J. Phys. Chem. 1995, 99, 4380-4385
4380
Transient States of an Intramolecular Proton Transfer Cycle Studied by Degenerate Four-Wave Mixing Thomas Hdfer, Peter Kruck, Thomas ElsaesserJ and Wolfgang Kaiser* Physik Department E 11, Technische Universitat Miinchen, 0-85748 Garching, Germany Received: December 9, 1994@
The kinetics of an intramolecular proton transfer cycle, in particular the population of the various transient states, is investigated by a new experimental technique: degenerate four-wave mixing in combination with excitation of the molecules by picosecond (2 x lo-” s) UV pulses. The molecular hyperpolarizability y(3) of different excited electronic states is measured at five spectral positions between 10 000 and 30 000 cm-’. Resonance enhancement up to 3 orders of magnitude near electronic transitions is observed. The high sensitivity of y(3)to molecular resonances allows the study of extremely short-lived states. As an example, data are presented on the dynamics of the intramolecular proton transfer reaction in 2-(2’-hydroxyphenyl)benzothiazole where three transient states are elucidated.
1. Introduction In a previous paper’ an experimental technique, degenerate four-wave mixing (DFWM) plus optical excitation, was introduced to study the dynamics of photochemical reactions by the time-dependent change of the nonlinear susceptibility x ( ~ )( w;w,--w,w). Regarding each molecular state-populated during the photochemical reaction-as a different species, the molecular hyperpolarizability y(3) of the individual states contributes differently to the measured total susceptibility. Prior to excitation ~ ( is~determined 1 for the molecular ground state, and after excitation several subsequent molecular states may be distinguished by their dynamical behavior and by the magntitude of their hyperpolarizability y(3)at different frequencies. In this article, we investigate the previously studied, photoinduced, intramolecular proton transfer of the aromatic molecule 2-(2’-hydroxypheny1)benzothiazole (HBT).2-10 The change of structure of the molecule after excitation is accompanied by a modification of the n-electron system, resulting in substantial alternation of the electronic spectra. Because of the strong dependence of the nonlinear susceptibility on the momentary state of the system, one expects distinct changes of x ( ~after ) photoexcitation. Special attention is given to the application of this experimental technique to the study of short-lived molecular electronic states. HBT undergoes a closed reaction cycle comprising proton transfer upon excitation, deactivation of the reaction product, and proton return to the initial molecular geometry. The following scheme summarizes the relevant reaction steps according to ref 2:
Additional information-the molecular structures, the time constants, and the absorption spectra-of the proton transfer cycle important for the present discussion is depicted in Figure
* To whom correspondence should be addressed. Present address: Max-Bom-Institutfiir Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany. Abstract published in Advance ACS Abstracts, March 1, 1995. @
0022-365419512099-4380$09.OO/O
1.2-10 In nonpolar solvents the enol tautomer of HBT represents the dominant ground state species with a strong intramolecular hydrogen bond between the hydroxylic proton and the nitrogen atom (left side of Figure l).3,4Excitation of this species via the SO S1 absorption band at 28 000 cm-l leads to the rapid formation of the excited state S’1 of the keto tautomer with N-H and C=O groups (center of Figure l).5-7These new molecular groups-linked by an intramolecular hydrogen bond-have been investigated very directly by picosecond infrared spectroscopy.8 The formation time of the keto tautomer of 170 fs was measured for HBT dissolved in nonpolar tetrachloroethylene.6 The keto species shows a strongly Stokes-shifted fluorescence emission S’1- S’O with maximum around 540 nm. At room temperature, the first excited state of the keto-cis species is deactivated mainly by radiationless processes, resulting in a low fluorescence quantum yield of 0.02 in tetrachloroethyleneg and 0.01 in methylcyclohexane.2 The corresponding fluorescence decay times are 300 and 140 ps in C2C14 and methylcyclohexane, respectively. The mechanism of radiationless deactivation of the S’1 state and the pathway of the back-reaction to the enol geometry needs some discussion. Intersystem crossing to the triplet manifold, internal conversion to the ground state of the keto-cis tautomer, and an isomerization reaction forming the keto-trans tautomer could contribute to the overall decay rate of S‘1. The different relaxation mechanisms have been studied in time-resolved absorption measurements with both picosecondg and nanosecond2 time resolution. The data show that intersystem crossing makes a minor contribution to the radiationless decay and, thus, to the back-reaction at room temperature. A strong transient absorption band located between 400 and 500 nm was found to live for more than several hundred picoseconds9 and was attributed to the keto-trans state S ’ O , ~The ~ . ~keto-cis ground state S‘O is expected to return rapidly (approximately picoseconds) to the enol ground state The relative populations of the keto-cis and keto-trans state as a result of the isomerization reaction-which proceeds over the twisted intermediate state s’,,-are determined by the corresponding decay rates. Numbers have been reported in ref 2. In the present paper we adopt these numbers in our reaction model.
-
2. Experimental Section Picosecond light pulses with high pulse intensities of up to GWlcm2 are generated at five frequencies starting from the 0 1995 American Chemical Society
Transient States of a Proton Transfer Cycle
Enol
Keto-cis
J. Phys. Chem., Vol. 99, No. 13, I995 4381
Keto-trans
focal plane at z = 0; detection in the positive z direction). The difference between the transmission maximum and minimum AT allows to estimate the magnitude of Re($3)), where Re&(3)(-w;w,-w,w)) [m2N2]= (c€mo2/3)n2 [m2/W].13 For a Gaussian beam the nonlinear refractive index n2 is given by16
AT
Absorptlon
I
E Y
*
P Absorption
C W
Absorption
30000
20000
10000
0
Figure 1. Molecular structures and relevant energy levels of the enol form of HBT (left), of the keto-cis form (center), and of the ketotrans form (right). The SOabsorption and the S'I and the S ' O , transient ~ absorption spectra are shown schematically. The solid arrows mark the different photon energies where the nonlinear susceptibility of the molecule is investigated. The arrow at the very left indicates the exciting transition which initiates the photoreaction.
output of an active-passively mode-locked Nd:YAG laser system (5= 20 d, zp = 20 ps). The frequencies are the fundamental of the Nd:YAG laser at 9398 cm-l, the second harmonic at 18 797 cm-' generated in one KDP crystal, and the third harmonic at 28 193 cm-' generated with two KDP crystals." Two additional, intense, frequency-shifted picosecond light pulses are produced at 15 872 and 25 268 cm-l by transient stimulated Raman scattering in acetone (pump frequencies 18 797 and 28 193 cm-l).12 The applied five photon energies are marked in Figure 1 as solid arrows starting at the three molecular states SO, S'1, or S'O,~. The following measurements are performed: (1) Standard absorption and emission spectra are recorded by a spectrophotometer and a fluorimeter with single-photon counting detection. Fluorescence decay (the lifetime of the excited keto species) is measured and spectrally integrated, after excitation at 28 193 cm-I with the help of a single-shot streak camera (temporal resolution 4 ps). Pump and probe experiments are made to investigate the transient absorption spectra after proton transfer. The change of transmission of the sample after excitation is monitored by a weak probe pulse of variable time delay and at different frequen~ies.~ (2) In order to determine the sign as well as to estimate the magnitude of the nonlinear refraction of n-hexane, and with it the real part of - w;w, - w,w),l3 the self-focusing of the sample is in~estigated.'~In the straightforward experimental setup15 the sample under study is placed close to the waist of a focused Gaussian beam, which is directed onto a finite aperture. At higher intensities the sample acts as a lens due to the spatially nonuniform, intensity-dependent refraction index. Dependent upon the sign of the nonlinearity, one observes a negative or positive lensing effect, increasing or decreasing the intensity transmitted through the aperture as the sample is moved in relation to the focal plane (z: sample position relative to the
xz(
A ~ 2 ~ c 9 n I~ Z ~ Z ~ ~ ~
(1)
where A is a constant (-0.4) depending upon the diameter of the aperture, IO the pulse intensity in the focal plane, and leff the effective sample length. (Experimentally, the energy transmission is determined.) (3) The experimental system for degenerate four-wave mixing has been described previously.' In short, in the experimental configuration the incident, vertically polarized pulses are split into two nearly parallel beams of equal intensity, which are temporally and spatially (crossing angle 2") superimposed in the liquid sample contained in a cuvette of 500 p m length. In this geometry, the two coherent laser waves interact inside the nonlinear medium over a length of approximately 300 p m and produce a transient grating." The self-diffraction efficiency of the grating VM in first order defined by 7~ = IdZ1 is the observed quantity, where ID is the diffracted light intensity and I1 the intensity of one incident light pulse. VM is given by1*J9
where I1 and 12 are the input pulse intensities, d is the cell length, and a is the absorption coefficient of the investigated sample at the frequency Q. c denotes the vacuum light velocity and n the refractive index. Thus, by measuring 1 ; 1 ~at known incident - w ; w , - w,w)l. fields, one can determine
IxE(
The present measurements are performed with zero delay between the two pulses generating the transient polarization grating. For a frequency position of the pulses below 27 000 cm-l, the HBT sample is essentially transparent. Consequently, a population grating generated by the two interfering pulses can be neglected. At 28 193 cm-', Le., in the range of the SO S1 absorption band of enol-HBT, the intensity of the pulses (see Figure 5 ) is reduced to a value of 0.2 GW/cm2 to keep the excitation level low. For the study of the dynamics of VM, an additional intense pump pulse at 28 193 cm-l is introduced to excite up to 30% of the molecules within the volume of the grating to the enolS1 state. The resulting change of the susceptibility x(3)of the molecules due to their subsequent proton transfer reaction is observed as a function of time after excitation. Commerically available HBT (Aldrich) is purified by liquidphase chromatography. Spectrograde n-hexane serves as solvent without further purification. The concentration of the samples ranges between and 2 x M. All measurements are performed at room temperature.
-
3. Results
-
(1) Fluorescence Properties and Transient Absorption. The enol SO S1 absorption band of HBT peaks at 29 500 cm-' with a molar extinction coefficient of 1.4 x lo4M-I cm-' (Figure 1, left). Excitation in this band results in the strongly Stokes-shifted emission of the keto tautomer which is located between 14 000 and 20 000 cm-'. The spectrally integrated fluorescence kinetics of HBT in n-hexane is monitored by a streak camera. Figure 2 shows the observed decay of the emission intensity well fitted by a single-exponential curve, yielding a lifetime of the keto S'1 state of 160 f 20 ps.
Hofer et al.
4382 J. Phys. Chem., Vol. 99, No. 13, 1995
= 1.2 x M (closed circles). The transmission of the solvent has similar features at both frequencies, that is, a minimum followed by a maximum as the sample is moved from -z to +z. These data are indicative of a positive n2, Le., Re kEi( - w;w, - w,w)) > 0.I6 With the help of eq 1 we calculate an essentially frequency-independent nonlinear refraction index of approximately n2 = 2.3 x cm2/GW, corresponding to a real value of 4.7 x m2N2. The agree with the literature value sign and the magnitude of of ref 22. The same measurements with the HBT solution give slightly larger valley and peak amplitudes (see Figure 3). In fact, the enhanced nonlinear refraction index of the solution points to a positive n2 of the HBT molecule in the ground state. (3) Transient Grating Experiments. Two sets of investigations are reported here: Measurements of the solute molecule (i) in the ground state and (ii) in subsequent excited states. (i) Transient grating experiments without excitation are performed at five frequencies in the resonant as well as in the nonresonant part of the HBT spectrum. These frequencies are indicated in Figure 1 (left) by the arrows starting from the SO state. In solutions the macroscopic susceptibility x(3) has contributions of the solvent and the solute and can be expressed for noninteracting particles byz3
&; ::x
Tima [pa]
Figure 2. Kinetics of the keto fluorescence of HBT in n-hexane as measured by a streak camera. The spectrally integrated emission (logarithmically plotted) decays single exponentially with a time constant of 160 ps. Excitation is at 28 193 cm-'. z -20 1.1
-io
["I 0
10
20
: HBT I n n-hexan. n : n-hexan.
1 .o
x$;
-7
E
0.9
t
Y
+
c 0
-
'ii 1.2
Ei e
I-
1.0
0.8
-20
-10
0
io
20
["I Figure 3. Normalized aperture transmission of a self-focusing (Z-scan) measurement of n-hexane (open squares) and of a HBT solution ( c = 1.2 x lo-* M, closed circles) for the two frequencies (a) 9398 and (b) 18 797 cm-'. Intensities in the focal plane are (a) 25 and (b) 27 GW/ cm2. The valley-peak sequence of the transmission indicates a positive nonlinear refraction index n2 of the solvent. The enhancement of this feature in the HBT solution points to a positive nz of the HBT molecule in the ground state. Literature values are 300 and 140 ps for the solvent tetrachloroethylene9and methylcyclohexane,2respectively. Orientational relaxation is not seen in the fluorescence kinetics of Figure 2 and does not have to be considered here.20x21 The transient absorption spectrum from the keto tautomer was investigated in the frequency range from 9398 to 28 193 ~ m - l . ~ The S'1 S', spectrum-measured 15 ps after excitation-is depicted in the center of Figure 1. A strong absorption band at 25 000 cm-' is detected (cma = 3.7 x lo4 M-' cm-l). In a similar way, the transient absorption from the S ' O , ~state was measured 800 ps after excitation. This spectrum peaks at 23 OOO cm-I with cma = 3.2 x lo4 M-I cm-l (Figure 1, right). (2) Nonlinear Refraction. Figure 3 shows self-focusing (Zscan) measurements at 9398 and 18 797 cm-I with the pure solvent n-hexane (open squares) and with a HBT solution of c
-
where is the solvent susceptibility, xf: the solute susceptibility, and y e the orientationally averaged molecular hyperpolarizability of the solute molecules. Na is the Avogadro number, cs0 the molar concentration of the solute, and L(4)the correction factor for the local field given by L(4) = ('/3(n2 2))4.24 Measuring x(3)as a function of the solute concentration cso allows to determine the magnitude and the phase of the complex quantity yf) = lyf'l e-@. In Figure 4 the experimental 1x(3)1 values are depicted'as solid circles versus the molar concentration cso for four frequencies. The experimental accuracy is determined by uncertainties in the determination of the pulse intensities. The frequency-independent value of n-hexane (at cso = 0 M) of = 0.5 x m2N2is in agreement with the value of the Z-scan measurements and with the number of ref 22. The /y.el value of the solute is determined from the relationshp between 1$3)1 and cs0 observed at various frequencies. The calculated solid lines are adjusted to the experimental points with different phases: q = Oo (yf: positive) and lql = 90" so imaginary, sign undetermined). The concentration dependence of at low cs0 values (Le., with I l x ~ ~provides l) the phase of y e . The measured y e values of the ground state of HBT at the different frequences are summarized in Table 1. (ii) The time evolution of the diffraction efficiency VM after excitation at 28 193 cm-l is studied at the same five frequencies introduced above. In Figure 5 the dynamics of VM is shown for four DFWM frequencies. Approximately 20% of the molecules in the sample are excited by the pump pulse. In Figure 5a-c the magnitude of VM changes rapidly within the resolution of the apparatus at t = 0 ps. At 28 193 cm-' a decrease by 30% is observed, whereas at 25 268 and 18 797 cm-l VM increases by a factor of 35 and 1.4, respectively. The following recovery of VM occurs with a time constant of 160 z t 20 ps. In Figure 5a,b the initial value of l ; l ~is not reached even at long delay times. Approximately 400 ps after excitation
~2:
J. Phys. Chem., Vol. 99, No. 13, I995 4383
Transient States of a Proton Transfer Cycle 0.8
I
,
I
1
,
Delay Tlme [ p r ]
I
0
100 200 300 400 5 0 0
p=O
\
-
I
-20&100
0.7
N>
0.8
I
E 0.6
0.6
0.3
N
0
;;0.5
0.5
v0
Y Y
0.2
SO
--
