Femtosecond photoisomerization of azobenzene-derivative binding to DNA

Ultrafast photoisomerization and relaxation dynamics of trans (T) 4′-methylthioazobenzene (AzD) binding to double-strand DNA (T-AzD-dsDNA) as well as single-strand DNA (T-AzD-ssDNA) and T-AzD were investigated by the measurement of femtosecond absorbance changes on S 2 T excitation (400 nm) with the rate equation analysis. All the solutions showed the fast (τ1, A1) and slow (τ2, A2) decay components and the offset component (A3). The greatly different negative or positive absorbance change behaviors by the probe wavelength of 400 or 420 nm for the T-AzD solution were attributed to the remarkable dependence of the absorption cross-section difference between the T-isomer excited and ground states on the probe wavelength and of that between the cis (C)- and T-isomer ground states. The significantly shorter S 2 T state lifetimes τ1 for T-AzD-dsDNA and T-AzD-ssDNA were observed to be 30 and 60 fs, respectively, compared with that (220 fs) of T-AzD. This is presumably attributed to the intramolecular electron transfer from DNA bases to T-AzD in T-AzD-DNAs, suggesting the first observation of electron transfer in an ultrafast photoisomerization system interacting with DNA. While, the kinetic rate k 2,1 T , I from the S 2 T state to the bottleneck intermediate state I 1 T , C of the initial process in the T to C photoisomerization and the I 1 T , C state lifetime τ2 hardly changed like 1.3 × 1011, 1.4 × 1011 and 1.6 × 1011s−1 and like 6.7, 6.2 and 6.0 ps, respectively. The latter implies that the birth time of the C-isomer is almost the same for all the solutions. Furthermore, the T-to-C photoisomerization rate ηT,C by single-shot excitation was determined from A3 to be 1.2%, 0.36% and 0.22% at the 100-nJ pulse energy level, indicating that the T-AzD molecule is one of the most efficient T-to-C photoisomerization molecules. The decrease of ηT,C in T-AzD-DNAs is due to the dramatic shortening of the excited-state lifetime τ1.