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Article

Introduction of a 4-Hexyl-2-thienyl Substituent on Pyridine Rings as a Route for Brightly Luminescent 1,3-Di-(2-pyridyl)benzene Platinum(II) Complexes

by
Alessia Colombo
1,
Claudia Dragonetti
1,
Francesco Fagnani
1,*,
Dominique Roberto
1,*,
Simona Fantacci
2 and
Daniele Marinotto
3
1
UdR INSTM di Milano, Dipartimento di Chimica, Università degli Studi di Milano, via C. Golgi 19, 20133 Milan, Italy
2
Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto di Scienze e Tecnologie Chimiche “Giulio Natta” SCITEC, Consiglio Nazionale delle Ricerche (CNR), via Elce di Sotto 8, 06213 Perugia, Italy
3
Istituto di Scienze e Tecnologie Chimiche (SCITEC) “Giulio Natta”, Consiglio Nazionale delle Ricerche (CNR), via C. Golgi 19, 20133 Milan, Italy
*
Authors to whom correspondence should be addressed.
Molecules 2025, 30(22), 4410; https://doi.org/10.3390/molecules30224410
Submission received: 29 September 2025 / Revised: 7 November 2025 / Accepted: 12 November 2025 / Published: 14 November 2025
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Abstract

The synthesis and characterization of two new complexes, namely Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-mesitylbenzene)Cl and Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-(2-thienyl)benzene)Cl, are reported. Both exhibit luminescence quantum yields approaching unity (Φlum = 0.96–0.99) in the green region of the visible spectrum (534–554 nm) in diluted degassed dichloromethane solution. Similarly to other N^C^N platinum(II) complexes, a broad emission band grows in the deep red region (738–752 nm) upon increasing the concentration, due to the creation of bi-molecular emissive excited states. Interestingly, it appears that the introduction of a 2-thienyl group on the pyridine rings is a route to maintain excellent quantum yields even in concentrated solution. In order to have an insight into the electronic properties of the novel compounds, density functional theory (DFT) and time-dependent (TD)DFT approaches were employed to calculate the molecular geometry, the ground state, the electronic structure and the excited electronic states of the complexes, both as a monomers and dimers in solution.

1. Introduction

Today, a lot of research work is dedicated to platinum(II) complexes, due to their appealing properties in various fields of optoelectronics and photonics, like photocatalysis [1,2,3], nonlinear optics [4,5,6,7,8,9,10,11,12], sensing [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27], Organic Light Emitting Diodes [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56], and biomedicine [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71]. The platinum atom is associated with a strong spin–orbit coupling, promoting intersystem crossing to triplet excited states and their subsequent radiative decay with emission of light [72,73]. In addition, platinum(II) complexes are characterized by a square planar geometry which can allow Pt–Pt or ligand–ligand intermolecular interactions, leading to bi-molecular states in the excited (excimers) or in the ground (dimers) states. This aspect is of particular importance because the parallel emissions from mono-molecular and bi-molecular excited states are an efficient way to modulate the color of OLEDs by choosing the suitable quantity of the platinum complex in the host matrix used for the fabrication of the emissive layer [74].
Thus, a huge amount of square planar platinum(II) complexes has been designed. In particular, twenty-five years ago, Cardenas et al. reported that K2PtCl4 reacts with 1,3-bis(pyridin-2-yl)benzene (bpybH) in acetic acid, generating Pt(bpyb)Cl, the first Pt(II) compound with a cyclometalated terdentate N^C^N 1,3-bis(pyridin-2-yl)benzene ligand [75]. A few years later, Williams et al. observed that this complex is highly luminescent at room temperature in diluted deoxygenated solution of dichloromethane (Φlum = 0.60 and τ = 7.2 μs) [76]. The discovery of its amazing luminescent properties, in which the rigidity of the terdentate ligand and the presence of a cyclometalating carbon have a positive effect [77], has been a springboard for the design of many platinum(II) complexes with variously substituted 1,3-bis(pyridin-2-yl)benzene ligands.
An interesting aspect of these Pt(bpyb)Cl derivatives is that the absorption and emission wavelengths depend on the presence of electron-donating and electron-withdrawing groups on the cyclometalating benzene and on the pyridine rings. Thus, time-dependent density functional theory calculations predict that the introduction of electron-withdrawing groups in the benzene ring and/or electron-donating groups in the pyridine rings gives a blue-shift in the emission, whereas the addition of electron-donating groups in the benzene ring and/or electron-withdrawing groups in the pyridines leads to a red-shift [78]. This behavior has been confirmed experimentally, offering a useful way to tune the color of the emission [58,77,79,80].
Recently, we found that the introduction of a variously substituted phenyl group on the position 4 of the pyridine rings of the N^C^N 1,3-bis(pyridin-2-yl)benzene ligand opens the door to a new family of luminescent platinum(II) complexes with quantum yields approaching unity [39,81,82,83], which has already found application in the fabrication of efficient OLEDs [39,81]. This observation prompted us to study the influence of the introduction of a different polarizable aromatic substituent, namely a 4-hexyl-2-thienyl group, on the position 4 of the pyridine rings.
In addition, it was reported that the introduction of a bulky mesityl group at position 5 of the cycloplatinating benzene ring of the 1,3-bis(pyridin-2-yl)-benzene ligand allows the tendency to form bi-molecular states to decrease due to steric hindrance. which inhibits the face-to-face approach of two platinum complexes, decreases Pt–Pt interactions and/or π–π stacking, and reduces self-quenching at elevated concentrations [84]. Such a steric effect is an interesting tool for the fabrication of OLEDs, in which local concentrations can be elevated, because self-quenching is an energy sink that reduces device efficiencies [84]. Remarkably, it was shown that substitution of the mesityl group at position 5 of the cycloplatinating benzene ring with a 2-thienyl group leads to a strong red-shift in the emission in line with the influence of the aromatic substituents on the energy of the HOMO [84].
These reports, and the fact that the availability of complexes with tunable emission over a wide color range offers considerable scope in the development of OLEDs encouraged us to prepare and well-characterize Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-mesitylbenzene)Cl and Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-(2-thienyl)benzene)Cl (Chart 1). Both are characterized by excellent quantum yields.

2. Results and Discussion

2.1. Synthesis of the Platinum(II) Complexes

The new pro-ligands, 1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-mesitylbenzene (HL1) and 1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-(2-thienyl)benzene (HL2), were synthesized by Suzuki–Miyaura cross-coupling the corresponding boronic acid pinacol ester [39] with 2-chloro-4-(4-hexyl-2-thienyl)-pyridine (Scheme 1). Reaction in glacial acetic acid of these pro-ligands with K2PtCl4 led to the desired complexes in 61–68% yield. They were fully characterized by NMR (1H and 13C) and elemental analysis. Full details of the synthesis and characterization are provided in the Section 3 and in the Supporting Information.

2.2. Photophysical Properties

The electronic absorption spectra of complexes PtL1Cl and PtL2Cl in CH2Cl2 at different concentrations (1 · 10−6–5 · 10−5 M) are presented in Figure 1. Both complexes are characterized by bands of high intensity below 330 nm, attributed to intraligand 1 π–π * transitions of the N^C^N cyclometalating ligand, analogous to related platinum(II) complexes [84]. The absorption bands at 330–470 nm correspond to transitions of mixed charge-transfer/ligand-centered character, as reported in the literature [84,85]. Accordingly, they exhibit a pronounced hypsochromic effect typical of electronic transitions with an appreciable degree of charge-transfer character; the blue-shift in the peaks with increasing solvent polarity (from toluene to dichloromethane) is in agreement with a higher dipole moment in the ground state than in the excited state [84] (see Figures S20 and S21).
The normalized excitation and emission spectra at room temperature of PtL1Cl and PtL2Cl in degassed dilute (2 · 10−6 M) and concentrated (2 · 10−4 M) dichloromethane solution are shown in Figure 2. After excitation at 290 nm in dilute solution, the emission spectrum of PtL1Cl shows a structured broad band characterized by a high-energy emission maximum at 534 nm, with shoulders at 568 and 617 nm, attributed to emission from the T1 state [82,84]. Complex PtL2Cl is characterized by a high-energy emission maximum at 554 nm, with a shoulder at 589 nm, red-shifted with respect to PtL1Cl, in agreement with the substitution of the mesityl group with the more-donating 2-thienyl moiety [84]. The same hypsochromic effect observed in the absorption spectra in toluene vs. dichloromethane is also observed in the excitation spectra (see Figures S24 and S25).
It is worth pointing out that the high-energy emission maximum of both complexes is red-shifted compared with the parent complexes with unsubstituted pyridine rings [84], which highlights the effect of introducing 4-hexyl-2-thienyl substituents. The effect is stronger in the presence of the mesityl group, although it is slightly less pronounced than that observed in the case of a triphenylamine substituent on the pyridine rings [81] (Table 1).
As expected from the typical comportment of N^C^N platinum complexes [30,79,80,81,82,83,84], an increase in the concentration of [PtL1Cl] and [PtL2Cl] up to 2 · 10−4 M leads to the appearance of a new, broad, structureless emission band centered at 752 and 738 nm, respectively, which becomes progressively more intense at the expense of the shorter wavelength bands. This lower energy band, ascribed to bi-molecular emissive excited states (excimers/aggregates) of the platinum (II) complexes generated through π–π ligand–ligand and Pt–Pt intermolecular interactions [28], is much more intense for [PtL2Cl] than for [PtL1Cl] (see Figure 2), highlighting that the presence of the bulky mesityl group in para of the cyclometalating benzene is much more efficient than the thienyl group in hampering the formation of dimeric species.
Both new complexes exhibit excellent absolute quantum yields, 0.96 and 0.99 for PtL1Cl and PtL2Cl, respectively, as measured with an integrating sphere (see Table 1). When the dichloromethane solution is aerated, the Φlum diminishes drastically to 0.02 and 0.06, due to oxygen quenching, in agreement with the behavior of related complexes (Table 1) [81,84]. This represents an interesting aspect for photodynamic therapy, because an effective generation of singlet oxygen can be expected for these complexes [71].
It is well-established that an augmentation of concentration typically causes a strong diminution of the luminescence quantum yield of N^C^N cyclometalated platinum(II) complexes due to the formation of excimers and aggregates [77]. In the case of PtL1Cl and PtL2Cl, although the quantum yield decreases upon increasing the concentration of the degassed dichloromethane solution, it remains very high (Φlum = 0.50 and 0.42 at 2 · 10−4 M, respectively), as previously observed for the complex related to PtL1Cl bearing a triphenylamine instead of a 4-hexyl-2-thienyl substituent (Φlum = 0.44 at 2 · 10−4 M) [81] and for other N^C^N cyclometalated platinum(II) complexes bearing a variously substituted phenyl group in position 4 of the pyridine rings [39,82,83]. This observation suggests that the introduction of aromatic groups on the pyridine rings is a route to maintain excellent quantum yields even in concentrated solution, an aspect of particular interest for the fabrication of OLEDs, in which local concentrations can be elevated.
Excited state decay measurements of PtL1Cl in degassed dilute dichloromethane solution were carried out, exciting at 405 nm at the emission wavelength of 532 nm, evidencing a luminescence lifetime, τ, of 24.1 μs (Table 1). A similar lifetime was obtained at the emission wavelength of 725 nm (ESI, Figure S20), in agreement with the tail of the emission spectra of the complex at low frequency in dilute solution (Figure 2). A much lower τ (6.1 μs) is obtained in concentrated solution (ESI, Figure S21), as expected, because an increase in the concentration leads to the formation of bi-molecular states with a diminution of both lifetime and quantum yield. Similarly, PtL2Cl is characterized by a longer luminescence lifetime in dilute (τ = 5.9 μs, Table 1) than in concentrated (τ = 1.4 μs) solution (ESI, Figures S22 and S23). It is worth pointing out that PtL1Cl, bearing a mesityl group in position 5 of the cyclometalating benzene, is characterized by a much longer lifetime than the related complex with non-substituted pyridine rings [84], revealing the positive effect of the introduction of 4-hexyl-2-thienyl substituents, which remains less pronounced than that observed in the case of triphenylamine substituents (Table 1). However, such an effect, though of interest for many applications like bioimaging [81], cannot be generalized because it is not observed when the mesityl group is substituted by a 2-thienyl one.
Moreover, the two new complexes were investigated in poly(methyl methacrylate)—PMMA—thin films with a loading of 1% w/w, registering the absorption and luminescence spectra, and measuring absolute quantum yields and lifetimes in air. In agreement with the relatively low amount of compound in the polymer, the UV–Vis absorption spectra present the same features of those of diluted solutions. Emission and excitation spectra are reported in Figures S32 and S33 and resemble the corresponding spectra in dichloromethane solution, as expected as a consequence of the low loading of the complex in the polymer, so that no aggregate species are clearly visible. The luminescence quantum yields of the films are 0.50 and 0.40, for PtL1Cl and PtL2Cl, respectively, similar to that (0.56) previously reported for a related Pt(II) complex with triphenylamino substituents on the pyridine rings [81]; these quantum yields are remarkable since the measurements are carried out in air. With respect to solutions, the lifetimes decrease in the PMMA films (16.4 vs. 24.1 μs for PtL1Cl, and 5.1 vs. 5.9 μs for PtL2Cl). In addition, kr values of 3.03 · 104 s−1 (PtL1Cl) and 7.78 · 104 s−1 (PtL2Cl) were found, which are interesting values for the preparation of OLED devices (see Table S3 for complete data). The calculated kr are quite similar to those found for dilute dearated solutions, while knr are higher, due to the vibrational motions induced by the interaction with the PMMA matrix.

2.3. Theoretical Calculations

To give insight into the electronic and optical properties of the two investigated complexes, we employed computational modeling with methods based on the density functional theory (DFT) and its time-dependent version (TDDFT). We started by optimizing the molecular geometry of the two monomeric complexes and of their dimers by including dichloromethane solvation effects. The optimized geometries of the two investigated Pt complexes and their related dimers are reported in Figure 3. For the dimer of PtL1Cl, with the bulky mesityl group at position 5 of the cycloplatinating benzene ring, only one dimer molecular geometry characterized by the mesityl group on the opposite side has been optimized. For compound PtL2Cl, we explored the two possible reciprocal arrangements of the two complex units, finding that in dichloromethane solution, the most stable configuration by 1.7 kcal/mol is that one with the two 4-hexyl-2-thienyl groups on the same side. We consider the optimized monomers representative of the low-concentration solution and the dimers of the higher concentrations. We have seen for similar systems that the considered models (monomer and dimer), though simplified, are able to describe two different concentration conditions [39].
We therefore simulated the spectra of complexes PtL1Cl and PtL2Cl, both as monomers and dimers, by computing 60 TDDFT excitations for the monomers and 150 excitations for the dimers, thus, we are able to describe the 450–270 nm and 480–290 nm regions of the absorption spectra of the monomers and dimers, respectively. All the simulated absorption spectra are presented in Figure S36.
We observe that the spectra of the monomers of PtL1Cl and PtL2Cl are quite similar in the 400–320 nm region, while a few differences essentially due to a higher intensity of the transitions computed for the PtL2Cl monomer are evidenced in the 400–500 nm region and 320–290 nm region. The spectra of dimers show a few differences due to the higher intensity of the transitions of PtL1Cl dimer and for a larger number of transitions characterized by a comparable or lower intensity in the considered wavelength range. The reasonable agreement between the measured and simulated UV–Vis spectra prompted us to investigate the nature of the main transitions giving rise to the spectra in order to assign the spectral bands. To this end, we calculated the electron density differences between the most intense excited states and the ground state electron density for both monomers and dimers. This kind of analysis provides a valuable tool for assigning the character of spectral bands since it visualizes the directionality of the electron density from the initial state to the final excited state. In Figure 4, Figure 5, Figure 6 and Figure 7, the plots of the density difference between the excited states and the ground state are reported for both PtL1Cl and PtL2Cl in the monomer and dimer forms: in red, where the electronic density moves is reported, and in blue, where it arrives. We have considered the excited states with the highest oscillator strengths that mainly contribute to the main spectral features. It is interesting to notice that the main transition involves as starting states the Pt d orbitals mixed with π combinations on the substituted 2-pyridyl-benzene ligands, while the arriving states are delocalized on the antibonding π combinations of the Pt ligands. This observation brings us to assign the transitions generating the spectra as metal-to-ligand charge-transfer transitions (MLCT) and metal–ligand-to-ligand charge-transfer transitions (MLLCT).

3. Materials and Methods

All the reagents and the solvents were used as received from the supplier (Merck, Rahway, NJ, USA). The purifications were performed through column chromatography on silica gel (Merck Geduran 60, 0.063–0.200 mm).
The NMR characterizations were obtained recording on a Bruker AV III 300 MHz or AV III 400 MHz spectrometers (Billerica, MA, USA). Chemical shifts in 1H and 13C NMR spectra are reported in parts per million (ppm) and the coupling constants are measured in Hertz (Hz). The multiplicities of signals are listed as singlet (s), d (doublet), t (triplet), quartet (q), multiplet (m).
Elemental analyses were performed by the Department of Chemistry of the University of Milan.
Electronic absorption spectra in solution were obtained with a UV-3600i Plus UV-VIS-NIR spectrophotometer (Shimadzu Italia S.r.l., Milan, Italy). Luminescence measurements were carried out in CH2Cl2 solution after the Freeze–Pump–Thaw (FPT) procedure, necessary to remove dissolved oxygen. Absolute photoluminescence quantum yield, Φ, was measured using a C11347 Quantaurus Hamamatsu Photonics K.K spectrometer (Hamamatsu, Japan, See ESI). The details about the synthesis of intermediated I1 and I2, of the thienyl-pyridine P1 and of boronic esters B1 and B2 are reported in the ESI, together with the NMR spectra of all compounds.

3.1. Synthesis of PtL1Cl

3.1.1. Synthesis of Ligand HL1

Boronic ester B1 (32 mg, 0.069 mmol), pyridine P1 (58 mg, 0.207 mmol), Na2CO3 (54 mg, 0.514 mmol) and Pd(PPh3)4 (6 mg, 0.005 mmol) were added to 1,2-dimethoxyethane (1.0 mL) and water (1.0 mL) in a Schlenk tube and the mixture was stirred at reflux under Ar atmosphere. After 48 h, the reaction was cooled to rt, AcOEt and water were added, and the phases were separated. The organic phase was washed with water, and the aqueous phase was extracted with AcOEt; the organic phases were dried over Na2SO4 and evaporated at reduced pressure. The reaction mixture was purified by flash chromatography on silica gel (eluent: hexane/AcOEt 9:1), obtaining 21 mg of product (yield 45%).
1H-NMR (400 MHz, CD2Cl2) δ (ppm): 8.88 (1H, bs), 8.71 (2H, d, J = 5.2 Hz), 8.11 (2H, s), 7.95 (2H, d, J = 1.3 Hz), 7.57 (2H, s), 7.52 (2H, dd, J = 1.3 Hz, J = 5.2 Hz), 7.11 (2H, s), 7.04 (2H, s), 2.69 (4H, t, J = 7.6 Hz), 2.39 (3H, s), 2.15 (6H, s), 1.75–1.67 (4H, m), 1.43–1.32 (12H, m), 0.93 (6H, t, J = 6.8 Hz).
13C-NMR (100 MHz, CD2Cl2) δ (ppm): 149.52, 128.86, 128.09, 127.46, 124.24, 122.46, 118.56, 116.83, 31.63, 30.44, 30.40, 28.93, 22.59, 20.74, 20.64, 13.81.

3.1.2. Synthesis of Complex PtL1Cl

Ligand HL1 (21 mg, 0.031 mmol) and K2PtCl4 (15 mg, 0.036 mmol) were added to glacial AcOH (1.0 mL) in a Schlenk tube and the mixture was stirred at reflux under Ar atmosphere. After 48 h, the reaction was cooled to rt and water was added to precipitate more product. The orange precipitate was filtered on a Buchner funnel, washed with H2O, MeOH and Et2O, and dried, obtaining 17 mg of product (yield 61%) which was not further purified.
1H-NMR (400 MHz, CD2Cl2) δ (ppm): 9.20 (2H, d, J = 6.1 Hz, J(195Pt) = 40 Hz), 7.86 (2H, d, J = 2.0 Hz), 7.55 (2H, d, J = 1.1 Hz), 7.49 (2H, dd, J = 2.0 Hz, J = 6.1 Hz), 7.41 (2H, s), 7.19 (2H, d, J = 1.1 Hz), 7.04 (2H, s), 2.69 (4H, t, J = 7.6 Hz), 2.39 (3H, s), 2.16 (6H, s), 1.74–1.64 (4H, m), 1.42–1.32 (12H, m), 0.93 (6H, t, J = 6.9 Hz).
13C-NMR (100 MHz, CD2Cl2) δ (ppm): 151.73, 145.55, 139.24, 136.05, 128.68, 128.05, 125.26, 124.32, 122.59, 118.59, 115.01, 31.61, 30.37, 28.89, 22.57, 20.66, 13.81.

3.2. Synthesis of PtL2Cl

3.2.1. Synthesis of Ligand H2L

Boronic ester B2 (43 mg, 0.104 mmol), pyridine P1 (87 mg, 0.312 mmol), Na2CO3 (81 mg, 0.768 mmol) and Pd(PPh3)4 (8 mg, 0.007 mmol) were added to 1,2-dimethoxyethane (1.5 mL) and water (1.5 mL) in a Schlenk tube and the mixture was stirred at reflux under Ar atmosphere. After 48 h, the reaction was cooled to rt, AcOEt and water were added and the phases were separated. The organic phase was washed with water, and the aqueous phase was extracted with AcOEt; the organic phases were dried over Na2SO4 and evaporated at reduced pressure. The reaction mixture was purified by flash chromatography on silica gel (eluent: hexane/AcOEt 8:2); since the obtained product was not sufficiently pure, a preparative TLC was then performed eluent: hexane/AcOEt 9:1), obtaining 10 mg of product (yield 15%).
1H-NMR (400 MHz, CD2Cl2) δ (ppm): 8.74 (2H, d, J = 5.2 Hz), 8.86 (1H, t, J = 1.5 Hz), 8.42 (2H, d, J = 1.5 Hz), 8.12 (2H, d, J = 0.9 Hz), 7.63 (1H, d, J = 3.6 Hz), 7.58 (2H, d, J = 1,1 Hz), 7.54 (2H, dd, J = 0.9 Hz, J = 5.2 Hz), 7.43 (1H, d, J = 5.1 Hz), 7.21 (1H, dd, J = 3.6 Hz, J = 5.1 Hz), 7.13 (2H, s), 2.71 (4H, t, J = 7.6 Hz), 1.77–1.66 (4H, m), 1.46–1.33 (12H, m), 0.94 (6H, t, J = 7.1 Hz).
13C-NMR (100 MHz, CD2Cl2) δ (ppm): 156.99, 149.77, 145.14, 143.79, 143.01, 140.55, 140.11, 135.44, 128.19, 127.34, 125.38, 125.27, 124.80, 124.07, 122.41, 118.73, 116.86, 31.65, 30.43, 30.42, 28.96, 22.61, 13.84.

3.2.2. Synthesis of Complex PtL2Cl

Ligand HL2 (10 mg, 0.015 mmol) and K2PtCl4 (8 mg, 0.019 mmol) were added to glacial AcOH (0.5 mL) in a Schlenk tube and the mixture was stirred at reflux under Ar atmosphere. After 48 h, the reaction was cooled to rt and water was added to precipitate more product. The orange precipitate was filtered on a Buchner funnel, washed with H2O, MeOH and Et2O, and dried, obtaining 9 mg of product (yield 68%) which was not further purified.
1H-NMR (400 MHz, CD2Cl2) δ (ppm): 9.15 (2H, d, J = 6.1 Hz, J(195Pt) = 39 Hz), 7.89 (2H, s), 7.81 (2H, s), 7.59 (2H, s), 7.54 (1H, d, J = 3.5 Hz), 7.47–7.39 (3H, m), 7.23–7.18 (3H, m), 2.71 (4H, t, J = 7.7 Hz), 1.76–1.67 (4H, m), 1.47–1.34 (12H, m), 0.95 (6H, t, J = 6.7 Hz).

3.3. Preparation of PMMA Films

Thin films containing 1 wt% of complexes PtL1Cl and PtL2Cl in poly-methylmethacrylate (PMMA, Mw ≈ 15,000 g/mol) on quartz plate (thickness 1 mm) were obtained by spin-coating (Cookson Electronic Company P-6708D, Phoenix, AZ, USA). The parameters of spinning (RPM-revolutions per minute) were RPM 1: 800; Ramp 1: 1 s, Time 1: 5 s; RPM 2: 2000; Ramp 2: 1 s, Time 2: 120 s; RPM 3: 4000; Ramp 3: 2 s, Time 30 s. The solutions for the 1 wt% thin film were prepared with 1.33 mg of complex and 133 mg of PMMA in 1 mL of dichloromethane.

3.4. Theoretical Calculations

The geometry optimizations of monomers and dimers of complexes PtL1Cl and PtL2Cl were performed with the Gaussian09 program package (G09) [86] by using the B3LYP exchange–correlation functional [87] integrated with the D3-BJ model [88] to include the dispersion effects in the geometry optimizations. The dichloromethane solvation effects were included through the conductor-like polarizable continuum model as implemented in G09 [89,90,91,92]. All the atoms except Pt were described by 6–31G** basis set [93,94,95], while Pt was described with the LANL2DZ basis set, along with the corresponding pseudopotentials [96]. A total of 60 and 150 TDDFT excitations were computed for the PtL1Cl and PtL2Cl monomers and dimers, respectively, by including dichloromethane solvation effects.

4. Conclusions

In conclusion, two new complexes, namely Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-mesitylbenzene)Cl and Pt(1,3-bis(4-(4-hexyl-2-thienyl)-pyridin-2-yl)-5-(2-thienyl)benzene)Cl were easily synthesized and well-characterized. In degassed diluted dichloromethane solution, both exhibit luminescence quantum yield approaching unity (Φlum = 0.96–0.99) in the green region of the visible spectrum (534–554 nm) with lifetimes values of 24.1 and 5.9 µs, respectively. Interestingly, this work shows that the introduction of a 2-thienyl group on the pyridine rings is a route to maintain very good quantum yields (Φlum = 0.42–0.50) even in concentrated solution, a feature of particular interest for the preparation of OLEDs. Also, it confirms that a bulky mesityl group in position 5 of the cyclometalating benzene hampers the neighbouring of monomeric complexes better than a 2-thienyl group, as evidenced by the lower intensity of the low energy band characteristic of excimers/aggregates in concentrated solutions. These observations represent guidelines for the design of N^C^N platinum complexes, which are of importance for the development of luminescent materials [97,98]. As a next step, it would be of interest to use the novel complexes bearing a long alkyl chain, as prepared in the present work, as solution-processable emitters for the fabrication of solution-processed OLEDs.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30224410/s1, the full characterization of the two complexes (Synthetic procedures, 1H and 13C NMR spectra, photophysical characterization in solution and in thin films, calculated absorption spectra) have been included as part of the Supplementary Information. Reference [99] is cited in the Supplementary Materials.

Author Contributions

Conceptualization, A.C. and D.R.; methodology, D.M., C.D., F.F. and S.F.; investigation, D.M., F.F. and S.F.; resources, A.C. and C.D.; data curation, D.M. and F.F.; supervision, D.R.; writing—original draft, F.F.; writing—review and editing, D.M., A.C., C.D., F.F., D.R. and S.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Acknowledgments

Fondazione Cariplo and Regione Lombardia are acknowledged for the instrumentation bought during the SmartMatLab Centre project (2014). The work was supported by the National Interuniversity Consortium of Materials Science and Technology (Project TRI_25_073 Dragonetti and TRI_25_173 Colombo) and the University of Milan (PSR2025_DIP_005_GDICA). This work has been funded by the European Union—NextGenerationEU under the Italian Ministry of University and Research (MUR) National Innovation Ecosystem grant ECS00000041—VITALITY. CUP: B43C22000470005.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 04410 ch001
Chart 1. Chemical structures of the investigated Pt(II) complexes.
Chart 1. Chemical structures of the investigated Pt(II) complexes.
Molecules 30 04410 ch001
Molecules 30 04410 sch001
Scheme 1. Synthesis of complexes PtL1Cl and PtL2Cl. (i): Pd(PPh3)4, Ba(OH)2·8H2O, 1,2-dimethoxyethane/H2O, Ar, 80 °C, 20 h; (ii): Pd(PPh3)4, LiCl, toluene, Ar, reflux, 24 h; (iii): Pd(dppf)Cl2, AcOK, toluene, Ar, reflux, 24 h; (iv): Pd(PPh3)4, Na2CO3, dioxane/H2O, Ar, 100 °C, 48 h; (v): Pd(PPh3)4, Na2CO3, 1,2-dimethoxyethane/H2O, Ar, 100 °C, 20 h; (vi): AcOH, Ar, reflux, 48 h.
Scheme 1. Synthesis of complexes PtL1Cl and PtL2Cl. (i): Pd(PPh3)4, Ba(OH)2·8H2O, 1,2-dimethoxyethane/H2O, Ar, 80 °C, 20 h; (ii): Pd(PPh3)4, LiCl, toluene, Ar, reflux, 24 h; (iii): Pd(dppf)Cl2, AcOK, toluene, Ar, reflux, 24 h; (iv): Pd(PPh3)4, Na2CO3, dioxane/H2O, Ar, 100 °C, 48 h; (v): Pd(PPh3)4, Na2CO3, 1,2-dimethoxyethane/H2O, Ar, 100 °C, 20 h; (vi): AcOH, Ar, reflux, 48 h.
Molecules 30 04410 sch001
Molecules 30 04410 g001
Figure 1. Absorption spectra of PtL1Cl (A) and PtL2Cl (B) in CH2Cl2 at different concentrations at 298 K.
Figure 1. Absorption spectra of PtL1Cl (A) and PtL2Cl (B) in CH2Cl2 at different concentrations at 298 K.
Molecules 30 04410 g001
Molecules 30 04410 g002
Figure 2. Normalized excitation and emission spectra of PtL1Cl (A) and PtL2Cl (B) at different concentrations in deaerated CH2Cl2 at 298 K.
Figure 2. Normalized excitation and emission spectra of PtL1Cl (A) and PtL2Cl (B) at different concentrations in deaerated CH2Cl2 at 298 K.
Molecules 30 04410 g002
Molecules 30 04410 g003
Figure 3. Optimized geometries of the monomers and dimers of PtL1Cl and PtL2Cl in their top and front views.
Figure 3. Optimized geometries of the monomers and dimers of PtL1Cl and PtL2Cl in their top and front views.
Molecules 30 04410 g003
Molecules 30 04410 g004
Figure 4. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL1Cl monomer.
Figure 4. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL1Cl monomer.
Molecules 30 04410 g004
Molecules 30 04410 g005
Figure 5. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL1Cl dimer.
Figure 5. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL1Cl dimer.
Molecules 30 04410 g005
Molecules 30 04410 g006
Figure 6. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL2Cl monomer.
Figure 6. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL2Cl monomer.
Molecules 30 04410 g006
Molecules 30 04410 g007
Figure 7. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL2Cl dimer.
Figure 7. Electron density differences in the most relevant S0–Sn electronic transitions of the PtL2Cl dimer.
Molecules 30 04410 g007
Table 1. Key photophysical parameters of PtL1Cl and PtL2Cl in comparison with that of related complexes [81,84], at 298 K in dilute degassed CH2Cl2 solution.
Table 1. Key photophysical parameters of PtL1Cl and PtL2Cl in comparison with that of related complexes [81,84], at 298 K in dilute degassed CH2Cl2 solution.
λmax,em/nm
Monomer
[Excimer/Aggregate] a		Φlum
Degassed
[Aerated] 		τ/µs
Molecules 30 04410 i001534 b
[752]		0.96 b
[0.02]		24.1
Molecules 30 04410 i002554 b
[738]		0.99 b
[0.06]		5.9
Molecules 30 04410 i003501
[690–700]		0.62 c
[0.045]		7.9
Molecules 30 04410 i004548
[690–700]		0.54 c
[0.015]		20.5
Molecules 30 04410 i005548
[725]		0.89 b,d
[0.01]		50.0
a Excimer/aggregate in concentrated solution. b Absolute Φlum measured with an integrating sphere. c Luminescence quantum yields were determined by the method of continuous dilution, using quinine sulfate in 1 M H2SO4 (Φlum = 0.546) as the standard; the estimated uncertainty is 20% or better [84]. d From reference [81].
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Colombo, A.; Dragonetti, C.; Fagnani, F.; Roberto, D.; Fantacci, S.; Marinotto, D. Introduction of a 4-Hexyl-2-thienyl Substituent on Pyridine Rings as a Route for Brightly Luminescent 1,3-Di-(2-pyridyl)benzene Platinum(II) Complexes. Molecules 2025, 30, 4410. https://doi.org/10.3390/molecules30224410

AMA Style

Colombo A, Dragonetti C, Fagnani F, Roberto D, Fantacci S, Marinotto D. Introduction of a 4-Hexyl-2-thienyl Substituent on Pyridine Rings as a Route for Brightly Luminescent 1,3-Di-(2-pyridyl)benzene Platinum(II) Complexes. Molecules. 2025; 30(22):4410. https://doi.org/10.3390/molecules30224410

Chicago/Turabian Style

Colombo, Alessia, Claudia Dragonetti, Francesco Fagnani, Dominique Roberto, Simona Fantacci, and Daniele Marinotto. 2025. "Introduction of a 4-Hexyl-2-thienyl Substituent on Pyridine Rings as a Route for Brightly Luminescent 1,3-Di-(2-pyridyl)benzene Platinum(II) Complexes" Molecules 30, no. 22: 4410. https://doi.org/10.3390/molecules30224410

APA Style

Colombo, A., Dragonetti, C., Fagnani, F., Roberto, D., Fantacci, S., & Marinotto, D. (2025). Introduction of a 4-Hexyl-2-thienyl Substituent on Pyridine Rings as a Route for Brightly Luminescent 1,3-Di-(2-pyridyl)benzene Platinum(II) Complexes. Molecules, 30(22), 4410. https://doi.org/10.3390/molecules30224410

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