Energy Transfer in Near-Orthogonally Arranged Chromophores Separated through a Single Bond

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 Förster’s
ideal-dipole approximation ignores the contribution from significant
higher-order 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 kET = 2.2 × 1010 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.