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 pi-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-& sdot;-Pe(+)& sdot;, 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 (tau(CR)(a) =1.75 ns) than the corresponding monomer (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 (lambda(max)similar to 450 nm) and perylene dimer radical cation (lambda(max)similar to 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 ( tau(CR) =1.35 mu s) in non-polar solvents via delocalization of radical cation between the donors.