Department of Chemistry, University of Rochester, Rochester, New York 14627, USA.
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, USA.
Photochem Photobiol Sci. 2016 Nov 2;15(11):1417-1432. doi: 10.1039/c6pp00233a.
Although rhodamine dyes have been extensively studied for a variety of applications, many details of their photophysics are not yet fully understood, including the possible presence of a charge separated electronic state lying near the optically active excited singlet state and the role of twisting substituent groups in excited-state quenching. To address this, a large library of rhodamine dyes was studied in which the chalcogen is varied from O, to S and Se and the aryl group is either absent (in the pyronin series) or is a phenyl or thienyl substituent. Through an analysis of steady-state absorption spectroscopy, electrochemistry, X-ray crystallography, and quantum mechanical calculations, we show that the lowest unoccupied molecular orbital (LUMO) energy decreases in the O → S → Se series and when a phenyl or thienyl substituent is added. The reduction of the LUMO energy is larger for thienyl species in which the aromatic group has increased torsional flexibility. Excited state lifetimes and fluorescence quantum yields of these dyes in a high and low polarity solvent reveal dramatically different photophysics between chromophores with phenyl and thienyl substituents, due to a combination of torsional and inductive effects. In the pyronin and phenyl-substituted species, non-radiative decay can occur through an amine-to-xanthylium core charge separated state that is stabilized in a highly polar environment. In the thienyl derivatives, a lower energy excited state, which we term S', is accessed from Svia rotation of the aryl group and the excited state population rapidly equilibrates between S and S' in 6-30 ps. Preliminary photochemical hydrogen production data display the potential application of the thienyl derivatives for conversion of solar energy.
尽管罗丹明染料已经被广泛研究用于各种应用，但其光物理性质的许多细节仍未完全理解，包括可能存在位于光活性激发单线态附近的电荷分离电子态，以及扭转取代基在激发态猝灭中的作用。为了解决这个问题，我们研究了一个大型的罗丹明染料库，其中的杂原子从 O 变为 S 和 Se，芳基基团要么不存在（在吡咯啉系列中），要么是苯基或噻吩基取代基。通过稳态吸收光谱、电化学、X 射线晶体学和量子力学计算的分析，我们表明最低未占据分子轨道（LUMO）能量在 O → S → Se 系列中降低，并且当加入苯基或噻吩基取代基时。噻吩类物种的 LUMO 能量降低更大，其中芳香族基团具有增加的扭转灵活性。这些染料在高极性和低极性溶剂中的激发态寿命和荧光量子产率揭示了具有苯基和噻吩基取代基的生色团之间截然不同的光物理性质，这是由于扭转和诱导效应的组合。在吡咯啉和苯基取代的物种中，非辐射衰减可以通过胺-蒽酮核电荷分离态发生，该态在高极性环境中稳定。在噻吩衍生物中，通过芳基基团的旋转可以进入较低能量的激发态 S'，并且激发态种群在 6-30 ps 内迅速在 S 和 S'之间平衡。初步的光化学产氢数据显示了噻吩衍生物在太阳能转化中的潜在应用。
