Excited-State Dynamics in Segregated Donor-Acceptor Stacks Versus a Peri-Bisdonor-Acceptor System.

The investigation of impact of through-space/through-bond electronic interaction among chromophores on photoexcited-state properties has immense potential owing to the distinct emergent photophysical pathways. Herein, the photoexcited-state dynamics of homo-sorted π-stacked aggregates of a naphthalenemonoimide and perylene-based acceptor-donor (NI-Pe) system and a fork-shaped acceptor-bisdonor (NI-Pe 2 ) system possessing integrally stacked peri-substituted donors was examined. Femtosecond transient absorption (fsTA) spectra of NI-Pe monomer recorded in chloroform displayed spectroscopic signatures of the singlet state of Pe; 1 Pe*, the charge-separated state; NI - ⋅-Pe + ⋅, and the triplet state of Pe; 3 Pe*. The examination of ultrafast excited-state processes of NI-Pe aggregate in chloroform revealed faster charge recombination ( τ C R a ${{\tau }_{CR}^{a}}$ =1.75 ns) than the corresponding monomer ( τ C R m ${{\tau }_{CR}^{m}}$ =2.46 ns) which was followed by observation of a broad structureless band attributed to an excimer-like state. The fork-shaped NI-Pe 2 displayed characteristic spectroscopic features of the NI radical anion (λ max ~450 nm) and perylene dimer radical cation (λ max ~520 nm) upon photoexcitation in non-polar toluene solvent in the nanosecond transient absorption (nsTA) spectroscopy. The investigation highlights the significance of intrinsic close-stacked arrangement of donors in ensuring a long-lived photoinduced charge-separated state ( τ C R ${{\tau }_{CR}}$ =1.35 μs) in non-polar solvents via delocalization of radical cation between the donors.