Activating intramolecular singlet exciton fission by altering π-bridge flexibility in perylene diimide trimers for organic solar cells |
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Authors: | Benedetta Carlotti Ifeanyi K. Madu Hyungjun Kim Zhengxu. Cai Hanjie Jiang Angelar K. Muthike Luping Yu Paul M. Zimmerman Theodore Goodson III |
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Affiliation: | Department of Chemistry, University of Michigan, Ann Arbor MI 48109 USA.; Department of Chemistry Biology and Biotechnology, University of Perugia, via Elce di Sotto n.8, 06123 Perugia Italy ; Department of Chemistry, Incheon National University, Incheon 22012 Republic of Korea ; Department of Chemistry, The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago IL 60637 USA |
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Abstract: | In this study, two analogous perylene diimide (PDI) trimers, whose structures show rotatable single bond π-bridge connection (twisted) vs. rigid/fused π-bridge connection (planar), were synthesized and investigated. We show via time resolved spectroscopic measurements how the π-bridge connections in A–π–D–π–A–π–D–π–A multichromophoric PDI systems strongly affect the triplet yield and triplet formation rate. In the planar compound, with stronger intramolecular charge transfer (ICT) character, triplet formation occurs via conventional intersystem crossing. However, clear evidence of efficient and fast intramolecular singlet exciton fission (iSEF) is observed in the twisted trimer compound with weaker ICT character. Multiexciton triplet generation and separation occur in the twisted (flexible-bridged) PDI trimer, where weak coupling among the units is observed as a result of the degenerate double triplet and quintet states, obtained by quantum chemical calculations. The high triplet yield and fast iSEF observed in the twisted compound are due not only to enthalpic viability but also to the significant entropic gain allowed by its trimeric structure. Our results represent a significant step forward in structure–property understanding, and may direct the design of new efficient iSEF materials.We show via time resolved spectroscopy that triplet formation proceeds via intersystem crossing in a rigid-bridged perylene diimide trimer and via efficient and fast intramolecular singlet exciton fission in the analogous flexible-bridged trimer. |
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