Ramos Ricardo Campos, Fujiwara Takashige, Zgierski Marek Z, Lim Edward C
Department of Chemistry and The Center for Laser and Optical Spectroscopy, The University of Akron, Akron, Ohio 44325-3601, USA.
J Phys Chem A. 2005 Aug 18;109(32):7121-6. doi: 10.1021/jp050889b.
Excitation-energy dependence of fluorescence intensity and fluorescence lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN), and 1-naphthonitrile (NN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for S1 (pi pi*, L(b)) excess vibrational energy exceeding about 1000 cm(-1). This is confirmed by comparison of the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the TDDFT potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the aminobenzonitriles at higher energies is due to the crossing of the pi pi* singlet state by the lower-lying pi sigma*(C[triple bond]N) singlet state of very small radiative decay rate. The threshold energy for the fluorescence "break-off" is in good agreement with the computed energy barrier for the pi pi*/pi sigma* crossing. For NN, on the other hand, the observed decrease is in fluorescence yield at higher excitation energies can best be attributed to the crossing of the pi pi* singlet state by the pi sigma* triplet state.
在超声速自由射流中，已测量了4-二甲基氨基苯甲腈（DMABN）、4-氨基苯甲腈（ABN）、4-二异丙基氨基苯甲腈（DIABN）和1-萘甲腈（NN）的荧光强度和荧光寿命对激发能量的依赖性。在所有情况下，当S1（ππ*，L(b)）的过剩振动能量超过约1000 cm⁻¹时，荧光产率显著下降，而荧光寿命仅适度下降。通过将归一化荧光激发光谱与该化合物在气相中的吸收光谱进行比较，证实了这一点。结果表明，在较高能量下相对荧光产率的强烈下降主要是由于发射态辐射衰减率的降低。将实验结果与激发态的TDDFT势能曲线进行比较，强烈表明在较高能量下氨基苯甲腈辐射衰减率的降低是由于ππ单重态与辐射衰减率非常小的较低的πσ（C≡N）单重态交叉所致。荧光“截止”的阈值能量与计算得到的ππ*/πσ交叉的能垒吻合良好。另一方面，对于NN，在较高激发能量下观察到的荧光产率下降最好归因于ππ单重态与πσ*三重态的交叉。
