1. Introduction

Organic fluorescent emitters have received great attention due to their application potential in the field of organic light-emitting diodes (OLEDs) [1], fluorescent sensors [2], bioimaging [3], and organic lasers [4]. However, many conventional low-molecular-weight fluorescent molecules are subjected to weakened or quenched fluorescence emitting in aggregated states such as in solid-state thin-films (actually the favorable forms in optoelectronic devices application). In general, this behavior is defined as aggregation-caused quenching (ACQ) which originates from strong intermolecular interaction such as π- π stacking to decrease radiative decay rate [5]. Therefore, the frustrating results greatly hinder the applications of materials in organic solid-state lasers (OSLs). In the last decade, researchers have made much effort directly to alleviate the ACQ behavior in solid-state for organic emitters. Finally, some novel class of organic molecules have been developed based on, e.g., siloles [6], arylethylenes [7] and cyanostilbenes [8], demonstrating so-called aggregation-induced (enhanced) emission [9–11]. Besides, for organic emitters, the weakened fluorescence emitting inducing by self-absorption is a long-standing problem for OSLs applications as well. Generally, in organic semiconductor molecules, light can excite the molecule from its ground state to an vibrational level in the excited-state, followed by rapid relaxation to the bottom of the excited-state manifold. Then, lasing usually takes place from the zero-point vibrational level in the first excited-state to a vibrational level in the ground-state, followed by vibrational relaxation as well. The photocycle process behave as a quasi-four-level system [4]. Because of the small Stokes shift derived from the quasi-level system, organic semiconductor molecules suffer from severe self-absorption waveguiding loss, resulting in high lasing thresholds [12]. For organic molecules, in fact, some strategies have been proposed to overcome self-absorption by enlarging Stokes shift, for example, molecules with enhanced molecular rotation, enhanced intramolecular charge transfer, and intermolecular charge-transfer (CT) states, doped organic molecules with appropriate acceptors, molecules with excited-state intramolecular proton transfer (ESIPT) characteristics [13–16]. Among these, ESIPT molecules achieve tremendous attention due to their diverse structures and the largest achievable Stokes shift.

Above all, the suppressed fluorescence quenching and enhanced Stokes shift are essential for the realization of OSLs with outstanding amplified spontaneous emission (ASE) characteristics, which is the prerequisite of lasers. On account of the four-level cyclic scheme of enol (E)-keto (K) phototautomerization process, see Fig. 1(a), ESIPT have large Stokes shift and can avoid self-absorption behavior [17]. However, reported ESIPT molecules show high fluorescence emission in solution but weak fluorescence emission in the solid-state [18–20]. Recently, solid-state emitters with high fluorescence quantum efficiencies have been widely used in the field of optoelectronic devices and laser applications [21–24]. Here, we developed an ESIPT molecule with the intramolecular hydrogen bond between O-H∙∙∙O, 3-Dimethylamino-1-(2-hydroxy-phenyl)-propenone (DHP), which is an essential structural motif for the ESIPT molecules. DHP showed faint green emission in CHCl₃ solution with Φ_f = 0.08, however, greatly enhanced ESIPT fluorescence (Φ_f = 0.53) was observed from its doped amorphous film. For many reported ESIPT molecules, ASE was observed in single crystal thin-films [14,25–28], by comparison, the solid-state-induced fluorescence emission enhancement for DHP promote the occurrence of ASE behavior in thin-film for the compound dispersed in the inert host at a rather large (33 wt%) chromophore concentration.

 

Fig. 1 (a) Schematic diagram of ESIPT photocycle. (b) Normalized absorption (blue) and emission spectra (green) in CHCl₃ (1 × 10⁵ M) and emission spectra (red) for doped thin-film of DHP. The inset demonstrates the chemical structure of DHP.

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2. Experimental section and theoretical methodology

The molecules were dissolved in chloroform (CHCl₃) with 1 × 10⁵ M. In addition, we spin-coated the thin-film doped with polystyrene (PS) in a certain amount (33 wt%) of DHP onto quartz substrate. We used UV-Vis spectrophotometer (HITACHI U-3010, Japan) and Fluorescence Spectrometer (fluoromax-4 spectrofluometer) respectively to obtain the absorption and photoluminescence (PL) spectra. For ASE measurements, the thin-film sample was photopumped at normal incidence with strip light, The pump light came from a pulsed Nd:YAG laser (5.55 ns, 10 Hz) (Surelite I, Continuum Corp, USA), using the third harmonic (355 nm). The emitted light waveguided by thin-film was collected with Fiber Optic Spectrometer (Ocean Optics SpectraSuite, USB2000). In addition, we achieved the PL quantum yield (Φ_f) for 150 nm thickness doped films of the investigated molecule using an integrating sphere with an absolute PL quantum yield measurement system (Hamamatsu C11347). At the same time, PL transient decays were measured for the solution and thin-film samples using a time-correlated single photon counting system (Edinburgh FLS980) with excitation at 375 nm. All the measurements were carried under air.

Quantum chemical calculations of DHP were performed using the density functional theory/time-dependent density functional theory (DFT/TD-DFT) as implemented in the Gaussian 09 software package. Calculations on the ground state (S₀) and the lowest-energy excited singlet state (S₁) in the gas phase are carried out using the CAM-B3LYP functional with the 6-31G* basis set [29].

3. Results and discussion

The ESIPT-active and donor-accepter (D-A) structured molecule DHP is shown in Fig. 1(b), which was synthesized by heating a mixed dimethyl formamide- dimethylacetal solution of 2′-hydroxyacetophenone at 85 °C for 1 h according to the reported literature [30]. Although DHP is a reported molecule, its optical properties are not fully investigated. In aprotic solvents CHCl₃, the lowest-lying absorption band of DHP is located at 356 nm, as shown in Fig. 1(b) assigned to the enol absorption E→E* [31]. In Fig. 1(b), the fluorescence emission of DHP was featured as dual-peak emission bands in CHCl₃, namely a local emission (LE) band found at 411 nm and a large Stokes shifted ESIPT emission band found at 536 nm. However, the CHCl₃ solution (1 × 10⁵ M) of DHP displays nearly non-luminous with the Φ_f of 0.037. In the doped thin-film, the absorption spectra is similar with that in solution. Nevertheless, in solid-state, the fluorescence emission of DHP features broad band at 522 nm and is strongly bathochromically shifted relative to its absorption with a large Stokes shift of 1.11 eV, deriving from its ESIPT process and D-A structure [25]. No effective overlap between the absorption and the emission spectra indicates that complete ESIPT occurs and the emission is ascribed to K*→K, which thereby limits self-absorption losses to offer the prospect of low ASE threshold. Besides, DHP demonstrates brightly green emissive with a Φ_f of 0.53 in the solid-state (thin-film with thickness of 100 nm) compared with its solution.

We deduce that the small Φ_f in solution for DHP is caused by ineffective radiative deactivation. Aim at better understanding of the relaxation processes, we calculate the radiative k_r and nonradiative k_nr rate constants using the relation Φ_f = k_r/(k_r + k_nr) = k_r × τ. Additionally, we measured the fluorescence lifetime τ. As shown in Fig. 2, the lifetimes of DHP in solution are 2.75 ns and 0.78 ns for E* form and K* form respectively [32], and the lifetime of DHP in solid-state is 1.97 ns. It is revealed that the k_r ranged from 1.35 × 10⁷ s⁻¹ to 4.74 × 10⁷ s⁻¹ in solution, while, k_r dramatically increased to 2.69 × 10⁸ s⁻¹ in solid-state. However, k_nr in solution (from 3.51 × 10⁸ s⁻¹ to 12.34 × 10⁸ s⁻¹) and solid-state (2.38 × 10⁸ s⁻¹) are similar. Therefore, radiative decay is promoted more strongly in solid-state than that in solution for DHP, which is favor of ASE performance [33].

 

Fig. 2 (a) Normalized PL decay transients for DHP in doped thin-film. (b) Normalized PL decay transients for DHP in CHCl₃ solution (1 × 10⁵ M) of E* form. (c) Normalized PL decay transients for DHP in CHCl₃ solution (1 × 10⁵ M) K* form.

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The molecular equilibrium geometry and ESIPT process of DHP can be qualitatively investigated by DFT/TD-DFT calculations [14], and the results are shown in Fig. 3. See Fig. 3(a), in the ground state, E form of DHP shows strong innate intramolecular H-bonds (1.591 Å) between the carbonyl unit and hydroxy group, which increases the molecular rigidity greatly and is favor of activating the molecular planarization [34]. Thus, it leads to a rather π-conjugated planar conformation for DHP with dihedral angle of 2.51° between phenol and carbonyl units in ground-state, as shown in Fig. 3(a). In addition, the molecular geometries of DHP in the excited LE and ESIPT states present planar conformations as well with dihedral angles of 1.08° and 1.27° between phenol and carbonyl units respectively in Fig. 3(a)

 

Fig. 3 (a) Optimized geometries for DHP. (b) Potential energy surfaces in the gas phase and representation of the LE and ESIPT states of DHP .

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Geometry optimizations in S₁ base on the Franck-Condon state and the proton-transfer process generates two local minima in potential energy surface corresponding to the LE and ESIPT states respectively, and the calculated potential energy surfaces of S₀ and S₁ are shown in Fig. 3(b). The calculated vertical excitation energy for E→E* is 3.38 eV (367 nm), which is good agreement with the experimental value (λ_abs = 356 nm in CHCl₃), see Fig. 1(b). Relative to the optimized geometry in S₀, the energies of LE and ESIPT states are + 3.15 eV and + 2.77 eV. Therefore, the calculated vertical transition energies for LE and ESIPT states emission are 425 nm and 551 nm respectively, coinciding with the experimental results (λ_LE,PL = 411 nm and λ_ESIPT,PL = 536 nm in CHCl₃) as shown in Fig. 1(b).

Due to the inspiring optical properties above, we next investigate the ASE behavior of the designed molecules in doped thin-film. The solid-state thin-film of 33 wt% doped with PS were fabricated by spin-coating. As the pulse energy increased, in Fig. 4(a), a narrow ASE peak achieved to dominate the emission spectra rapidly and the ASE peak located at 559.14 nm. Figure 4(b) shows the full-width at half-maximum (FWHM) linewidth and relative output power of the emission spectra as a function of the pump intensity, and dramatically decreased FWHM and increased output power were observed. In order to quantify our observations, when the FWHM intensity of the emission spectrum drops to half of its PL value, the corresponding pulse energy of exciting light is the ASE threshold. According to this definition, the ASE threshold of DHP is 12.41 mJ/cm², which is comparable to that of other reported ESIPT molecules, such as imidazole-based [14] and 1,3-diaryl-β-diketones [25] compounds. Above all, the ASE result is consistent with the photophysical properties in solid-state thin-film of DHP, owing to the higher fluorescence quantum yield Φ_f, shorter excited state lifetime τ and enhanced radiative decay rate k_r in solid-state, in comparison to these in solution.

 

Fig. 4 (a) The edge-emission spectra of doped thin-films for DHP excited by different energy. Output intensity at λ_ASE. (b) Full Widths at Half-maximum (FWHM) as a function of pump energy for DHP.

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4. Conclusion

In conclusion, we have successfully synthesized a ESIPT molecule (DHP), which is nearly nonfluorescent in CHCl₃ (Φ_f = 0.037) but displays high fluorescence quantum yield in the solid-state thin-film (Φ_f = 0.53). The high radiative decay rate (2.69 × 10⁸ s⁻¹) in the solid-state thin-film is demonstrated to originate from the effect of the enlarged fluorescence quantum yield Φ_f, shortened excited state lifetime τ, ESIPT-active intramolecular H-bond, the molecular planarization and molecular D-A structure, revealing by combined photophysical properties and DFT/TD-DFT studies. Finally, the favorable four-level ESIPT photocycle and suppressed self-absorption losses of DHP in solid-state thin-film allow for the occurrence of efficient ASE behavior.