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 1 Ad* to An (ΔG et = −1.25 eV) and Ad to 1 An* (ΔG et = −0.12 eV) obtained via Weller analysis. Upon photoexcitation (λ ex = 266 and 355 nm), self-assembled AdAn reveals the formation of radical ion-pair intermediates (τ cr A = 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 hydrogenbonded 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.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor–Acceptor Conjugates

“…A near-orthogonal nucleobase−arene conjugate, 9-(adenin-9-yl)anthracene (AdAn) 24, provides another unique example of an alternate distichous arrangement in the crystal structure (Figure 26e). 152 The molecules crystallize in an alternatedistichous architecture wherein the columnar π-stacks of anthracene have protruding adenine units that extend the assembly sideways through hydrogen-bonding (Figure 26f). Photoinduced electron transfer from adenine (donor) to anthracene (acceptor) in the self-assembled structures in solution results in the formation of long-lived radical ion pair intermediates having a lifetime of τ CR = 120 ms by virtue of delocalization of electrons among the anthracene π-stacks and holes along the hydrogen-bonded adenine units (Figure 26g, h).…”

“…Circumventing the Coulombic interaction directed interdigitating arrangement requires careful engineering of molecular structure and appropriate functionalization of the units. , The π-stacking propensity of D–A units in conjunction with hydrogen bonding, − metal-coordination bonding, − ion templates, − chiral auxiliaries, ,,, self-organizing surface-initiated polymerization, , side-chain functionalization induced amphiphilicity, , and halogen interactions have been utilized to promote segregated D and/or A motifs. Several hierarchical architectures including parallel bicontinuous, coaxial, ,, hexagonal, slip-stacked, helical, ,,, helix wrapped, nonparallel, and alternate distichous possessing segregated arrays have been achieved utilizing such strategies (Figure ).…”

Functional Systems Derived from Nucleobase Self‐assembly

“…The spectroscopic results are consistent with hole sharing among guanines within the G‐quadruplex that is kinetically competitive with the formation of GEAn .+ . Such a result reveals the possibility of nucleobase‐arene assembly for lifetime engineering by using supramolecular design . This also suggests that G‐quadruplexes can serve as a hole trap mitigating oxidative DNA stress and effective hole conduits in ordered donor−acceptor assemblies (see Section 2.2).…”

Direct Tracking Excited-State Intramolecular Charge Redistribution of Acceptor–Donor–Acceptor Molecule by Means of Femtosecond Stimulated Raman Spectroscopy

“…Functional organic molecular materials with intramolecular charge transfer (ICT) character have received significant attention in past few decades because of their potential in numerous applications in two photon absorption, organic light emitting diodes, and organic photovoltaics, etc. − As is well-known, molecules exhibiting ICT characters are normally composed of an electron-donor (D) and an electron-acceptor (A) linked by a π-bridge, and the ICT properties of conventional dipolar D–A molecules are easily tuned by solvent polarity and D/A variations, − which manifest in obvious solvatochromism along with the permanent dipole moment changes between excited state and ground state. The formation of delocalized excitons may provide a low bandgap system with long exciton lifetime, leading to reduced barrier for the ICT state dissociation.…”