One‐dimensional molecular co‐crystal alloys capable of full‐color emission for low‐loss optical waveguide and optical logic gate

The luminescence color of molecule‐based photoactive materials is the key to the applications in lighting and optical communication. Realizing continuous regulation of emission color in molecular systems is highly desirable but still remains a challenge due to the individual emission band of purely organic molecules. Herein, a novel alloy strategy based on molecular co‐crystals is reported. By adjusting the molar ratio of pyrene (Py) and fluorathene (Flu), three types of molecular co‐crystal alloys (MCAs) assemblies are prepared involving Py‐Flu‐OFN‐x%, Py‐Flu‐TFP‐x%, Py‐Flu‐TCNB‐x%. Multiple energy level structure and Förster resonance energy transfer (FRET) process endow materials with tunable full‐spectra emission color in visible region. Impressively, these MCAs and co‐crystals can be successfully applied to low optical loss waveguide and optical logic gate by virtue of all‐color luminescence from blue across green to red, together with smooth surface of one‐dimensional microrods, which show promising applications as continuous light emitters for advance photonics applications.

tetracyanoquinodimethane (TCNQ), [30,31] and so on).Binary donor/acceptor (D/A) systems would be also beneficial for continuous and regular molecular packing, which would provide additional electron transfer pathway for optoelectronic systems. [32]However, the typical luminescent co-crystals usually possess individual emission band.In other word, tunable emission color and high signal transmission capability could only be achieved by selecting lots of raw materials, which will result in a significant increase in cost.Hence, constructing continuous full-color emitters based on co-crystals still remains a huge hindrance.
[35][36] Owing to the continuous tunable energy levels and high structural compatibility, OAs have drawn great attention in the field of organic semiconductors, [37][38][39] organic photovoltaics (OPVs), [40][41][42][43][44][45] organic heterostructures (OHSs), [46][47][48][49][50][51][52] and among many others.Notably, as for luminescent materials, continuous change of composition could not only tune the emission color, [53,54] but also further regulate the quantum yield and excited-state lifetime. [55]Benefiting from advantages of two categories of materials above, effective combination of both co-crystals and alloys could provide a new idea for the design of photofunctional materials, named as molecular co-crystal alloys (MCAs).Typically, MCAs as ternary alloys can be abbreviated to A x B y C x+y (A and B are electron donor while C is acceptor, and the distribution of A and B is random), in which the energy levels can be controlled between A-C and B-C co-crystals.However, the synthesis method of MCAs is harsher for the following two reasons.Firstly, owing to the high requirement for crystallinity of OAs, molecular sizes of A and B need to possess high similarity to avoid breaking lattice drastically.Secondly, the binding energy between A-C and B-C needs to be close, or phase separation will occur easily to form A-C and B-C co-crystals separately. [56,57]Based on these dilemmas, the effective MCAs are still rather rare to date.
In this work, we report a generalizable electron donor substitution strategy for the construction of new types of MCAs.We have selected three kinds of pyrene-based (Py-based) co-crystals including Py-OFN, Py-TFP, Py-TCNB as basic units, and fluorathene (Flu) as substitute for Py because of the similar molecular size with Py, and close binding energy with OFN, TFP and TCNB.The formation of three kinds of MCAs (Py-Flu-OFN-x%, Py-Flu-TFP-x%, Py-Flu-TCNB-x%, in which x represents the substitution ratio of Flu) is driven by both arene-perfluoroarene (AP) and charge-transfer (CT) interactions through a mixed solution self-assembly method.Owing to the different energy level characteristics of these two electron donors, continuously tunable emission wavelength could be achieved by the highly adjustability of stoichiometry.Furthermore, MCAs could maintain the high crystalline degree and novel optical properties of the pristine co-crystals, including anisotropic polarization, high photoluminescence quantum yield (PLQY), 1D optical waveguide, and so on.By virtue of the full-color luminescence of MCAs, new types of low-loss optical waveguide and optical logic gate systems could be further realized.This work thus not only proposes an effective pathway for the construction of new photofunctional MCAs, but also provides an idealized platform for in-depth investigation of solid-state luminescent materials with integrated and tunable photonic performances.

Co-assembly of one-dimensional co-crystals and MCAs
We have selected Py and Flu as electron donor, while OFN, TFP and TCNB were selected as electron acceptor (Figure S1).Single-crystal X-ray diffraction (SCXRD) analysis shows that Py-OFN, Py-TFP, Py-TCNB, Flu-TFP, and Flu-TCNB crystallize in a monoclinic crystal system with the space group of P2 1 /c, while Flu-OFN crystallizes in the space group of P2 1 (Tables S1,S2, Figures S2-S7).The powder Xray diffraction (PXRD) data were in good agreement with the single crystal simulation results, which show high phase purity of co-crystals (Figure S8).By means of the crystallographic data of pristine co-crystals, the growth morphologies were also simulated (Figure S9).The crystal faces shown in PXRD have a high degree of consistency with corresponding simulated morphologies (Table S8).In these co-crystals, strong CT and AP interactions allow D/A pairs to arrange orderly, which are self-assembled into a parallel manner.According to the calculation result of electrostatic potential (ESP), the minimal point of ESP is located on the surface of Py and Flu, while the maximal point of ESP is located on the surfaces of OFN, TFP and TCNB, which provide the prerequisites for parallel arrangement for D/A pairs (Figure S10).Furthermore, we used independent gradient model based on Hirshfeld partition (IGMH) method to get a deeper insight into intermolecular interactions. [58]As shown in Figure S11, van der Waals force plays an important role in co-crystals.The shape of isosurface has a high consistency with acceptor molecules, which indicates that the mutual attraction between D/A pairs is the basis of parallel configuration.
As aforementioned, similar molecular size and binding force are two prerequisites for forming MCAs.The transverse length of Py is 8.89 Å and the longitudinal length is 6.65 Å (Figure 1A), while the transverse length of Flu is 8.36 Å and the longitudinal length is 6.36 Å (Figure 1B).They are very similar in size and both have high axisymmetric symmetry, suggesting the rationality of the formation of MCAs.The π-π stacking distance in OFN-based co-crystals is about 3.35 Å, while the π-π stacking distance of TFP and TCNBbased co-crystals is about 3.55 Å.The distance between D/A pairs with the same acceptor is pretty close.Furthermore, according to the calculation result of interaction energy, we could conclude that the binding energies between Py, Flu and three electron acceptors are very close, whose difference is less than 0.4 kcal/mol (Figure S12).These conditions lay the foundation for the replacement of Flu in Py-based co-crystals.Therefore, we propose a mixed solvent evaporation method to synthesize MCAs.Typically, 1 mL monomer solutions containing different acceptors and different molar ratio of Flu and Py in tetrahydrofuran (THF) were quickly injected into 2 mL isopropyl alcohol (IPA) and then stirred uniformly.The evaporation time of mixed solvent is significantly longer than that of single solvent system, which can be attributed to the strong hydrogen bond between THF and IPA (Figure 1C,D).Compared with THF or dichloromethane (DCM) with higher saturated vapor pressure, this method could enable multiple components to self-assemble into one crystal slowly and evenly, which would improve the quality of crystal and prevent phase separation efficiently.
Nuclear magnetic resonance (NMR) spectroscopy was performed to detect the multi-component content of MCAs.Firstly, we tested the 1 H-NMR spectra of pristine Py and Flu.As shown in Figure S13, the 1 H spectrum of Py shows a single peak at 8.20 ppm, a double peak at 8.29 ppm and a triple peak at 8.09 ppm.And the 1 H spectrum of Flu shows five peaks located at 7.43, 7.72, 7.98, 8.06, and 8.15 ppm, respectively (Figure S14).Then, we further carried on the composition analysis to these six MCAs (Figures S15-S21).By comparing the characteristic peak integral area of Flu at 7.43 ppm and Py at 8.29 ppm, it could be concluded that the content of Flu is basically the same as the feeding ratio (Table S3).Large quantity of Flu is one of the most important evidences for the formation of MCAs.We further performed the PXRD data after grinding crystals to further reveal the molecular arrangements of the possible MCAs configuration.Taking OFN-based MCAs as an example, the corresponding PXRD patterns of Py-Flu-OFN-25% and Py-Flu-OFN-50% show a great consistency with Py-OFN but an obvious difference with Flu-OFN (Figure 1E), which indicates that the alloys are almost isomorphic to the pure Py-OFN co-crystals.Random occupation of Flu at the lattice site of Py-OFN would not disturb its crystal structure.Furthermore, the peak represented (020) crystal plane of Py-OFN is located at 10.58 • , while the peaks represented (020) crystal plane of Py-Flu-OFN-25% and Py-Flu-OFN-50% shift to 10.52 • and 10.38 • , respectively, showing a slight displacement with the increasing of Flu ratio.This can be attributed to that the increasing feeding ratio of Flu breaks the homogeneous environment of Py-OFN, and the occupied volume of Flu in OFN-based MCAs is larger than that of pyrene.Similarly, the interplanar spacing of (0-11) crystal planes for TFP-based MCAs and TCNB-based MCAs are enlarged (Figure 1F,G).However, the full width at half maxima of MCAs remains almost unchanged, which indicates that the MCAs possess high crystallinity.Therefore, different from the traditional doping materials, [59,60] wide range of substituted composition would not destroy the crystallinity of MCAs system.

Polarized luminescence and FRET
Moreover, the needle-like crystal morphology of both cocrystals and MCAs indicates that they may possess high anisotropy.It further inspired our study on polarized emis-

F I G U R E 3 (A)
The PL spectra of Py-OFN@300 nm, Py-Flu-OFN-25%@365 nm, Py-Flu-OFN-50%@365 nm and Flu-OFN@365 nm.(B) The PL spectra of Py-TFP@365 nm, Py-Flu-TFP-25%@365 nm, Py-Flu-TFP-50%@365 nm and Flu-TFP@365 nm.(C) The PL spectra of Py-TCNB@365 nm, Py-Flu-TCNB-25%@365 nm, Py-Flu-TCNB-50%@365 nm and Flu-TCNB@365 nm.sion properties.The PL intensity have a strong correlation with the polarization angle and it could be well fitted by using a cos 2 θ function. [61]The anisotropic value r is further used to evaluate the linear polarization based on the formula r = (I max −I min )/(I max +I min ) (I max presents the maximum polarized PL intensity and I min presents the minimum polarized PL intensity). [62,63]As shown in Figure 3D 1).It is expected that such high anisotropy endows both 1D co-crystals and MCAs with high potential for use as all-color polarized emitting materials.
Because of the random occupation of electron D/A pairs in MCAs, two kinds of electron donors (A, B) and electron acceptor (C) would be described as ACAC, ACBC and BCBC.Among them, the chromophores ACAC and BCBC are derived from co-crystals while ACBC is assigned to the MCAs (Figure 4A).To further reveal the interactions between chromophores, we tested the UV-vis absorption spectra of Flu-OFN, Py-TFP and Py-TCNB (Figure 4B).It is noteworthy that the emission peaks of Py-OFN, Flu-TFP and Flu-TCNB overlap with the absorption spectra of Flu-OFN, Py-TFP and Py-TCNB.Furthermore, MCAs provide close distance (<100 Å) for Förster resonance energy transfer (FRET) between chromophores.Moreover, we measured the fluorescence lifetimes of three kinds of MCAs.Compared with the Py-OFN (@400 nm), Flu-TFP (@485 nm) and Flu-TCNB (@575 nm), the lifetime values of MCAs reduce significantly (Figure 4C-E), which prove the occurrence of FRET from Py-OFN/Flu-TFP/Flu-TCNB to Flu-OFN/Py-TFP/Py-TCNB. [64] We further quantified the FRET efficiency according to the equation Ep = 1−τ DA /τ D , in which τ DA and τ D represent the PL lifetime values of the energy donor in the presence and absence of the energy acceptor, respectively (Table S5). [65]The FRET efficiency for the six MCAs Py-Flu-OFN-25%, Py-Flu-OFN-50%, Py-Flu-TFP-25%, Py-Flu-TFP-50%, Py-Flu-TCNB-25% and Py-Flu-TCNB-50% are 56.2%,52.2%, 82.9%, 82.7%, 50.2% and 42.9%, respectively.Owing to the occurrence of efficient FRET between multiple chromophores, the efficiency of photon utilization for MCAs is improved so the absolute quantum yield (QY) of MCAs remains at a high level (Table S4).
To get a deep insight into the emission mechanism of MCAs, we tested the emission spectra under different excitation wavelength.It is noteworthy that Flu-OFN, Py-TFP and Flu-TCNB show excitation wavelength-dependent properties, in which the fluorescence emission undergoes a red-shift with the increase of excitation wavelength.However, Py, Flu, Py-OFN, Flu-TFP, Py-TCNB and MCAs have not possessed the excitation wavelength-dependent properties (Figures S23-S26).Loss of excitation wavelength-dependent property for MCAs could be related to FRET (Figure 4F).Upon irradiating the MCAs, photon would be absorbed by all three kinds of chromophores.Then the chromophore at the lower energy level would quickly absorb the energy of chromophore at the S n level, and photons would return to the lowest excitation level (S 1 ) through internal conversion (IC).Therefore, loss of excitation wavelength-dependent property would be explained in two ways.If the chromophore with excitation wavelength dependence is an energy donor, due to efficient FRET, the photon energy at S n level of co-crystals A will be rapidly absorbed by other chromophore, which would lead to the disappearance of the excitation wavelength dependence because the energy of high-energy luminescence center is absorbed by other chromophores.In contrast, if the chromophore with excitation wavelength-dependent property is an energy acceptor, energy transfer can only be transferred from a higher energy level to a lower energy level, and various emission behaviors would be eliminated because of single and fixed input energy.Therefore, the complex FRET behavior provides novel optical emission capabilities for MCAs.
To better understand the energy levels and electron transition processes of co-crystals and MCAs from a theoretical view, we performed time-dependent density functional theory (TD-DFT) calculations on these materials.Firstly, for co-crystals, we selected the minimum repeated unit to depict electron transition behavior during photoexcitation/emission (Table S6).As shown in Figure 5A, charge density difference (CDD) was used to analyze the transition paths of electrons. [66]During the process of the electron transition from S 1 to S 0 , it can be concluded that the frontier orbitals associated with Flu-OFN are localized on Flu, while the electron transition behavior of other co-crystals is charge transfer from electron donor to acceptor.Their frontier orbitals are also shown in Figure S27.
As aforementioned, the MCAs give rise to a new chromophore ACBC.However, the electron transition process of ACBC still remains unclear.Therefore, to investigate the electron transition process of ACBC, three kinds of MCAs were modeled and further calculated (Table S7).As shown in Figure S28, the frontier orbitals of MCAs are recombined drastically.The process of S 1-5 back to ground state S 0 is further described by CDD (Figure 5B-D).As for OFNbased MCAs, charge transfer from OFN to Flu happens in the process of S 1 back to S 0 , which shows a great difference with Flu-OFN co-crystal.In addition, charge transfer from Py to Flu emerges.Similarly, TFP-based MCAs and TCNBbased MCAs exhibit long-range electron transfer from Flu to the farthest acceptor molecule.Therefore, we figure out that the change of electron transfer routes during the emission process is the fundamental reason for the difference in the luminescence of MCAs.

One-dimensional optical waveguide and optical logic gate
Co-crystals and MCAs with regular one-dimensional (1D) needle-like morphology have great potential in optical waveguide materials and integrated optoelectronic device.Herein, we evaluated the optical waveguide properties by performing spatially resolved luminescent tests.By using the micro-area excitation laser (λ ex = 375 nm), light signal could be received at the end of the needle-like crystals.Hence, the optical-loss coefficient (R) was calculated according to the exponential decay formula I tip /I body = Aexp(−RD), where I tip and I body represent the emission intensities of the emitting tip and excited position, respectively.A refers to the ratio of energy escaping and propagating, and D is the propagation distance between the emitting tip and excited position. [67]s shown in Figure 6A-C, the R values of Py-OFN, Py-TFP and Py-TCNB were 3.30 × 10 −5 dB/μm, 1.33 × 10 −2 dB/μm and 9.99 × 10 −3 dB/μm, while those of Flu-based co-crystals were 6.30 × 10 −4 dB/μm, 2.33 × 10 −3 dB/μm and 3.53 × 10 −4 dB/μm (Figure 6G-I).Although the homogeneous environment has been broken in MCAs, they still possess low optical-loss coefficient as 3.26 × 10 −4 dB/μm, 9.64 × 10 −4 dB/μm, 6.71 × 10 −4 dB/μm (Figure 6D-F).There are two reasons for this phenomenon.Firstly, slow evaporation rate of mix solvent system would significantly reduce the crystal interfacial energy so the formation probability of defect would decrease greatly.As shown in the transmission electron microscope (TEM, Figure S29), the co-crystals and MCAs show 1D needle morphology with smooth surface, and the photon consumed by Rayleigh scattering will be cut down.Secondly, the main emission peak are slightly overlapping with absorption spectra, which indicates that crystals would not absorb the optical signal pro-duced by excitation light source (Figure S30).The optical loss coefficient of these materials is lower than those of stateof-the-art reported molecular waveguide materials (Table S9), which provides great potential for photonic communications.
As shown in Figure 7A, the output signal of optical waveguide materials is changed with the increase of excitation wavelength.Thus, excitation wavelength-dependent multi-channel photonic signals can be conveyed within the individual 1D microrod, significantly enhancing information storage and transmission capability.Therefore, taking the advantages of both all-color emission and excitation wavelength-dependent properties, we developed a high-level security optical logic gate prototype.The basic model structure of the optical logic gate is illustrated in Figure 7B.Once we start the optical logic gate system, two kinds of excitation wavelengths (λ 1 , λ 2 ) can be input simultaneously into the slab waveguiding coupler, [68] which contains two identical crystalline materials.Correspondingly, we will collect two signals at the output port (λ 3 , λ 4 ).If the output signal λ 3 and λ 4 show great consistency, the output is "1", otherwise the output is "0".Taking Flu-TFP and Flu-TCNB as examples, while Flu-TFP materials are irradiated by 375 nm and 405 nm, respectively, the output signal shows consistency so the output is "1".In contrast, Flu-TCNB possess excitation-wavelength dependent property so the output signals of different excitation wavelength show inconsistency.The output signal is "0" (Figure 7C).Therefore, we exemplified the application of the photonic logic gate by employing the excitation-dependent optical waveguide properties (Figure 7D).

CONCLUSIONS
In summary, we propose an electron donor substitution strategy based on high similarity between Py and Flu.Three new types of color-tunable MCAs (Py-Flu-OFN-x%, Py-Flu-TFP-x%, Py-Flu-TCNB-x%) were further synthesized via a mixed-solvent self-assembly method.By adjusting the molar ratio of Py and Flu, continuous fluorescence emission could be achieved covering RGB regions.Moreover, from the experimental and theoretical perspectives, the new energy level structures of MCAs are related to the efficient FRET and the recombination of the frontier orbitals.Combined with high crystallinity and all-color luminescence properties, the co-crystals and MCAs could achieve multi-color optical waveguides with rather low optical loss coefficient than most existing materials.By virtue of excellent optical waveguide and excitation wavelength-dependent properties, we further designed optical logic gate with higher security.Therefore, this work not only provides an original approach to continuously modulate the emission color for molecular co-crystals, but also supplies a strategy for optical waveguide materials with low-loss coefficient.It can be expected that these design principles can be further extended to similar MCAs system to contribute to the development of multicolor optoelectronics.

Synthesis of Py-OFN, Py-TFP, Py-TCNB
10 mL of monomer solution containing pyrene (0.05 mmol, 101.1 mg) and OFN (0.05 mmol, 136.0 mg), TFP (0.05 mmol, 100.0 mg) or TCNB (0.05 mmol, 89.0 mg)) in THF were quickly injected into 10 mL IPA.The mixed solution was sonicated for 20 min to obtain a homogeneous solution.Then the rod-shape co-crystals could be obtained within a week by slow evaporation of mixed solvent.Crystals with smaller size could be obtained by dropping 40 μL mixed solution onto the quartz substrate.Py-OFN 1

Synthesis of Py-Flu-OFN-x%, Py-Flu-TFP-x%, Py-Flu-TCNB-x%
x mmol Flu, 0.05-x mmol Py and 0.05 mmol OFN (TFP or TCNB) were dissolved in 10 mL THF (x was less than 0.025).Then THF solution was ultrasonicated for 20 min and injected into 10 mL IPA.After stirring for 20 min, the homogeneous could be obtained within a week by slow evaporation of mixed solvent.Crystals with smaller size could be obtained by dropping 40 μL mixed solution onto the quartz substrate.

Equipment and characterization
Single-crystal X-ray diffraction data of these samples were carried out on Rigaku Oxford Diffraction Supernova X-ray source diffractometer equipped with monochromatized Cu-Kα radiation (λ = 1.5406Å) at room temperature.Powder X-ray diffraction data of these samples were carried out on Shimadzu XRD-7000 (3 KW) X-ray diffractometer.Solid UV−vis absorption spectra were collected on a Shimadzu UV-3600 spectrophotometer at room temperature.Data were recorded in the wavelength range of 250−800 nm, and BaSO 4 powder was used as a standard (100% reflectance).All the relevant photoluminescence (PL) tests and time-resolved lifetime were conducted on an Edinburgh FLS980 fluorescence spectrometer.The fluorescent lifetime was measured using pulse laser with 372 nm.The PLQY values of all co-crystals were recorded by using a Teflon-lined integrating sphere (F-M101, Edinburgh, diameter: 150 mm and weight: 2 kg) accessory in FLS980 fluorescence spectrometer. 1H NMR spectra were recorded by a Bruker BBFO-400 spectrometer.Fluorescence microscopic images of crystals were taken under OLYMPUS IXTI fluorescence microscope.The ISS Q2 FLIM/FFS confocal system (ISS Inc.) was used to acquire the PL images.The system was attached to a Nikon inverted microscope, equipped with the Nikon 4×/0.2NA,10×/0.3NA,20×/0.75NAand 40×/0.95NAobjective lens and diode lasers of 375 and 405 nm were used.The images were acquired using CMOS detector from TUCSEN (model MIchrome 6) and Mosaic V2.1 software.

Calculation details
In this work, on the basis of Gaussian 16 program, [69] density functional theory (DFT) and time dependent den-sity functional theory (TDDFT) were employed for all the calculation.Structural optimization of all co-crystals were evaluated at DFT/PBE0/6-311G(d)-D3 [70] level while the energy level of singlet is carried out by TDDFT/PBE0/6-311G(d)-D3 level and sent out through wave function analysis program Multiwfn. [58]The highest occupied molecular orbitals and lowest unoccupied molecular orbitals were visualized through Vesta software. [71]Furthermore, independent gradient model based on Hirshfeld partition (IGMH) were calculated by Multiwfn program. [58]Crystal structure prediction was performed by Materials Studio package.The intermolecular interaction is carried out through E Interaction = E AB −E A −E B +E BSSE (E Interaction is the energy of intermolecular interaction between A and B, E AB is the energy of the whole system, E A is the energy of A part while E B is the energy of B part, E BSSE is the energy of basis set superposition error). [72]MCAs were modeled by Materials Studio package based on Py co-crystals and further optimized by Gaussian 16 program.

A C K N O W L E D G E M E N T S
This work was supported by the Beijing Municipal Natural Science Foundation (Grant No. JQ20003), and the National Natural Science Foundation of China (Grant No. 22275021).

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 authors declare no conflicts of interest.
The corresponding polarization angle-dependent PL intensities of (D) OFN-based co-crystals and MCAs, (E) TFP-based co-crystals and MCAs and (F) TCNB-based co-crystals and MCAs.

F
I G U R E 4 (A) Schematic diagram of the composition of MCAs.(B) Solid-state UV-vis absorption spectra of Flu-OFN, Py-TFP, Py-TCNB.Timeresolved PL-decay profiles of (C) OFN-based MCAs at 400 nm, (D) TFP-based MCAs at 480 nm, (E) TCNB-based MCAs at 575 nm.(F) Diagram of FRET mechanism in MCAs system.F I G U R E 5 (A) Charge density difference (CDD) between S 0 and S 1 for Py-OFN, Flu-OFN, Py-TFP, Flu-TFP, Py-TCNB, Flu-TCNB.(B) CDD of OFN-based MCAs.(C) CDD of TFP-based MCAs.(D) CDD of TCNB-based MCAs (Cyan represents the electron and yellow represent the hole.Electrons will transfer from the electron to the hole).

F
I G U R E 7 (A) Diagram depicting multi-excitation wavelength waveguides.(B) Basic structure of optical logic gate system.(C) The PL image of Flu-TFP (the two image above) and Flu-TCNB (the two image bottom) excited by 375 nm and 405 nm.(D) Scheme of operation of the optical logic gates.
Anisotropy value of co-crystals and OCAs.