Phosphorescence azanaphthalenes

The benzene and azabenzenes form iso-electronic series of molecules, as naphthalene and the azanaphthalenes also do. The ground state electronic and geometric structures are therefore quite similar within one series. The substitution of CH groups with nitrogens introduces lone-pair to 7r transitions, and lowers the benzene and naphthalene symmetries. Small and systematic trends are found for linear response properties of the azabenzenes [189]. Each molecule is, however, very specific with respect to phosphorescence due to the delicate nature of the SOC and electric dipole activity interactions. 

In the following we discuss the phosphorescence lifetimes of 3(n, x ) and 3(x,x ) states of the azabenzenes and the azanaphthalenes, which can differ by several orders of magnitude. The symmetry axes and geometries of the discussed compounds are reproduced in Fig. 14. Since benzene and naphthalene lack (n,x ) transitions their triplet state radiative lifetimes are, as for other hydrocarbons, considerably longer. 

Two of the four investigated naphthyridines (1,8- and 2,7-naphthyridine with C%v symmetry) have one spin sublevel that is radiatively forbidden, the other two have all spin sub-levels active. Phosphorescent emission have been observed for all naphthyridines, but the radiative lifetimes are not available. The measured lifetime of 1,5-naphthyridine (0.02 s) is much shorter, than our radiative value. This again indicates that vibronic coupling (leading to non-radiative or radiative decay) is a strong contributor to the lifetimes of the triplet states of azanaphthalenes, in a perturbative sense thus stronger than the action of dipole and spin-orbit coupling. 

Table 20 Triplet excitation energies (eV) and phosphorescence lifetimes r (s) of azaben-zenes and azanaphthalenes calculated for different state symmetries and spin sublevel components by random phase approximation (HF) and MCQR with double zeta (DZ) basis set.

The feasibility of ENDOR-like experiments in zf using optieal detection was suggested by Schmidt and van der Waals (1969). The observation of spectra of this type was reported nearly simultaneously by Harris et al (1969) and Chan et al (1969). The focus in these studies was on the nuclear spin sublevels of phosphorescent quinoxalines. The mechanisms responsible for the intensity of the ENDOR transitions in zf have been discussed (Harris and Buckley, 1976). The technique was extended to both chlorine isotopes by Buckley and Harris (1970). Chan and van der Waals (1973) used the ENDOR technique to determine the N quadrupole frequencies in perdeuteroquinoline. The analysis given rests on the assumption of the coincidence of principal axes, information not normally available from the zf experiment. C and H ENDOR transitions were observed in the low-field experiments of Hochstrasser et al (1973,1978) on (nn ) benzophenone. It has also been shown that certain components of the proton hyperfine tensors can be obtained from a careful analysis of the zf ENDOR spectra of azanaphthalenes (Dennis and Tinti, 1975). Chan and Walton (1977) have observed proton ENDOR signals in zf. More recently, Dinse and Winscom... 

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