Dynamics in the excited electronic state of periodic mesoporous biphenylylene-silica studied by time-resolved diffuse reflectance and fluorescence spectroscopy

Abstract

The excited state dynamics of periodic mesoporous organosilica (powder) bearing biphenylylene moieties densely in the silica framework (Bp-PMO) is investigated for the first time using femtosecond time-resolved diffuse reflectance (TDR) and picosecond time-resolved fluorescence spectroscopies. The TDR spectra revealed the excitation-relaxation process of the biphenylylene moieties, including the relaxation of the twisted Frank-Condon (FC) state to the lowest singlet excited state (S₁) with a time constant of 730 ± 95 fs, and efficient quenching of the S₁ state by excimer (E) formations with two time constants of 7.0 ± 0.2 ps (E₁: ca. 64%) and 170 ± 47 ps (E₂ and E₃: ca. 36%). The individual absorption spectra of the FC, S₁, and E states were reconstructed by the TDR spectral analysis. The time-resolved fluorescence spectra showed that the excimers decayed with three time constants of 1.3 ± 0.2 ns (E₁), 8.2 ± 0.7 ns (E₂) and 27 ± 2 ns (E₃). The fluorescence quantum yields of the excimers are suggested to be almost zero for E₁, and unity for E₂ and E₃, which implies that the fluorescence quantum yield of Bp-PMO (Φ_F = 0.38) can be explained by the fraction of the highly emissive excimers (E₂ and E₃). The excited state dynamics of Bp-PMO is quite different from those of a solution of 4,4′-bis-(triethoxysilyl)biphenyl precursor and a biphenyl molecular crystal (powder).