Self‐Assembly of Hydrogen‐Bonded Organic Crystals on Arbitrary Surfaces for Efficient Amplified Spontaneous Emission

Organic lasers attract much attention due to their high efficiency, low energy consumption, and structural flexibility. However, long‐term stability and the creation of the lasers on arbitrary surfaces remain a challenge. Here, a synthesis of amide‐based organic molecules that provides packing into hydrogen‐bonded organic crystals (OCs) is reported. The resulting OCs demonstrate an amplified spontaneous emission (ASE) regime with 0.55 μJ cm−2 threshold under the normal conditions due to 5%–13% quantum yield and high emission rate (1.02 ns). The simple process of self‐assembly of the hydrogen‐bonded OCs and highly stable ASE (over 30 min of continuous operation) allow fabricating fibers, flexible polymers, and hard planar periodic optical systems based on them, which paves the way to creating organic laser diodes of an arbitrary design.


Introduction
Since the discovery of conductive polymers, [1a] organic solar absorbers, [1b] and laser dyes, [1c] organic solids have a special place among the materials for micro-and optoelectronic applications.Compared to most of their inorganic counterparts, the low cost and simple liquid-phase manufacturing of organic solids are accompanied with high efficiency of operation and clean energy transition. [2]Along with the chemical composition, the intermolecular interactions directly determine the functionality of the organic solids through the managing of molecule packing into a crystal.An increase of this energy leads to a change in the electronic structure of the crystal compared to a single molecule, [3a,b] which opens up the possibility to tune the optical, electronic, and transport properties of the resulting organic crystal (OC).3f-k] On the contrary, a decrease in this energy by introducing hydrogen bonds [3c-e] preserves the electronic features of individual molecules and their photoemission properties, as well as expands the fashion of molecular packing and provides the control of the OC growth on arbitrary surface.1c] Organic lasers attract much attention due to their high efficiency, low energy consumption, and structural flexibility.However, long-term stability and the creation of the lasers on arbitrary surfaces remain a challenge.Here, a synthesis of amide-based organic molecules that provides packing into hydrogen-bonded organic crystals (OCs) is reported.The resulting OCs demonstrate an amplified spontaneous emission (ASE) regime with 0.55 μJ cm À2 threshold under the normal conditions due to 5%-13% quantum yield and high emission rate (1.02 ns).The simple process of self-assembly of the hydrogen-bonded OCs and highly stable ASE (over 30 min of continuous operation) allow fabricating fibers, flexible polymers, and hard planar periodic optical systems based on them, which paves the way to creating organic laser diodes of an arbitrary design.
3c-e] Possessing excellent liquid-phase self-assembly, managed by external stimuli, [4] as well as flexible low-energy hydrogen bonds, the resulting OCs offer novel solids of desired molecular packing, crystal shapes, and environmental stability [5] for optical application.4a,b,7d] However, compared to a big family of optically active OCs, also possessing ASE [8] and lasing effects (Table S4, Supporting Information), [9] the stable and highly efficient operation of the hydrogen-bonded OCs are yet in their infancy.
Here we report on the liquid-phase self-assembly of OCs based on ethyl 2-((1-((adamantan-1-yl)amino)-4-(4-methoxyphenyl)-1,4-dioxobut-2-en-2-yl)amino)-4, 5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate molecules packed into a single crystal of centrosymmetric space group P2 1 /c.Structural analysis, numerical modeling, and optical spectroscopy confirmed the hydrogen nature of molecular packing, which ensures the optical absorption and photoluminescence (PL) of individual molecules packed into a single crystal.An efficient PL (5%-13%) with high relaxation time (1.02 ns), as well as regular geometric shape allow observing ASE with a threshold of 0.55 μJ cm À2 at ambient conditions.An endurance of the obtained crystals during ASE regime, confirmed during 30 min of continuous operation, together with the crystal growth on arbitrary surfaces such as flexible polymer substrates, periodic silicon structures, and optical fibers, opens the way to the easy fabrication of scalable coherent light sources.

Synthesis
Substituted derivatives of Gewald's 2-aminothiophenes have been chosen as model structural units to build the OC with promising optical properties.10a-g] Moreover, their low toxicity and high biocompatibility [10h,i] make them perspective also for "green" optoelectronics.

Modeling
Based on structural analysis, we have performed numerical modeling using the density functional theory (DFT) to confirm the molecular packing driven by hydrogen bonding, as well as to predict the optical properties of the resulting OCs (Figure 1c-e, and Figure S7-S12, Supporting Information).DFT revealed that the presence of neighboring molecules 9 insignificantly affects the density-of-state (DOS) spectra and the PL of the OCs as well (see below).Moreover, based on the Hirshfeld surface analysis (Figure 1e, S13, S14, Supporting Information), we confirmed that the adamantan part of the molecule 9 contributed significantly to the packing of the molecules into the OC via hydrogen interactions.

Optics
Optical transmission spectroscopy confirmed the DFT results.As shown in Figure 1b, S15 (Supporting Information), the single OCs are optically transparent with an estimated bandgap of 2.3 AE 0.1 eV.Herein, both the single OC and the solution of the molecule 9 in DMF demonstrate similar optical absorption behavior.This correlates well with the structural and DFT data predicting the hydrogen interaction between the packed molecules.In turn, this interaction prevents the electron transfer between the molecules 9 and explains the agreement between the absorption spectra of the OC and the molecule solution.
An excitation of both the single OC and the molecule solution by 3.55 eV photons (350 nm, see Experimental Section) stimulated an efficient photoexcitation of electrons between the allowed energy states (Figures S7-S12, Supporting Information), leading to an intense PL signal.Herein, the PL of the OC is centered at 565 nm, while the molecule solution demonstrates PL at 533 nm with the same (100 nm) full width at half maxima (FWHM).
A detailed study of the PL showed the following (Figure 2).During excitation by 350 nm (150 fs pulse duration, 80 MHz repetition rate) of a varied laser fluence (0.01 to 0.9 μJ cm À2 ), we detected a linear increase of the PL intensity without changing the FWHM (Figure 2a,b).A further increase in the laser fluence (from 0.7 μJ cm À2 ) led to deviation from the linear slope and the irreversible PL intensity drop.Such behavior has been also analyzed by confocal Raman spectroscopy: As shown in Figure 2c, the positions of the Raman peaks and the ratio of intensities were maintained, while the intensity of the Raman peaks dropped upon the action of laser radiation with 0.7 μJ cm À2 fluence.This can be explained by the thermal decomposition of the OCs under the action of laser radiation, which possibly heats the OC above the decomposition temperature (see thermogravimetric data in Figure S5, Supporting Information) due to almost 100% absorption at 350 nm.In addition, the measurement of the PL decay showed that the single OC re-emits photons quite fast (1.02 ns compared to 1.4 ns for the solution, see Figure 2d, S18, Supporting Information), while the PL emission mapping demonstrated the homogeneous PL decay time over the OC surface (Figure 2e, Supporting Information).

ASE
An analysis of the quantum yield (QY) for the OCs showed that the QY value varies from 5% for 400 nm laser pump to 13% for 260 nm (Figure 2f ), [11] which is possibly due to an increase in the absorption coefficient in the ultraviolet region.These results made it possible to estimate the radiative (k r ) and nonradiative decay rates (k nr ).According to the definition of τ = 1/(k r þ k nr ) and φ = k r /(k r þ k nr ), [1c] where τ is PL decay time (1.02 ns in our case) and φ is the PL QY (0.058 for 350 nm laser pump, Figure 2f ), we obtained the values of k r = 0.057 ns À1 and k nr = 0.92 ns À1 .Despite the fact that the nonradiative decay rate is 16 times higher than k r , a regular geometric shape of the OC and a relatively high thermal stability (Figure S5, Supporting Information) should ensure the transition from the PL to ASE regimes.To prove it, we have utilized 354 nm laser pump at a low repetition rate (see Experimental Section).Figure 3a demonstrates that with an increase in the laser fluence (0.55 μJ cm À2 threshold, Table S4, Supporting Information), the PL intensity grows nonlinearly with a parallel decrease in FWHM (Figure 3b): 6-fold narrowing of the PL spectrum (Figure S19, Supporting Information) and 15-fold increase in its intensity directly indicate the ASE regime. [3]Important is that the endurance of ASE regime has been confirmed during 30 min of continuous photoemission (with 4% error) at ambient conditions (Figure 3d).
We have also estimated the gain parameter of the OCs: The Q factor, describing an ability of any feedback material to retain light, can be expressed as Q = λ r /Δλ, where the resonance wavelength λ r corresponds to ASE maximum (600 nm in our case), and Δλ is the FWHM value (14 nm in our case, see Figure S19, Supporting Information).In our case Q equals 42, which is not enough for lasing, [1c] but still meets the criteria for creating the laser diodes.
Finally, taking into account the simple process of self-assembly of the hydrogen-bonded OCs and their promising thermal and optical properties, we assumed that the OC could serve as an efficient optical emitter not only as independent objects but also as an integrated part of the optical systems.To check our assumption we have performed the OC self-assembly on different surfaces such as flexible polymer substrates (Figure 4a), optical fibers (Figure 4b,c), and periodic silicon structures (Figure 4d).As can be seen from the corresponding PL and ASE spectra (Figure 4e-g), the OC crystals demonstrate efficient PL/ASE under normal conditions on any surfaces.Herein, no any crumbling or degradation of the OCs during several weeks of continuous operation can be detected.

Experimental Section
Materials: All the chemical reagents were purchased from commercial sources and used without further purification unless otherwise specified.The synthesis and NMR analysis of the molecules are described in detail in Supporting information.
Single-Crystal X-Ray Diffraction: The unit cell parameters and the X-Ray diffraction intensities were measured on a Xcalibur Ruby diffractometer.
The empirical absorption correction was introduced by a multiscan method using SCALE3 ABSPACK algorithm.Using Olex2, the structures were solved with the SHELXT or olex2.solveprograms and refined by the full-matrix least-squares method in the anisotropic approximation for all nonhydrogen atoms with the SHELXL program.Hydrogen atoms bound to carbon were positioned geometrically and refined using a riding model.The hydrogen atoms of NH groups were also refined independently with isotropic displacement parameters.The structures were refined using HKLF5 format file as twins with two components.CCDC numbers: 2262701, 2262703, 2262702.
Optical Transmittance: For the transmittance spectra measurements, the OCs were irradiated with a white halogen lamp of 360-2500 range (Avantes).The light was focused with an objective (Mitutoyo 10Â/0.28NA) and further the signal was collected by an objective (Mitutoyo 50Â/0.42NA).The transmittance response was analyzed with a HORIBA LabRAM confocal spectrometer with a water-cooled charge-coupled device (CCD, Andor DU 420A-OE 325) with 150 g mm À1 diffraction grating.
Raman Spectroscopy: For the Raman scattering spectra measurements, the OCs were irradiated by a He-Ne laser with 632.8 nm wavelength in a confocal LabRAM HR Horibo system.Laser radiation was focused with an objective (Mitutoyo 100Â/0.9NA), collected, and analyzed by confocal Horiba LabRam spectrometer (1800 g mm À1 diffraction grating).
Photoluminescence: For the PL measurements, the OCs were irradiated with the third harmonic of femtosecond laser with 1050 nm central wavelength (Yb 3þ active medium, TeMa, Avesta Project, a pulse length of 150 fs, a repetition rate of 80 MHz).The laser radiation was focused with an objective (NUV Mitutoyo 20Â/0.4NA).The PL signal was collected with the objective (Mitutoyo 50Â/0.42NA) and then analyzed with a Horiba LabRam confocal spectrometer with a water-cooled CCD (Andor DU 420A-OE 325) with 150 g mm À1 diffraction grating.
Amplified Spontaneous Emission: For the ASE regime spectra measurements, single OCs were irradiated with third-harmonic EKSPLA PL2140, Nd:YAG laser with 1064 nm central wavelength (and 354 nm as a third harmonic).The laser repetition rate was 10 Hz with 27 ps impulse duration.Laser radiation was focused with an objective (20Â/0.4NA), while ASE signal was detected with Avantes spectrometer and then analyzed.The intensity of laser radiation was varied from 0.1 to 1 μJ cm À2 .
PL Lifetime: Time-resolved PL measurements were performed on a confocal microscope MicroTime 100 (PicoQuant) equipped with an objective (40Â/0.65 NA) and a 405 nm pulsed diode laser.The laser repetition Quantum Yield Measurements: The QY of prepared samples was measured using an absolute quantum yield spectrometer (C11347-11, Hamamatsu Photonics K.K.) in automated scanning mode (5 nm step).All measurements were performed at room temperature.

Figure 1 .
Figure 1.a) Crystal structure of the OC and its optical image.Scale bar, 50 μm.b) Absorption (red curve) and PL (blue curve) spectra for single OC and the solution of the molecules 9 in DMF (concentration of 0.5 mg ml À1 ).c,d) Shape and energies of frontier molecular orbitals in the OC and the total DOS (TDOS).e) Hirshfeld surface of the OC, demonstrating the predominant contribution of hydrogen interactions (see also Figure S13, Supporting Information).
part of the compound.The planar shape can be explained by π-electron conjugation and the three-centered intramolecular hydrogen bond between the amino group and two oxygen atoms of ketone and ester carbonyls [N-H•••O 2.07(3) Å and 2.09(3) Å].In the resulting OC, the molecules 9 are linked into centrosymmetric dimers (Figure S4, Supporting Information) via hydrogen bonds (Figure 1e) between the amide NH group and the ketone carbonyl [N-H•••O 2.30(3) Å].In this dimer, the molecules are arranged in an antiparallel manner, while the thiophene and aryl rings are close to each other.Nevertheless, the large distance between them (3.6-3.8Å) indicates that π-π interactions are relatively weak (Figure S4, Supporting Information).The adjacent dimers are organized in a slip-stacked structure.The absence of strong π-π stacking interactions and the presence of several short C-H•••O and C-H•••H bonds between adjacent stacks involving aliphatic hydrogen atoms of the adamantyl and cyclohexane moieties play an important role in maintaining the PL of the resulting OC (see below)

Figure 2 .
Figure 2. a,b) Evolution of the PL from single OC upon the 350 nm (150 fs, 80 MHz) laser radiation of varied fluence (from 0.01 to 0.9 μJ cm À2 ).c) Confocal Raman analysis of the OC before and after the action of 350 nm (laser fluence of 0.7 μJ cm À2 ).d) Time-resolved PL measurements for the OC and the solution of molecules 9 in DMF, as well as e) the fluorescence lifetime imaging microscopy of the OC performed on a confocal microscope MicroTime 100 (PicoQuant) equipped with an objective (40Â/0.65 NA) and laser source of 405 nm (5MHz, 70 ps).f ) PL quantum yield analysis for the powder of OCs (red dot at 350 nm corresponds to the QY of the solution of molecules 9 in DMF with the concentration of 0.5 mg ml À1 ).Red arrow indicates the wavelength of pumping laser light for ASE regime (Figure3).

Figure 3 .
Figure 3. a,b) ASE regime achieved by the 10 ps laser pump (354 nm).c) Optical image of the OC at ASE regime.Scale bars, 50 μm.d) Endurance of ASE regime in OC under the action of 0.6 μJ cm À2 laser fluence (354 nm).