Luminescence of Biphenyl and Geometry of the Molecule in Excited Electronic States

Abstract

Fluorescence and phosphorescence spectra of biphenyl, biphenyl‐d₁₀, and related compounds were studied in n‐heptane at 77°K. The fluorescence spectrum of each of these compounds displays a long progression involving odd quanta of a low‐frequency vibration, while the phosphorescence spectrum exhibits a short progression involving even quanta of the same vibrational mode. The fundamental frequencies in these progressions are 635 cm⁻¹ for biphenyl, 595 cm⁻¹ for biphenyl‐d₁₀, and 626 cm⁻¹ for 4,4′‐difluorobiphenyl. From their appearance in Raman spectra (but not in infrared), the ratio of the frequencies in the isotopic species, their dependence on the position of substitutions, and their absence in the fluorescence spectrum of crystalline biphenyl, these frequencies are concluded to be the torsional frequencies in the ground state. The prominence of odd quanta of the torsional mode in fluorescence and even quanta in phosphorescence is shown to be a strong indication that the molecule of biphenyl is planar in the lowest excited states (singlet and triplet).