Influence of Substituents on the Vectorial Difference Static Dipole Upon Excitation in Synthetic Bacteriochlorins

Organic dye aggregates have been shown to exhibit exciton delocalization in natural and synthetic systems. Such dye aggregates show promise in the emerging area of quantum information science (QIS). We believe that the difference in static dipole (Δ<i>d</i>) is an essential dye parameter in the development of molecular QIS systems. However, a foundational understanding of the structural factors influencing Δ<i>d</i> remains elusive. Bacteriochlorins play a vital role in photosynthesis due to their exceptional photophysical properties. Therefore, bacteriochlorins are particularly suitable dyes for the construction of aggregate systems for QIS. Synthetic bacteriochlorins further offer stability and tunability via chemical modifications. Here, the influence of substituents on the Δ<i>d</i> of monomeric (nonaggregated) dyes was investigated via density functional theory (DFT) and time-dependent (TD)DFT in a set of 5-methoxybacteriochlorins progressively substituted with ethynyl, phenyl, and phenylethynyl substituents at the 3,13 and 3,13,15 positions of the macrocycle. Symmetrically substituted 5-methoxybacteriochlorins were shown to have the largest Δ<i>d</i>. The increase in Δ<i>d</i> in the series of dyes was largely due to changes in the orientation of the static dipole upon excitation rather than large changes in magnitude. In addition, the transition dipole (<i>μ</i>) and the angle between Δ<i>d</i> and <i>μ</i> (<i>ζ</i>) were calculated. Three 5-methoxybacteriochlorins with large predicted Δ<i>d</i> and <i>μ</i> values were synthesized and characterized spectroscopically. The trend in Δ<i>d</i> values empirically determined using the solvatochromic Stokes shift method was comparable to the DFT calculations.