Excited State Kinetics in Crystalline Solids: Self-Quenching in Nanocrystals of 4,4鈥?Disubstituted Benzophenone Triplets Occurs by a Reductive Quenching Mechanism
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- 作者：Gregory Kuzmanich ; Sabrina Simoncelli ; Matthew N. Gard ; Fabian Sp盲nig ; Bryana L. Henderson ; Dirk M. Guldi ; Miguel A. Garcia-Garibay
- 刊名：Journal of the American Chemical Society
- 出版年：2011
- 出版时间：November 2, 2011
- 年：2011
- 卷：133
- 期：43
- 页码：17296-17306
- 全文大小：1039K
- 年卷期：v.133,no.43(November 2, 2011)
- ISSN：1520-5126
文摘
We report an efficient triplet state self-quenching mechanism in crystals of eight benzophenones, which included the parent structure (1), six 4,4鈥?disubstituted compounds with NH2 (2), NMe2 (3), OH (4), OMe (5), COOH (6), and COOMe (7), and benzophenone-3,3鈥?4,4鈥?tetracarboxylic dianhydride (8). Self-quenching effects were determined by measuring their triplet鈥搕riplet lifetimes and spectra using femtosecond and nanosecond transient absorption measurements with nanocrystalline suspensions. When possible, triplet lifetimes were confirmed by measuring the phosphorescence lifetimes and with the help of diffusion-limited quenching with iodide ions. We were surprised to discover that the triplet lifetimes of substituted benzophenones in crystals vary over 9 orders of magnitude from ca. 62 ps to 1 ms. In contrast to nanocrystalline suspensions, the lifetimes in solution only vary over 3 orders of magnitude (1鈥?000 渭s). Analysis of the rate constants of quenching show that the more electron-rich benzophenones are the most efficiently deactivated such that there is an excellent correlation, 蟻 = 鈭?.85, between the triplet quenching rate constants and the Hammet 蟽+ values for the 4,4鈥?substituents. Several crystal structures indicate the existence of near-neighbor arrangements that deviate from the proposed ideal for 鈥渘-type鈥?quenching, suggesting that charge transfer quenching is mediated by a relatively loose arrangement.