From symmetry breaking to symmetry swapping: is Kasha's rule violated in multibranched phenyleneethynylenes?

• Published in: Chemical science (Cambridge) Vol. 14; no. 8; pp. 1986 - 1996
• Main Authors: Swathi, K, Sujith, Meleppatt, Divya, P. S, P, Merin Varghese, Delledonne, Andrea, Phan Huu, D. K. Andrea, Di Maiolo, Francesco, Terenziani, Francesca, Lapini, Andrea, Painelli, Anna, Sissa, Cristina, Thomas, K. George
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 22.02.2023; The Royal Society of Chemistry
• Subjects: Broken symmetry; Chemistry; Excitation; Fluorescence; Optoelectronics; Photon absorption; Symmetry
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc05206g

The phenomenon of excited-state symmetry breaking is often observed in multipolar molecular systems, significantly affecting their photophysical and charge separation behavior. As a result of this phenomenon, the electronic excitation is partially localized in one of the molecular branches. However, the intrinsic structural and electronic factors that regulate excited-state symmetry breaking in multibranched systems have hardly been investigated. Herein, we explore these aspects by adopting a joint experimental and theoretical investigation for a class of phenyleneethynylenes, one of the most widely used molecular building blocks for optoelectronic applications. The large Stokes shifts observed for highly symmetric phenyleneethynylenes are explained by the presence of low-lying dark states, as also established by two-photon absorption measurements and TDDFT calculations. In spite of the presence of low-lying dark states, these systems show an intense fluorescence in striking contrast to Kasha's rule. This intriguing behavior is explained in terms of a novel phenomenon, dubbed "symmetry swapping" that describes the inversion of the energy order of excited states, i.e. , the swapping of excited states occurring as a consequence of symmetry breaking. Thus, symmetry swapping explains quite naturally the observation of an intense fluorescence emission in molecular systems whose lowest vertical excited state is a dark state. In short, symmetry swapping is observed in highly symmetric molecules having multiple degenerate or quasi-degenerate excited states that are prone to symmetry breaking. Highly symmetric multibranched phenyleneethynylenes exhibit intense fluorescence despite the presence of low-lying dark states. The inversion of the energy order of excited states is explained in terms of a novel phenomenon dubbed "symmetry swapping".