Single-Component Organic Light-Harvesting Red Luminescent Crystal

Highly efficient photoinduced energy transfer is observed
in an
orthogonal bichromophore naphthalenimide-perylenimide (NP), leading to strong solid-state luminescence (Φ = 0.5 ±
0.04) in the red region (λem = 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.