Crystallization-Enhanced Emission and Room-Temperature Phosphorescence of Cyclic Triimidazole-Monohexyl Thiophene Derivatives

Abstract

The development of organic room-temperature phosphorescent (ORTP) materials represents an active field of research due to their significant advantages with respect to their organometallic counterparts. Two cyclic triimidazole (TT) derivatives bearing one and three hexyl-thiophene moieties, TT-HThio and TT-(HThio)₃, have been prepared and characterized. Both compounds display enhanced quantum yields in their crystalline form with respect to those in a solution state, revealing crystallization-enhanced emissive (CEE) behavior. Importantly, while single fluorescence is observed in solution, crystalline powders also feature dual ORTP, whose respective molecular and aggregate origins have been disclosed through X-ray diffraction analysis and DFT/TDDFT calculations. The relation between the photophysical properties of TT-HThio and its crystallinity degree has been confirmed by a decrease in photoluminescent quantum yield (Φ) and loss of vibronic resolution when its crystals are ground in a mortar, revealing mechanochromic behavior and confirming CEE features.

1. Introduction

Single-component organic materials characterized by rich emissive behavior, including room-temperature long-lived features, are receiving growing attention from the scientific community due to the benefits they offer with respect to their widely used metal-containing phosphorescent counterparts, including biocompatibility and low cost. Applications of organic room-temperature phosphorescence (ORTP) in different fields spanning bioimaging [1,2], anti-counterfeiting [3,4], catalysis [5] and displays [6] have been assessed. Many strategies, including π-π stacking interactions [7,8,9,10], host-guest systems [11,12,13], co-assembly based on macrocyclic compounds [14], crystallization [15,16] and cocrystallization [17], halogen bonding [18,19] and doping in a polymer matrix [20], have been developed to realize ORTP materials. In parallel, knowledge of the mechanisms involved in the photophysics of multi-emissive ORTP systems is evolving forward, step by step, aiming at the materials’ optimization.

In this context, in the last few years, our group has been involved in the preparation and characterization of various members of a family of compounds with triimidazo[1,2-a:1’,2’-c:1’’,2’’-e][1,3,5]triazine, TT [9], as a prototype. TT is characterized by crystallization-induced emissive (CIE) behavior, displaying, in particular, ultralong phosphorescence (ORTUP) (up to 1 s) under ambient conditions associated with the presence of strong π-π stacking interactions in its crystalline structure [7]. The presence of one or multiple heavy (Cl, Br and I) atoms or chromophoric fragments, namely 2-fluoropyridine, 2-pyridine or pyrene, on the TT scaffold greatly modifies both its molecular and solid-state photo-physical behavior, resulting in a complex, excitation-dependent photoluminescence with emissions comprising dual fluorescence, molecular phosphorescence and supramolecular ORTP and ORTUP [10,21,22,23,24,25,26,27]. Applications of TT derivatives in different fields including bio-imaging [26] and explosive detection [28] have been reported, and the identification of new members with different functionalities appears highly desirable.

Herein, we describe the synthesis and structural and photophysical characterization of 3-(4-hexylthiophen-2-yl)-triimidazo[1,2-a:1’,2’-c:1”,2”-e][1,3,5]triazine, TT-HThio, and 3,7,11-tri(4-hexylthiophen-2-yl)-triimidazo[1,2-a:1’,2’-c:1”,2”-e][1,3,5]triazine, TT-(HThio)₃ (Scheme 1), with the hexylthiophene moiety being chosen for both its electron-donor properties and the ability to produce organic crystals that can respond to mechanical stimuli [29,30].

TT-HThio and TT-(HThio)₃ display enhanced properties in the crystalline form with respect to their solutions revealing CEE (crystallization-enhanced emissive) behavior. Importantly, while single fluorescence is observed in solution, crystals are also characterized by dual phosphorescence, whose origin has been disclosed through X-ray diffraction analysis and DFT/TDDFT calculations.

2. Results

3-(4-hexylthiophen-2-yl)-triimidazo[1,2-a:1’,2’-c:1”,2”-e][1,3,5]triazine, TT-HThio, and 3,7,11-tri(4-hexylthiophen-2-yl)-triimidazo[1,2-a:1’,2’-c:1”,2”-e][1,3,5]triazine, TT-(HThio)₃, have been synthetized by Stille coupling between tributyl(4-hexylthiophen-2-yl)stannane and the corresponding mono- (TTBr) or tri-brominated (TTBr₃) TT derivative (Scheme 1).

In diluted DCM solutions (1 × 10⁻⁵ M), TT-HThio displays a single fluorescence (FL) with photoluminescent quantum yield, Φ, equal to 11%; wavelength of the maximum peak (λ_em) = 370 nm and lifetime (τ) = 0.81 ns at 298 K; and λ_em = 365 nm and τ =1.71 ns at 77 K (Figure 1, Figures S5 and S6). In PMMA-blended films (0.5 w% TT-HThio), one fluorescence (FL, Φ = 15%, λ_em = 365 nm, τ = 0.72 ns at 298 K, Figure 1 and Figure S19) is observed in exactly the same position as that measured in DCM at 77 K, as expected for a molecular emission in rigidified media.

Crystals of TT-HThio show, both at 298 (Φ = 26%) and 77 K, an excitation-dependent PL behavior comprising one fluorescence and two phosphorescences (Figure 2). In particular, at 298 K, one fluorescence at λ_em = 376 nm (FL, τ = 1.15 ns, Figure S7) is observed by exciting at 300 nm, one high-energy phosphorescence (HEP, with vibronic replica at λ_em = 425 and 451 nm, τ = 5.91 ms, Figure S8) is visible by exciting at 376 nm and one low-energy phosphorescence (LEP, at λ_em = 497, 530 and 578 nm, τ = 50.22 ms, Figure S9) appears when exciting at 450 nm. At 77 K, the three components are better resolved vibronically, in almost the same positions but with longer lifetimes with respect to those measured at room temperature (FL, λ_em = 368 nm, τ = 1.48 ns; HEP, λ_em = 420 and 448 nm, τ = 10.61 ms; LEP, λ_em = 494, 533 and 577 nm, τ = 132.01 ms, Figures S10, S11 and S12, respectively). Intriguingly, the corresponding excitation profiles are characterized at low energy by three contributions recognizable at both 298 and 77 K. Specifically, one low-energy state (332, 354 nm at 298 K and 335, 347 at 77 K) assigned to singlet molecular excited state S₁ in agreement with structural and theoretical studies (see later), one triplet excited state of molecular origin (389 nm at 298 K and 390, 403 nm at 77 K), T_mol, and one triplet excited state of aggregate origin (446 nm at 298 K and 448, 471 nm at 77 K), T_ag, are observed. A mirroring relationship with the corresponding emission is clearly visible at 77 K.

Based on the observation that Φ increases from 11%, in diluted DCM solution, to 26%, measured for TT-HThio crystals, we hypothesized possible aggregation-enhanced emissive (AEE) features; therefore, solvent (THF)/non-solvent (water) experiments have been performed. Addition of increasing water fractions to TT-HThio in THF (keeping the concentration equal to 1 × 10⁻⁵ M) results, instead, in emission quenching (Figure 3), indicating that aggregation itself is not sufficient to intensify luminescence, and that proper molecule organization (i.e., crystallization) is an indispensable condition to enhance emission from TT-HThio. Similarly, in TT itself, crystallization is a necessary condition to switch on luminescence [9].

The relation between the photophysical properties of the sample and its crystallinity degree has been confirmed by a decrease in Φ (18%) and the loss of vibronic components when TT-HThio crystals are ground in a mortar, revealing mechanochromic behavior and confirming CEE features (Figure 4).

In diluted DCM solutions (1 × 10⁻⁵ M), TT-(HThio)₃ displays a single fluorescence (FL, Φ = 5%, λ_em = 380 nm, τ = 1.17 ns at 298 K and λ_em = 370 nm, 1.42 ns at 77 K, Figure 5, Figures S20, S21). In PMMA-blended films (0.5 w% TT-(HThio)₃), one fluorescence (FL, Φ = 17%, λ_em = 372 nm, τ = 0.90 ns at 298 K, Figure 5 and Figure S28) is observed in exactly the same position as that measured in DCM at 77 K.

The solid-state photophysical characterization of TT-(HThio)₃ has been performed on crystalline powders (Φ = 22%), while different attempts to prepare single crystals suitable for XRD analysis failed. Similarly to TT-HThio, TT-(HThio)₃ displays at both 298 and 77 K an excitation-dependent PL behavior, comprising FL, HEP and LEP (Figure 6). In particular, at 298 K, FL at λ_em = 382, 400 nm (τ = 0.42 ns, Figure S22) is observed by exciting at 300 nm together with HEP at λ_em = 428, 453 nm (τ = 15.10 ms, Figure S23). The latter can be selectively activated by exciting at 384 nm. LEP at λ_em = 514, 550 nm (τ = 41.51 ms, Figure S24) becomes visible by exciting at sufficiently low energy (450 nm) to exclude the otherwise more intense FL and HEP. At 77 K, the three components are still present at λ_em = 364, 381 nm (FL, τ = 1.23 ns, Figure S25), λ_em = 423, 447 nm (HEP, τ = 153.42 ms, Figure S26) and λ_em = 492, 513 nm (LEP, τ = 327.16 ms, Figure S27). Again, in excitation profiles, one S₁ state (360 nm at 298 K and 336, 354 at 77 K), one T_mol (387, 408 nm at 298 K and 386, 407 nm at 77 K), and one T_ag (448 nm at 298 K and 438, 468 nm at 77 K) are visible both at 298 and 77 K. The photophysical parameters of TT-HThio and TT-(HThio)₃ are summarized in

Table 1. Photophysical parameters of TT-HThio and TT-(HThio)₃.

Compound			298 K			77 K
			Φ (%)	λem	τav	λem	τav
TT-HThio	DCM		11	370	0.81 ns	365	1.71 ns
	PMMA		15	365	0.72 ns
	Solid state	Crystals	26	376	1.15 ns	368	1.48 ns
				425, 451	5.91 ms	420, 448	10.61 ms
				497, 530, 578	50.22 ms	494, 533, 577	132.01 ms
		Ground Crystals	18	376	1.11 ns	375	1.91 ns
				428, 441	5.20 ms	423, 443	9.56 ms
				500	29.81 ms	492, 513	60.84 ms
TT-(HThio)3	DCM		5	380	1.17 ns	370	1.42 ns
	PMMA		17	372	0.90 ns
	Crystalline powders		22	382, 400	0.42 ns	364, 381	1.23 ns
				428, 453	15.10 ms	423, 447	153.42 ms
				514, 550	41.51 ms	478, 512, 544	327.16 ms

.

Single crystals of TT-HThio suitable for X-ray diffraction analysis have been obtained by the slow evaporation of methanol solutions. TT-HThio crystallizes in the P-1 space group with two molecules in the asymmetric unit, A and B, forming very long colorless needles. In both A and B molecules, the TT and Thio moieties are almost coplanar, the least-squares planes through the two units are 14.12 and 6.00° in A and B, respectively (Figure 7a). The hexyl chain assumes in both cases an all-trans conformation. A and B molecules are almost orthogonal to each other (Figure 7b), both forming segregated head-to-head π-stacked aggregates with identical distance, equal to 5.599 Å, between the triazinic centroids of adjacent molecules. The shortest intermolecular contacts along the stacks are C1A⋅⋅⋅C8A_1+x,y,z, 3.353(3), S1A⋅⋅⋅C9A_1+x,y,z, 3.344(2) and C5A⋅⋅⋅C5A_{1−x,1−y,1−z}, 3.376(3) Å (A stack) and S1B⋅⋅⋅C9B_1+x,y,z, 3.388(2) Å (B stack). The TT moiety of molecule A is connected from both sides to other two centrosymmetry-related A molecules through quite short C–H⋅⋅⋅N hydrogen bonds (H5A⋅⋅⋅N5A_{−x,1−y,1−z}, 2.38 Å; H8A⋅⋅⋅N1A_{1−x,−y,1−z}, 2.43 Å), forming infinite ribbons from which the thio-hexyl chains depart (Figure 7c). The columnar aggregates of B molecules develop inside the hexyl chains of A molecules. A weak C–H⋅⋅⋅N hydrogen bond (H7A⋅⋅⋅N1B_{−x,1−y,1−z}, 2.72 Å) connects A and B molecules with each other.

3. Discussion

As previously observed for a series of TT derivatives bearing one to three chromophoric substituents (i.e., 2-pyridine [25], fluoropyridine [24] or pyrene [26,27]) on the TT scaffold, the presence of the chromophore switches on fluorescence which is otherwise absent in TT itself in solution. From DFT/TDDFT calculations, it was inferred that TT lacks low-energy singlet states with non-null oscillator strength (f) due to the high symmetry of its electronic system. Analogously, “silent” low-energy singlet states are present in trihalogenated TT derivatives [21,23]. However, in monosubstituted TT derivatives, the disruption of the molecular symmetry results in S₁ and upper levels with medium-to-high oscillator strength depending on the nature of the substituent itself. This is also noted for TT derivatives functionalized with three equal chromophoric fragments owing to the adopted non-planar conformation [27]. For TT-HThio, the S₁ level is computed at 258 nm (f = 0.49), quite similar to the mono-pyridine TT derivative for which S₁ is computed at 256 nm with f = 0.48. In both cases, S1 is mainly (88%) a HOMO→LUMO transition of (π,π*) type and very weak CT character (in the present case from thiophene to TT) (Figure 8 and Table S2). Moving to TT-(HThio)₃ (Figure 9 and Table S3), it should be noted that the presence of three, rather than one, chromophoric substituents does not significantly affect the absorption and emission properties of the compound, as also observed for the mono-, di- and three-pyrene series of TT derivatives [27]. The absence of inter-chromophoric communication in multiple-substituted TT derivatives, evidenced also by electrochemical studies on TT and its halogenated derivatives [31], is associated with a splitting of the MO levels of the mono-derivative in two or three almost degenerate levels, originating excited S_n and T_n levels nearly isoenergetic with those of the mono-derivative (Table S3). For example, the first singlet levels of TT-(HThio)₃ are computed at 258 (S₁, f = 0.005), 257 (S₂, f = 0.86) and 257 nm (S₃, f = 0.86), all of them of (π,π*) type with very weak CT character.

The observation of HEP in the solid state for both TT-HThio and TT-(HThio)₃ can be explained by examining the singlet–triplet energy gap, ΔE_ST, separating S₁ and the closer, lower-energy triplet state (T_n) and the nature of this level. In the DFT freely optimized geometry of TT-HThio, starting from its X-ray molecular structure, a rather high (0.53 eV) energy gap is obtained. However, compared with S₁, the closer triplet state, T₅ (a 50% HOMO-2→LUMO and 20% HOMO-2→LUMO+9 (π,π*) transition) has a much more marked CT character, as deduced by looking at the involved HOMO-2 (Figure 8), which is essentially localized on thiophene only. The corresponding excited state dipoles, in fact, are 1.90 D (S₁) and 3.37 D (T₅). The different character of S₁ and T₅ suggests an easy intersystem crossing (ISC) which explains the HEP observed in crystals. Moreover, it should be noted that the optimized structure is characterized by much larger twisting than that observed in the X-ray structure. In fact, the C1-C2-C10-S1 torsion angle (ω, see Figure 7a for the atom numbering scheme) measures 38.3° in the optimized geometry, to be compared with the corresponding experimental torsions, 11.1(3)° and 7.4(3)° for molecules A and B, respectively, of the asymmetric unit. By fixing ω to the values assumed in the crystal structure, ΔE_ST decreases from 0.53 to 0.28 (ω = 11.1°) and 0.27 eV (ω = 7.4°), making the singlet-to-triplet ISC even easier. HEP is therefore associated with radiative decay from T₅ itself or a lower triplet state T_mol after internal conversion (IC) from T₅. It is visible in crystals thanks to the rigidifying effect and protection from oxygen quenching ascribable to intermolecular interactions.

The additional phosphorescent emission, LEP, can be associated with the presence of H-aggregates in the crystal structure, in agreement with previous findings on compounds with the same triazinic scaffold [9,10,21,22,23,24,25,26,27]. The schematic representation of the photophysical processes involved in the emissive behavior of both TT-HThio and TT-(HThio)₃ is reported in the Jablonski diagram below (Figure 10). It is interesting to compare the relative simplicity of this diagram with that, much more complicated, depicted for the analogous derivative with 2-pyridine, TT-Py [25]. Though the thiophene and pyridine chromophoric groups share rather similar electronic features, reflected for example in the similar emissive properties of their TT derivatives in DCM solution (Φ = 11%, λ_em = 370 nm for TT-HThio; Φ = 17%, λ_em = 351 nm for TT-Py), they are responsible for a different emissive behavior in their solid state. While crystals of both compounds display a high-energy phosphorescence of molecular origin (at 425, 451 nm for TT-HThio and at 408-418 nm, according to the crystalline phase, for TT-Py), associated with emission from a ³(π,π*) low-energy triplet level, TT-Py shows an additional long-lived molecular component, almost overlapped with the fluorescence. This latter emission was ascribed to radiative decay from a triplet state of mixed ³(σ/π,π*) symmetry, which is responsible for the observed dual anti-Kasha phosphorescence [32]. Such triplet states are absent in TT-HThio, whose frontier MOs have all π symmetry. Moreover, crystals of TT-Py display an additional low-energy fluorescence, associated with the higher mobility of the chromophoric pendant compared with the hexylthiophene group.

In summary, two new members of the photophysically intriguing TT family have been isolated and characterized. The compounds, bearing one or three hexylthiophene fragments, display CEE behavior comprising fluorescence and heavy-atom-free dual phosphorescence, associated with molecular and supramolecular features. This work adds a new building block to the knowledge of this family and to RTP organic phenomena in general. From these results, the preparation of new multicomponent emitters can be envisaged.

4. Materials and Methods

4.1. General Information

All reagents were purchased from chemical suppliers and used without further purification unless otherwise stated. TTBr and TTBr₃ were prepared according to published procedures [10,21]. Tributyl(4-hexylthiophen-2-yl)stannane was prepared according to a published procedure [33]. ¹H and ¹³C NMR spectra were recorded on a Bruker AVANCE-400 instrument (400 MHz). Chemical shifts are reported in parts per million (ppm) and are referenced to the residual solvent peak (CH₂Cl₂, ¹H: δ = 5.32 ppm, ¹³C: δ = 54.0 ppm); coupling constants (J) are given in hertz (Hz) and are quoted to the nearest 0.5 Hz. Peak multiplicities are described in the following way: s, singlet; d, doublet; t, triplet; p, pentet; m, multiplet.

Films of TT-HThio and TT-(HThio)₃ dispersed in polymethylmethacrylate (PMMA) were prepared by spin coating (2000 rpm, 60 s) a dichloromethane solution (TT-HThio or TT-(HThio)₃/PMMA = 0.5 wt %; PMMA = 10 wt % with respect to the solvent) on a quartz substrate.

4.2. Synthesis of TT-HThio

TT-HThio was prepared by Stille coupling between TTBr and tributyl(4-hexylthiophen-2-yl)stannane. In a typical reaction, TTBr (0.580 g; 2.10 mmol), tributyl(4-hexylthiophen-2-yl)stannane (0.959 g, 2.10 mmol), LiCl (1.000 g, 23.60 mmol), Pd(PPh₃)₂Cl₂ (0.160 g, 0.210 mmol) and dry toluene (10 mL) are transferred inside a 100 mL dry Schlenk flask equipped with a magnetic stirrer. The mixture is de-aerated by means of three freeze–pump–thaw cycles. The system is heated under a static nitrogen atmosphere at 120 °C for 16 h. The reaction is then cooled to room temperature, filtered on Buchner and the solvent removed in vacuum. The crude reaction mixture is purified by flash chromatography on SiO₂ with DCM/ACN as eluents (R_f = 0.53 in DCM/ACN = 8/2). The product is further purified by precipitation from DCM with hexane to give TT-HThio as a white solid (0.535 g, 1.47 mmol, Yield: 70%). Crystals suitable for X-ray diffraction analysis were obtained by slow evaporation from a MeOH solution.

NMR data (9.4 T, CD₂Cl₂, 298 K, δ, ppm): ¹H NMR 7.80 (m, 2H), 7.53 (d, J = 1.2 Hz, 1H), 7.26 (m, 2H), 7.22 (d, J = 1.6 Hz, 1H), 7.07 (s, 1H), 2.69 (t, J = 7.1 Hz, 2H), 1.73 (p, J = 7.2 Hz, 2H), 1.46–1.30 (m, 6H), 0.95 (t, J = 6.9 Hz, 3H). ¹³C NMR 144.3 (C), 136.4 (C), 136.0 (C), 135.8 (C), 132.3 (CH), 129.6 (CH), 129.1 (CH), 129.1 (CH), 128.1 (C), 123.1 (C), 122.3 (CH), 112.1 (CH), 111.7 (CH), 32.3 (CH₂), 31.0 (CH₂), 29.6 (CH₂), 23.2 (CH₂), 14.4 (CH₃).

4.3. Synthesis of TT-(HThio)₃

TT-(HThio)₃ was prepared by Stille coupling between TTBr₃ and tributyl(4-hexylthiophen-2-yl)stannane. In a typical reaction, TTBr₃ (0.400 g; 0.920 mmol), (tributyl(4-hexylthiophen-2-yl)stannane (1.264 g, 2.769 mmol), LiCl (0.390 g, 9.20 mmol), Pd(PPh₃)₂Cl₂ (0.065 g, 0.092 mmol) and dry toluene (10 mL) are transferred inside a 100 mL dry Schlenk flask equipped with a magnetic stirrer. The mixture is de-aerated by means of three freeze–pump–thaw cycles. The system is heated under a static nitrogen atmosphere at 120 °C for 16 h. The reaction is then cooled to room temperature, filtered on Buchner and the solvent removed in vacuum. The crude reaction mixture is purified by flash chromatography on SiO₂ with DCM/hexane as eluents (R_f = 0.72 in hexane/AcOEt = 8/2). The product is further purified by precipitation from DCM with MeOH to give TT-(HThio)₃ as a white solid (0.390 g, 0.56 mmol, Yield: 61%). Crystals suitable for X-ray diffraction analysis were obtained by slow evaporation from a MeOH solution.

NMR data (9.4 T, CD₂Cl₂, 298 K, δ, ppm): ¹H NMR NMR data (9.4 T, CD₂Cl₂, 298 K, d, ppm): ¹H NMR 7.43 (d, 1.10 Hz, 3H), 7.18 (s, 3H), 7.09 (d, 1.10 Hz, 3H), 2.68 (t, J = 7.5 Hz, 6H), 1.68 (p, J = 7.5 Hz, 6H), 1.48–1.30 (m, 18H), 0.90 (t, 6.9, 9H). ¹³C NMR 143.9 (C), 136.4 (C), 132.3 (CH), 129.2 (CH), 128.0 (C),122.3 (CH), 122.3 (C), 32.1 (CH₂), 30.8 (CH₂), 29.40 (CH₂), 23.0 (CH₂), 14.3 (CH₃).

4.4. Single-Crystal X-ray Studies

X-ray data of TT-HThio were collected on a Bruker Apex II diffractometer (Bruker AXS Inc., Madison, WI, US) using MoKα radiation [34]. The structure was solved using direct methods and refined with SHELXL-14 [35] using a full-matrix least-squares procedure based on F² using all data. Hydrogen atoms were placed at geometrically estimated positions. Details relating to the crystal and the structural refinement are presented in Table S1. Long bars of TT-HThio were grown at room temperature in a methanol solution of the compound. Full details of crystal data and structure refinement, in CIF format, are available as Supplementary Information. CCDC reference number: 1913040.

4.5. Computational Details

DFT and TDDFT calculations on TT-HThio and TT-(HThio)₃ were performed with the Gaussian 16 program (Revision A.03) [36] using the 6-311++G(d,p) basis set. The geometry of TT-HThio was optimized starting from the experimental molecular structure, as derived from X-ray studies. For comparison purposes, we adopted the same functional ωB97X [37] used for calculations of the previously reported parent cyclic triimidazole and its halogenated derivatives.

4.6. Photophysical Characterization

Photoluminescence quantum yields were measured using a C11347 Quantaurus–Absolute Photoluminescence Quantum Yield Spectrometer (Hamamatsu Photonics K.K, Shizuoka, Japan) equipped with a 150 W Xenon lamp, an integrating sphere and a multichannel detector. Steady-state emission, excitation spectra and photoluminescence lifetimes were obtained using an FLS 980 (Edinburgh Instrument Ltd., Livingston, United Kingdom) spectrofluorimeter. The steady-state measurements were recorded with a 450 W Xenon arc lamp. Photoluminescence lifetime measurements were performed using an EPLED-300 (Edinburgh Instrument Ltd.) and microsecond flash Xe-lamp (60W, 0.1 ÷ 100 Hz) with the data acquisition devices time-correlated single-photon counting (TCSPC) and multi-channel scaling (MCS) methods, respectively. Average lifetimes were obtained as τ_av = $\frac{{\displaystyle \sum }{A}_i{\tau }_{i }^2}{{\displaystyle \sum }{A}_i{\tau }_i}$ from bi-exponential or three-exponential fits. Low-temperature measurements were performed by immersion of the sample in a liquid N₂ quartz dewar.