Photoinduced Changes in Aromaticity Facilitate Electrocyclization of Dithienylbenzene Switches.

Baswanth Oruganti,Bo Durbeej,Péter Pál Kalapos,Varada Bhargav,Gábor London

doi:10.1021/jacs.0c06327

Abstract

The concepts of excited-state aromaticity and antiaromaticity have in recent years with increasing frequency been invoked to rationalize the photochemistry of cyclic conjugated organic compounds, with the long-term goal of using these concepts to improve the reactivities of such compounds toward different photochemical transformations. In this regard, it is of particular interest to assess how the presence of a benzene motif affects photochemical reactivity, as benzene is well-known to completely change its aromatic character in its lowest excited states. Here, we investigate how a benzene motif influences the photoinduced electrocyclization of dithienylethenes, a major class of molecular switches. Specifically, we report on the synthesis of a dithienylbenzene switch where the typical nonaromatic, ethene-like motif bridging the two thienyl units is replaced by a benzene motif, and show that this compound undergoes electrocyclization upon irradiation with UV-light. Furthermore, through a detailed quantum chemical analysis, we demonstrate that the electrocyclization is driven jointly and synergistically by the loss of aromaticity in this motif from the formation of a reactive, antiaromatic excited state during the initial photoexcitation, and by the subsequent relief of this antiaromaticity as the reaction progresses from the Franck–Condon region. Overall, we conclude that photoinduced changes in aromaticity facilitate the electrocyclization of dithienylbenzene switches.

Highlights

The concept of aromaticity is widely used in organic chemistry to predict and rationalize structures and reactivities of cyclic conjugated molecules residing in their electronic ground state
The UV−vis absorption spectra of 1a shown in Figure 1 display high-energy bands and do not exhibit any solvent dependence, which is further corroborated by spectra recorded in additional solvents given in Figure S1 of the Supporting Information (SI)
Through a detailed computational analysis, we have found that the electrocyclization process is facilitated by photoinduced changes in the aromaticity of the benzene motif of 1a

Summary

INTRODUCTION

The concept of aromaticity is widely used in organic chemistry to predict and rationalize structures and reactivities of cyclic conjugated molecules residing in their electronic ground state. Through transient absorption spectroscopy and time-dependent density functional theory (TD-DFT) studies of dihydroazulene-vinylheptafulvene photoswitches operated through electrocyclization reactions, it was found that the loss of aromaticity in a benzene motif from the initial electronic excitation hampers the photochemical reactivity by increasing an already existing barrier along the excited-state potential energy surface (PES).[26] In this light, it is worthwhile to investigate whether a similar conclusion is applicable to other types of photoswitches utilizing electrocyclization reactions and featuring a benzene motif To this end, we here consider dithienylethene switches, which are one of the most extensively studied class of electrocyclization-based photoswitches,[31−39] with applications in photopharmacology,[40,41] molecular electronics,[34,38,42,43] and in the design of complex functional materials.[44−46] we report on the synthesis and the experimental and computational characterization of a dithienylbenzene switch where the typical nonaromatic, ethene-like motif[31−39] bridging the two thienyl units, is replaced by an aromatic benzene motif.

RESULTS AND DISCUSSION

CONCLUSIONS

EXPERIMENTAL AND COMPUTATIONAL DETAILS

■ ACKNOWLEDGMENTS

■ REFERENCES