Charge separation, charge recombination and intersystem crossing in orthogonal naphthalimide-perylene electron donor/acceptor dyad.

Abstract

We prepared an orthogonal electron donor/acceptor dyad (NI-Py) with perylene (Py) as electron donor and 4-aminonaphthalimide (NI) as an electron acceptor. The molecule adopts orthogonal geometry due to the steric hindrance exerted by the 4-amino substituents on the NI moiety. The photophysical properties of dyad were studied by steady-state UV-Vis absorption and fluorescence spectroscopies, femtosecond/nanosecond transient absorption spectroscopies and DFT computations. Ground state interaction between the NI and Py units is negligible; however, charge separation occurs upon photoexcitation, indicated by the quenching of the fluorescence of the dyad in polar solvents, i.e. fluorescence quantum yield (ΦF) is 61.9% in toluene and ΦF = 0.2% in methanol. Spin-orbit-coupled charge transfer-induced intersystem crossing (SOCT-ISC) was confirmed by femtosecond transient absorption spectroscopy (charge separation takes 1.7ps and charge recombination takes 6.9ns, in CH2Cl2). Nanosecond transient absorption spectra indicated the formation of perylene-localized triplet state, and the triplet state lifetime (175μs) is much longer than that accessed with the heavy atom effect (3-bromoperylene; 16μs). The singlet oxygen quantum (ΦΔ) yield of the dyad is 2.2% in hexane and 9.5% in dichloromethane. The low SOCT-ISC efficiency as compared to the previously reported analogue (ΦΔ = 80%) is attributed to the mismatch of the 1CT/Tn state energies, and/or the orientation of the NI and Py units, i.e. orthogonal geometry is not sufficient for achieving efficient SOCT-ISC in compact electron donor/acceptor dyads.