Long-Alkyl Chain Functionalized Imidazo[1,5-a]pyridine Derivatives as Blue Emissive Dyes

Abstract

A series of boron difluoride compounds with 2-(imidazo[1,5-a]pyridin-3-yl)phenols bearing alkylic chains at the 1-position has been synthesized and characterized both with ¹H and ¹³C NMR and infrared spectroscopy. This series of compounds displayed blue emission in solution and in thin polymeric films, with interesting features like large Stokes shifts and good fluorescence quantum yields. Time-Dependent Density Functional Theory (TD-DFT) calculations allowed for the identification of the main electronic transitions as intra ligand transitions (¹ILT), as corroborated by the Natural Transition Orbitals (NTOs) shapes.

1. Introduction

Imidazo[1,5-a]pyridines are heterocyclic compounds widely known in the literature due to their use in the fabrication of optoelectronic devices or as ligands in the synthesis of many coordination compounds [1,2]. They also displayed a wide range of different biological activities, such as antitumoral [3,4] and antibacterial effects [5,6], and for the treatment of Alzheimer’s disease [7]. These compounds are also well known for their luminescence properties; in fact, they have been used as probes even in living cells, towards several analytes, such as Hg²⁺ [8], H₂S [9] and sulfites [10]. Moreover, it has been recently reported by Zhang that a coordination polymer derived from imidazo[1,5-a]pyridine is capable of detecting Cu²⁺, CrO₄²⁻ and Cr₂O₇²⁻ ions in water solution [11]. Even more studied is the ability of imidazo[1,5-a]pyridines to act as ligands with a large variety of metal centers. This allows us to obtain luminescent coordination compounds that can, in principle, be used in the fabrication of optoelectronic devices, such as Organic Light Emitting Diodes (OLEDs). These ligands, depending on the substituent on the main core, can bind the coordination center in various ways, starting from bidentate ligands [12], to tridentate [13] and even tetradentate [14] (Figure 1).

One of the most common substituent on the main core is the pyridinyl ring on the position 1: this permits a typical N,N bidentate coordination mode, which can be suitable for binding to many transition metal ions, such as Mn²⁺, Ir³⁺, Cu⁺, Ag⁺, Zn²⁺, Ru²⁺ and Os²⁺ [15,16,17].

On the other hand, a N,O coordination is possible when alcoholic [18] or phenolic residues are used as substituents. This, upon the deprotonation of the hydroxyl group, has led to the preparation of another series of coordination compounds, even with semimetals, including B(III) centers [19,20].

Usually, zinc and copper coordination compounds of imidazo[1,5-a]pyridines show remarkable luminescence properties. As an example, Volpi [21] recently reported a series of N∩N zinc complexes of 1,3-substituted imidazo[1,5-a]pyridines characterized by emission maxima in the blue region of the visible spectrum (410–460 nm) and with a maximum fluorescence quantum yield of 33%.

While imidazo[1,5-a]pyridine compounds are renowned for their luminescence properties, especially due to their usually large Stokes shifts [22], there are very few examples reporting these compounds as emissive layers in OLEDs [23], as well as in light-emitting electrochemical cells [16]. However, many of these derivatives showed high quantum yields and blue emission which, together with the aforementioned large Stokes shift, are very important properties. Moreover, it is possible to tune all these properties as well as their emission wavelength by changing the substituents on the core moiety.

On the other hand, another class of extremely well-known luminescent compounds is constituted by the boron dipyrromethene (BODIPY) derivatives. Interestingly, these species have very high quantum yields (up to 90%) and excellent chemical and photophysical stability in solution as well as in a solid state, with fluorescence lifetime in the range of 1–10 ns. A large portion of these species finds its application as a molecular sensor [24], in particular as pH indicators in organic, aqueous and mixed aqueous-organic media.

Thanks to these outstanding properties, such as high photo and chemical stability, high absorption coefficients, high quantum yields, and good solubility even in aqueous media, this family of molecules is used in the fabrication of optoelectronic devices, including OLEDs [25,26]. Unfortunately, in the solid state, BODIPY derivatives show very small Stokes shifts (5–20 nm), leading to serious self-quenching.

In our continuing research on coordination compounds with nitrogen ligands [27,28,29,30], we recently focused our attention on imidazo[1,5-a]pyridine scaffolds [31,32], utilized both as N,N- [33,34,35] or N,O- [36,37,38,39] bidentate ligands. Specifically, we combined the properties of these heterocycle and BODIPY, mimicking the latter upon the coordination of a BF₂ fragment using N∩O substituted (imidazo[1,5-a]pyridin-3-yl)phenolates [40,41]. These compounds showed excellent fluorescent properties and photostability and they have been used in preliminary tests as emissive materials in the fabrications of some blue emissive OLEDs [42]. Unfortunately, they displayed poor film forming properties when spin coated from a solution, thus affecting the overall performances of the devices. For these reasons, we present here a series of new developed BF₂-(imidazo[1,5-a]pyridin-3-yl)phenolates functionalized with long alkyl chains, in order to increase their solubility in organic solvents and their capacity to form even and homogeneous films.

2. Materials and Methods

2.1. General Remarks

Infrared Spectra were acquired on a Shimadzu Prestige-21 spectrophotometer with a 1 cm⁻¹ resolution. Elemental analyses were obtained with a Perkin-Elmer CHN Analyzer 2400 Series II. NMR spectra were recorded with an AVANCE 400 Bruker spectrometer operating at 400 MHz for ¹H NMR and 100 MHz for ¹³C{¹H} NMR. Chemical shifts are given as δ values in ppm relative to residual solvent peaks as the internal reference. J values are given in Hz. The UV-vis, excitation and emission spectra were measured using a fluorescence spectrometer (Edinburgh Instruments FS5) equipped with a 150 W continuous Xenon lamp as a light source and were corrected for the wavelength response of the instrument; lifetime measurements were performed on the same FS5 Edinburgh Instruments fluorimeter equipped with an EPLED-320 (Edinburgh Instruments) as the pulsed source. Absolute fluorescence quantum yields in solution were determined using a PhotoMed GmbH K-Sphere Integrating Sphere (3.2 inch. diameter). Fluorescence quantum yields at low temperatures were estimated indirectly using the following equation:

$${\Phi }_T={\Phi }_{RT}\frac{I_T}{I_{RT}}$$

(1)

where ${\Phi }_T$ is the fluorescence quantum yield calculated at a given temperature, ${\Phi }_{RT}$ is the fluorescence quantum yield obtained experimentally at room temperature, $I_T$ is the maximum intensity of the emission spectrum at a given temperature and $I_{RT}$ is the maximum intensity of the emission spectrum recorded at room temperature. The correction depending on the refractive index has been neglected due to a lack of experimental data regarding the refractive indexes of dichloromethane at all the temperatures; for this reason, the calculated fluorescence quantum yields have to be intended as esteems and not as absolute values.

Analysis of the lifetime decay curve and determination of absolute quantum yields were done using Fluoracle^® Software package (Version 1.9.1) which runs the FS5 instrument. C1 and C1-BF₂ have been synthesized using an already reported method [41]. All procedures have been done under inert atmosphere following the conventional Schlenk’s technique. Films were obtained by spin coating on a quartz substrate 200 μL of a 5% m/m dichloromethane solution of the compound of interest and poly (methyl methacrylate) (PMMA) for one minute at a rotation speed of 5500 rpm.

2.2. General Synthesis of Pyridyl Ketones

0.5 g (20.6 mmol) of magnesium was heated under vacuum before 15 mL of anhydrous THF was added with 10 mg of iodine. The mixture was refluxed for 30 min and then a solution of 12 mmol of the 1-bromoalkane (C3: 1-bromopropane, C5: 1-bromopentane, C10: 1-bromodecane, C18:1-bromooctadecane) in 5 mL of anhydrous THF were added dropwise. The mixture was refluxed for 1 h and then cooled to room temperature. The excess of magnesium was eliminated by filtration and the resulting yellow solution was added dropwise to a solution of 10 mmol of 2-cyanopyridine in 20 mL of anhydrous THF at 0 °C. The solution was left stirring at 0 °C for 1 h and then at room temperature overnight. The solution was then quenched with 30 mL of HCl 2 M, then the pH was adjusted to 8 with an aqueous solution of NaOH 2M. The mixture was extracted with dichloromethane, washed with brine, dried over Na₂SO₄ and the solvent was removed under reduced pressure to give a brown oil which was finally purified using column chromatography (ethyl acetate:hexane 1:10) to give the final product.

2.3. General Synthesis of CX Ligands

In 30 mL of glacial acetic acid, 7.39 mmol of pyridyl ketone, 14.78 mmol of salicylaldehyde and 36.96 mmol of ammonium acetate were added. The resulting mixture was left stirring at room temperature for 7 days. The solution was then diluted with 150 mL of water and extracted with 3 × 70 mL of dichloromethane. The organic phase was washed with a saturated solution of NaHCO₃, dried over Na₂SO₄ and the solvent was removed under reduced pressure to give a yellow oil, which was then triturated with methanol to give the final product as a yellow solid.

C3: mp: 217°. Yield: 0.67 g (36%). Anal. Calcd (%) for C₁₆H₁₆N₂O: C, 76.16; H, 6.39; N, 11.10. Found (%): C, 75.86; H, 6.21; N, 11.44. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.44 (t, J = 7.4, 1H), 7.73 (d, J = 7.8, 1H), 7.43 (t, J = 11.4, 1H), 7.31–7.21 (m, 1H), 7.14 (t, J = 8.6, 1H), 7.02–6.90 (m, 1H), 6.67 (m, 1H), 6.59 (t, J = 6.4, 1H), 2.87 (t, J = 7.4, 2H), 1.94–1.65 (m, 2H), 0.99 (t, J = 7.4, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 156.59, 134.59, 131.71, 129.50, 127.31, 123.90, 122.10, 118.95, 118.78, 117.89, 117.84, 114.58, 113.88, 28.86, 23.08, 14.08.

C5: mp: 123°. Yield: 0.73 g (35%). Anal. Calcd (%) for C₁₈H₂₀N₂O: C, 77.11; H, 7.19; N, 9.99. Found (%): C, 77.23; H, 7.89; N, 9.84. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.42 (d, J = 7.3, 1H), 7.66 (d, J = 7.8, 1H), 7.44 (d, J = 9.1, 1H), 7.18 (t, J = 7.1, 1H), 7.14 (d, J = 7.5, 1H), 6.89 (t, J = 7.0, 1H), 6.69 (m, 1H), 6.52 (t, J = 7.2, 1H), 2.89 (t, J = 7.5, 2H), 1.80 (m, 2H), 1.42–1.30 (m, 4H), 0.90 (t, J = 7.0, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 156.57, 134.51, 131.81, 129.56, 127.16, 124.03, 122.14, 118.97, 118.75, 117.93, 117.83, 114.49, 113.92, 31.70, 29.49, 26.74, 22.65, 14.18.

C10: mp: 83°. Yield: 0.83 g (32%). Anal. Calcd (%) for C₂₃H₃₀N₂O: C, 78.82; H, 8.63; N, 7.99. Found (%): C, 79.13; H, 8.50; N, 7.67. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = δ 8.40 (d, J = 7.2, 1H), 7.73 (d, J = 7.7, 1H), 7.45 (d, J = 7.7, 1H), 7.25 (t, J = 7.5, 1H), 7.14 (d, J = 8.1, 1H), 6.96 (t, J = 7.5, 1H), 6.71 (m, 1H), 6.62 (t, J = 6.7, 1H), 2.91 (t, J = 7.5, 2H), 1.80 (m, 2H), 1.45–1.12 (m, 14H), 0.87 (t, J = 6.6, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 156.59, 134.44, 129.75, 127.17, 122.28, 119.02, 118.75, 118.12, 117.89, 114.05, 110.04, 32.05, 29.84, 29.77, 29.75, 29.62, 29.56, 29.47, 26.75, 22.85, 14.26.

C18: mp: 41°. Yield: 1.26 g (37%). Anal. Calcd (%) for C₃₁H₄₆N₂O: C, 80.47; H, 10.02; N, 6.05. Found (%): C, 80.01; H, 10.12; N, 5.74. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.36 (d, J = 7.3, 1H), 7.67 (d, J = 7.9, 1H), 7.37 (d, J = 9.1, 1H), 7.22–7.16 (m, 1H), 7.06 (d, J = 7.9, 1H), 6.89 (t, J = 7.3, 1H), 6.61 (m 1H), 6.53 (t, J = 6.5, 1H), 2.81 (t, J = 7.5, 2H), 2.01–1.93 (m, 2H), 1.76–1.67 (m, 2H), 1.21 (m, 28H), 0.81 (t, J = 6.7 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃, 298 K, J [Hz]): δ = 156.65, 131.98, 129.51, 127.18, 123.91, 122.13, 118.94, 118.82, 117.88, 117.83, 114.60, 114.22, 113.87, 32.10, 29.88, 29.84, 29.54, 22.86, 14.26.

2.4. General Synthesis of BF₂ Compounds

2.5 mmol of CX was dissolved in 2 mL of dichloromethane. Then, a solution of 7.4 mmol of BF₃· Et₂O in 1 mL of dichloromethane was added dropwise. After the solution turned red, 3.6 mmol of Et₃N was added. The mixture was left stirring overnight and the resulting solid was filtered and washed with cold dichloromethane to give the final products as white solids.

C3-BF₂: mp: 194°. Yield: 0.30 g (40%). Anal. Calcd (%) for C₁₆H₁₅N₂OBF₂: C, 64.03; H, 5.04; N, 9.33. Found (%): C, 64.44; H, 4.88; N, 9.81. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.51 (d, J = 6.1, 1H), 7.80 (d, J = 7.9, 1H), 7.57 (d, J = 7.5, 1H), 7.4 (t, J = 7.7, 1H), 7.22 (d, J = 8.2, 1H), 7.00 (t, J = 7.6, 1H), 6.97–6.84 (m, 2H), 3.14 (t, J = 7.6, 2H), 1.94–1.78 (m, 2H), 1.01 (t, J = 7.3, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ =155.19, 131.98, 127.71, 127.40, 122.41, 122.18, 120.76, 120.61, 119.86, 119.38, 117.26, 111.13, 26.41, 23.08, 14.15. FT-IT (ATR) (cm⁻¹): ṽ = 1034–1147 (BF₂), 2877–2980 (aliphatic C-H).

C5-BF₂: mp: 143°. Yield: 0.41 g (50%). Anal. Calcd (%) for C₁₈H₁₉N₂OBF₂: C, 65.88; H, 5.84; N, 8.54. Found (%): C, 65.15; H, 5.61; N, 8.66. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.41 (d, J = 7.2 1H), 7.78 (d, J = 7.9, 1H), 7.62–7.48 (m, 1H), 7.32 (t, J = 7.6, 1H), 7.19 (d, J = 8.2, 1H), 7.04–6.83 (m, 3H), 3.14 (t, J = 7.7, 2H), 1.81 (m, 2H), 1.38 (m, 4H), 0.89 (t, J = 6.7, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 155.10, 131.88, 131.11, 127.61, 127.54, 122.40, 122.16, 120.74, 120.53, 119.81, 119.28, 117.25, 111.11, 31.78, 29.45, 24.47, 22.46, 14.07. FT-IT (ATR) (cm⁻¹): ṽ = 1005–1131 (BF₂), 2869–2960 (aliphatic C-H).

C10-BF₂: mp: 113°. Yield: 0.48 g (48%). Anal. Calcd (%) for C₂₃H₂₉N₂OBF₂: C, 69.36; H, 7.34; N, 7.03. Found (%): C, 69.20; H, 7.58; N, 6.88. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.49 (d, J = 7.7, 1H), 7.78 (d, J = 7.2, 1H), 7.60 (m, 1H), 7.35 (t, J = 7.2, 1H), 7.27 (d, J = 9.4, 1H), 7.11–7.02 (m, 1H), 6.99–6.84 (m, 2H), 3.28–3.11 (m, 5H), 1.88–1.76 (m, 2H), 1.38 (m, 8H), 1.35–1.17 (m, 13H), 0.87 (t, J = 6.7, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 155.34, 132.15, 127.90, 127.58, 122.42, 122.17, 120.81, 120.61, 119.93, 119.54, 117.24, 111.25, 47.22, 32.03, 29.84, 29.73, 29.71, 29.47, 29.45, 24.61, 22.82, 14.25. FT-IT (ATR) (cm⁻¹): ṽ = 1012–1154 (BF₂), 2845–2949 (aliphatic C-H).

C18-BF₂: mp: 106°. Yield: 0.63 g (49%). Anal. Calcd (%) for C₃₁H₄₅N₂OBF₂: C, 72.93; H, 8.89; N, 5.49. Found (%): C, 72.43; H, 9.15; N, 5.11. ¹H NMR (400 MHz, CDCl₃, 298 K, J [Hz]): δ = 8.54 (d, J = 5.9, 1H), 7.83 (d, J = 8.0, 1H), 7.62–7.55 (m, 1H), 7.39 (t, J = 7.8, 1H), 7.26 (t, J = 6.6, 1H), 7.02 t, J = 7.8, 1H), 6.99–6.86 (m, 2H), 3.17 (t, J = 7.7, 2H), 1.83 (m, 2H), 1.48–1.16 (m, 30H), 0.89 (t, J = 6.7, 3H). ¹³C NMR (100 MHz, CDCl₃, 298 K): δ = 155.27, 132.04, 131.26, 127.80, 127.56, 122.42, 122.16, 120.71, 120.64, 119.88, 119.44, 117.24, 111.17, 32.07, 29.84, 29.80, 29.74, 29.72, 29.50, 29.48, 24.59, 22.83, 14.25. FT-IT (ATR) (cm⁻¹): ṽ = 1041–1152 (BF₂), 2845–2956 (aliphatic C-H).

2.5. Computational Details

All calculations were carried out at the density functional (DFT) level of theory with the ADF2022.101 program package [43,44,45]. The BLYP functional plus a D3(BJ) Grimme dispersion correction energy term (BLYP-D3(BJ)) [46] was employed for all calculations. Frequency analyses were performed for all optimized structures to establish the nature of the stationary points. TD-DFT implemented in the ADF package was used to determine the excitation energies: the 20 lowest singlet-singlet excitations were calculated by using the optimized geometries. For geometry optimizations, B, C, N, O and F atoms were described through TZ2P basis sets [triple-ξ Slater-type orbitals (STOs) plus two polarization functions]. For hydrogen atoms, the TZP basis set was employed. The corresponding augmented basis set was employed in TD-DFT calculations [47]. Restricted formalism, no-frozen-core approximation (all-electron) and no-symmetry constrains were used in all calculations. Solvent effects (CH₂Cl₂) were simulated employing the conductor-like continuum solvent model (COSMO) [48,49,50], as implemented in the ADF suite.

3. Results and Discussion

3.1. Syntheses and Characterization

Pyridyl ketones were prepared by following an already published method [51], by reaction between 2-cyanopyridine and the Grignard reagent derived from 1-bromoalkane. They were then used in the synthesis of the CX ligands through a condensation reaction with salicylaldehyde, in the presence of ammonium acetate as a source of the imidazolic nitrogen for the formation of the imidazo[1,5-a]pyridine ring (Scheme 1). The boron difluoride coordination compounds were obtained by reaction of L^R with boron trifluoride diethyl etherate, in the presence of triethylamine, in dichloromethane at room temperature (Scheme 1).

The purity of the products was confirmed by elemental analysis and solution ¹H and ¹³C{¹H} NMR using CDCl₃ as solvent.

¹H NMR of CX ligands (Figures S1–S8) displayed the aromatic signals in the range between 8.50–6.50 ppm, whereas the peaks relative to the alkylic chain could be found at 3.50–0.50 ppm. ¹³C NMR displayed instead the resonances of the aromatic carbon atoms between 160 and 110 ppm and at about 35–14 ppm for the alkyl chain carbons (Figures S1–S8).

¹H NMR of CX-BF₂ recorded in CDCl₃ (Figures S9–S16) showed the same signal pattern in a similar region.

The infrared spectrum (ATR) (Figures S17–S20) shows a series of broad signals between 1000 and 1150 cm⁻¹, in the typical range attributed to the presence of a BF₂ fragment.

3.2. Optical Properties in Solution

Ligands CX displayed a weak fluorescence and were not thoroughly investigated, while all the boron compounds showed blue fluorescence emission in solution (Figure 2). C1 and C1-BF₂ have been studied previously [41] and are used here as a comparison for the luminescence properties of the novel compounds.

Firstly, in order to evaluate a possible solvent effect, we started the evaluation of the photophysical properties of this series of compounds from measuring the fluorescence parameters of C3-BF₂, taken as a representative example of the series, in different solvents (

Table 1. Photophysical data of C3-BF₂ recorded in different solvents (5 × 10⁻⁵ M).

Solvent	λabs (nm)	ε (M−1 cm−1)	λem (nm)	λex (nm)	τ (ns)	ΦPL
Dichloromethane	348	139,605	446	349	3.39	0.19
Methanol	344	115,184	442	346	3.65	0.18
Ethyl acetate	346	20,759	446	348	3.59	0.20
Toluene	351	179,357	450	351	3.68	0.19
Tetrahydrofuran	348	23,484	447	349	3.84	0.26

) (Figures S21–S23).

The absorption maxima fall in a narrow range of 5 nm, with the excitation spectra being very similar to the relative absorption spectra. The spectra recorded in toluene and ethyl acetate displayed a single absorption maximum, suppressing the peak at around 250 nm due to the strong absorption of the solvent in that region. The emission is weakly influenced as well, with a very slight bathochromic shift with decreasing the solvent polarity. Moreover, it is possible to observe the roto-vibrational structure of the band in the spectrum recorded in toluene (Figure S22). Fluorescence lifetimes are very similar, falling in the range between 3.39 and 3.84 ns. No dramatic quenching of the fluorescence was observed, thus confirming only a negligible solvent effect for this series of compounds.

Dichloromethane was then used as solvent for all the following analyses. All the photophysical data are reported in

Table 2. Photophysical data of CX and CX-BF₂ recorded in dichloromethane solution (5 × 10⁻⁵ M).

Compound	λabs (nm)	ε (M−1 cm−1)	λem (nm)	λex (nm)	τ (ns)	ΦPL
C1	339	129,282	472	340	2.97	0.06
C1-BF2	347	117,956	446	348	3.53	0.22
C3	337	94,620	471	340	3.04	<0.05
C3-BF2	348	139,605	446	349	3.39	0.19
C5	339	118,515	476	341	3.05	<0.05
C5-BF2	348	198,650	448	350	3.41	0.22
C10	338	104,460	473	339	2.86	0.05
C10-BF2	348	71,097	449	349	2.94	0.19
C18	340	193,810	477	344	2.65	0.06
C18-BF2	348	193,189	449	348	2.95	0.20

. We recorded the absorption, emission and excitation spectra of both ligands (Figure S24) and boron compounds. A net increase in the emission intensity could be observed when comparing a ligand with the respective BF₂ compound, accompanied by a slight hypsochromic shift of 20–25 nm.

Focusing on the boron complexes, the absorption spectra displayed two maxima, the first one at around 235 nm and the second one at 348 nm. The emission spectra are very similar among the series, with emission maxima between 446 and 449 nm (Figure 3) in the blue region, as can be observed from the CIE1931 chromaticity plot (Figure 4). The very low superposition between the emission spectra and the corresponding absorption spectra is a key feature of this class of compounds, and is fundamental in order to avoid self-absorption processes that could affect the luminescence performances. The excitation spectra largely reproduce the respective absorption traces.

All the BF₂ species displayed mono-exponential lifetime decays, in the range of 2.94 and 3.53 ns, with good fluorescence quantum yields up to 22%, with no sensible quenching effect from the alkyl chains, with the luminescence intensity not affected considerably by the presence of such groups with respect to C1-BF₂. Interestingly, even if the long chains could in principle influence the luminescence performances, they are not involved in the electronic transitions responsible for the absorption process (vide infra), as also corroborated by the nearly identical absorption and emission spectra throughout the series (Figure 5).

Notably, the coordination to the BF₂ fragment greatly increased the luminescent quantum yield of the ligands, reasonably by giving more rigidity to the whole system and thus preventing non-radiative decay paths given by the rotation of the phenolic ring with respect to the imidazolic portion. This is also confirmed by the orbital topology, which clearly showed no contribution from the BF₂ moiety (vide infra), as previously observed for C1 and C1-BF₂ [41].

Fluorescence emission analysis at lower temperatures was performed in order to establish if, by thermally preventing the vibrations of the alkyl pendants, the emission intensity could be affected. The measurements were performed in a temperature range from 0 to −80 °C: upon lowering the temperature, the luminescence intensity increased, with no shifting of the emission maxima (Figure S25). This can be better rationalized by looking at the estimated fluorescent quantum yields calculated at lower temperatures (Figure 6,

Table 3. Photoluminescence quantum yields of CX-BF₂ in dichloromethane solution (5 × 10⁻⁵ M) at different temperatures. ^a experimentally recorded; ^b calculated.

LRBF2	298 K a	273 K b	233 K b	193 K b
C1-BF2	0.22	0.22	0.31	0.30
C3-BF2	0.19	0.19	0.23	0.24
C5-BF2	0.22	0.23	0.26	0.28
C10-BF2	0.19	0.20	0.24	0.25
C18-BF2	0.20	0.22	0.25	0.24

).

The free ligands instead did not show an interesting increase in their emission even by lowering the temperature.

3.3. Optical Properties PMMA Films

A 5% (m/m) mixture of CX-BF₂ in poly (methyl methacrylate) (PMMA) was dissolved in dichloromethane; 200 μL of the resulting solution was then spin coated on a quartz glass at a rate of 5500 rpm for 1 min. The resulting samples were used for the analysis of the fluorescence performances in thin polymeric films (Figure 7).

The fluorescence emission is still in the blue region, with a very small hypsochromic shift of less than 10 nm with respect to the emission recorded in solution. The excitation maxima are also slightly shifted towards the blue, but very close to the maxima recorded in solution. All the fluorescence lifetimes are fitted with a biexponential curve: the first lifetime is between 1.49 and 1.79 ns and the second lifetime is between 3.90 and 4.13 ns (

Table 4. Photophysical data of CX-BF₂ recorded in PMMA films.

Compound	λem (nm)	λex (nm)	τ (ns)	ΦPL
C1-BF2	440	344	1.70 (22.8%) 4.13 (77.2%)	0.11
C3-BF2	440	344	1.66 (14.0%) 3.90 (86.0%)	0.11
C5-BF2	440	344	1.74 (22.0%) 4.03 (78.0%)	0.07
C10-BF2	440	344	1.49 (14.0%) 3.99 (86.0%)	0.11
C18-BF2	440	343	1.79 (21.8%) 4.06 (78.2%)	0.14

). This could be ascribed to some interaction within the polymeric matrix, which could promote different decay pathways. This is also indirectly corroborated by the lower fluorescence quantum yields (7–14%) when compared to solution.

Interestingly, the performances of the compounds with long alkyl chains are similar to those of C1-BF_2, and can therefore be tested in the fabrication of optoelectronic devices.

3.4. DFT Calculations

In analogy with the previously reported optimization of C1-BF₂ from its X-ray data [41], the ground state (S₀) geometries of all other CX-BF₂ compounds were optimized at the DFT level of theory. TD-DFT calculations were performed to rationalize the nature of the transitions responsible for the absorption processes.

The contributions of single orbital transitions to the absorption at lower energy are reported in

Table 5. Contributions of single orbital transitions for CX-BF₂ compounds.

	H/L + 1	H/L	H − 1/L	H − 1/L + 1
C1-BF2	84.3%	7.7%	2.2%	1.9%
C3-BF2	85.0%	7.7%	1.7%	1.7%
C5-BF2	84.3%	8.1%	1.8%	1.8%
C10-BF2	83.5%	8.2%	2.5%	1.8%
C18-BF2	83.9%	8.3%	2.0%	1.7%

. The absorption centered at about 340 nm is composed of four electronic transitions (Table 5), mainly having a homo-lumo + 1 (>83.5%) character and a lower contribution (7–8%) from homo-lumo transition. The frontier molecular orbital topologies are very similar for all the compounds (Figure 8): the HOMO is delocalized over the whole molecule, whereas LUMO is mainly localized on the imidazopyridine moiety. The HOMO−1 has a strong contribution from the phenol residue, while LUMO + 1 is again distributed over the entire ligand. It is worth noting that neither the BF₂ fragment nor the alkyl chain contribute to the frontier orbitals shapes, so that they play no part in the absorption process; the BF₂ portion is giving rigidity to the system by blocking the rotation of the phenolic ring with respect to the imidazopyridine skeleton; this enhances the fluorescence properties limiting the non-radiative decay paths and indeed increasing the quantum yields of CX-BF₂ compounds compared to the free ligands. The long alkyl chain does not affect the emission process as well; thus, it has an effect only on the physical properties of the compounds, such as solubility and film-forming. In summary, the main electronic transitions observed for CX-BF₂ compounds can be described as intra ligand transitions (¹ILT), as also demonstrated by the Natural Transition Orbitals (NTOs) depicted in Figure 9 for C5-BF₂, taken as a representative example of the series.

4. Conclusions

In this work, we presented the synthesis, characterization and study of the photophysical properties of a series of boron difluoride compounds of (imidazo[1,5-a]pyridine-3-yl)phenols functionalized with alkylic chains. This functionalization helped to enhance the solubility and film formation properties of these compounds, with little to no alteration to their luminescence performances. Nevertheless, all these compounds displayed a blue fluorescence emission, with large Stokes shifts and good absolute quantum yields, thus paving the way for their use as emissive materials in blue emissive optoelectronic devices. In particular, they will be tested in the fabrication of blue organic light emitting diodes (OLEDs), continuing a preliminary study where C1-BF₂ was already successfully tested. Moreover, the almost complete inertness of the luminescence performances towards the substitution could be further exploited functionalizing these compounds with several other substituents.