Room temperature charge-transfer phosphorescence from organic donor-acceptor Co-crystals

Abstract

Engineering the electronic excited state manifolds of organic molecules can give rise to various functional outcomes, including ambient triplet harvesting, that has received prodigious attention in the recent past. Herein, we introduce a modular, non-covalent approach to bias the entire excited state landscape of an organic molecule using tunable 'through-space charge-transfer' interactions with appropriate donors. Although charge-transfer (CT) donor-acceptor complexes have been extensively explored as functional and supramolecular motifs in the realm of soft organic materials, they could not imprint their potentiality in the field of luminescent materials, and it still remains as a challenge. Thus, in the present study, we investigate the modulation of the excited state emission characteristics of a simple pyromellitic diimide derivative on complexation with appropriate donor molecules of varying electronic characteristics to demonstrate the selective harvesting of emission from its locally excited (LE) and CT singlet and triplet states. Remarkably, co-crystallization of the pyromellitic diimide with heavy-atom substituted and electron-rich aromatic donors leads to an unprecedented ambient CT phosphorescence with impressive efficiency and notable lifetime. Further, gradual minimizing of the electron-donating strength of the donors from 1,4-diiodo-2,3,5,6-tetramethylbenzene (or 1,2-diiodo-3,4,5,6-tetramethylbenzene) to 1,2-diiodo-4,5-dimethylbenzene and 1-bromo-4-iodobenzene modulates the source of ambient phosphorescence emission from the ³CT excited state to ³LE excited state. Through comprehensive spectroscopic, theoretical studies, and single-crystal analyses, we elucidate the unparalleled role of intermolecular donor-acceptor interactions to toggle between the emissive excited states and stabilize the triplet excitons. We envisage that the present study will be able to provide new and innovative dimensions to the existing molecular designs employed for triplet harvesting.

Fig. 1. Schematic of the modular donor–acceptor co-assembly strategy to tune the excited state manifold of pyromellitic diimide phosphor. Molecular structure of PmDI (acceptor) and its tunable emission with different aromatic donors; locally excited fluorescence (¹LE) in THF solution, charge-transfer (¹CT) fluorescence in electron rich-aromatic solvents (p-xylene, mesitylene and TMB), locally excited phosphorescence (³LE) in 1-bromo-4-iodobenzene (D₄), 1, 4-dibromobenzene (D₅) and charge-transfer phosphorescence (³CT) in 1,4-diiodo-2,3,5,6-tetramethylbenzene (D₁), 1,2-diiodo-3,4,5,6-tetramethylbenzene (D₂). Simplified Jablonski diagram showing various emission processes possible in the excited state (photographs of ¹LE, ¹CT emission obtained by 340 nm and 370 nm Xe lamp excitation and ³LE and ³CT phosphorescent co-crystals under 365 nm UV lamp).

Fig. 2. Solution state CT fluorescence of PmDI in electron rich aromatic solvents: (a) molecular structure of PmDI (acceptor) and different non-aromatic and aromatic solvents with varying electronic character. (b) Normalized absorption spectra of PmDI in various solvents, which shows the presence of ground state CT band in electron rich aromatic donors. (c) Normalized steady-state emission spectra showing ¹LE emission in THF, benzene and broad, red-shifted CT emission band in electron-rich aromatic solvents (λ_exc = 340 nm). (d) Normalized emission spectra showing CT emission in electron-rich aromatic solvents upon selective excitation at the CT band (λ_exc = 420 nm). (e) Normalized emission spectra showing distinct ¹LE emission in THF (λ_exc = 340 nm) and ¹CT emission in TMB (λ_exc = 420 nm). (f) Fluorescence lifetime decay profiles of ¹LE emission in THF (λ_exc = 340 nm, λ_collected = 420 nm) and ¹CT emission in TMB (λ_exc = 442 nm, λ_collected = 500 nm), IRF is the instrument response function. (g) Plot of emission maximum versus the ionization potential of various aromatic solvents under study, which shows that emission maximum becomes red-shifted upon decreasing the ionization potential of the solvent.

Fig. 3. ³CT phosphorescence studies of A+D₁ and A+D₂ co-crystals: (a) molecular structures for A+D₁ and A+D₂ donor–acceptor complexes. (b) Normalized excitation spectra of individual donors (D₁ and D₂, λ_monitored = 420 nm), acceptor (A, λ_monitored = 560 nm) and donor–acceptor co-crystal (A+D₁ and A+D₂, λ_monitored = 560 nm), which shows the red-shifted band for donor–acceptor co-crystal compared to individual components suggesting the formation of CT complex. (c) Steady-state emission spectra of the acceptor (A) and donor–acceptor co-crystal (A+D₁ and A+D₂), which shows the weakly emissive nature of bare acceptor and highly emissive nature of donor–acceptor pair (λ_exc = 340 nm). (d) Normalized delayed emission spectra of acceptor (A) doped in PMMA matrix at 20 K (1 wt% with respect to PMMA) and donor–acceptor co-crystal at room temperature, which shows a red-shift in the emission maximum of donor–acceptor pair compared to the ³LE emission of acceptor hinting towards the ³CT emission (λ_exc = 340 nm, delay time = 1 ms for A and 50 μs for A+D₁ and A+D₂). (e) Lifetime decay profile for A+D₁ and A+D₂ co-crystal upon excitation at 340 nm and selective excitation at CT band (λ_exc = 430 nm, λ_collected = 560 nm). Temperature-dependent (f) steady-state emission spectra and (g) lifetime decay profile (λ_collected = 560 nm) of A+D₁ showing ³CT phosphorescence nature of the emission upon selective excitation at the CT band (λ_exc = 430 nm).

Fig. 4. (a) Single-crystal X-ray diffraction data of A+D₂ co-crystal: donor–acceptor arrangement of A+D₂ pair, driven by various halogen bonding and weak π⋯π interactions, such as (a) I⋯π (marked with red lines), π⋯π (marked with black lines) in a stack and (b) I⋯CO (marked with green lines) in the same plane. (c) Theoretical calculations of A+D₁: natural transition orbitals (NTOs) of A+D₁ pair for first excited singlet (S₁) and triplet state (T₁), calculated using TD-CAM-B3LYP level in conjunction with 6-31+g(d) basis set for C, N, O, H and LANL2DZ basis set showing CT character.

Fig. 5. ³LE phosphorescence studies and single-crystal X-ray diffraction data of A+D₄ co-crystal: (a) excitation spectra of individual donor (D₄, λ_monitored = 420 nm), acceptor (A, λ_monitored = 560 nm) and donor–acceptor co-crystal (A+D₄, λ_monitored = 560 nm), which suggest absence of CT band (inset shows molecular structure for A+D₄ pair). (b) Steady-state emission spectra of bare acceptor (A) and donor–acceptor pair (A+D₄) shows weakly emissive nature of acceptor and highly emissive nature of donor–acceptor co-crystal (λ_exc = 340 nm). (c) Normalized delayed emission spectra of acceptor doped in PMMA matrix at 20 K (1 wt% with respect to PMMA) and donor–acceptor pair at room temperature show the same emission maximum, hinting towards the ³LE emission (λ_exc = 340 nm). Donor–acceptor arrangement of A+D₄ pair, driven by halogen carbonyl interactions, (d) I⋯CO (marked with red lines) and Br⋯CO (marked with green lines) in the same plane, (e) I⋯CO (marked with light-green and brown lines) and Br⋯CO (marked with blue and orange lines) in the parallel plane.