Spectroscopy and Photophysics of Monoazaphenanthrenes III. Luminescence of Phenanthridine and 7,8-benzoquinoline in Crystalline State

Abstract

Fluorescence and phosphorescence spectra and the decay profiles of both these emissions have been investigated for the polycrystals of phenanthridine and 7,8-benzoquinoline, in the liquid helium (5 K) -room temperature range. These two monoazaderivatives of phenanthrene, which differ only by the position of N-heteroatom in the aromatic ring skeleton of phenanthrene, were found to exhibit very different fluorescence spectra, which also differ greatly in their temperature behavior. Supplementary investigations of the fluorescence of single crystals of 7,8-benzoquinoline have supported classification of observed fluorescence as an excimer fluorescence (caused by the specific arrangement of molecules of 7,8-benzoquinoline in the crystal). In contrary fluorescence of phenanthridine crystals is of the monomeric type. Phosphorescence spectra observed for the crystals of both molecules are very similar, but their temperature dependence is also different. This may be considered as an indication of a different physical mechanism of nonradiative intersystem crossing processes, which are operating between the lowest excited singlet state and the lowest excited triplet state in the crystals of both molecules.

Donor–Acceptor Boron-Ketoiminate Complexes with Pendent N-Heterocyclic Arms: Switched-on Luminescence through N-Heterocycle Methylation

“…At 77 K, all five compounds display two sets of vibrational bands, one in the region 360–420 nm and the other roughly between 460–600 nm. These are consistent with the fluorescence and phosphorescence bands, respectively, displayed by phenanthridine at low temperature . This assignment is supported by very different emission decay times (τ) of the two sets of bands (9.6 ns and 750 ms, in the case of [ 3 Me]­OTf) corresponding to spin-allowed and formally spin-forbidden transitions respectively.…”

“…These are consistent with the fluorescence and phosphorescence bands, respectively, displayed by phenanthridine at low temperature. 42 This assignment is supported by very different emission decay times (τ) of the two sets of bands (9.6 ns and 750 ms, in the case of [3Me]OTf) corresponding to spin-allowed and formally spinforbidden transitions respectively. The lifetimes of the phosphorescence bands of 1, 3, and 4 were similar (τ = 870, 880, 820 ms; the estimated uncertainty is ±10%), while that of 2 was a little longer at 1300 ms. Interestingly, the ratio of the integrated intensities of the fluorescence to phosphorescence bands is much higher for the methylated compound (ratios are approximately 3.5:1 for [3Me]OTf and 1:4 for 3), which might indicate that the rate of intersystem crossing (ISC) has been retarded upon methylation.…”

Zero Kinetic Energy Photoelectron Spectroscopy of Benzo[h]quinoline

“…In this work, we report resonantly enhanced multiphoton ionization (REMPI) spectroscopy of the first electronically excited state S 1 as well as ZEKE spectroscopy of the cationic state D 0 of benzo­[ h ]­quinoline (B h Q). We compare the spectroscopic features of B h Q with those of two isoelectronic systems phenanthrene and anthracene and comment on the unique contribution of the nitrogen atom. ,,− We observe that in terms of molecular orbitals, the first two electronic states of B h Q are in reverse order relative to those of phenanthrene, a not-so-subtle effect of the heteroatom in the molecular frame. − The low molecular symmetry has relaxed the selection rule in vibronic transitions, allowing all in-plane modes to be observable. Nevertheless, the ZEKE spectra are sparse, implying structural similarities between the cation and the first excited electronic state.…”

Fluorescence enhancement of quinolines by protonation

“…15 The excited-state proton transfer in N-heterocycles, including isoquinoline and benzo[h]quinoline, has been previously studied in acidic media. 9,[16][17][18] Benzo[h]quinoline and acridine (benzo[b] quinoline), belonging to a group of polycyclic aromatic Nheterocycles, have been discovered to be particularly interesting since they possess different pK a values in the ground and excited state. [19][20][21][22] However, there has been no studies that are based on comparing different acids and the effects of the resulting counter-anion.…”