Phenoxazine-Quinoline Conjugates: Impact of Halogenation on Charge Transfer Triplet Energy Harvesting via Aggregate Induced Phosphorescence.
Saheli KarmakarSuvendu DeyManoj UpadhyayDebdas RayPublished in: ACS omega (2022)
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 ( 3 CT) 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 3 CT 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 ( 1 CT) and triplet CT ( 3 CT) states under VLE, (ii) energetic nondegeneracy of 1 CT and 3 CT states ( 1 CT- 3 CT, 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 3 CT caused by increased rigidity of the molecular backbone. This design principle reveals a method to understand nondegeneracy of 1 CT and 3 CT states, and RTP with a concentration-dependent AIP effect using halogen substituted twisted donor-acceptor conjugates.
Keyphrases
- dual energy
- room temperature
- image quality
- computed tomography
- contrast enhanced
- energy transfer
- positron emission tomography
- air pollution
- magnetic resonance imaging
- molecular docking
- molecular dynamics
- machine learning
- high resolution
- drug delivery
- density functional theory
- diabetic rats
- dna methylation
- particulate matter
- gene expression
- deep learning
- high glucose
- endothelial cells
- mass spectrometry
- visible light
- drug discovery
- electron microscopy