Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
Phys Chem Chem Phys. 2009 Oct 21;11(39):8706-13. doi: 10.1039/b909113k. Epub 2009 Jul 29.
The photophysics and spectroscopic properties of six naphthalene-oligothiophene derivatives, three oligothiophenes (n = 2, 3 and 4) end-capped with naphthalene chromophores and three additional oligomers derived from the former with additional thiophene and bithiophene units in their terminal positions, were investigated in solution at 293 and 77 K. Detailed results are presented on absorption, emission and triplet-triplet absorption spectra together with all relevant quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation), excited state lifetimes, singlet energies and the overall set of deactivation rate constants: k(F), k(IC) and k(ISC). From these, three important conclusions could be drawn: (1) the main deactivation channel for the molecules in solution is the radiationless S(1) approximately approximately --> T(1) intersystem crossing; (2) the incremental addition of thiophene units leads to an increment of the fluorescence at the expense of a decrease of the S(1) approximately approximately --> S(0) internal conversion yield and (3) from time-resolved fluorescence experiments, in the picosecond time domain, the decays were found to be monoexponential, excluding decay processes involving energy transfer or conformational relaxation between the chromophoric units. This is explored, by comparison with recent studies on conjugated thiophene and para-phenylene-vinylene (PPV) copolymers and oligomers, to establish that with polythiophenes the energy transfer process is responsible for the fast (few ps) component(s) whereas with PPV polymers a relaxation of the first singlet excited state ought to be considered. DFT theoretical calculations (B3LYP/3-21G* level) support the S(1) state of the compounds with a quinoidal-like structure (within the thiophene central moiety) with a degree of coupling with the terminal naphthalene rings. Singlet oxygen yields were determined and the triplet energy transfer to (3)O(2) to produce (1)O(2) was found to be highly efficient with values of S(Delta) (= phi(Delta)/phi(T)) varying from 0.61 to 0.75, thus revealing to be an important deactivation path for the triplet state of these compounds.
六种萘并噻吩寡聚物衍生物的光物理和光谱性质，其中三种寡聚物（n = 2、3 和 4）在末端带有萘发色团，另外三种寡聚物则是在其末端位置上具有额外的噻吩和并噻吩单元，在 293 和 77 K 下在溶液中进行了研究。详细结果包括吸收、发射和三重态-三重态吸收光谱，以及所有相关的量子产率（荧光、系间窜越、内转换和单线态氧形成）、激发态寿命、单线态能量和整体失活速率常数集：k(F)、k(IC) 和 k(ISC)。从中可以得出三个重要结论：（1）分子在溶液中的主要失活途径是无辐射的 S(1) 约 --> T(1) 系间窜越；（2）噻吩单元的递增添加会导致荧光的增加，而 S(1) 约 --> S(0) 内转换产率的降低；（3）从皮秒时间域的时间分辨荧光实验中，发现衰减是单指数的，排除了涉及发色团之间能量转移或构象松弛的衰减过程。通过与最近关于共轭噻吩和对苯撑乙烯基（PPV）共聚物和寡聚物的研究进行比较，探索了这一点，以确定对于聚噻吩，能量转移过程是快（几个皮秒）组分的原因，而对于 PPV 聚合物，应该考虑第一单线态激发态的弛豫。DFT 理论计算（B3LYP/3-21G* 水平）支持化合物的 S(1) 态具有醌式结构（在噻吩中心部分），与末端萘环有一定的耦合。测定了单线态氧产率，并发现三重态能量转移到（3）O(2) 产生（1）O(2) 非常有效，S(Delta)（= phi(Delta)/phi(T)）的值从 0.61 到 0.75 不等，这表明对于这些化合物的三重态，这是一种重要的失活途径。
