Structural Volume Changes upon Photoisomerization: A Laser-Induced Optoacoustic Study with a Water-Soluble Nitrostilbene

Resumo

The trans to cis photoisomerization of 4,4′-dinitro-2,2′-disulfonylstilbene (DS) was studied by laser-induced optoacoustic spectroscopy (LIOAS) in aereated neat water and in aereated aqueous solutions of various monovalent cations (NH4+, N(CH3)4+, Na+, K+, and Cs+) and the respective cis to trans photoisomerization only in the presence of NH4+. In every case, two single-exponential components were required to fit the data, one with an unresolved lifetime (<20 ns) for the appearance of the triplet state mixture T (the perpendicular triplet state 3p* in equilibrium with the lowest trans triplet state 3t*) and one with a longer lifetime of (75 ± 20) ns at 5.5 °C for the decay of the T mixture. The temperature dependence of the LIOAS amplitudes in combination with the determined isomerization quantum yields afforded a contraction of -(1.4 ± 0.15) ml/mol for the trans to T transition, whereas a smaller contraction of -(0.15 ± 0.15) ml/mol was obtained for the cis to T transition. The different values of the contraction indicate a greater similarity between the average structures of the T components with the cis ground singlet state than with the trans ground singlet. The total structural volume change for the trans to cis transition is in average ΔVtc = - (1.2 ± 0.1) ml/mol. The calculated contribution of electrostriction is at most 50% of this value. However, the nature of the countercation had no influence on the data as would be expected for changes in the specific interaction with the H-bond network in water upon photoisomerization. Thus, ca. 50% of the total contraction is attributed to intrinsic effects, related to a shorter C=C bond and a smaller accessible volume in the cis isomer. The LIOAS data show that cis-DS lies (28 ± 35) kJ/mol above trans-DS in agreement with the calculated value of 37 kJ/mol.