A combined experimental and theoretical study shows a significant barrier (ca. 100 kJ/mol) to rotation through the interchromophoric carbon−carbon single covalent (1.49 Å) bond between the naphthalenimide and perylenimide units that prevents coplanarization of the two units in the dyad NP, thereby forcing them to act as independent chromophores/redox centers. Upon photoexcitation, highly efficient energy transfer is observed from the naphthalenimide (energy donor) to the perylenimide (energy acceptor) moiety predominantly through Coulombic coupling, completely isolating the orbital overlap (Dexter-type) interaction between the chromophoric units at such short separation by virtue of their orthogonal arrangement. Because Forster's ideal-dipole approximation ignores the contribution from significant higherorder Coulombic interactions at such short distances between donor and acceptor moieties, the complete coupling was computed from the transition densities, giving an estimate of the energy-transfer rate from the naphthalenimide donor to the perylenimide acceptor of k ET = 2.2 × 10 10 s −1 , in agreement with observations. Ultrafast excitation energy (ca. 40 ps, 90%) and electron (<0.5 ps, 10%) transfer from the singlet excited state of naphthalenimide to the perylenimide moiety competes with further delayed processes in the conjugate NP. Upon excitation at 345 nm, conjugate NP exhibits near-quantitative energy transfer in conjunction with solvent-polarity-dependent (solvatochromic) perylenimide fluorescence, resulting in a remarkable Stoke's shift of ca. 175−240 nm. Favorable photophysical properties such as high fluorescence quantum yield, wide excitation range, ultrafast energy transfer, marginal electron transfer, and large Stoke's shift make this conjugate a potential candidate for biological applications.

With an increase in temperature, an unprecedented restoration of symmetry in the symmetry breaking excited state charge transfer is observed in a geminal pair of near-orthogonally connected perylenimide dimers. Such restoration of symmetry could be attributed to the interchromophoric planarization and/or loss of solvation asymmetry at elevated temperature resulting in enhanced fluorescence quantum yield.

This work highlights the utility of π–π stacked self-assembly for enhanced survival time of charge transfer intermediates upon photoexcitation of donor–acceptor systems.

Highly efficient photoinduced energy transfer is observed in an orthogonal bichromophore naphthalenimideperylenimide (NP), leading to strong solid-state luminescence (Φ = 0.5 ± 0.04) in the red region (λ em = 631 nm). Steric hindrance imparted by orthogonal naphthalenimide and diisopropyl phenyl units prevents the association of perylenimide moieties, thereby retaining a high quantum yield of fluorescence emission even in the crystalline state. Upon photoexcitation at 340 nm, the intramolecular parallel orientation of the transition dipoles permits efficient Coulombic coupling between naphthalenimide and perylenimide in combination with weak H-type excitonic interactions between the intermolecular perylenimide units that results in intense red fluorescence in the crystalline state of dyad NP. Negligible photoinduced electron transfer from the singlet excited state of the perylenimide to the naphthalenimide unit (ΔG = 0.44 eV) and a marginal feasibility (ca. 10%) of photoinduced electron transfer from the singlet excited state of the naphthalenimide to the peryleninimide unit (ΔG = −0.17 eV) makes the dyad strongly fluorescent when excited at both 340 and 475 nm, corresponding to the naphthalenimide and perylenimide units, respectively. A narrow emission at 631 nm, wide absorption window (300−600 nm), and high fluorescence quantum yield in the crystalline state make the dyad NP a potential candidate for optoelectronic and photonic applications.

Circular dichroism spectra for a series of structurally analogous hairpin oligonucleotides, tethered at the 5′-end with an axially chiral naphthalenimide-perylenimide dyad (NP), is dependent on the nature (AT vs GC) and the orientation (5′-C vs 5′-G) of the adjacent base pair that stacks with the dyad. Charge transfer (CT) interaction between the naphthalenimide unit of the dyad NP and the adjacent guanine–cytosine (5′-C) base pair has been characterized by UV–vis absorption and fluorescence measurements. Molecular dynamics simulations of the dyad end-capped hairpin DNA ODN6i and TD-DFT calculations of the naphthalenimide and perylenimide units confirm that the CT transition dipole orients perpendicular to the perylenimide transition dipole. The orthogonality of the cross product of the CT and the perylenimide transition dipoles with the displacement vector connecting the two dipoles in space results in the zeroing out of the rotational strength of the perylenimide transition dipole, subsequently leading to the DNA-induced nonexciton coupled circular dichroism corresponding to perylenimide, in concurrence with experimental CD spectrum. Singular value decomposition of thermal denaturation of the hairpin DNA having CT interaction (ODN6i) revealed the denaturation proceeded through an additional intermediary stage compared to the hairpin DNA without the CT interaction (ODN6). Dissimilar CD (induced CD for ODN6i vs exciton-coupled CD for ODN6) spectra corresponding to perylenimide unit obtained for similar NP end-capped hairpin DNA sequences cautions against the indiscriminate use of the exciton chirality method, particularly in systems like DNA and proteins containing polarizable chromophores that can interact with reporter transition dipoles.