Revisiting organic charge-transfer cocrystals for wide-range tunable, ambient phosphorescence

Simple and efficient designs that enable a wide range of phosphorescence emission in organic materials have ignited scientific interest across diverse fields. One particularly promising approach is the cocrystallization strategy, where organic cocrystals are ingeniously formed through relatively weaker and dynamic non-covalent interactions. In our present study, we push the boundaries further by extending this cocrystal strategy to incorporate donor–acceptor components, stabilized by various halogen bonding interactions. This non-covalent complexation triggers ambient, charge-transfer phosphorescence (3CT), which can be precisely tuned across a broad spectrum by a modular selection of components with distinct electronic characteristics. At the core of our investigation lies the electron-deficient phosphor, pyromellitic diimide, which, upon complexation with different donors based on their electron-donating strength, manifests a striking array of phosphorescence emission from CT triplet states, spanning from green to yellow to reddish orange accompanied by noteworthy quantum yields. Through a systematic exploration of the electronic properties using spectroscopic studies and molecular organization through single-crystal X-ray diffraction, we decisively establish the molecular origin of the observed phosphorescence. Notably, our work presents, for the first time, an elegant demonstration of tunable 3CT phosphorescence emission in intermolecular donor–acceptor systems, highlighting their immense significance in the quest for efficient organic phosphors.


Synthetic Scheme and Procedure
Scheme S1.Molecular Structures of A, D1, D2 and D3.
Pyromellitic diimide (A) was synthesized according to the literature procedure. [S1]   The donors D1, D2 and D3 were synthesized as follows.
Synthesis of 1,2 diiodo-4,5-dimethoxybenzene (D2): Periodic acid (2.11 g, 9.26 mmol) was dissolved in methanol (20 mL).Iodine (4.77 g, 18.82 mmol) was added, and the mixture obtained was stirred for 20 min under room temperature.1,2-dimethoxybenzene (2 g, 14.45 mmol) was added and the mixture was refluxed at 70 °C for 4 h.The reaction mixture was allowed to cool at room temperature and poured into a solution of sodium sulfite (10 g in 100 mL water).The white precipitate was isolated by filtration, washed with cold methanol (20 mL) and air-dried.To obtain the desired white solid D2 (5.48 g, 97% yield

Experimental Procedures
Protocol for cocrystal synthesis: Saturated solutions of both donor and acceptors in chloroform were prepared and mixed in 1:1 molar ratio.Then ethanol was added into it as a bad solvent.The mixture was heated at 70 °C for ten minutes and then kept at room temperature for crystallization.
Protocol for sample preparation: For thin films, of D-A complexes, the saturated solutions of D and A was drop casted on to a clean quartz substrate and kept for drying.For preparing 1 wt% thin films, acceptor or donor molecules (1 mg) were mixed with 100 mg of PMMA.This mixture was then heated at 50 °C for 10 minutes followed by sonication (5 minutes) to dissolve all the components thoroughly.Then, 0.5 mL of this solution was drop-casted on a clean quartz substrate.Finally, the drop-casted thin films were dried at 60 °C for 30 minutes before performing the photophysical studies.For the phosphorescence studies of the cocrystals, a small amount of the cocrystal was placed in between two quartz plates and is properly sealed.

Note:
The red-shifted absorption band of cocrystals, when compared to acceptor alone, suggested the ground-state charge-transfer complexation in the cocrystals.Note: Emission profile upon exciting at locally-excited band is same as that when excited at the chargetransfer band for all the cocrystals (Fig. 2a).

Note:
The increase in the emission intensity and lifetime under vacuum compared to that of air suggested the involvement of triplet states.The increasing emission intensity with decreasing temperature confirmed the nature of emission to be phosphorescence.

Note:
The increase in the emission intensity and lifetime under vacuum compared to that of air suggested the involvement of triplet states.The increasing emission intensity with decreasing temperature confirmed the nature of emission to be phosphorescence.Note: Spectroscopic characteristics in film state is similar to that of the cocrystals.

Fig. S6 .
Fig. S6.Air and vacuum studies of A-D3 .a) Emission spectra and b) lifetime decay plot of A-D3 cocrystals.c) Emission spectra of A-D3 cocrystals at varying temperatures (λexc.= 480 nm for emission studies and, λexc.= 405 nm, λcollected = 595 nm for lifetime decay measurements).Note:The increase in the emission intensity and lifetime under vacuum compared to that of air suggested the involvement of triplet states.The shorter component of lifetime increased and the

Table S1 .
Summary of lifetime decay profiles of A-D1, A-D2 and A-D3 cocrystals under ambient conditions.

Table S2 .
Summary of lifetime decay profiles of A-D1 cocrystals in air and vacuum.

Table S3 .
Summary of lifetime decay profiles of A-D2 cocrystals in air and vacuum.

Table S4 .
Summary of lifetime decay profiles of A-D3 cocrystals in air and vacuum.

Table S5 .
Summary of lifetime decay profiles of A-D1 cocrystals at varying temperatures.

Table S6 .
Summary of lifetime decay profiles of A-D2 cocrystals at varying temperatures.

Table S7 .
Summary of lifetime decay profiles of A-D3 cocrystals at varying temperatures.

Table S8 .
Summary of lifetime decay profiles of A-D1, A-D2 and A-D3 CT complexes in film state.