4P070
Kinetics and Equilibrium of cis-trans Isomers in the Ground State of4-Amino Azobenzene(Mol. Photosci. Res. Cent., Kobe Univ.) °Neeraj Kumar Joshi, Akihide Wada
Introduction] Photo-induced studies provide the information regarding the various radiativeand non-radiative relaxation processes by which an excited molecule returns to its groundstate.

For the control on such photo-induced reactions, prior detail information of all

deactivation processes is highly vital.

In azobenzene (AB) derivatives, trans to cis

photoisomerization occurs following irradiation with UV-visible light.1 Cis to trans thermal
isomerization occurs spontaneously in the dark owing to the thermodynamic stability of thetrans isomer.1, 2 Substitution by different groups at different positions in the parent moiety(AB) can alter spectroscopic properties and dynamics / kinetics of isomerisation.3,4 Moreover,mechanism of photoisomerization has also been a subject of debate due to the involvement of
more than one higher singlet electronic excited state (i.e, S1 and /or Sn)2. On the other hand,by the virtue of participation of only one state, i.e., the ground state in thermal isomerization,
it may become easier to probe the mechanism of isomerization. Therefore, to explore themechanism of isomerization and to know one of the deactivation processes over a complete
photoreaction cycle, we address the kinetics of thermal isomerisation of 4-aminoazobenzene(AAB) and effect of solvent polarity, viscosity and temperature on it.
Experimental] AAB obtained from Wako Pure Chemical Industries, Ltd., Japan (99% pure)
was used as such. All the solvents used were of spectroscopic grades. Steady state absorption
spectra were recorded by dual beam Shimadzu UV-2400PC spectrophotometer. To generatecis-AAB, cw UV laser (405 nm), ~200 mW was used. To record the shorter ground state
decay times, transient time resolved setup was build-up.
Results and Discussion]

In diethyl ether (DEE) and acetone (ACTN), AAB exhibits

absorption maximum at around 380 nm with long wavelength absorption tail (Fig. 1). To
generate cis-AAB, UV laser (405 nm) was irradiated on the sample, until it reached atequilibrium (photo-stationary state).

Figure 1b shows the absorption spectrum ofphotostationary state. Difference absorption spectrum of AAB in DEE is depicted in Fig. 1c.
It is clearly shown that prominent photobleaching at 380 nm and the transient absorption
peaks of the photostationary state at around 340 nm and 450 nm are observed. The transientabsorption peaks can be assigned to the cis-AAB. These curves indicate that most of transAAB molecules are converted to the cis-configuration.

As the laser irradiation is stopped,cis-AAB isomers do not remain
stable and return towards more stabletrans-configuration, and thus one canfollow the course of cis to transconversion (Fig. 2). Rate of thermal
isomerization form cis to trans-isomer
is mainly governed by activationenergy and generally follows a firstorder kinetics.

Fig. 1 Absorption spectra of AAB in DEE (298 K)

In order to estimate the activationenergy of cis-AAB, we followed the course
1.2of isomerization at different temperatures.

1.0

isomerization reaction in dark obeys a0.8

singleexponential

behaviour.

Forestimation of activation energy, Eyring andArrhenius equation was employed and theobtained values of barrier height are 0.80
eV and 0.78 eV, respectively. From thetemperature dependent measurement on
AAB in ACTN and methanol (MeOH)

O. D. (380 nm)

The uni-molecular thermal cis-to-trans

0.6

310 K
305 K
300 k
295 k
290 k

DEE

0.4
0.2
0

1

2

3

4

56

78

9Time (hrs)Fig. 2 Temperature dependence of groundstate decay of cis-AAB in DEE.

solutions, activation energy was estimated at 0.6 eV and 0.3 eV, respectively.
From the above observation, dependence of polarity on activation energy was noticed as it
varies from 1.06 eV in gas phase5 to 0.3 eV in MeOH. In gas phase, inversion mechanism has
been proposed for AAB.5,6 DEE being non-ploar in character, inversion mechanism can beconsidered. On the other hand, a polar intermediate can lower the activation energy and favorrotation, and thus accelerate the rate.

Therefore, it is worth considering the rotation

mechanism in solvents of higher polarity.
References(1) H. Rau, et al., J. Am. Chem. Soc., 104, 1616 (1982)(2) H. M. D. Bandara, et al.,Chem. Soc. Rev., 41, 1809 (2012).
(3) J. G. Amoros, et al., Phys. Chem. Chem. Phys., 12, 13238 (2010).
(4) Y. Hiorse, et al., J. Phys. Chem. A, 106, 3067 (2002).
(5) J. Dokic, et al., J. Phys. Chem. A, 113, 6763 (2009).
(6) L. Wang, et al., J. Photochem. and Photobiol. A, 205, 104 (2009).