Measurements of the Relaxation Kinetics of Photo-Excited Enol Form of Dibenzoylmethane

The decay rate of the excited state of the enol form of dibenzoylmethane (DBM) in different solvents is measured directly using nanosecond light pulses. The observed relaxation kinetics is explained by the formation of three intermediate forms of the excited cis-enol form of DBM. The decay rates ofthe intermediates and their absorption cross sections for A 266 nm are reported too.


INTRODUCTION
The photochemical behaviour of enolizable/3-dicarbonyl compounds, their derivatives and analogous have been a subject of recent intensive studies. As a result of a photoinduced hydrogen transfer, the photoac- Experimental -3 and theoretical -6 data give evidence to suggest that excitation of the chelated enol form leads to the formation of more than one non-chelated short-lived species. We are reporting here the results of the determination of the decay time of the excited dibenzoylmethane enol form and the probable energy scheme of the relaxation process.

EXPERIMENTAL Apparatus
The experimental set-up is shown in Figure 1. It consists of two synchronized Nd:YAG lasers A and B. The laser A described in detail elsewhere, 7 produced light pulses with duration of 5 ns at X 1.06 Im. The laser B was Q-switched to produce pulses of 7 ns duration at the same wavelength. The lasers operated with a pulse repetition rate up to 10 Hz in TEMoo mode. The delay time between the output pulses of both lasers was varied in the range 0-10 s by a proper electronic control of the electrooptic switches used in both lasers. The jitter of the delay time was 10 ns within the range of 0-1 s and less than 1% within the range of s-10 s. 10 -4 mol 1-1). It was found that the absorption did not depend on the energy density of the radiation when it was below 0.1 J. cm -2 (i.e., the range of linear absorption) and that the saturation of the absorption took place for energy densities above 1.5 J. cm-2. During the measurements the energy densities of the probe and exciting pulses were chosen to be within these two ranges, respectively.

Solutions
Dibenzoylmethane was "analytical purity" Merck commercial product. Isooctane, cyclohexane and methanol (Merck "spectroscopic grade") were used as solvents. Solutions with concentration of 10 -4 mol. 1-1 were prepared and kept in the dark until thermodynamic equilibrium between two tautomeric forms was established.

Irradiation
The wavelength of the exciting pulse Ae 355 nm was chosen to be in the absorption band of the enol dibenzoylmethane form (Figure 2).

EXPERIMENTAL RESULTS
The probe pulse transmission through the sample with isooctane solution of DBM as a function of delay time between the exciting and probe pulse is shown in Figure 3a (hp 355 nm) and Figure 3b (hp 266 nm). As it is seen while the transmission at 355 nm decreases to its initial (before irradiation) value, the transmission at 266 nm keeps the reverse trend.
We tried to match the experimental results (dots on Figure 3) with an analytical expression which involves a sum of exponential terms.
It was found that a sum of three exponential terms produces a good fit. Therefore the normalized transmission may be presented as" 3 In  Table I. A13 and q13 could not be determined for the same reason.

MODEL AND DISCUSSION
Before presenting a model of the relaxation process we point out three effects which may influence the transmission process but should be excluded due to their slow recovery time. These three effects are formation of keto tautomer molecules, diffusion and thermal effects. First, the formation of keto tautomer molecules during the exciting pulse should be excluded since the reverse process keto-enol is very slow and takes several hours. Second, one may assume that the excited molecules leave irradiated volume due to diffusion. However, our estimation has shown that the diffusion from a beam waist of 100 m diameter is important in a time scale of about 1-2 seconds. Third, the study of far field beam divergence did not indicate any beam distortion due to thermal effects or microbubbles. Note that, in the experiment ms after the exciting pulse, the transmission of both probe pulses (Ap3 or Ap4) is practically the same as before the excitation. Now we consider the experiment in which a probe pulse with Ap= 355 nm was used (Figure 3a). The lifetime of the first excited state $ is too small (10--10 -9 s), in comparison to the time resolution in this experiment, therefore we may assume in our model that immediately after the end of the exciting pulse $ is empty and the molecules occupy the lower levels. In this case the probe pulse transmission is determined by the number of molecules at the ground level So. Since the three exponential law fits very well the experimental results we can assume that there are three levels situated between ground state So and first excited state $1. The molecules which occupy these three excited levels are returning back to So level with different rates.
Here 0" 3 is the absorption cross section for hp 355 nm and is the cell thickness.
Substitution of (11) in (12) gives: In [T3( t)/T3(oo)] 0"31[N2"exp (-k2,t) ,+ C,.exp (-k4,t)+ C2.exp (-kt)] (13) After comparing (13) and (2a) we find that A3 0"31N2 A23 0"31C2 A33=0"31C (14) and q13 k21 q23 k31 + k34 qa3 k4 The experimentally derived coefficients Ai3 and qi3 permit the values of kay, k34 and their ratio k3/k34 to be determined. For isooctane solution the ratio k3/k3, is 2:0.9, for cyclohexane k31/k34 3 : 1.3 and for methanol ka/k34=O.35q=O.16. In order to explain the second part of the experiment, in which a probe pulse was Ap 266 nm, we use the same energy diagram ( Figure  4) and the same rate equations (4-7). As non-excited enol form of DBM has a small absorption at this wavelength (see Figure  where O'i4 is the absorption cross-section for Ap 266 nm from level. After substituting ni from (8), (9) and (10)  Ap--266 nm were also determined and are presented in Table II. Now we try to associate the energy states shown in Figure 4 with possible transformations of the chelated enol form of the DBM molecule (Scheme le). PPP calculations [9] show that when the molecule is at S state the 2-3 bond order decreases and has nearly the same magnitude as the 3-4 bond. This means that a rotation round the 2-3 bond is highly probable. On the other hand, the quantum chemical calculations of the energies of S and T1 states indicate that these energies become equal at 90 rotation round the 2-3 bond (Scheme b). Because of this there is a fast intersystem crossing  Another possibility for relaxation of the excited molecules is formation of a nonstable rotamer (Scheme la) formed by rotation round 1-2 bond. The possibility of this rotation has been discussed elsewhere. 4 This rotamer may be related with the state 2 in the energy diagram.
In conclusion, using nanosecond pulses we were able to study the relaxation of DBM excited enol form. The presented model is in good agreement with the experimental results. However, additional experiments with shorter pulses and within a wider wavelength range are required to obtain enough information for understanding the complete mechanism of photoinduced enol-keto transformation.