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Controlling the S1 Energy Profile by Tuning Excited-State Aromaticity

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JournalJournal of the American Chemical Society
DateAccepted/In press - 10 Aug 2020
DateE-pub ahead of print - 10 Aug 2020
DatePublished (current) - 2 Sep 2020
Issue number35
Volume142
Number of pages8
Pages (from-to)14985–14992
Early online date10/08/20
Original languageEnglish

Abstract

The shape of the lowest singlet excited state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10–20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

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