Ultrafast Excited State Dynamics of Forward and Reverse trans-cis Photoisomerization of Red-Light-Absorbing Indigo...

Femtosecond time-resolved transient absorption (TRTA) spectroscopy was carried out to investigate the ultrafast excited state dynamics of both trans → cis and trans ← cis photoisomerization of red-light-absorbing indigo derivatives. For N,N'-bis(tert-butyloxycarbonylmethyl)indigo (tBOMI), the excited state lifetime of the trans-form was measured to be 41 ps while that of the cis-form was as short as 730 fs in acetonitrile (Acn). The excited state lifetime of trans-N,N'-dimethylindigo (DMI) in Acn was also measured to be as short as 10 ps. These values are much shorter than those of the blue-light-absorbing trans-forms of indigo derivatives such as N,N'-diacetylindigo (DAI) and thioindigo (ThI). The chromophore of indigo is composed of two pairs of electron donor and acceptor substituents conjugated in the shape of a letter "H" (so-called "H-chromophore"), although DFT and TDDFT calculations suggest that the charge transfer (CT) character is not very significant. Nevertheless, when a weak CT within the H-chromophore is promoted, the absorption band shifts to longer wavelengths and the excited state lifetime shortens. For the photoisomerization of DAI and ThI, a relatively long excited state lifetime is required for the photoisomerization, while for tBOMI and DMI, a vibrationally hot ground state that overcomes the energy barrier in the ground state is produced by rapid nonradiative decay through conical intersection. In the case of cis-tBOMI, the repulsion between the two adjacent negatively charged carbonyl groups and the weakening of the central C═C double bond in the S₁ state twist the molecule, shorten the excited state lifetime, and increase the quantum yield of the trans ← cis photoisomerization.