Synthesizing Covalent L-Shaped Bacteriochlorin Dimers To Isolate Purely Coulombic Excitonic Coupling

Abstract Using de novo bacteriochlorins, we employed a click-chemistry strategy to synthesize elusive oblique dimers with L-oriented transition dipole moments (TDM) designed to isolate purely Coulombic excitonic coupling. The resulting covalent bis-bacteriochlorins (bis-BC1 and bis-BC2) utilize a non-conjugating bridge to prevent spatial orbital overlap between the constituent nonidentical chromophores. To enable excitonic coupling, we incorporated 3- and 13-auxochromic groups, which enhanced the TDM magnitudes of the constituent monomers. As a result, both bis-bacteriochlorins exhibited the split absorption band of comparable intensities characteristic of L-shaped oblique dimers. Excitonic coupling was confirmed by fluorescence excitation and anti-Stokes experiments. The Coulombic nature of the observed excitonic coupling was examined through a combination of computational calculations (molecular orbitals), NMR, temperature-dependent absorption, and absorption and fluorescence spectroscopies in polar solvents. Consequently, bis-BC1 was shown to exhibit purely Coulombic excitonic coupling, manifesting in an intermediate coupling regime with substantial exciton delocalization between likely thermally equilibrated exciton states. In contrast, bis-BC2 appeared to possess an excited state susceptible to nonradiative decay in DMF due to minor orbital overlap and remained primarily in a weak-mixing regime owing to a large site-energy asymmetry of its constituent chromophores. These observations were ascribed to the coplanarity of its macrocycles, presumably secured by an intramolecular hydrogen bond. Ultimately, with isolated, solely Coulombic excitonic coupling, bis-BC1 serves as a rigorous benchmark for fundamental studies and for engineering molecular exciton-based materials, while bis-BC2 provides a valuable limiting-case reference for the boundaries of the weak-mixing regime and the onset of minor orbital overlap.