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

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.