Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular Charge Transfer in Single-Component Phenoxazine–Quinoline Conjugates and Chemical Sensing

Achieving red phosphorescence
from purely organic system is a challenging
feat due to the predominant thermal nonradiative decay pathways of
the excited electrons. Here, we design single-component charge-transfer
(CT) complexes based on phenoxazine–quinoline conjugates (PQ1–PQ3), in which the phenoxazine ring
is covalently attached to the quinolinyl fragment via a C–N
bond. These conjugates in concentration-dependent absorption studies
show a new low-energy CT absorption band along with the parent π–π*
band with binding constants of up to 102 M–1 due to self-association via intermolecular CT (I2CT).
Steady-state emission and phosphorescence decay transient measurements
of all of the conjugates in solutions, thin films, and crystals reveal
the signature of I2CT that leads to orange-red phosphorescence
(ORP) at ambient conditions. Theoretical calculations show the existence
of dimer with stronger I2CT characteristics and reduced
energy gap between the lowest singlet (S1) and triplet
(T1) states (ΔEST = 0.05–0.14
eV), which is in line with emission measurements. These conjugates
are used for solid-state dichloromethane vapor sensors, and PQ3 can be transformed into oxygen sensor. These studies give
an insight into the ORP properties and provide a rational strategy
for the design of single-component self-CT complexes with ORP feature
at ambient conditions.