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

Abstract

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 (f _w = 90%, v/v) that both PQCl (10^-5 M) and PQBr (10^-5 M) show concentration-dependent AIP with phosphorescence quantum yields (ϕ_P) of ∼25% and ∼28%, respectively, whereas aggregate induced quenching (ACQ) was observed in PQ. The phosphorescence lifetimes (τ_P) of the PQCl and PQBr aggregates were shown to be ∼22-62 μs and ∼22-59 μs, respectively. The ϕ_P 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, f _w = 90%, v/v). Film (Zeonex, 0.1 wt %) studies revealed that ϕ_P of PQ (7.1%) is relatively high, while PQCl and PQBr exhibit relatively low ϕ_P 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.

Figure 1

Energy diagram with proposed molecular structures.

Figure 2

Representative torsions in the (a) PQ and (b) PQCl. H atoms are removed for the sake of clarity.

Figure 3

The energy diagram of PQCl with natural transition orbitals describing the excitation characters of the ¹CT and ³HLCT states (M02X-631G (d,p)).

Figure 4

Absorption (UV–vis) and emission spectra of PQ, PQCl, and PQBr along with donor (Phx) and acceptor (QPP) components in toluene solutions (10 μM) at ambient conditions.

Figure 5

Lifetime analysis of (a) PQ, (b) PQCl, and (c) PQBr in different conditions at RT (λ_ex = 425 nm). (d) Phosphorescence spectra (0.1 ms delay) of donor (Phx), acceptor (QPP), PQCl, and PQBr in MCH at 77 K.

Figure 6

Aggregate induced emission (AIE) spectra of (a) PQCl and (b) PQBr with increasing water content in THF (10^–6 M) at RT (λ_ex = 425 nm). Aggregate induced phosphorescence (AIP) spectra of (c) PQCl and (d) PQBr aggregates (f_w = 90%, THF-H₂O) at ambient conditions (0.1 ms delay) (λ_ex = 425 nm). Phosphorescence lifetime of (e) PQCl and (f) PQBr aggregates at ambient conditions (λ_ex = 390 nm).

Figure 7

Steady state and RTP spectra (0.1 ms detector delay) of (a) PQCl and (b) PQBr at ambient conditions in Zeonex films (0.1 wt %) (λ_ex = 425 nm). Steady state and RTP (0.1 ms detector delay) spectra of (c) PQCl and (d) PQBr in powder (λ_ex = 425 nm). Inset shows RTP decays (λ_ex = 390 nm) of PQCl and PQBr in Zeonex films as well as powder at ambient conditions.