| Photochemical reaction and properties of the excited state of aryl azides(AR-N=N=N) have been studied extensively using ultrafast time-resolved spectroscopy and with the tools of computational chemistry. The decomposition of the aryl azides upon photoexcitation leads to formation of aryl nitrenes which have been widely applied in different fields such as physics, chemistry, materials, and biological. Some reports mainly focused on the quantum yields of light-induced decomposition of the aryl azides, lifetimes of the corresponding singlet aryl nitrenes and the formation of the corresponding further reaction products in different solvents. But little have been known about the relationship between the quantum yields of the photodecomposition of the aryl azides and the decay channels of the formation of the corresponding niterenes. In this paper, the excited state and nonadiabatic decay dynamics in different solvents of phenyl azide(PA), 2-naphthyl azide(2NA) and 1-naphthyl azide(1NA) have been studied by resonance Raman spectra and complete active space self-consistent field calculation(CASSCF). The relationship between the quantum yields of the photodecomposition of the aryl azides and the decay channels of the formation of the corresponding niterenes was studied, the results as follow:(1) The vibrational spectra, electron spectra and resonance Raman spectra of PA, 2NA and 1NA azide were assigned. Their UV(Ultraviolet) absorption spectra, Fourier transform FT-Raman, FT-infrared(IR) and resonance Raman spectra were obtained. And they were also studied by density functional theory(DFT) and CASSCF calculation. We obtained the vibrational spectra of ground states, geometric structures and electronic absorption bands characteristic transitions of the excited states and the curve-crossing points.The intensity pattern of the resonance Raman spectra of the same material at a certain excitation wavelength in three different solvents was very similar to one another. Most of the C-band and B-band resonance Raman spectra of PA could be assigned as strong fundamental modes n7 and n11. C-band resonance Raman spectra of PA was assigned as weak peaks ν9, ν10 and ν19. B-band resonance Raman spectra of PA could be assigned as weak peaks ν9, ν14, ν19, ν20, ν21, ν22, ν23 and ν24. Owing to the very intense fluorescence interference, A-band resonance Raman spectra were not obtained. C-band resonance Raman spectra of 2NA was assigned as strong peaks ν9, ν11, ν15 and ν18, weak peaks ν12, ν13, ν20, ν21, ν23 and ν30. B-band resonance Raman spectra of 2NA could be assigned as strong peaks ν9, ν11, ν13, ν15 and ν18, weak peaks ν10, ν20, ν21, ν23, ν25, ν29 and ν30. A-band resonance Raman spectra of 2NA could be assigned as strong peaks ν8, ν9, ν11, ν15, ν18 and ν29, weak peaks ν12, ν13, ν20, ν21, ν23, ν25, ν31 and ν33.(2) We solved some problems in the vibrational spectra assignment. It had a single-peak ν6(N=N=N stretch) at ~2100 cm-1 in the computed Raman spectrum of PA, but had a dual-peak in the corresponding FT-IR and FT-Raman spectra. However, 2NA and 1NA had no this phenomenon. The dual-peak could not be assigned to dimer and monomer of PA by DFT calculation of many dimers, but according to previous reports, it was assigned to Fermi resonance.(3) The characteristic transitions of electronic absorption bands of three molecules were studied.The excited states S2(A’) of 2NA and 1NA were caused by πH→πL*(0.61) and πH→πL*(0.69) electronic transitions. But the excited states S2(A’) of PA was caused by πH→πL+2*(0.52) + πH→πL+1*(0.34) + πH-1→πL+1*(0.31) electronic transitions. Three absorption bands of PA and 2NA in the region of 200-400 nm were assigned as S2(A’), S3(A’), S6(A’) and S2(A’), S5(A’), S7(A’) respectively. Two absorption bands of 1NA in the region of 200-400 nm were assigned as S2(A’) and S7(A’).(4) The relationship between the quantum yields of the photodecomposition of the aryl azides and the decay channels of the formation of the corresponding niterenes were researched by discussing concomitant fluorescent in resonance Raman experiment. The 294 nm concomitant fluorescence spectrum of PA was assigned as the S2,min→S0 transition, which served as the efficient radiative decay channel of PA in the populated S2 state. The 385 nm concomitant fluorescence spectrum of 1NA was assigned as singlet 1-naphthyl nitrene, which was an effective decay channel. The concomitant fluorescence spectrum of 2NA were very different in different solvents. This indicates that two or more transient species had been formed.(5) The excited state decay mechanisms of moleculars mentioned above were proposed by discussing the relationship between the intensity mode of resonance Raman spectra and CASSCF calculations in detail. The major S2(A’) excited state decay channel of PA and 2NA were S2,FC→S2,min→S0( radiative decay channel) and S2,FC→S2S1→S1( nonradiative decay channel), and 1NA also had similar radiation decay channel. And then the relationship betweenthe quantum yields of the photodecomposition of the aryl azides and the decay channels of the formation of the corresponding niterenes were studied. The energy barrier(4 kcal?mol-1) of S2S1 conical intersection of PA was higher than 2NA(0.7 kcal?mol-1). This suggested that the formation of singlet phenyl nitrene was more difficult than singlet 2-naphthyl nitrene. |
