An experimental study is presented that leads to the elucidation of the whole band structure for the triplet exciton state of 1,4-dibromonaphthalene. We show that the band states are essentially those of a linear chain characterized by nearest neighbor interactions along the c crystallographic axis. Although the crystal is topologically a three-dimensional network, for the practical purpose of energy transfer and trapping the crystal behaves as a set of linear chains. Heavy doping of DBN-h6 (host) with up to 18% DBN-d6 (guest) yielded the expected discrete spectra of a random linear array corresponding to a nearest neighbor interaction of 7.4± 0.1 cm−1 and total bandwidth of 29.6± 0.4  cm−1 for the zero-zero transition. A symmetric mode at 0+520 cm−1 was shown to have an exciton bandwidth of 15 cm−1, and an unsymmetric vibrational level at 0+250  cm−1 was shown to have a vanishing bandwidth in accordance with expectations from weak coupling theory. The quasiresonance interactions of host and guest were significant and were included in establishing the agreement between experiment and calculation. The total bandwidth is estimated to be 90± 20  cm−1. This type of doping has an effect on line shape even at very low concentration and even in undoped crystals the line shape is asymmetrical, perhaps due to impurities and imperfections that have small zero-order shifts from the band center. The intensity of transitions to the carbon-13 isotopic traps is shown to be distributed over the band states in the origin and is clearly visible in the narrow band nontotally symmetric vibrational state. The vibrational analysis of the crystal spectrum (electronic or vibrational states) is shown to be dependent on exciton effects even in the case where no Davydov splitting is observed. Spectra of DBN-h6 (guest, H) in DBN-d6 (host, D) were also studied and resonance multiplets were seen and identified in absorption and emission. We identified ··· DHD···,  ··· DHHD···,  ··· DHHHD···, and probably ··· DHDHD··· in the phosphorescence spectra at 2°K. We present a brief discussion of the effect of dimensionality on the dynamical features of energy transfer and trapping, and we show experimentally, that heavy doping (d6 in h6) enhances the phosphorescence of the crystal about 60-fold. In addition the existence at 2°K of a Boltzmann distribution over resonance multiplets leads us to propose a long-range trap-to-trap migration as part of the thermalizing process.

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.

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