Reducing the internal reorganization energy via symmetry controlled π-electron delocalization. Issue 24 (6th June 2022)

• Record Type: Journal Article
• Authors: Wu, Chi-Chi; Li, Elise Y.; Chou, Pi-Tai
• Is Part Of: Chemical science. Volume 13:Issue 24(2022)
• Page Start: 7181
• Page End: 7189
• Publication Date: 2022-06-06
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d2sc01851a
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : The reorganization energies may be significantly reduced by molecular symmetry effect. Abstract : The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor–acceptor (D–A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor–acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S0 and S1 states. Mixing a certain degree of local excitation (LE) with CT in the S1 state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.