Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, Perugia, Italy.
Chemphyschem. 2012 Feb;13(3):724-35. doi: 10.1002/cphc.201100674. Epub 2012 Jan 27.
Detailed investigations by time-resolved transient absorption and fluorescence spectroscopies with nano- and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes (1-3) and three ethynyl anthracenes (4-6) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1-4, thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge-transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO(2) derivative 1. No push-pull character is found for 5 and 6, which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.
通过时间分辨瞬态吸收和荧光光谱学的详细研究，具有纳秒和飞秒时间分辨率，旨在描述三种乙炔芴（1-3）和三种乙炔蒽（4-6）在不同极性溶剂中的最低激发单线态和三线态的特性。发现溶剂会改变化合物 1-4 的最低激发单线态的失活途径，从而改变它们的荧光、系间窜越和内转换效率。随着溶剂介电常数的增加，荧光和三重态产率逐渐降低，而内转换量子产率增加。这些实验结果，加上明显的氟溶剂化效应，表明涉及到一个具有电荷转移（CT）特征的发射态，该态被极性溶剂强烈稳定。这一点通过超快光谱研究得到了证明，在超快光谱研究中，检测到两个瞬态，它们的特征光谱形状分别分配给局域激发（LE）和 CT 态，CT 态是高度极性溶剂中寿命更长且荧光更强的态。在高度极性介质中起作用的分子内 LE→CT 过程在 NO 2 衍生物 1 中变得特别快（高达≈300 fs）。5 和 6 没有推拉性质，它们表现出不同的光物理行为；事实上，溶剂极性不会显著改变最低激发单线态的动力学。在 TDDFT 水平上进行的量子力学计算也用于确定最低激发单线态和三线态的态序和性质，并解释氟和蒽衍生物的不同光物理行为，特别是关于系间窜越过程。
