Low energy magnetic and electric dipole transitions of the biphenyl crystal

The absorption and fluorescence spectra of biphenyl-h10 and biphenyl-d10 neat crystals, and of lightly and heavily doped isotopically mixed crystals have been studied at 4.2 °K. The first singlet excited state of biphenyl is assigned as 1B3g (33128.2 cm−1 in biphenyl-h10, 33270.4 cm−1 in biphenyl-d10) and transitions to it are found to be consistent with a magnetic dipole mechanism by polarization measurements and calculations. The bulk of the intensity in the B3g1⇄Ag(X) transitions is electric dipole arising from Herzberg-Teller coupling through a mode b2u (9) at 626 cm−1 (h10) and ≈ 593 cm−1 (d10). The next excited state is assigned as 1B2u (M-polarized) and the splitting of the 1B3g and 1B2u states of 746.8 ± 3 cm−1 provides the inter-ring resonance interaction corresponding to the splitting of the 1B2u (D6h) state of benzene through excitation exchange interactions and second-order shifts. The isotopic mixed crystal spectra provide evidence that the width of the biphenyl crystal exciton band (from 1B3g) is less than 4 cm−1. The observation of doublets in the 1B2u excited states of biphenyl-d10 and doublets in the absorption and emission of biphenyl-h10 in biphenyl-d10 suggest that the crystal structure of d10 is changed, at 4.2°K, from the 300°K structure having two centrosymmetric molecules per unit cell. For the first time for organic crystals we report the phonon-induced spectrum: These spectra are electric dipole dipole sidebands of the magnetic zero-phonon transitions.