Benniston Andrew C, Harriman Anthony, Rostron James P
Molecular Photonics Laboratory, School of Natural Sciences, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK.
Phys Chem Chem Phys. 2005 Aug 21;7(16):3041-7. doi: 10.1039/b506776f. Epub 2005 Jul 13.
The photophysical properties of the target compound are extremely sensitive to changes in solvent polarity since the lowest-energy excited states possess considerable charge-transfer character. Excitation results in a greatly increased dipole moment, with the resultant excited singlet state retaining a lifetime of ca. 1 ns in all solvents. Radiative decay involves coupling between the lowest-energy excited singlet state and both the ground state and an upper excited singlet state. The level of coupling to the upper singlet decreases in non-polar solvents, presumably due to symmetry factors. The radiative rate constant decreases smoothly with increasing solvent polarity function as the molecule acquires an ever increasing dipolar character. Non-radiative decay includes both intersystem crossing and internal conversion, but the former process dominates in polar solvents. The excited singlet state lifetime is very weakly dependent upon temperature in the solid state. However, in polar solutions where the Stokes' shift decreases with decreasing temperature, there is clear evidence for an activated process. This is believed to involve coupling to the upper-lying singlet excited state.
目标化合物的光物理性质对溶剂极性的变化极为敏感，因为最低能量激发态具有相当大的电荷转移特性。激发会导致偶极矩大幅增加，由此产生的激发单重态在所有溶剂中的寿命约为1 ns。辐射衰变涉及最低能量激发单重态与基态以及较高激发单重态之间的耦合。在非极性溶剂中，与较高单重态的耦合程度降低，这可能是由于对称因素所致。随着分子获得越来越大的偶极特性，辐射速率常数随溶剂极性函数的增加而平稳下降。非辐射衰变包括系间窜越和内转换，但在前一种过程在极性溶剂中占主导地位。激发单重态寿命在固态下对温度的依赖性非常弱。然而，在极性溶液中，斯托克斯位移随温度降低而减小，有明确证据表明存在一个活化过程。据信这涉及与较高的单重激发态的耦合。
