Electronic spectrum of 1,5-naphthyridine: Crystal spectra

Polarized spectra (4 K) of the following systems are recorded in the region 26 000 – 29 000 cm^?1 ($\text{π}{}^1\text{← n}$ transition): 1,5-naphthyridine-d₀ in durene, p-xylene, neat crystal and naphthalene; 1,5-naphthyridine-d₆ in durene and in naphthalene. The spectra in naphthalene differ radically from the others, which resemble the vapor spectrum in being built principally upon two main origins interpreted as the true origin 0 and a vibronic origin (6a_u)₀¹ near 0 + 183 cm^?1 (0 + 163 cm^?1, -d₆). The favored interpretation of the naphthalene spectrum, which is polarized in the molecular plane, invokes an origin (L-polarized), but (6a_u)₀¹ is missing, and there are then three further vibronic origins, each with distinctive polarization at 0 + 309, 0 + 362, 0 + 516 cm^?1 (-d₀), 0 + 312, 0 + 384, 0 + 492 cm^?1 (-d₆). The changed appearance of the spectrum in naphthalene is attributed to a diminution, because of an increased spacing between the interacting states, of the strong vibronic coupling which is considered to exist between the n, $\text{π}{}^1$ state and two higher π, $\text{π}{}^1$ states.