Triplet Formation Involving a Polar Transition State in a Well-Defined Intramolecular Perylenediimide Dimeric Aggregate

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• 作者：Dirk Veldman ; Sté ; phanie M. A. Chopin ; Stefan C. J. Meskers ; Michiel M. Groeneveld ; René ; M. Williams ; René ; A. J. Janssen
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：July 3, 2008
• 年：2008
• 卷：112
• 期：26
• 页码：5846 - 5857
• 全文大小：442K
• 年卷期：v.112,no.26(July 3, 2008)
• ISSN：1520-5215

文摘

A cofacially stacked perylenediimide (PDI) dimer with a xanthene linker was studied under a variety of conditions (solvent, temperature) and serves as a model for the molecular interactions occurring in solid films. Intrinsically, the PDI units have a fluorescence quantum yield (Φ_F) close to unity, but Φ_F is lowered by a factor of 6−50 at room temperature when two PDI moieties are held in a cofacial arrangement, while the decay time of the most emissive state is increased significantly (τ_F = 27 ns in toluene) compared to a monomeric PDI molecule (τ_F = 4 ns). Fluorescence measurements show a strong solvent and temperature dependence of the characteristics of the emissive excited state. In a glassy matrix of toluene (TOL) or 2-methyltetrahydrofuran (2-MeTHF), Φ_F is high, and the decay time is long (τ_F = ~50 ns). At higher temperature, both Φ_F and τ_F are reduced. Interestingly, at room temperature, Φ_F and τ_F are also reduced with increasing solvent polarity, revealing the presence of a polar transition state. Photoinduced absorption of the stacked molecules from the picosecond to the microsecond time scale shows that after photoexcitation reorganization occurs in the first nanoseconds, followed by intersystem crossing (ISC), producing the triplet excited state. Using singlet oxygen (¹Δ_g) luminescence as a probe, a triplet quantum yield (Φ_T) greater than 50% was obtained in air-saturated 2-Me-THF. Triplet formation is exceptional for PDI chromophores, and the enhanced ISC is explained by a decay involving a highly polar transition state.