The impact of twisting on the intersystem crossing in acenes: an experimental and computational study

Due to their unique excited state dynamics, acenes play a dominant role in optoelectronic and light-harvesting applications. Their optical and electronic properties are typically tailored by side-group engineering, which often result in distortion of the acene core from planarity. However, the effect of such distortion on their excited state dynamics is not clear. In this work, we investigate the effect of twisting on the photophysics of acenes, which are helically locked to a defined twist angle by tethers of different lengths. Ultrafast transient absorption and time resolved fluorescence show a clear dependence of the rate of intersystem crossing with twisting. This trend is explained using quantum chemical calculations, showing an increase of spin-orbit coupling (SOC). At much earlier times, structural reorganization in S₁, including coherent vibrational wave packet motions, is reflected in transient spectral changes. As predicted by theory, decreasing the length of diagonal tether induces enhanced activity and frequency blue-shifting of a normal vibration consisting of anthracene twisting against restraint of the tethering chain. Overall, these results serve as design principles for tuning photophysical properties of acenes via controlled twisting of their aromatic core.