Excited-State Antiaromaticity in Nonbenzenoid Aromatics: Examining the Dynamics of Intramolecular Proton Transfer With a Small Driving Force.

The relief of excited-state antiaromaticity (ESAA) is a powerful driving force that underpins the photochemical behavior of photoexcited aromatic molecules. Photo-induced isomerization, cycloaddition, or proton transfer pathways alleviate the large energetic destabilization caused by ESAA. However, the excited-state dynamics of annulenes displaying only weak ESAA remain largely unexplored. Here, we fine-tune the electronic character of a series of annulenes to minimize their ESAA. Excited-state proton transfer (ESPT) converts hydroxytropyliums (HTs) and hydroxybenzotropyliums (HBTs) to their nonaromatic tautomers, which are only slightly lower in energy than the excited-state antiaromatic forms. As a result of this near-degeneracy, we find that HBTs exhibit weak photoacidity (pKa drop of 0.3-1.2 units) and unusually slow, reversible ESPT, proceeding on the ns timescale rather than the typical ps timescale. The fluorescence spectra of these HBTs display the hallmarks of ESPT emitters: (i) dual fluorescence; (ii) excitation-dependent emission; and (iii) large Stokes shifts. The driving force for this ESPT is increased by the addition of electron-donating groups, which promote intramolecular charge transfer. Our results demonstrate that judicious structural changes can moderate the ESAA of nonbenzenoid annulenes, minimizing the driving force for ESPT, which leads to slow proton transfer kinetics and weak photoacidity.