D-π-A type conjugated pyrene derivatives: Third-order nonlinear absorption and excited-state dynamics

Two novel conjugated pyrene derivatives (C1 and C2) with D-pi-A structures were synthesized to realize broadband reverse saturable absorption (RSA) under femtosecond laser excitation (ranging from 515 to 700 nm) resulting from two-photon absorption (WA) and WA-induced excited-state absorption. Furthermore, these two derivatives showed RSA on the picosecond and nanosecond time regions (532 nm). Quantum chemical calculations indicated that two mechanisms for the HOMO-LUMO excitation of the two molecules exist simultaneously: intramolecular charge transfer (which is considered to trigger the nonlinear optical response) and pi-pi* transition. It was found that the pi-pi* transition localized in the pyrene group can contribute to the increase in the nonlinear absorption of pyrene derivatives. Femtosecond time-resolved transient absorption spectrum provided information on the dynamics of the locally excited singlet state ((LE)-L-1) and the charge-transfer state ((CT)-C-1) for both compounds. The results show that these pyrene derivatives are promising candidates for future optoelectronic and RSA applications.