Flexible Förster resonance energy transfer‐assisted optical waveguide based on elastic mixed molecular crystals

Flexible molecular crystal waveguides based on elastic molecular crystals (EMCs) are essential in flexible and compact optical materials. An increased loss coefficient α due to self‐absorption is often a problem in optical waveguides (OWGs) of fluorescent chemical materials waveguiding photons in active mode. Herein, the development of anthracene‐based elastic mixed molecular crystals (EMMCs) is reported for Förster Resonance Energy Transfer (FRET)‐assisted OWG. To yield a FRET crystal system based on elastic molecular crystals, 1%–5% acceptor doping for fluorescent molecular crystals of 9,10‐dibromoanthracene 1 was successful by selecting the same regioisomer having electron‐withdrawing group, 9,10‐diformylanthracene 2, as a dopant. In addition to conversion to the mixed system, there is a difference in the elastic modulus and hardness in EMC C1 and EMMC C2@1. However, the elastic behaviour was also shown in a few percent doping of the acceptor. The α value of this EMMC, composed of 1 including 1% of 2 (0.0077 dB/μm), is much lower than that of EMC composed of 1 (0.1258 dB/μm) because of reducing self‐absorption in the FRET system. An efficient and flexible OWG was successfully developed by selecting an appropriate acceptor molecule and its low doping rate for mixed crystal construction. This method is a practical approach in various functional and flexible crystal systems.


INTRODUCTION
π-Conjugated molecular crystals work for a variety of optical applications because of the advantages of anisotropic molecular arrangement and shape, as well as the possibilities of controlling optical properties. [1]On one hand, molecular arrangements in molecular crystals can be predicted in many ways evaluating the intermolecular interactions of designed molecules. [2]Moreover, the realization of anisotropic physical properties has been reported by controlling the molecular orientation by co-crystallization. [3]The photophysical properties of molecular crystals are largely due to the πconjugation length and charge distribution inherent to the molecule.Molecular design consisting of electron-donors (D) and electron-acceptors (A) is involved in regulating the HOMO-LUMO gap based on donor-acceptor interactions. [4]owever, it is possible to control crystal properties by energy transfer from D to A by mixing different molecules or con-structions of nano-or micro-sized heterostructures. [5]To produce more versatile, adaptable, and effective photonic applications, utilizations of Förster resonance energy transfer (FRET), which is a distance-dependent energy transfer process that occurs between two fluorescent molecules, termed the D and A, is proposed.Based on the distance sensitive characteristics of FRET, fluorescent molecules have been conventionally utilized as probes in bioapplications. [6]On the other hand, tuning of optical characteristics in organic solids is becoming a big topic, recently.The emission colour tuning of optical waveguides is also an important topic. [7]FRET also is expected to have advantages for the tuning. [8]However, preparing crystals consisting of two different molecular components with a desired composition is difficult, and few reports are available. [9]lexible molecular crystals (FMCs) have received much attention in recent years. [10]  π-conjugated molecules. [11]This makes it possible to convert optical properties accompanying crystal deformation and is expected to be applied to flexible optoelectronic devices and sensors. [12]Zhang's group realized an OWG application by red emission EMC and is developing various OWG applications. [13]Hayashi et al. also developed an OWG application with green-emitting and orangeemitting EMCs. [14]Naumov et al. reported OWG with plastically bendable molecular crystals (PMCs). [15]They also applied OWG in EMC and realized sensing. [16]In addition, Zhang et al. reported OWG by elastic mixed molecular crystals (EMMCs) obtained by doping luminescent molecules to the non-fluorescent EMC. [13]More recently, Zhang reported the fabrication of doped crystals for roomtemperature phosphorescence. [17]In OWG, it is known that self-absorption in the crystal raises the value of the loss coefficient, that is, lowers OWG efficiency (Figure 1A).FRET is a potential approach to reduce this self-absorption efficiency, resulting in higher efficiency to flexible OWG (Figure 1B).To realize a highly efficient flexible waveguide based on FRET, it is necessary to design a method for creating a tunable EMMC system.11b] We assumed that molecular doping in this anthracene crystal system would be the key to realizing EMMC for FRET.We report herein the fabrication of EMMCs composed of 9,10-dibromoanthracene as host crystal and 1%-5% of 9,10-diformylanthracene as a randomly doped guest molecule.By carefully selecting the combinations of regioisomers, a statistical distribution of the dopant was obtained, and efficient flexible OWG based on FRET between the different anthracenes was achieved.

RESULTS AND DISCUSSION
The chemical structures of 9,10-dibromoanthracene 1 and 9,10-diformylanthracene 2 are shown in Figure 2A.To discuss the energy levels and transition energies for C1 and C2@1, we performed density functional theory (DFT) calculations on these anthracenes by the time-dependent density functional theory (TD-DFT) method (B3LYP/6-311G, Gaussian 03 Program).For the calculation of C1, a model generated by the crystal structure containing the adjacent three molecules was used, and the calculation results were summarized in Figure S1 and Table S1.The calculation was also performed for a model of C2@1.To prepare the calculation model, the aforementioned crystal structure was used, and the sandwiched molecule was replaced with a 9,10-diformylanthracene molecule.The calculation results were summarized in Figure S2 and Table S2.The simulated UV-vis spectra shown in Figures S1 and S2 suggested that the optical transitions in C2@1 are slightly lower energy than that of C1.Because C2@1 possibly shows red-shifted luminescence as a result of doping of 9,10-diformylanthracene, energy transfer can be expected from 9,10-dibromoanthracene to 9,10-diformylanthracene in a solid state.The crystals, C1 (triclinic, P 1), C2@1 (triclinic, P 1), and C2 (monoclinic, P2 1 /c), were prepared in a solution phase.The powders of 1, 2 (mole fraction of the mixed 2: 0.00, 0.05, 0.06, 0.08, 0.10, and 0.11), and chloroform (1.5 mL) were put into a sample tube, then heated until all solids were dissolved.The sample tubes remained at room temperature (20 ± 3 • C) to yield precipitated crystals (Figure 2B). 1 H NMR spectra of the dissolved mixed crystals reveal that the actual mole fraction of doped 2 slightly decreases from the ideal value as the mixing fraction of 2 increases (Figure 2C).The actual mole fraction could be accurately modulated because of the linear relationship between the feed ratio and the actual mole fraction of 2 as shown in Figure 2C.Microscope images are shown in Figure 2D.The crystals obtained have a needle shape with a side length (L) of 70-140 μm and a height (h)-to-L ratio of around 1/10.There is no difference in the shape of C1 and C2@1.However, branched crystals were obtained when the doping ratio was more than 10% (Figure S3).This is probably because of the appearance of another preference on epitaxial crystal growth caused by the dopant molecules.It is noted that different isomers (1,8-, 1,5-, and 2,6-disubstituted anthracenes) were not mixed in C1 during the crystallization process.Our results indicated that the regioisomers such as 9,10-dimethylanthracene and 9,10-dicyanoanthracene were doped into the crystal of 1 as in this result (Table S3).An appropriate selection of molecules based on the similarity of chemical structures and mixing ratio is required for tuning molecular crystals (Figures S4-S6).
Figure S7 shows the DSC data for C2@1.A melting point was observed as one peak at any doping ratio.The melting points of C1, C2@1 (1%−2%), and C2@1 (4%−5%) are 227 • C, 226 • C, and 225 • C, respectively.The melting point decreases with an increasing doping ratio of 2. Powder XRD data for C2@1 is shown in Figure S8.These profiles of the crystals at 1%−5% doping of 2 reveal that the same pattern as C1 was observed.
Crystallographic data of C1, C2@1, and C2, are summarized in Table 1.As reported previously, the crystal structure of C1 is triclinic P 1, a = 3.9739 • .It should be noted that there is no significant difference in the positional relationship between the molecules.However, a more detailed observation of the crystal structure reveals slightly different overlap rates in the π-π stacking between anthracenes in columns 1 and 2 (Figure 3).This is probably because doping of 2 slightly changes the stable configuration of the 9,10-dibromoanthracene-based crystal.
Figure 4A shows the absorption and PL spectra of C1 and C2@1.C2@1 shows lower energy absorption and PL spectra (λ abs onset = 525 nm, λ PL = 560.5 nm) than C1 (λ abs onset = 470 nm, λ PL = 492.0nm).It is noteworthy that PL spectra of EMMC show unimodal bands, suggesting efficient FRET from 1 to 2 in the crystal.This result makes a dramatic difference in that C2@1 exhibits orange PL while C1 emits green PL (Figure 4B).8a] We further measured PL lifetime and absolute PL quantum yield was measured for supporting FRET.The PL decay profiles shown in Figure S9 and the yields and TD-DFT simulation indicate FRET behaviour (See Supporting Information).
Upon applying and releasing stress, the needle-shaped crystal can be bent and recovered to the original shape (Figure 4C).The mixed crystal, C2@1, is also capable of bending in both directions of the b-and c-faces.Mechanical bending-relaxation motions could be performed over many cycles.The bending angle reaches more than 90 • upon stress (Figure 4C).In the bending tests, we performed the deformation for a part of the crystal (Figure 4C).Bending angles [11c,d] were estimated as angles between two straight crystal parts of the bent crystal.Nanoindentation experiments were performed for C2@1 and C1. Figure 4D shows load-depth curve measured for both crystals.To show the curve clearly for C1, the graph in different scale is shown as Figure S10.The measured elastic modulus (E) and hardness (H) of C2@1 were found to be about 6.9 and 0.049 MPa, respectively (Figure 4D).However, E and H of C1 were found to be about 1.7 GPa and 5.2 MPa, respectively.This result indicates that the stability of the crystal structure by doping of 2 had a significant effect on the elastic modulus and hardness of the crystal.We believe the decrease of H and E values is prob- ably derived from the slight decreasing of the intermolecular interaction between columns of anthracene π-π stacking.
As further characterizations of crystal morphologies, SEM images were taken for C1 and C2@1 (1%, 2%, 3%, 4%, and 5%) (Figure S11).As shown, each crystal possesses a smooth surface, which is ideal for efficient optical waveguiding based on the elimination of the possibilities of Rayleigh scattering.
Figure 5 displays the OWG performance of the crystals estimated by a spatially resolved μ-PL spectral measurement system (Figure S12).The fluorescence intensities at the excited position (I EX ) and the emitting tip (I WG ) were recorded to calculate the optical loss coefficient by single exponential fitting (I WG /I EX = Aexp(−αX).Here, X is the distance between the excited position and the emitting tip and α is the loss coefficient.The four plots of C1 were well-fitted as a single exponential decay curve with small value of error bar.The α of C1 was thus calculated to be 1258 dB/cm (Figure 5A-C).The coefficient and red-shift accompanied by the increasing of the propagation distance are mainly affected by the reabsorption during the propagation of the light along the crystal.In the region of shorter wavelengths, the PL band overlaps with the absorption band of the crystal, and the excitation spectrum of the crystal (Figure 4A), which is harmful to light propagation and results in a high loss coefficient.Waveguiding efficiency is improved by doping the small amount of acceptor 2 due to its small reabsorption.It is noteworthy that the α value estimated for C2@1 in the smallest doping ratio of 1% is 0.0077 dB/μm which is dramatically lower than that of C2@1 in the doping ratio of 2%−5% (Figures S13-S17).Since the optical loss in active OWGs is mainly due to the self-absorption processes, these results suggest that values of α decreased as a result of decreased reabsorption.In the FRET system, the main PL and absorption bands of the EMMC are well separated, which is beneficial to propagating light in the crystal and results in a low loss coefficient.
Interestingly, a lower doping rate of C2@1 (1%) results in a dramatically lower α (0.0077 dB/μm) than that of C2@1 (2%−5%) (Figures S9-S13 and Table S3).Furthermore, plots of α are estimated at each doping rate of 2 (0%, 1%, 2%, 3%, 4%, and 5%) as shown in Figure 6 and Table S4.With increasing the doping rate, α was also increased definitely, which could be interpreted as the increased self-absorption of 2 or decreased crystallinity.Han et al. achieved efficient optical waveguiding based on the redshifted emissions by CT complexes formed in mixed crystals, and the waveguiding efficiency is gradually increased with increasing the dopant ratio, which is a reasonable example to interpret our improved optical waveguiding. [9]

SUMMARY
EMMCs were successfully prepared by doping 9,10-diformylanthracene acceptor 2 into a 9,10dibromoanthracene 1 crystal.It is important to select the same regioisomer having the electron-withdrawing group as dopant and the ratio of dopant.This acceptor contributes to the tuning of physical properties showing the difference in fluorescence and elastic modulus (and hardness) by FRET in the EMMC system.Furthermore, we found that FRETmediated OWG became more efficient as a result of the decreased overlap between the absorption and PL bands.
Although the α estimated for C1 was larger than 0.1 dB/μm, those estimated for C1@2 (1%−5% doping) were one order smaller because of the red-shifted emission accompanied by FRET.We believe that this insight will become a useful guideline for constructing functional EMMC with efficient active OWG systems.

Crystal growth
Anthracenes (9,10-dibromoanthracene and 9,10diformylanthracene) purchased from Tokyo Chemical Industry Co. Ltd. (TCI) were used as received.Crystal growth was performed in solution phase.First of all, 1 (50 mg, 0.15 mmol) and 2 were mixed in a vial (20 mL).To grow C2@1, mole mixing ratio of 18:1, 15:1, 12:1, 9:1, 8:1 were used to yield crystals with dopant ratio of X = 1, 2, 3, 4, 5, respectively.Both molecular powders and 1.5 mL chloroform were mixed in a vial and heated on a hot plate to dissolve all powders, then remained at room temperature to precipitate crystals.Next, crystals were obtained by carefully removing the solution.

Measurements
UV-vis absorption spectra were obtained on V-650 (JASCO corp.).Optical microscope images were obtained by SHI-MADZU moticam 1080BMH with WUBEN E19 UV 365 nm.Data collection for X-ray crystal analysis was performed on Rigaku/R-AXIS RAPID (CuKα λ = 1.54187Å) and Rigaku/XtaLAB Synergy-S/Cu (CuKα λ = 1.54187Å) diffractometers.The X-ray measurement was performed at 93.15 or 103.15K.The structures were solved by direct methods (SHELXT) and refined through full-matrix leastsquares techniques on F 2 using SHELXL and OLEX2 crystallographic software packages.PXRD measurements were performed using a Rigaku SmartLab SE diffractometer Cu-Kα radiation (X-Ray wavelength: 1.5418 Å).DSC measurements were conducted using Shimadzu DSC-60 plus, in N 2 atmosphere with a heating or cooling rate of 20 • C/min.Liquid-state 1 H (400 MHz) nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECZ400S.Nanoindentation experiments were performed using a DUH-W201 (Shimadzu), equipped with a Berkovich tip (triangular115).Samples for indentations were observed through an objective lens (×20 or ×10) and CCD camera.Indents were performed for an isolated crystal sample placed on a glass substrate at the conditions of 0.1422 mN/s rate for loading and unloading, 3 seconds of holding time, 0.981 mN of maximum loading force.Spatially resolved μ-PL spectra were measured by an analyzing system [405 nm UV laser OptoSigma LDU33-405-3.5 (Sigma-Koki), micro-PL spectra using a USB400 and a R200-7-UV-vis probe] recorded by a CCD (detector: Sony ILX511B linear silicon CCD array).The excitation laser was filtered with a band-pass filter (YIF-BA460IFS) and focused on the microfibers with an objective (5×, NA = 0.15, or 50×, NA = 0.80).The collected emission was then guided to a spectrometer (Lambda Vision SA-100A) and recorded by a CCD (detector: Hamamatsu photonics S11151-2048 CCD linear image sensor).SEM images were obtained by HITACHI FE-SEMSU8020.

Theoretical calculation
DFT calculation was performed using Gaussian 03 suit of programs by optimization using the B3LYP/6-311G (d, p)

F I G U R E 1
Illustrations of crystal fluorescence and OWG.(A) EMCs.(B) EMMCs.Elastic FRET-assisted OWG has the potential to reduce selfabsorption and lower loss coefficient, α.

F I G U R E 2
Fabrication of the mixed molecular crystals.(A) Chemical structures of 1 and 2. (B) An illustration of the crystal growth method.(C) Plots of 2 content in the crystal on feed ratio of 2 estimated by 1 H NMR measurements.(D) Photographs of C1 and C2@1 taken under ambient light.

F I G U R E 5 F I G U R E 6
Optical waveguiding characteristics of the crystals.(A) Fluorescence microscope images taken for C1 at varied D, (B) PL spectra measured at the edges of the crystal at varied D, (C) plots of I WG /I EX with a single exponential fitting decay curve.(D) Fluorescence microscope images taken for C2@1 (1%) at varied D, (E) PL spectra measured at the edges of the crystal at varied D, (F) plots of I WG /I EX with a single exponential fitting decay curve.Plots of α estimated for each doping ratio of 2.