Solvent dependent trans → cis photoisomerization of N, N′-diacetylindigo studied by femtosecond time-resolved transient absorption spectroscopy

Abstract

Solvent dependence of trans→cis photoisomerization of N,N′-diacetylindigo (DAI) was investigated by femtosecond transient absorption (TA) spectroscopy. A strong and sharp singlet excited state absorption band centered at 595nm was observed in the TA spectrum and the differential absorbance (ΔAbs) and the wavelength at the maximum of the band were periodically modulated by coherent vibrational wavepacket motions. However, even in the “reactive solvent”, ethyl acetate (EtAc), where the isomerization takes place, the formation of cis-DAI was unobservable in our experimental time window of 2ns which means that isomerization is occurring in a longer time scale. Preceding studies suggest that intermolecular hydrogen-bond (H-bond) with the solvent quenches the excited state and blocks the isomerization. Interestingly, we have observed that dimethylformamide (DMF) blocked the isomerization more efficiently than methanol. The excited state lifetime was shorter in DMF compared to EtAc solution suggesting that weak H-bonding with the formyl group of DMF is sufficient to prevent the slow trans→cis photoisomerization of DAI.