Unsolicited Photoexcited-State Pathways Relegate the Long-Lived Charge Separation in Self-Assembled Nucleobase–Arene Conjugate

Abstract

We report a comprehensive analysis of the ultrafast photoexcited-state processes of a nucleobase–arene conjugate, 9-(adenin-9-yl)anthracene (AdAn), in organic solvents. By virtue of the strong base-pairing and π–π interactions, AdAn forms an alternate distichous assembly, wherein the adenines are near-orthogonally flanked to the columnar anthracene-on-anthracene (An–An) stacks. The preliminary photophysical and redox analysis of AdAn in organic solvent reveals a summation of adenine (Ad) and anthracene (An) spectral features with minimal perturbation in the ground-state properties. The frontier molecular orbital analysis suggests favorable charge transfer characteristics in the dyad, which is further corroborated by the negative value of the free energy for electron transfer from 1Ad* to An (ΔGet = −1.25 eV) and Ad to 1An* (ΔGet = −0.12 eV) obtained via Weller analysis. Upon photoexcitation (λex = 266 and 355 nm), self-assembled AdAn reveals the formation of radical ion-pair intermediates (τcrA = 120 ms), which display a solvent polarity dependence as demonstrated via nanosecond and femtosecond transient absorption spectroscopy. The “emergence upon assembly” approach offers segregated trajectories of the charge carriers, i.e. delocalization of the electron through the columnar π-channels of anthracenes and the diffusion of the holes across hydrogen-bonded adenines, resulting in persistent radical ion-pair intermediates. Though the unsolicited photoexcited-state pathways of AdAn lead to undesirable photoproducts, the long-lived charge-separated state from a favorable arrangement of DA stack proposes an elegant DA assembly design for emergent solution processable photofunctional devices.