Photophysical properties of oligophenylenes end-capped with naphthyls in solution and solid state

Abstract

Naphthyl-terminated oligophenylenes (DNpOPhs) were investigated to clarify how substitution topology and π-conjugation length regulate excited-state properties in linear aromatic systems. A comprehensive photophysical analysis by combining steady-state and time-resolved spectroscopy in solution and the solid state reveals pronounced para–meta contrasts in absorption spectra, fluorescence efficiencies, and radiative rates. para-DNpOPhs exhibit high fluorescence quantum yields and enhanced radiative rates, kf whereas meta-analogues show blue-shifted spectra and reduced oscillator strengths due to localized frontier orbitals. All compounds display deep-blue emission, with solid-state spectra red-shifted relative to solution, and phosphorescence enabling determination of triplet energies localized on the terminal naphthyl units. TD-DFT calculations reproduce the experimental trends, demonstrating a linear correlation between computed oscillator strengths, f and measured kf in solution, consistent with the Strickler–Berg relationship. In the solid state, however, kf saturates at ~5 × 108 s-1 for molecules with large f, indicating that intermolecular interactions impose an upper limit on the effective radiative rate in the condensed phase. These results provide mechanistic insight into how intrinsic electronic structure and condensed-phase effects jointly define emission behavior, offering design principles for deep-blue emitters and related aromatic chromophores.