A new insight into aggregation structure of organic solids and its relationship to room‐temperature phosphorescence effect

In order to improve the performance of organic luminescent materials, lots of studies have been carried out at the molecular level. However, these materials are mostly applied as solids or aggregates in practical applications, in which the relationship between aggregation structure and luminescent property should be paid more attention. Here, we obtained five phenothiazine 5,5‐dioxide (O‐PTZ) derivatives with distinct molecular conformations by rational design of chemical structures, and systematically studied their room‐temperature phosphorescence (RTP) effect in solid state. It was found that O‐PTZ dimers with quasi‐equatorial (eq) conformation tended to show stronger π‐π interaction than quasi‐axial (ax) conformers in crystal state, which was more conducive to the generation of RTP. Based on this result, a multi‐level structural model of organic solids was proposed to draw the relationship between aggregation structure and RTP effect, just like the research for the structure‐property relationship of proteins. Using this structural model as the guide, boosted RTP efficiency from 1% to 20% was successfully achieved in the corresponding host‐guest doping system, showing its wide applicability.

Thus, for the further development of related materials, a deep understanding of the relationship between luminescent property and aggregation structure is of great importance.However, the structures of aggregates are usually rather complex, which has largely hindered the clarification of corresponding structure-property relationship.
In recent years, the research of organic room-temperature phosphorescence (RTP) materials has been a hot topic for their great values in both theoretical research and practical application.[44][45][46][47][48][49][50][51][52][53][54] Similarly, the research of RTP materials also suffered a lot from the complex aggregation structure of organic solids.If the aggregation structure can be classified as multi-levels, the corresponding structure-property relationship will be much easier to be revealed, just like the study of proteins with similar complex structure (Figure 1A).
Herein, we designed and synthesized five 10-phenyl-10Hphenothiazine 5,5-dioxide derivatives, and carefully studied their relationship between aggregation structure and RTP property in crystal state (Figure 1B).Particularly, in the design of their chemical structures, the methyl substituent that could provide steric constraints was introduced to different positions of target compounds, which then led to the distinct aggregation structures and RTP performances.Among them, no methyl group or the methyl on the o-position of 10-phenyl substituent led to the quasi-equatorial (eq) conformation, which caused shorter π-π distance and larger π-π overlap within molecular dimers, while the methyl on the 1-position of phenothiazine 5,5-dioxide core resulted in quasi-axial (ax) conformation, then induced longer π-π distance and much smaller π-π overlap in crystal state.Although similar phosphorescence property was obtained for these target compounds in solution at 77 K, the eq-conformers were found to show much better RTP performance than ax-ones in crystal state for their different aggregation structures.Further on, by utilizing eq-conformer of phenothiazine 5,5-dioxide (O-PTZ-H (eq), O-PTZ-H-1Me (eq) or O-PTZ-H-2Me (eq)) as host to construct doping systems with different phenothiazine guests, much better RTP performance was obtained in the systems with both eq-conformations for host and guest, which was considered to be derived from the more efficient host-guest interactions in aggregation.[57][58] The low-level structure influences the formation and composition of higher-level structures, while the tertiary structure determines the final luminescent property, just like proteins.Based on this model, the relationship between aggregation structure and RTP property in this work was well clarified, which also showed great implications for the study of other functional materials in solid state.

RESULTS AND DISCUSSION
Five 10-phenyl-10H-phenothiazine 5,5-dioxide derivatives of O-PTZ-1Me-H (ax), O-PTZ-2Me-H (ax), O-PTZ-H (eq), O-PTZ-H-1Me (eq) and O-PTZ-H-2Me (eq) were designed by introducing different amounts of methyl group to the 1position of phenothiazine 5,5-dioxide core or the o-position of benzene substituent.It was expected that the different steric effects of the methyl group in different positions could lead to the distinct molecular conformations for these target compounds.As shown in Scheme S1, these compounds could all be easily synthesized within four steps, regardless of the relatively fine chemical structure regulation.The molecular structure and purity of them were certified by the combination of 1 H/ 13 C NMR spectra, high resolution mass spectra (HRMS), single crystal structures and high performance liquid chromatography (HPLC).Then, the relationship between aggregation structure and RTP property was systematically studied for their crystals.Firstly, we cultured the crystals of these five phenothiazine 5,5-dioxide (O-PTZ) derivatives by solvent evaporation, and analyzed their single crystal structures in detail (Table S1).Just like our previous work about phenothiazine (PTZ) derivatives, [59] these target compounds tended to form two distinct molecular conformations in solid state, due to the different positions of methyl (Figures 2A,B, S2 and S3).Specifically, when one or two methyl groups were introduced into the 1-position of the O-PTZ core, the resultant compounds O-PTZ-1Me-H (ax) and O-PTZ-2Me-H (ax) presented ax-conformation in the crystals.As for O-PTZ-H (eq), O-PTZ-H-1Me (eq) and O-PTZ-H-2Me (eq) with no methyl group or methyl group in the o-position of phenyl substituent, they gave typical eq-conformation.Thus, the regulation of molecular conformation for phenothiazine 5,5dioxide derivatives was successfully realized through rational design of chemical structure.To the best of our knowledge, this should be the first time to observe and regulate these two characteristic conformations in phenothiazine 5,5-dioxide derivatives.
Besides, the relationship between chemical structure and molecular conformation was certified in theory.The calculations of potential surface scanning for these five target compounds were carried out with the torsion angle between phenothiazine 5,5-dioxide and benzene substituent acting as scan coordinate (Figure S1).As shown in Figures 2C,D, compound O-PTZ-2Me-H (ax) tended to present ax-conformation at the minimum potential energy points, while O-PTZ-H-2Me (eq) favored eq-one.Similar results were obtained in O-PTZ-H-1Me (eq) and O-PTZ-H (eq) (Figures S2d and S3b).For compound O-PTZ-1Me-H (ax), the intramolecular hydrogen bond (-O⋅⋅⋅H-) between phenothiazine 5,5-dioxide and benzene substituent made it to deviate a little bit from typical ax-conformation in single molecular state, while its influence would diminish in crystal (Figure S2c).This suggested that different positions of methyl group would play different constraint effects on the rotation between phenothiazine 5,5-dioxide and benzene groups, then resulting in distinct molecular conformations.Therefore, the design of chemical structure to control molecular conformation of phenothiazine 5,5-dioxide derivatives was well verified in theory.
Subsequently, the molecular packing states corresponding to different conformations were obviously different (Figure S4).Taking O-PTZ-H-2Me (eq) as an example, the dihedral angle between two benzene rings in phenothiazine 5,5-dioxide core was measured to be as large as 154.60 • in crystal state, while that of O-PTZ-2Me-H (ax) was just 131.83 • .This indicated the more planar phenothiazine 5,5dioxide group in eq-conformer, which would be conducive to enhance the intermolecular π-π stacking.According to our previous research results, the strong π-π stacking would lead to a longer RTP lifetime by decreasing the radiative decay rate (k P ) and non-radiative decay rate (k P,nr ) from T 1 to S 0 state (Figure S5). [60]Therefore, this is a packing mode heavily related to RTP effect.As a consequence, much shorter π-π distance and larger π-π overlap within molecular dimer were observed in O-PTZ-H-2Me (eq) crystal, in comparison with its analogue of O-PTZ-2Me-H (ax) (Figure 2A).Similar results could be found in other target compounds (Figure S4).Based on them, it could be concluded that the molecular F I G U R E 3 (A) Phosphorescence spectra and (B) time-resolved phosphorescence-decay curves of O-PTZ-1Me-H (ax) (@416 nm), O-PTZ-2Me-H (ax) (@418 nm), O-PTZ-H (eq) (@405 nm), O-PTZ-H-1Me (eq) (@406 nm) and O-PTZ-H-2Me (eq) (@395 nm) solutions at 77 K (concentration: 10 −5 M). (C) Phosphorescence spectra and (D) time-resolved phosphorescence-decay curves of O-PTZ-1Me-H (ax) (@520 nm), O-PTZ-2Me-H (ax) (@513 nm), O-PTZ-H (eq) (@518 nm), O-PTZ-H-1Me (eq) (@510 nm) and O-PTZ-H-2Me (eq) (@525 nm) crystals at 298 K. packing and the corresponding aggregation structure of organic solids could be determined by the molecular conformation to some extent.Thus, if the resultant RTP properties well correspond to aggregation structures, the development of efficient RTP materials would be achieved by direct design of molecular chemical structure.
Accordingly, the phosphorescence properties of these five compounds in different states were studied carefully (Figure 3 and Figures S6-S7).As shown in Figure 3A, when these compounds were dissolved in dichloromethane (DCM) solution at 77 K, deep blue phosphorescence around 400 nm could be detected.Time-resolved phosphorescence decay curves indicated that these solutions showed similar phosphorescence lifetimes around 0.20 s, regardless of their distinct molecular conformations (Figure 3B and Figure S7).When it turned to crystal state, much different RTP performances were presented for the different conformers (Figures S8-S13, Tables S2).Among them, eq-conformers (O-PTZ-H/O-PTZ-H-1Me/O-PTZ-H-2Me) showed obvious green afterglow after turning off the UV (ultraviolet) irradiation for their long RTP lifetimes (@∼510 nm) of 60.57 ms, 5.07 ms and 54.69 ms (Figures 3C and 3D).As for the ax-ones of O-PTZ-1Me-H/O-PTZ-2Me-H, their RTP lifetimes (@510 nm) were just 11.74 μs and 11.45 μs, thus no afterglow could be observed.After careful comparison between solutions and solids, it could be concluded that much different RTP properties of these organic solids were mainly caused by their different aggregation structures, especially for the different intermolecular π-π overlaps, rather than the only chemical structure or molecular conformation.
Besides, dual RTP emissions were observed for their solids (Figure 3C).Coupled with their single crystal structures, it could be inferred that the phosphorescence peak around 510 nm should come from the molecular dimer with π-π stacking and the one around 420 nm from monomer.Also, the lifetimes of monomer RTP emissions are all shorter than the ones of dimers, especially for the O-PTZ derivatives with eq-conformation.This indicated the formation of molecular dimer with π-π stacking was much beneficial to the realization of persistent RTP emission.Besides, the ratios of monomer phosphorescence in ax-conformers were obviously larger than the crystals with eq-conformations, for their much weaker intermolecular π-π stacking.
Further on, the phosphorescence properties of their crystals were measured at 77 K.[63][64] Specifically, their main phosphorescence peaks all located at around 450 nm, which corresponded to the monomer phosphorescence.It was thought that the low temperature restrict the formation of excimer, thus leading to the large decrease in dimer phosphorescence (∼510 nm), just like previous works. [16]urther on, the samples with poor crystallinity were prepared through grinding their crystal or heat melting-rapid cooling (Figures S16-S19).The measurements of their luminescent properties found that dual RTP emissions could still be observed for nearly all of them, and no obvious change occurred for the corresponding lifetimes.This indicates the short-range intermolecular interactions (i.e., π-π interaction) should be main responsible for their RTP property, and the dual RTP emissions should be the intrinsic behavior of molecular dimer with π-π stacking, rather than defect sites or inhomogeous crystal.
To paint a clear picture of the relationship between aggregation structure and RTP property, a model of multilevel structures was proposed for organic solid, namely chemical structure/molecular conformation/molecular aggregation, corresponding to the primary/secondary/tertiary structure of proteins (Figure 4).Among them, chemical structure involves the atomic structure and corresponding bonding in the molecule itself.The symmetry, topology, etc., exhibited by a molecule are denoted as molecular conformation.Molecular aggregation combines the characteristics of molecular conformation, crystal packing and the state of matter etc.In this model, the low-level structure influences the formation and composition of higher-level structures, while the tertiary structure determines the final luminescent property, just like proteins.
To further certify the wide applicability of this structural model, the doping systems were constructed, in which eqconformers of phenothiazine 5,5-dioxide with better RTP performance acted as host and phenothiazine derivatives with distinct molecular conformations as guest (Figure 5, Figures S20-S25 and Table S3).The preparation of doping system was to dissolve the host and guest molecules with a certain proportion in the mixed solvent of dichloromethane (DCM) and petroleum ether (PE), then slowly evaporating to obtain co-crystals.According to our previous work, efficient intermolecular interactions between phenothiazine 5,5-dioxide and phenothiazine derivatives in doping system could lead to the formation of exciplex, thus promoting the resultant RTP efficiency. [7]It was a pity that the evidence to demonstrate the host-guest interactions in doping system was hard to be obtained.Since the host and guest with similar chemical structure and molecular conformation are easier to interact with each other, the exciplex-based RTP will be more likely to be produced in the doping system with eq-type phenothiazine as guest.With it, the host-guest interactions can also be well certified.
Just as expected, efficient RTP emission was more likely to be obtained in the system with host and guest molecules both in eq-conformations, for the efficient intermolecular interaction between them (Figure 5 and Figure S23).For example, when O-PTZ-H-2Me (eq) acted as host, boosted RTP efficiency up 20% could be obtained in the doping sys-tem with its analogue of PTZ-H-2Me (eq) as guest, while the system based on PTZ-2Me-H (ax) guest just presented low efficiency of 1%.Powder X-ray diffraction (PXRD) results showed that pure O-PTZ-H-2Me (eq) solids gave similar molecular packing to the doping system with trace of PTZ-H-2Me (eq) or PTZ-2Me-H (ax) (Figure S26).However, in comparison with pure host and guest, an additional UV-Vis absorption band ranging from 350 to 400 nm appeared in the doping system with PTZ-H-2Me (eq) as guest, while that based on PTZ-2Me-H (ax) guest was not (Figure S27).Moreover, the newly formed absorption band could well correspond to the phosphorescence excitation spectrum of this doping system, indicating the formation of triplet exciplex (Figure S28).
Besides, the temperature-dependent phosphorescence behaviors of the doping system based on PTZ-H-2Me (eq)@O-PTZ-H-2Me (eq) were studied (Figure 5C).It was found that two main phosphorescence bands existed at 100 K, in which the one at about 450 nm corresponded to the monomer of O-PTZ-H-2Me (eq) and the one at about 500 nm from the dimer of PTZ-H-2Me (eq)/O-PTZ-H-2Me (eq).Since the formation of exciplex was a typical kinetic process, increasing temperature could promote the phosphorescence emission of PTZ-H-2Me (eq)/O-PTZ-H-2Me (eq) dimer.Thus, as the temperature increased from 100 to 300 K, the ratio of the phosphorescence band at about 450 nm gradually decreased, while that at about 500 nm gradually increased.Then, their cyclic voltammetry (CV) curves were measured and the HOMO/LUMO energy levels were calculated accordingly (Figure S29).As shown in Table S4, the HOMO and LUMO levels of PTZ-H-2Me (eq) were both higher than those of O-PTZ-H-2Me (eq), suggesting PTZ-H-2Me (eq) acted as donor and O-PTZ-H-2Me (eq) as acceptor in the exciplex (Figure S30).This could be further certified by the HOMO/LUMO orbital distributions of PTZ-H-2Me (eq)/O-PTZ-H-2Me (eq) dimer, in which HOMO mainly located on PTZ-H-2Me (eq) and LUMO on O-PTZ-H-2Me (eq) molecule (Figure 5F, Figures S31 and S32).Because of the formation of exciplex with intermolecular charge transfer, the intersystem crossing (ISC) between triplet and singlet states could be largely enhanced, thus resulting in the much promoted RTP efficiency (Figures 5D,E and Figure S33).Similar results could be observed for the host-guest doping systems based on the hosts of O-PTZ-H-1Me (eq) and OPTZ-H (eq) (Figure S34).To further clarify the mechanism for the enhanced phosphorescence of the doped sample, the photophysical parameters of the systems based on O-PTZ-H-2Me (eq) crystal, PTZ-2Me-H (ax)@O-PTZ-H-2Me (eq), PTZ-H-2Me (eq)@O-PTZ-H-2Me (eq) were calculated.As shown in Table S5, O-PTZ-H-2Me (eq) crystal gave a small radiative rate (k P ) of 0.16 s −1 and moderate non-radiative rate (k P,nr ) of 18.12 s −1 for RTP emission.When O-PTZ-H-2Me (eq) acted as host and PTZ-H-2Me (eq) as guest, the resultant doping system gave a much increased k P of 4.75 s −1 for the formation of triplet exciplex, while its k P,nr retained at 18.95 s −1 .As for the doping system with O-PTZ-H-2Me (eq) as host and PTZ-2Me-H (ax) as guest, its k P,nr largely increased to 456.59 s −1 for the mismatched molecular conformation and consequent inferior intermolecular interactions.Thus, the wide applicability of multi-level structural model for organic solids could be further certified based on these doping systems, in which the low-level structure influences the formation and composition of higher-level structures, and the tertiary structure of molecular aggregation determines the final RTP property.

CONCLUSIONS
In this work, by introducing different amounts of methyl groups into the 1-position of phenothiazine 5,5-dioxide core or the o-position of benzene substituent to regulate the molecular conformation, we successfully obtained five phenothiazine 5,5-dioxide derivatives with ax-and eq-conformations respectively (Figures S35-S48).Through careful analyses of their single crystal structures, it was found that the dimers of eq-conformers tended to form shorter π-π distance and larger π-π overlap than those of ax-conformers, which was more conducive to the generation of RTP.Based on in-depth understanding of the relationship between the tertiary structure and RTP performance of organic solids, the doping systems with eq-conformers of phenothiazine 5,5-dioxide acting as host and phenothiazine derivatives with distinct molecular conformations as guest were constructed.When phenothiazine derivatives with similar eq-conformation as the guest, they could lead to the much promoted RTP performance for the efficient intermolecular interaction between host and guest.Therefore, this work carefully and systematically studied the decisive relationship between the tertiary structure and RTP performance of organic solids for the first time, which would be much helpful for the further development of other luminescent materials in solid state. [65,66]

A C K N O W L E D G E M E N T S
The authors are grateful to the National Natural Science Foundation of China (grant number: 52273191, 22235006), the Natural Science Foundation of Tianjin City (grant number: 22JCYBJC00760), the Open Project Program of Wuhan National Laboratory for Optoelectronics (grant number: 2020WNLOKF013), the starting Grants of Tianjin University and Tianjin Government, and Independent Innovation Fund of Tianjin University (grant number: 2023XPD-0014), for financial support.They thank for the support of AIE Institute, Guangzhou.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The author declares no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

F
I G U R E 1 (A)Multi-level structures of proteins; (B) multi-level structures of organic solids and resultant room-temperature phosphorescence (RTP) performances for the five target compounds.

F I G U R E 2
Single-crystal structures and HOMO/LUMO (highest occupied molecular orbital/lowest unoccupied molecular orbital) distributions of (A) O-PTZ-2Me-H (ax) and (B) O-PTZ-H-2Me (eq).Different dihedral angles based on two benzene rings in phenothiazine 5,5-dioxide resulted in different π-π distances and π-π overlaps between dimers of these two crystals.The calculations of potential surface scanning for (C) O-PTZ-2Me-H (ax) and (D) O-PTZ-H-2Me (eq), in which the torsion angles between phenothiazine 5,5-dioxide and benzene substituent acted as scan coordinates.

F I G U R E 4
The multilevel structural model of organic solids.