Ultrafast intramolecular charge transfer in N-annulated complex with donor-acceptor conjugation

초록

With the development of metal-free, donor-acceptor dyes for dye-sensitized solar cells (DSSCs), it is important to understand the character of charge transfer states because charge transfer is a critical step that determines the efficiency of electron injection to the electrode in DSSCs. Accordingly, the dynamics of excitons and charge carriers in donor-acceptor dyes have been widely studied in many related fields. Recently, photosensitizers based on N-annulated rylene derivatives, which have outstanding optical properties, have been utilized in DSSCs exhibiting considerable power conversion efficiencies, but the charge carrier dynamics in those materials have not been investigated yet. Here, we explore ultrafast dynamics of intramolecular charge transfer (ICT) occurring in the excited states of diphenylamine N-annulated naphthalene monoimide (DPA-NNMI) dye. By using steady-state absorption/emission spectroscopy, femtosecond broadband transient absorption (TA) spectroscopy, and DFT calculations, we found that the ICT dynamics of DPA-N-NMI vary sensitively depending on the solvent polarity. In particular, according to the TA spectra on ultrashort time scale, the Franck-Condon (S1/FC) state evolves into the S1/ICT state, which then undergoes non-radiative relaxation accompanying conformational rearrangement in polar solvents. The rapid transition from the S1/FC state to the S1/ICT state and the relaxation of the S1/ICT state are correlated with the solvation dynamics, which were identified by extracting the solvation time-correlation function. This correlation underlines that the ultrafast charge carrier dynamics are strongly correlated with the solvation dynamics, accounting for the dependence of the ICT dynamics on the solvent polarity. The understanding of the ICT dynamics unveiled in this study may render comprehensive insights to the carrier dynamics in DSSCs, contributing to the effective design of dyes based on N-annulated rylene complexes.