Enhanced intersystem crossing in core-twisted aromatics

Abstract

We describe the design, bottom-up synthesis and X-ray single crystal structure of systematically twisted aromatics <b>1c</b> and <b>2d</b> for efficient intersystem crossing. Steric congestion at the cove region creates a nonplanar geometry that induces a significant yield of triplet excited states in the electron-poor core-twisted aromatics <b>1c</b> and <b>2d</b>. A systematic increase in the number of twisted regions in <b>1c</b> and <b>2d</b> results in a concomitant enhancement in the rate and yield of intersystem crossing, monitored using femtosecond and nanosecond transient absorption spectroscopy. Time-resolved absorption spectroscopic measurements display enhanced triplet quantum yields (<i>Φ</i> _T = 10 ± 1% for <b>1c</b> and <i>Φ</i> _T = 30 ± 2% for <b>2d</b>) in the twisted aromatics when compared to a negligible <i>Φ</i> _T (<1%) in the planar analog <b>3c</b>. Twist-induced spin-orbit coupling <i>via</i> activated out-of-plane C-H/C[double bond, length as m-dash]C vibrations can facilitate the formation of triplet excited states in twisted aromatics <b>1c</b> and <b>2d</b>, in contrast to the negligible intersystem crossing in the planar analog <b>3c</b>. The ease of synthesis, high solubility, access to triplet excited states and strong electron affinity make such imide functionalized core-twisted aromatics desirable materials for organic electronics such as solar cells.