Two-electron transfer stabilized by excited-state aromatization

Jinseok Kim,Seongchul Park,Jose L Zafra,Dongho Kim,Justin R Defrancisco,John D Tovar,Juwon Oh,Juan Casado,Manho Lim,David Casanova

doi:10.1038/s41467-019-12986-w

Abstract

The scientific significance of excited-state aromaticity concerns with the elucidation of processes and properties in the excited states. Here, we focus on TMTQ, an oligomer composed of a central 1,6-methano[10]annulene and 5-dicyanomethyl-thiophene peripheries (acceptor-donor-acceptor system), and investigate a two-electron transfer process dominantly stabilized by an aromatization in the low-energy lying excited state. Our spectroscopic measurements quantitatively observe the shift of two π-electrons between donor and acceptors. It is revealed that this two-electron transfer process accompanies the excited-state aromatization, producing a Baird aromatic 8π core annulene in TMTQ. Biradical character on each terminal dicyanomethylene group of TMTQ allows a pseudo triplet-like configuration on the 8π core annulene with multiexcitonic nature, which stabilizes the energetically unfavorable two-charge separated state by the formation of Baird aromatic core annulene. This finding provides a comprehensive understanding of the role of excited-state aromaticity and insight to designing functional photoactive materials.

Highlights

The scientific significance of excited-state aromaticity concerns with the elucidation of processes and properties in the excited states
(1.0, 9.0, and 15 ps in CH2Cl2 and 0.8, 2.5, and 19 ps in CH3NO2). This decrease in the excited-state lifetime of TMTQ upon increasing the solvent polarity suggests the existence of an excited state with a charge-localized electronic structure, which is more stabilized under more polar solvent conditions and accelerates the relaxation processes in the excited state dynamics[29,30]
As mentioned, Baird’s rule describes the reversed aromaticity in the excited triplet state, which reminds that the core 8π annulene of TMTQ should be triplet in the charge transfer (CT) state

Summary

Introduction

The scientific significance of excited-state aromaticity concerns with the elucidation of processes and properties in the excited states. Biradical character on each terminal dicyanomethylene group of TMTQ allows a pseudo triplet-like configuration on the 8π core annulene with multiexcitonic nature, which stabilizes the energetically unfavorable two-charge separated state by the formation of Baird aromatic core annulene This finding provides a comprehensive understanding of the role of excited-state aromaticity and insight to designing functional photoactive materials. Despite the lack of SOMO–SOMO interaction in [4n]π-electron systems, the interaction of a SOMO with energetically adjacent symmetric occupied and unoccupied molecular orbitals stabilizes the [4n]π cyclic conjugation interaction This completely reverses the criteria for excited-state aromaticity (that is, aromaticity/antiaromaticity for [4n]/[4n + 2]π-electron systems in the triplet states) and has been intensively investigated on a theoretical basis[5,6,7,8]. Time-resolved optical spectroscopies enable us to monitor (anti)aromaticity-driven changes of electronic structure and molecular conformations, which will fundamentally help to understand the role and effect of aromaticity in the excited states

Methods

Results

Conclusion