Printable Block Molecular Assemblies with Controlled Exciton Dynamics

Creating hierarchical molecular block heterostructures, with the control over size, shape, optical, and electronic properties of each nanostructured building block can help develop functional applications, such as information storage, nanowire spectrometry, and photonic computing. However, achieving precise control over the position of molecular assemblies, and the dynamics of excitons in each block, remains a challenge. In the present work, the first fabrication of molecular heterostructures with the control of exciton dynamics in each block, is demonstrated. Additionally, these heterostructures are printable and can be precisely positioned using Direct Ink Writing‐based (DIW) 3D printing technique, resulting in programable patterns. Singlet excitons with emission lifetimes on nanosecond or microsecond timescales and triplet excitons with emission lifetimes on millisecond timescales appear simultaneously in different building blocks, with an efficient energy transfer process in the heterojunction. These organic materials also exhibit stimuli‐responsive emission by changing the power or wavelength of the excitation laser. Potential applications of these organic heterostructures in integrated photonics, where the versatility of fluorescence, phosphorescence, efficient energy transfer, printability, and stimulus sensitivity co‐exist in a single nanowire, are foreseen.


Introduction
Rational design and synthesis of heterostructures with different functional building blocks is a critical step in the fabrication of nanoscale or microscale integrated organic optoelectronics. [1]rganic heterostructures were demonstrated to have wide In the present work, we use metal-free imidazole cationic molecules as building blocks to fabricate various organic heterostructures (Figure 1a).By simply adjusting the counteranion of I − , PF 6 − , ClO 4 − or BF 4 − , imidazole crystals can display shortlived prompt fluorescence (PF) with a lifetime as low as nanoseconds, or delayed fluorescence (DF) with a lifetime on the order of microseconds, or long-lived room temperature phosphorescence (RTP) with a lifetime up to milliseconds (Figure 1b). [6,7]By utilizing crystallization-driven self-assembly methods, we fabricated a series of heterostructures composed of light-emitting PF, DF, or RTP building blocks (Figure 1c).To the best of our knowledge, the molecular heterostructures prepared and described in this work represent the first examples of simultaneously displaying PF, DF, or RTP in different building blocks.These organic crystals reported here also exhibit interesting stimuli-responsive RTP properties and energy transfer process in heterojunction. [8]In addition, we demonstrated the utilization of a 3D printing technique to precisely control the location of molecular assemblies, creating a series of programable heterostructures with fluorescent and phosphorescent blocks (Figure 1d,e).
The emission of pure crystals of 1I and 2I with I − as counteranion exhibits a large Stokes shift and RTP with lifetimes in the millisecond time regime (Figure 2b-d).As shown in Figure 2b, the emission of crystal 1I-B exhibits a significant Stokes shift of 0.8 eV, calculated from the energy difference between the absorption band at 378 nm and the emission band at 500 nm.This large Stokes shift indicates that the emission is phosphorescence in nature.The time-resolved emission spectra of 1I-B and 2I crystals were measured, as depicted in Figure 2c,d, showing their long emission lifetimes of 1.8 ms and 2.2 ms, respectively.Similarly, crystal 1I-A exhibits the same RTP emission profile as crystals 1I-B and 2I, with an emission lifetime of 3 ms (Figure S1, Supporting Information).The emission spectra of 1I and 2I crystals exhibit two vibronic peaks at 500 nm and 540 nm, respectively, which are attributed to the 3 * excited state.
This long-lived phosphorescence in the crystalline state disappears when the counteranions of compounds 1 and 2 crystals are changed from I − to ClO 4 − , PF 6 − , or BF 4 − .1X and 2X (X = ClO 4 − , PF 6 − or BF 4 − ) show blue prompt fluorescence with emission peaks at 380 and 400 nm and lifetimes in the nanosecond time regime (Figure 2e; Figure S2, Supporting Information).These results suggest that the I − counteranion plays an important role in inducing solidstate long-lived RTP.The external heavy atom effect of I − counteranion [9] is thought to turn on the RTP properties of compounds 1 and 2. 1I-A and 1I-B crystals exhibit different phosphorescence emission quantum yields, which is 1% and 12% respectively.The distance between anions and cations is closer in 1I-B crystal compared to 1I-A (Figure 2a).The closer cation-anion distance is thought to enhance the external heavy atom effects and therefore increase the emission (phosphorescence) quantum yield of 1I-B crystal compared to 1I-A.
Interestingly, the 2I+2ClO 4 and 2I+2BF 4 co-crystals exhibit multiple emission components of PF, DF, and RTP in the solidstate at room temperature (Figures 3 and S3, Supporting Information).As shown in Figure 3a-c, for co-crystal of 2I+2ClO 4 , the respective time-resolved emission spectra collected from 0-360 ns, 1-12 μs, and 1-12 ms are characteristic of PF (emission lifetime of ≈10.3 ns), DF (emission lifetime of ≈2.1 μs) and RTP (emission lifetime of ≈2.8 ms).To study the effect of I − doping on photo-physical properties, we mixed 2I and 2BF 4 compounds in different ratios (from 1:2 to 2:98, n:n, I − :BF 4 − ) to obtain cocrystals.As shown in Figure 3d, by decreasing the doping ratio of I − counteranions in the co-crystal, the relative intensity of phosphorescence to fluorescence decreases.This observation further points to the essential role of I − counteranion in the RTP process.
According to the fluorescence and phosphorescence spectra, the energy difference between the S 1 and T 1 states of the cocrystal 2I+2ClO 4 /2I+2BF 4 is calculated to be 0.8 eV.The large energy difference of 0.8 eV indicates that the DF does not originate from a TADF (thermally activated DF) process, which relies on a small ΔE S 1 −T 1 .The emission lifetimes of DF and RTP for these co-crystals do not show significant changes when the temperature changes, further indicating that the delayed emission does not originate from a thermally activated process (Figures S4 and S5, Supporting Information).The DF emission is attributed to a TTA process in the 2I+2ClO 4 co-crystal, which is based on the exciton-exciton interaction and exciton diffusion process in the solid state. [11]In co-crystals containing both I − and ClO 4 − counteranions, triplet excitons are expected to be generated in the portion containing the I − counteranion.Subsequently, the triplet exciton will diffuse into the portion containing the ClO 4 − counteranion. [10]Since the 2ClO 4 compound lacks an efficient T 1 →S 0 radiative decay channel, subsequent exciton diffusion and TTA processes will take place in this portion, producing DF emission.The emission intensity of 2I+2BF 4 and 2I+2ClO 4 co-crystals was measured by changing the excitation laser intensity.As shown in Figure S6 (Supporting Information), we observed that the slope of  em ∼ ex (emission intensity vs. excitation intensity) on the log scale is ≈0.5, showing that an effective TTA process occurs. [11]he TTA process is highly dependent on the excitation power because high excitation power can generate more excitons and promote the exciton-exciton annihilation process.Therefore, we envision that the excitation power might be an external stimulus by which the relative fluorescence and RTP intensities of 2I+2ClO 4 and 2I+2BF 4 co-crystals can be adjusted.As shown in Figure 3e,f and Figure S7 (Supporting Information), the relative emission intensities of blue fluorescence and green phosphorescence in the co-crystals can be effectively modulated by changing the excitation power from 2 mW to 212 mW using a 355 nm excitation laser.Compared with phosphorescence, high-power laser excitation produces stronger fluorescence intensity.In addition, we found that the relative fluorescence and phosphorescence intensities could be further tuned by changing the wavelength of the excitation laser (266 or 355 nm).As shown in Figure 3f, by changing the laser excitation wavelength from 355 to 266 nm and keeping the excitation power at 2 mW, a larger fluorescence peak appeared in the co-crystal.As shown in Figure S8 (Supporting Information), the 2I+2BF 4 co-crystal has stronger absorptivity at 266 nm than at 355 nm, indicating that 266 nm laser excitation can produce excitons with higher concentration than 355 nm laser excitation at the same excitation laser power.A high exciton concentration in the solid state increases the probability of exciton-exciton annihilation and thus increases the DF intensity.

Fabrication and Characterization of Heterostructures
Based on these compounds with counteranion regulated emission properties (Figure S9, Supporting Information), we fabricated a series of heterostructures that individually emit PF, DF, or RTP in each corresponding block.As shown in the PXRD patterns (Figure S10, Supporting Information), 2I and 2BF 4 compounds exhibit similar packing patterns in the crystalline states.This observation indicates lattice matching between these compounds and suggests that heterostructures can be fabricated.Through a step-by-step crystal growth approach (details in Experimental Section), we successfully fabricated multiblock hetero-architectures using compounds with I − , ClO 4 − and BF 4 − counteranions, as shown in Figures 4 and S11 (Supporting Information).For the crystallization process, we first prepared pure micro-crystals of green-light emitting 2I and blue-light emitting 2BF 4 (Figure S9).We used 2BF 4 micro-crystals as seeds to induce the growth of compound 2I.As shown by the optical and confocal microscopy images in Figure 4, we obtained micron-sized tri-block heterostructures comprising a green phosphorescent 2I shell and a blue fluorescent 2BF 4 core.We also used the initial 2I microcrystals as the seeds to induce the growth of 2I+2ClO 4 cocrystals, thus forming a heterostructure composed of 2I (core) and 2I+2ClO 4 (shell) (Figure S11, Supporting Information).As shown in the fluorescent microscopy image in Figure S11a (Supporting Information), the inner core 2I micro-crystal emits green phosphorescence, and the outer shell of 2I+2ClO 4 co-crystal emits DF, PF, and RTP simultaneously, showing aquamarine color.We then excited the blue-emitting block with a focused laser (Figure 5c, spot 1), and then recorded the PL images and PL decay kinetics (Figure 5d-f) at the excited location (Figure 5c, spot 1) as well as two additional sites (Figure 5c, spots 2 and 3).We used filters to obtain the PL images and PL decay kinet-ics of the heterostructure with fluorescent blocks in the wavelength range 393-425 nm (PF) and phosphorescent blocks in the wavelength range 490-550 nm (RTP).As shown in the PL intensity changes in Figure 5d,e, the blue fluorescent emission gradually disappears in the heterojunction at x ≈40 μm, while the green phosphorescent emission increases significantly at the heterojunction (PL intensity in the red bracket of Figure 5e), indicating efficient blue-to-green emission energy transfer process.1a] (b) Singlet excitons in 2BF 4 crystal lead to a blue light emission.Blue light propagates at the crystal interface through the optical waveguide effect, which is achieved through light reflection.Then, a reabsorption process occurs at the 2BF 4 -2I heterojunction, where the green 2I crystal reabsorbs the blue light and subsequently emits low-energy green phosphorescence. [3]To differentiate these two mechanisms, we recorded and compared the PL kinetics obtained at spots 1, 2, and 3 by exciting spot 1.As shown in Figure 5f, the rising edge of the PL dynamics of the shell (spot 3) does not appear any time delay compared to the PL dynamics recorded by the body (spot 1) excitation.This suggests that the energy transfer process from fluorescent block to the phosphorescent block is due to the waveguide effect (mechanism b) and the process is close to the speed of light.The fast PL decay observed at spots 2 and 3 in Figure 5f is attributed to the waveguided fluorescence of the 2BF 4 core, which has a minor contribution in the emission range of 490-520 nm (Figure 3a). Figure 5g depicts a mechanistic schematic of the energy transfer process in the heterojunction of the 2BF 4 @2I heterostructure.

3D Printing
The ability to construct complex microstructures at designated positions is crucial for the development of future highperformance photonic circuits.Herein, we further demonstrate the sequential 3D printing of microscale heterostructures composed of 2BF 4 , 2I crystals, and 2BF 4 +2I co-crystals by utilizing the rapid evaporation of solvent in the femtoliter volume-limited meniscus without adding additives.
Figure 6a shows the corresponding snapshot during the printing process taken from the recorded video (Video S1, Supporting Information).We used borosilicate micro nozzles (controlled size of 1 to 8 μm diameter) prepared by thermal-pulling (thermal heating and pulling) as a reservoir and carrier (Figure 6a1).A meniscus is formed when the micro nozzle contacts the glass substrate (Figure 6a2).During the movement of the micro nozzle, the rapid evaporation of the solvent within the confined space enables the printing of free-form structures without the help of additives (Figure 6a3).The micro nozzle abruptly moves at a speed of 500 μm s −1 , momentarily terminating the printing process (Figure 6a4).The printed diblock microwires shown in Figure 6a are composed of 2BF 4 at the bottom and 2I at the top, respectively.
Control of solution wetting and printing speed is a prerequisite for reliable multi-step printing of heterostructures.We checked the printability of the solution based on the solvent composition and printing speed (Figure 6b).Stable printing is achieved under specific conditions (O).However, when using 40:60% (v/v) acetonitrile-water solution, the grown microstructure detached from the surface as the micro nozzle moved, as shown in Figure S12 (Supporting Information).On the other hand, when acetonitrile was used as a single solvent, it was found that the printing process was unable, presumably because the wetting (X) of acetonitrile solvent was too large to form solidified structure in the micro nozzle-confined space.We hypothesize that the wetting of the prepared solution on the substrate and the supersaturation of the solution in the meniscus determine the printability, as summarized (Figures S13 and S14, Supporting Information).
Our technique enables precise control over the length, width, shape, and array patterns (Figure 6c).We demonstrate printing with controlled lengths ranging from 20 to 40 μm and widths ranging from 1 to 8 μm (Figure 6d; Figure S15, Supporting Information).In addition, the programed branched structures (Figure 6e-g) and 3 × 5 diblock heterostructure arrays (Figure 6h,i) demonstrate the feasibility of 3D printing to construct complex structures and patterns.Under confocal microscope, the printed heterostructures in a 3 × 5 array showed blue emission from the bottom 2BF 4 crystals and green emission from the top 2I crystals (Figure 6j; Video S3, Supporting Information).The 2I and 2BF 4 blocks in the printed heterostructures were further characterized by the Transmission Electron Microscope (TEM) images and corresponding Selected Area Electron Diffraction (SAED) patterns, as shown in Figure S16 (Supporting Information).The planes of SAED patterns were carefully indexed based on the obtained d-spacing and PXRD results.The color mixing in Figure 6g,j is believed to occur due to the diffusion process of molecules (2I) when the micro nozzle containing the solution comes into contact with the printed 2BF 4 block.The heterostructure fabricated using self-assembly methods and 3D printing techniques exhibits waveguided light (Figure S17, Supporting Information), with sub 100 nm surface roughness, as shown in Figures S18 and S19 (Supporting Information), highlighting its application potential in the field of integrated optical waveguides.

Conclusion
In summary, we achieved PF, DF, and RTP in metal-free organic crystals by modulating the counteranions of a series of imidazole cationic molecules.The relative intensity between the fluorescent and phosphorescent emission components can be manipulated by external stimuli, including the power and wavelength of the laser excitation.A series of heterostructures based on these organic cations have been fabricated through controllable crystallization-driven self-assembly method or 3D printing technique.These heterostructures display PF, DF, or RTP in different building blocks, where efficient energy transfer processes in the heterojunction are probed.We expect these findings to expand the potential and practical applicability of molecular heterostructured materials.

Experimental Section
Synthesis of Compound 1 and 2: All starting materials were purchased from commercial sources and used as received.Unless otherwise stated, the solvents used for the synthesis were of analytical grade.Electrospray Ionization (ESI) mass spectra were obtained on a Bruker maXis II high resolution Q-TOF mass spectrometer (samples were dissolved in HPLC grade acetonitrile).NMR spectra were measured at 298 K on a Bruker DPX-500 or DPX-400 spectrometer.Elemental analyses were performed at the Institute of Chemistry, Chinese Academy of Sciences, Beijing.All reactions were performed under air atmosphere unless otherwise stated.Detailed experimental procedures and characterizations of the samples are provided in Supporting Information.
Seed preparation of 2BF 4 and 2I: the blue-emitting 2BF 4 and greenemitting 2I seeds (core part) were prepared by slowly diffusing diethyl ether vapor into concentrated DMF solutions of 2BF 4 and 2I, respectively.Micro-seeds of 2BF 4 and 2I were obtained after the process of washing and ultra-sonication.
Suspension Preparation of 2I and 2I+2ClO 4 : The 2I or 2I+2ClO 4 solution was prepared by dissolving 2I or 2I+2ClO 4 (5mg) into 2 mL of CH 3 CN, respectively.A cloudy suspension was then obtained by adding 0.5 mL of H 2 O (poor solvent) into their respective CH 3 CN solution.
Preparation of the Heterostructures: The dried 2BF 4 and 2I crystalline seeds were added to suspensions containing 2I and 2I+2ClO 4 , respectively.The suspensions were then left to stand at room temperature for two days.During this time, the shell blocks gradually grew at both ends of the micro-crystal core, resulting in the formation of a core-shell heterostructure.
Time-Resolved Photoluminescence (PL) Spectrum Measurements: PL measurements were performed on a home-made PL-scanning imaging microscope equipped with a time-correlated single photon counting (TCSPC) module.For focused illumination imaging measurements, the excitation laser beam was focused on the sample at 375 nm wavelength (PDL 800-B, PicoQuant, Germany) through a 100 × air objective lens (NA = 0.95, Olympus PLFLN 100 ×).By parking the excitation laser spot at a specific position, rapid scanning of the galvanometer mirror ensured that photons emitted from the entire crystal were collected.For wide-field illumination PL imaging measurements, the excitation laser beam was un-collimated in front of a 10 × (NA = 0.4) objective lens to form a uniform excitation spot to excite the entire crystal.Each scanned image contained 256 × 256 pixels.Fluorescent signals were collected using two high speed detectors (HPM-100-50 and HPM-100-40, Hamamatsu, Japan).Band pass filters 393-425 nm were selected for PF and 490-550 nm for RTP PL collection.
3D Printing Methods: 3D printing of freestanding of 2BF 4 and 2I heterostructures was performed on glass substrate at room temperature, 40% RH (relative humidity), using a home-built 3D printing platform.2BF 4 or 2I at a concentration of 1.5 mg mL −1 in various acetonitrilewater mixtures was used as solution link. Figure 1d shows a schematic diagram of the 3D printing platform.Heat-pulled borosilicate micro nozzles were used as reservoirs and carriers for the solutions required to print heterostructures.A motorized stage (resolution: 100 nm) precisely positioned the micro nozzle and substrate at specified x-, y-, and z-axis coordinates.A charge-coupled device (CCD) camera attached to a 50 × objective lens aligned with light illuminating the substrate in parallel provided real-time images, enabling sequential printing of heterogeneous organic crystal structures.
Borosilicate glass micro nozzles with different diameters (1 to 8 μm) were fabricated using a commercialized heat-pulling machine (P-97, Sutter Instruments).Custom software controlled the assembled motorized stages (Khouzu precision) to move the solution-filled micro nozzles and the substrate to the desired position at specified speeds to perform the 3D printing process.A CCD camera (Blackfly, Teledyne FLIR) attached to a 50 × infinity-corrected objective lens (NA = 0.55, Mitutoyo) allowed observation and recording of the printing process.

Figure 1 .
Figure 1.a) Crystal structures of compound 1 and 2. Grey atom: C, blue atom: N, yellow-green atom: F. H atoms were not shown here for clarity.b) Scheme showing PF, RTP and DF of compound 2 obtained by changing the counteranions, and photographs of the crystals upon photo-excitation ( ex = 365 nm).The energy level of the S 1 and T 1 states of compound 2 are determined by the 0-0 transition energies measured by time-resolved PL spectroscopy.TTA: triplet-triplet annihilation, ISC: intersystem crossing.c) Scheme of preparing organic heterostructures by crystallization-driven self-assembly method.d) Schematic diagram of the 3D printing platform, including the moving part of the micro nozzle and substrate controlled by the assembled motorized stages, and the observation part with objective lens and CCD camera that collects the transmitted light.e) Schematic of the two-step printing process for 2BF 4 and 2I diblock heterostructures (The arrow indicates the movement direction of the micro nozzle).

Figure 6 .
Figure 6.a) Optical microscopy images of the two-step printing process for 2BF 4 (bottom) and 2I (top) diblock heterostructure.b) Printability chart depending on printing speed and acetonitrile concentration.c) Schematic illustration of the length-, width-, shape-, and array pattern-controlled microstructures prepared using a 3D printing technique.d-g) Optical and confocal images of printed micro-wires with controlled length, width, shape, and pattern made from compound 2I, 2BF 4 , and 2I+2BF 4 .h) SEM, i) optical, and j) confocal images of 3 × 5 heterostructure arrays printed using compounds 2I (top) and 2BF 4 (bottom).The image j of samples at different rows was recorded by adjusting the focal plane of the microscope.
,b were both fitted by a biexponential decay function in the form ofI = A 1 * exp(− t  1 ) + A 2 * exp(− t  2

Table 1 .
Fitting parameters of the PL kinetics shown in Figure5a(core) and 5b (shell).