Phenoxazine–Quinoline Conjugates: Impact of Halogenation on Charge Transfer Triplet Energy Harvesting via Aggregate Induced Phosphorescence

Room-temperature
phosphorescence (RTP) from organic compounds has
attracted increasing attention in the field of data security, sensing,
and bioimaging. However, realization of RTP with an aggregate induced
phosphorescence (AIP) feature via harvesting supersensitive excited
charge transfer triplet (3CT) energy under visible light
excitation (VLE) in single-component organic systems at ambient conditions
remains unfulfilled. Organic donor–acceptor (D–A) based
orthogonal structures can therefore be used to harvest the energy
of the 3CT state at ambient conditions under VLE. Here
we report three phenoxazine–quinoline conjugates (PQ, PQCl,
PQBr), in which D and A parts are held in orthogonal orientation around
the C–N single bond; PQCl and PQBr are substituted with halogens
(Cl, Br) while PQ has no halogen atom. Spectroscopic studies and quantum
chemistry calculations combining reference compounds (Phx, QPP) reveal
that all the compounds in film at ambient conditions show fluorescence
and green-RTP due to (i) radiative decay of both singlet charge transfer
(1CT) and triplet CT (3CT) states under VLE,
(ii) energetic nondegeneracy of 1CT and 3CT
states (1CT– 3CT, 0.17–0.21 eV),
and (iii) spatial separation of highest and lowest unoccupied molecular
orbitals. Further, we found in a tetrahydrofuran–water mixture
(fw = 90%, v/v) that both PQCl (10–5 M) and PQBr (10–5 M) show concentration-dependent
AIP with phosphorescence quantum yields (ϕP) of ∼25%
and ∼28%, respectively, whereas aggregate induced quenching
(ACQ) was observed in PQ. The phosphorescence lifetimes (τP) of the PQCl and PQBr aggregates were shown to be ∼22–62
μs and ∼22–59 μs, respectively. The ϕP of the powder samples is found to be 0.03% (PQ), 15.6% (PQCl),
and 13.0% (PQBr), which are significantly lower than that of the aggregates
(10–5 M, fw = 90%, v/v).
Film (Zeonex, 0.1 wt %) studies revealed that ϕP of
PQ (7.1%) is relatively high, while PQCl and PQBr exhibit relatively
low ϕP values (PQCl, 9.7%; PQBr, 8.8%), as compared
with that of powder samples. In addition, we found in single-crystal
X-ray analysis that multiple noncovalent interactions along with halogen···halogen
(Cl···Cl) interactions between the neighboring molecules
play an important role to stabilize the 3CT caused by increased
rigidity of the molecular backbone. This design principle reveals
a method to understand nondegeneracy of 1CT and 3CT states, and RTP with a concentration-dependent AIP effect using
halogen substituted twisted donor–acceptor conjugates.