.3 = -0.014
0.84 lll,l,
0.00
r;= a.=
16 ps 0.015
‘I= X=
::;:
I
“s
1
~.
480
960
I,,,,!
1440 1,1,,1,1,11 time/w
1920
2400
Fig. 4. Time-resolved fluorescence decay of HNB in acetonitrile: (a) ,I,=400 nm; (b) I,=640 nm; and (c) the rise and decay of the fluorescence recorded at 640 nm (A.,= 325 nm).
nm might be assigned to a charge-transfer transition, from the naphthol ring to the benzimidazole moiety.
In that case, a quasi-planar conformation of the ground and excited states brought about by the intramolecular hydrogen bond would facilitate this process. On the other hand, the maximum and bandwidth of the red fluorescence band are not affected by the viscosity of the solvent (methanol, ethylene
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glycol, fig. 2), and the fluorescence of HNB in ethylene glycol shows a risetime of 15 ps, a value similar to that found in methanol ( 17 ps). All these observations suggest that there are no large solvent-dependent geometrical rearrangements in the form initially excited and in the tautomer, and that only a single emission occurs under this band. As noted above, it is likely that the electronic configuration of the phototautomer approaches a quinonoid structure with a fairly rigid conformation. Hence, its fluorescence would not be affected by the viscosity of the solvent, as is actually observed in methanol and ethylene glycol. Therefore, it is unlikely that the risetime of the keto tautomer fluorescence is due to a kinetic barrier in the ESIPT process [ 1,13- 19,27 1. If specific interactions between the keto tautomer and solvent molecules were the origin of the observed risetime, one would expect different values of this kinetic component in acetonitrile (weak protonaccepting solvent) and methanol (strong proton-donating and proton-accepting solvent) solutions, as is actually observed in the nanosecond decay times. The similar values recorded in the three solvents used here indicate that the 16 ps risetime is not due to specific interactions between the keto tautomer and the solvent. The other alternative is that the risetime is due to the vibrational relaxation of the excess energy of the phototautomer. Taking into account the Stokes shift ( 11000 cm- * ), the energy of the exciting photon (30770 cm-‘), and of the O-O absorption band (227000 cm-‘), this excess energy can be estimated as z 9000 cm-l and, therefore, a high vibrational temperature should be expected. Energy redistribution in hot molecules of large aromatic compounds by collisions with solvent molecules takes place in the picosecond time scale (5-50 ps) (for a recent review, see ref. [ 33 ] ) . In addition, a cooling time of 14 ps has been determined [ 19 ] for the vibrationally excited tautomers formed in the ESIPT reaction of 2- ( 2’-hydroxyphenyl )imidazo [ 1,2-a ] pyridine in cyclohexane. For this compound, it was observed that the intensity of the red tail of the transient absorption spectra decreases with time, while the position of the maximum remained constant [ 19 1. Recently, Frey and Elsaesser reported [ 15 ] a cooling time of 30 ps for the vibrationally excited closed-enol conformer of 2- (2’-hydroxy-5’-methyl623
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phenyl)-benzotriazole, generated by the back proton-transfer in the ground state. Accordingly, it is likely that the risetime of the fluorescence of HNB determined here in acetonitrile ( 16 ps), methanol ( 17 ps) and ethylen glycol ( 15 ps) is due to the kinetics of the energy redistribution of the vibrationally excited (hot) tautomer formed in the ESIPT reaction. This is also consistent with the observed transient spectra and kinetic parameters of the absorption and stimulated emission of HNB in acetonitrile at different wavelengths, recorded by picosecond techniques [ 321. In scheme 1, the main results of the present work and some photophysical data of HNB in acetonitrile are summarized. In this scheme, the reverse ground-state intramolecular proton-transfer (GSIPT ) reaction regenerates the closedenol form. Although there is not yet information concerning this process, the time constant of the GSIPT reaction in compounds showing ESIPT has been found to be in the nanosecond-subpicosecond range, depending on the molecule [ 13-l 5,19,34]. The risetime value ( 16 ps) of fluorescence in acetonitrile is independent on the temperature in the 273-334 K range. However, the fluorescence lifetime at x 600 nm decreases significantly in the same range (1.87, 1.68, 1.51 and 1.38 ns, at 273, 293, 313 and 333 K, respectively), with an apparent activation energy of 1 kcal/mol. This, and the low emission quantum yield ( x 10V4) indicate that most of the relaxed phototautomers return to the ground state by radiationless channels. Acknowledgement This work was supported, in part, by Projects PB90-0102 and MAT93-0369 from the Spanish Direccion General de Investigation Cientifica y TCcnica. AD thanks the Japanese Society for the Promotion of Science for a fellowship.
References [ 1] P.F. Barbara and H.P. Trommsdorff eds., Spectroscopy and dynamics of elementary proton transfer in polyatomic systems, Chem. Phys. 136 (1989) (special issue); Papers presented at the Conference on Photoinduced Proton Transfer in Chemistry, Biology and Physics, in honour of M. Kasha, J. Phys. Chem. 95 ( 199 1).
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[2] W. Kliipfer, Advan. Photochem. 10 (1977) 311.
[ 31 P.T. Chou, D. McMorrow, T.J. Aartsma and M. Kasha, J. Phys. Chem. 88 (1984) 5652. [4] A.U. Acufia, A. Costela and J.M. Muiioz, J. Phys. Chem. 90 (1986) 2807. [ 5] A.U. Acuila, F. Amat, J. Catalan, A. Costela, J.M. Figuera and J.M. Mutioz, Chem. Phys. Letters 132 (1986) 567. [6] A. Costela, F. Amat, J. Catalan, A. Douhal, J.M. Figuera, J.M. Munoz and A.U. Acuiia, Opt. Commun. 64 (1987) 457. [ 71 A. Costela, J.M. Muiioz, A. Douhal, J.M. Figuera and A.U. Acutia, Appl. Phys. B 49 (1989) 545. [ 8 ] A.U. Acutia, F. Amat-Guerri, A. Costela, A. Douhal, J.M. Figuera, F. Florida and R. Sastre, Chem. Phys. Letters 187 (1991) 98. [9] A. Douhal, F. Amat-Guerri, M.P. Lillo and A.U. Act&q J. Photochem. Photobiol. A, in press. [ 10 ] J. Sepiol, H. Bulska and A. Grabowska, Chem. Phys. Letters 140 (1987) 607. [ 111 P.T. Chou, M.L. Martinez and J.H. Clements, Chem. Phys. Letters 204 (1993) 395. [ 121 D. Haarrer, Japan J. Appl. Phys. 26 (1986) 3362. [ 131 T. Elsaesser and W. Kaiser, Chem. Phys. Letters 128 (1986) 231; T. Elsaesser, W. Kaiser and W. Ltittke, J. Phys. Chem. 90 (1986) 2901. [ 141 W. Wiechmann, H. Port, F. Laermer, W. Frey and T. Elsaesser, Chem. Phys. Letters 165 (1990) 28. [ 15 ] W. Frey and T. Elsaesser, Chem. Phys. Letters 189 ( 1992) 565. [ 161 B.J. Schwartz, L.A. Peteanu and C.B. Harris, J. Phys. Chem. 96 (1992) 3591. [ 171 T. Arthen-Engeland, T. Bultmann, N.P. Emsting, M.A. Rodriguez and W. Thiel, Chem. Phys. 163 ( 1992) 43. [ 18 ] J.L. Herek, S. Pedersen, L. Bafiares and A.H. Zewail, J. Phys. Chem. 97 ( 1992) 9046. [ 191 A. Douhal, H. Kandori, K. Yoshihara and F. Amat-Guetri, to be published. [ 201 T. Yamazaki, N. Tamai, H. Kume, H. Tsuchiya and K. Oba, Rev. Sci. Instrum. 56 (1985) 1187. [21] J.M. Beechem, E. Gratton and W. Mantulin, Globals Unlimited, University of Illinois, Champain-Urbana, IL, 1990. [22] H.K. Sinha and S.K. Dogra, Chem. Phys. 102 (1986) 337. [23] K. Das, N. Sarkar, D. Majumdar and K. Bhattacharyya, Chem. Phys. Letters 198 ( 1992) 443. [24] P.F. Barabara, P.K. WalshandL.E. Brus, J. Phys. Chem. 93 (1989) 29. [25] T.C. Swinney and D.F. Kelly, J. Phys. Chem. 95 (1991) 10369. [26] G.A. Bruker, T.C. Swinney and D.F. Kelly, J. Phys. Chem. 95 (1991) 3190. [27]T.P. Smith, K.A. Zaklika, K. Thakur, G.C. Walker, K. Tominaga and P.F. Barbara, J. Phys. Chem. 95 ( 1991) 10465. [28] W.R. Laws and L. Brand, J. Phys. Chem. 83 (1979) 795.
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[29] CM. Harris and B.K. Selinger, J. Phys. Chem. 84 (1980) 891. [ 301 G.J. Woolfe and P.J. Thistlethwaite, J. Am. Chem. Sot. 103 (1981) 3849. [31] G.W. Robinson, P.J. Thistlethwaite and J. Lee, J. Phys. Chem. 90 (1986) 4224.
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[ 32 ] H. Kandori, A. Douhal, F. Amat-Guerri, A.U. Act&a and K. Yoshihara, to be published.
[ 331 T. Elsaesser and W. Kaiser, Ann. Rev. Phys. Chem. 42 (1991) 83.
[ 341 F. Laermer, T. Elsaesser and W. Kaiser, Chem. Phys. Letters 148 (1988) 119.
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