Ghosh Rajib, Palit Dipak K
Radiation & Photochemistry Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.
J Phys Chem A. 2015 Nov 12;119(45):11128-37. doi: 10.1021/acs.jpca.5b08203. Epub 2015 Oct 29.
Significant effect of coupling between the electron donor and acceptor groups in intramolecular charge transfer (ICT) dynamics has been demonstrated by comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone derivatives (namely, DMAC-A and DMAC-B). In the case of the DMAC-B molecule, the distance between the donor (N,N-dimethylaniline or DMA) and the acceptor (carbonyl) groups is larger by one ethylene unit as compared to that in the case of DMAC-A. The excited singlet (S1) states of both the isomers have strong ICT character but their photophysical properties are remarkably different. In polar solvents, fluorescence quantum yields (and the lifetimes of the S1 state) of DMAC-A are more than 2 orders of magnitude lower (and shorter) than those of DMAC-B. Remarkable differences in the photophysical properties of these two isomers arise due to occurrence of the ultrafast twisting of the DMA group (or the TICT process) during the course of deactivation of the S1 state of the DMAC-A molecule, but not in the case of DMAC-B. In the later case, because of the presence of a large energy barrier along the twisting coordinate(s), TICT is not a feasible process, and hence, the S1 state of DMAC-B has the planar ICT structure. In the DMAC-A molecule, the strength of coupling between the donor and acceptor groups is relatively stronger because of a shorter distance between these groups. Femtosecond transient absorption spectroscopic measurements and DFT/TDDFT calculations have been adopted to establish the above aspects of the relaxation dynamics of the S1 states of these two isomeric chalcones.
通过比较两种同分异构的N，N-二甲基氨基查尔酮衍生物（即DMAC-A和DMAC-B）的光物理性质，已证明分子内电荷转移（ICT）动力学中电子供体与受体基团之间耦合的显著影响。在DMAC-B分子的情况下，与DMAC-A相比，供体（N，N-二甲基苯胺或DMA）和受体（羰基）基团之间的距离大一个乙烯单元。两种异构体的激发单重态（S1）都具有很强的ICT特性，但它们的光物理性质却有显著差异。在极性溶剂中，DMAC-A的荧光量子产率（以及S1态的寿命）比DMAC-B低2个数量级以上（且短）。这两种异构体光物理性质的显著差异是由于DMAC-A分子S1态失活过程中DMA基团发生超快扭转（或TICT过程），而DMAC-B则不然。在后一种情况下，由于沿扭转坐标存在较大的能垒，TICT不是一个可行的过程，因此，DMAC-B的S1态具有平面ICT结构。在DMAC-A分子中，由于供体和受体基团之间的距离较短，它们之间的耦合强度相对较强。已采用飞秒瞬态吸收光谱测量和DFT/TDDFT计算来确定这两种同分异构查尔酮S1态弛豫动力学的上述方面。
