Search for vibronic spin–orbit interactions in the triplet-manifold of phenanthrene N-heterocyclic analogues

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After detecting vibronic spin–orbit interactions in the triplet-manifold of some anthracene heterocyclics, we were interested in detecting the presence of these interactions in phenanthrene N-heterocyclics. These interactions appear to reduce triplet–triplet (T–T) intensities (oscillator strength f T values) in aromatic heterocyclics. This effect is important to laser dye technology. Reducing T–T absorption intensities in laser dyes results in reduced triplet-state losses and therefore, in higher laser-action efficiencies. To obtain more experimental evidence on this effect, the T–T absorption spectra of phenanthrene, 5,6- and 7,8-benzoquinoline, phenanthridine, and 1,7- and 4,7-phenanthroline over the violet/green spectral region were measured. This is the spectral region where the 3 A 1 −← 3 B 2 + transition of phenanthrene is located. To obtain these T–T absorption spectra, the steady-state method was employed, using the 337.4 nm line from a cw ion krypton laser for excitation. From the recorded triplet optical densities OD T( λ), the triplet extinction coefficients ε T( λ) were obtained by McClure’s intensity variation method. The triplet oscillator strengths f T of the 3 A 1 −← 3 B 2 + transition of phenanthrene and the corresponding transitions in the N-heterocyclics were also obtained by integration. Due to measurement errors, the uncertainty of integration limits needed to obtain the f T values, and overlap with neighboring transitions, inconclusive results were obtained for 5,6- and 7,8-benzoquinolines and phenanthridine. However, the f T values for 1,7- and 4,7-phenanthroline were considerably smaller than the f T value obtained for phenanthrene. This suggests the presence of vibronic spin–orbit interactions in these two compounds.