Abstract:
Interaction of light with electron donor−acceptor π-conjugated
systems leading to intramolecular charge transfer (ICT) plays an essential role
in transformation of light energy. Here the cascade of photoinduced ICT
processes is directly observed by investigating the excited state relaxation
dynamics of cyano and mono/di methoxy substituted diphenyl acetylene
derivatives using femtosecond pump−probe spectroscopy and nanosecond
laser flash photolysis. The femtosecond transient absorption spectra of the
chromophores upon ultrafast excitation reveal the dynamics of intermediates
involved in transition from initially populated Frank−Condon state to local
excited state (LE). It also provides the dynamic details of the transition from
the LE to the charge transfer state yielding the formation of the radical ions.
Finally, the charge transfer state decays to the triplet state by geminate charge
recombination. The latter dynamics are observed in the nanosecond transient
absorption spectra. It is found that excited state relaxation pathways are controlled by different stages of solvation and
intramolecular relaxation depending on the solvent polarity. The twisted ICT state is more stabilized (978 ps) in acetonitrile than
cyclohexane where major components of transient absorption originate from the S1 state.
