Rhenium(I) complexes with phenanthrolines bearing electron-withdrawing Cl and electron-donating CH₃ substituents – synthesis, photophysical, thermal, and electrochemical properties with electroluminescence ability

Abstract

Five rhenium(I) tricarbonyl complexes incorporating 1,10-phenanthroline derivatives with electron-withdrawing Cl and electron-donating CH₃ substituents were synthesized, and their photophysical, thermal, and electrochemical properties, with electroluminescence ability were examined. The melting temperature of these complexes was found to decrease from 391 to 281 °C upon adding methyl groups and decreasing the number of chlorine atoms. Compounds bearing methyl substituents could form amorphous molecular materials with glass transition temperatures in the range 118–127 °C. Cyclic voltammetry measurements demonstrated that the complexes are electrochemically active with low energy band gaps in the range 2.24–2.46 eV. All complexes are photoluminescent, form films in the solid state, and blend with poly(9-vinylcarbazole) (PVK) and PVK:(2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole) (PBD), emitting light with λ_em from 560 to 599 nm. They exhibited large Stokes shifts of up to 323 nm in solutions and up to 220 nm as film on a glass substrate. They were tested as guests in organic light-emitting diodes. As hosts, PVK and a binary matrix consisting of PVK with PBD were applied. The ability of the investigated complexes to emit light under an applied voltage was shown.

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Introduction

The photochemistry and photophysics of coordination compounds have been the subject of intense research efforts for many years. The presence of a transition metal in molecules introduces new types of excited states, which differ in their localization within the molecule, energy, dynamics, and reactivity. Compared to organic molecules, the excited state density is much higher and several different excited states can occur within a narrow energy range. Consequently, excited states of different character (metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), σ-bond-to-ligand charge transfer (σ → π*), and intraligand (IL)) can interact with each other, leading to complex photobehavior.¹ Having large spin–orbit coupling constants, transition metals substantially accelerate singlet → triplet intersystem crossing (ISC), producing the lowest triplet state on ultrafast time scales and yielding strong triplet-based photoluminescence of the order of microseconds in most cases.²

The continuing development of luminescent coordination compounds has largely been motivated by their possible use in catalysis,³ solar energy conversion,⁴ luminescence sensing,⁵ light-emitting diodes (LEDs),⁶ nonlinear optical materials,⁷ and molecular sensors.⁸

Since the first observation of electronically excited luminescence in [ReCl(CO)₃(phen)], reported by Wrighton and Morse in 1974,⁹ particular attention has been devoted to rhenium(I) diimine tricarbonyls [Re(CO)₃L_a(N–N)]ⁿ⁺ (L_a – axial ligand; n = 0 or 1). So far, a great number of investigations on the [Re(CO)₃L_a(bipy)]ⁿ⁺ and [Re(CO)₃L_a(phen)]ⁿ⁺ chromophores have appeared in the literature.¹⁰ The influence of structural variations of diimines, and the effect of the axial ligand on the photophysical and photochemical properties of rhenium(I) tricarbonyls have been investigated, both experimentally and computationally.¹¹ For example, Rillema et al.¹² examined the photobehavior of a series of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide and 5- and 6-derivatized phenanthroline ligands. Cardinaels¹³ and Pope¹⁴ demonstrated the effect of substitution patterns on the photoluminescent properties of rhenium(I) complexes incorporating substituted imidazo[4,5-f]-1,10-phenanthrolines.

Due to their remarkable photochemical and photophysical properties, diimine rhenium(I) tricarbonyls are still considered as excellent candidates for various applications in areas ranging from electronic and energy transfer to catalysis and medicinal chemistry. In this context, there is a need for the development of new luminescent rhenium(I) coordination compounds.

Herein, we report the synthesis, characterization, photophysical, thermal, and electrochemical properties of three novel rhenium(I) tricarbonyl complexes incorporating 1,10-phenanthroline derivatives: 2,5,9-trimethyl-4,7-dichloro-1,10-phenanthroline (2,5,9-(Me)₃-4,7-Cl₂phen), 2,9-dimethyl-4,7-dichloro-1,10-phenanthroline (2,9-(Me)₂-4,7-Cl₂phen), and 4,5,7-trichloro-1,10-phenanthroline (4,5,7-Cl₃phen). For a better understanding of the effect of the number and location of chloro substituents in 1,10-phenanthroline, rhenium(I) tricarbonyls with 4,7-dichloro-1,10-phenanthroline (4,7-Cl₂phen)^2a,2b,9,15e and 5-chloro-1,10-phenanthroline (5-Clphen)^2a,2b,9 were also included in the studies.

The luminescent properties of Re(I) complexes were studied in solution (CHCl₃ and CH₃CN) and solid state at room temperature, as well as in 4 : 1 EtOH–MeOH glass matrix at 77 K. To obtain a detailed insight into their electronic structures and spectroscopic properties, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed.

Results and discussion

Synthesis and general characterization of Re(I) complexes

The complexes [ReCl(CO)₃(2,5,9-(Me)₃-4,7-Cl₂phen)] (1), [ReCl(CO)₃(2,9-(Me)₂-4,7-Cl₂phen)] (2), [ReCl(CO)₃(4,5,7-Cl₃phen)] (3), [ReCl(CO)₃(4,7-Cl₂phen)] (4), and [ReCl(CO)₃(5-Clphen)]·CH₃CN (5) were synthesized by heating equimolar mixtures of [Re(CO)₅Cl] and the corresponding 1,10-phenanthroline derivative in acetonitrile.

The infrared spectra of 1–5 display three intense bands ν(CO) in the region 1876–2025 cm⁻¹ with a typical pattern for complexes incorporating the fac-[Re(CO)₃]⁺ moiety, two overlapping lower-energy bands (1937–1876 cm⁻¹), and a sharp intense band at higher wavenumbers (2018–2025 cm⁻¹). The presence of the phenanthroline ligand is shown in the IR spectra by medium intensity bands in the range 1617–1560 cm⁻¹, corresponding to the ν(CN) and ν(C=C) modes.

The ¹³C NMR spectra of 1–5 exhibit characteristic resonances around 198 and 189 ppm, corresponding to the carbonyl groups. Distinctive signals of the alkyl protons of the ligands 2,5,9-(Me)₃-4,7-Cl₂phen (in 1) and 2,9-(Me)₂-4,7-Cl₂phen (in 2) occur in the range 3.23–3.14 ppm, while the aromatic protons of the complexes of 1–5 were detected between 8.10 and 9.52 ppm (details are given in the Experimental part).

Molecular structures of rhenium(I) complexes

The crystallographic data for [ReCl(CO)₃(2,5,9-(Me)₃-4,7-Cl₂phen)] (1), [ReCl(CO)₃(4,5,7-Cl₃phen)] (3), and [ReCl(CO)₃(5-Clphen)]·CH₃CN (5) are summarized in Table S1 (ESI†). Perspective views showing the molecular structures of 1, 3, and 5 with the atom numbering are presented in Fig. 1. The crystal structure of 1 comprises two independent molecules per asymmetric unit, and only small variations in bond lengths and bond angles can be noticed between these molecules (Table S2†). The rhenium ions of 1, 3, and 5 adopt a distorted octahedral coordination geometry with the largest deviations from the expected 90° bond angles being due to the geometrical constraints imposed by the five-member chelate ring of the phen ligand, with N–Re–N bite angles equal to 74.7(3) and 74.6(3)° for 1, 74.4(4)° for 3, and 75.28(14)° for 5. The three carbonyl groups are in a facial arrangement and the Re–CO bonds are nearly perpendicular to each other, with the angles OC–Re–CO falling in the range 83.8(5)–92.2(4)°. The Re–C (1.881(11)–1.938(12) Å) and Re–N (2.158(4)–2.213(7) Å) distances are unexceptional and they correlate well with values reported for the related tricarbonyl Re(I) complexes.¹⁵

Fig. 1 A perspective view showing the asymmetric units of 1, 3, and 5 with the atom numbering. Displacement ellipsoids are drawn at 50% probability.

The structures 1, 3, and 5 are stabilized by short contacts, C–H⋯Cl, C–H⋯O, and C–H⋯N (Table S3†), which can be considered as weak hydrogen bonds as well as by π–π type interactions. As demonstrated by the crystal packing analysis (Mercury 2.4 program¹⁶), the molecules in structures 1 and 3 are linked into dimers by π–π type interactions with centroid-to-centroid separations of 3.883(5) Å and 3.986(5) Å in 1 and 3.869(7) Å in 3 (Fig. S1†). In the crystal lattice of 5, the molecules of [ReCl(CO)₃(5-Clphen)] are extended into a 1D supramolecular network through two different π⋯π type interactions with centroid-to-centroid separations of 3.755(3) Å and 3.744(3) Å (Fig. S1†).

DSC and TGA investigations

In the first heating scan of DSC measurements of all the rhenium(I) complexes, one endothermic peak corresponding to melting can be distinguished. The DSC thermograms of all complexes are presented in Fig. S16.† Their melting temperature (T_m) was in the range 281–391 °C. The influence of the presence and number of methyl groups and number of chlorine atoms in the studied complexes on their thermal behavior is clearly pronounced. In the case of compounds without methyl groups bearing two and three chlorine atoms (3 and 4), melting, together with decomposition, was observed. Along with the extension of the number of chlorine atoms from one (5) to two (4) or three (3), a significant increase in T_m was seen (about 80 °C). Compounds with a methyl group (1 and 2), which were isolated as crystalline substances, were found to be able to convert into a glassy state by fast cooling of their melts. Thus, they could form amorphous materials, so-called molecular glasses, which are an interesting class of materials combining the advantages associated with small molecules with the potential to form glassy phases.¹⁷ When the isotropic liquid was cooled down and heated again, a glass transition in the range 118–127 °C was observed. On further heating, in the case of compound 2, peaks due to crystallization at 167 °C and melting at 283 °C appeared, contrary to compound 1, in which a DSC thermogram with only a T_g crystallization endotherm at 291 °C was registered. An increase in the number of methyl groups results in decreases in both T_m and T_g of about 20 and 9 °C, respectively. TGA revealed that the obtained complexes demonstrated high thermal stability. The 5% weight loss temperature (T₅), which is usually considered as a criterion for determining the thermal stability, was in the range 295–395 °C (cf. Experimental section). Thus, a lack of thermal decomposition is observed below the melting temperature, except in compounds 3 and 4. These complexes melt with decomposition, as was mentioned above.

Electrochemical properties

The electrochemical properties of the Re(I) complexes were investigated using cyclic voltammetry (CV). From the onset of the oxidation and the first reduction peak, the ionization potential (IP) and electron affinity (EA), respectively, were estimated. Electrochemical data derived from CV measurements are summarized in Table 1, and a representative voltammogram is depicted in Fig. 2.

Table 1 Electrochemical data for Re(I) complexes^a

Code	Eox [V]	Eox,onset [V]	Ered1 [V]	Ered2 [V]	Ered3 [V]	Ered,onset [V]	IP [eV]	EA [eV]	Eg [eV]
a IP = −5.1 − Eox,onset, EA = −5.1 − Ered,onset, Eg = IP − EA.
1	1.06	0.95	−1.56	−1.73	−1.86	−1.47	−6.05	−3.63	2.42
2	1.06	0.95	−1.59	−2.19	—	−1.51	−6.05	−3.59	2.46
3	1.03	0.94	−1.38	−1.89	−2.06	−1.30	−6.04	−3.80	2.24
4	1.02	0.93	−1.49	−2.09	—	−1.42	−6.03	−3.68	2.35
5	0.99	0.91	−1.64	−2.12	—	−1.56	−6.01	−3.54	2.47

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Fig. 2 Cyclic voltammogram of 5 [scan rate 100 mV s⁻¹, electrolyte 0.2 M Bu₄NPF₆ in acetonitrile].

The data from Table 1 indicate that the oxidation potentials were relatively insensitive to changes in the phenanthroline substituents, and a slight increasing tendency was observed in the order 5 < 4 < 3 < 2 < 1. The oxidation onsets of the Re(I) complexes are very close to each other. Based on the obtained data, a reduction process can be associated with the phenanthroline ligand, while the anodic wave can be associated with the Re(II/I) oxidation process. The second oxidation peak of rhenium Re(III/II) was not observed, probably due to the electrochemical window of acetonitrile. The oxidation process of the Re(I) complexes was irreversible, excluding the complexes with electron-donating group (CH₃), that is, compounds 1 and 2. The first reduction process was quasi-reversible for all Re(I) complexes. To provide insight into the electroluminescent device fabrication of the investigated complexes, the IP/HOMO and the EA/LUMO were estimated. The EA decreases as the number of electron-withdrawing substituents increases, resulting in a decreased band gap in the order 5 > 4 > 3. Electron-donating –CH₃ groups increase the EA of 1 and 2 compared to 4; however, the energy band gaps (E_g) obtained for complexes 1, 2, and 4 are very similar. The band-gap decreases in the order 5 > 2 > 1 > 4 > 3. The HOMO is determined mainly by the orbitals of the metal unit, while the LUMO is determined mainly by the orbitals of the ligand unit.^15e It should be stressed that all Re(I) complexes exhibited a value of E_g useful for optoelectronic applications.

Absorption spectroscopy and DFT calculations

The UV-Vis spectra of rhenium(I) complexes have been studied in two solvents, which differ in polarity, (acetonitrile (ε = 37.5) and chloroform (ε = 4.8)), and in the solid state as films on glass substrates, and their spectral parameters are summarized in Table 2.

Table 2 Electronic spectral data for complexes 1–5

Complex	Medium	λ [nm] (10^3 ε [M^−1 cm^−1])
1	CH3CN	387.2 (6.16), 332.5 (9.10), 285.0 (28.57), 215.9 (55.60), 200.1 (53.08)
	CHCl3	404.6 (2.98), 339.6 (3.92), 282.2 (17.80), 246.2 (18.88)
	Film	396
2	CH3CN	382.1 (5.66), 330.3 (7.15), 270.7 (38.85), 231.4 (42.39), 215.6 (66.61), 196.1 (53.04)
	CHCl3	373.8 (3.68), 272.8 (24.72)
	Film	364
3	CH3CN	393.1 (8.85), 313.7 (12.5), 274.9 (43.77), 229.1 (46.99), 214.9 (64.11), 197.5 (55.48)
	CHCl3	408.9 (4.48), 313.9 (6.8), 277.8 (28.04)
	Film	405
4	CH3CN	380.4 (6.30), 268.3 (47.30), 212.8 (63.24)
	CHCl3	399.9 (4.60), 270.9 (36.92)
	Film	393
5	CH3CN	378.8 (6.05), 290.5 (19.72), 269.4 (40.40), 218.0 (48.16), 200.3 (63.46)
	CHCl3	393.9 (3.32), 268.0 (37.24), 240.3 (37.04)
	Film	385

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With reference to previous spectroscopic studies for diimine rhenium(I) tricarbonyls, the intense absorptions at high energy (200–320 nm) are predominately attributable to intraligand (IL) transitions of the diimine moieties, while the moderately intense lowest energy absorption band is typically from the spin-allowed metal d_π(Re) to ligand charge transfer.

As can be expected, the position and intensity of the lowest energy absorption of 1–5 is solvent dependent, shifting to higher energy in more polar media by 14 nm for 1, 10 nm for 2, 16 nm for 3, 16 nm for 4, and 15 nm for 5, and the MLCT band is sensitive to the number and nature of substituents in the 1,10-phenanthroline ring. The extent of the red shift was in the order 3 > 4 > 5, with an increasing number of chloro substituents, which draw electron density from the phenanthroline ring.

To obtain a deeper understanding of the nature of excited states involved in absorption processes, time-dependent DFT (TDDFT) calculations at the B3LYP/def2-TZVPD level were performed for 1–5. The calculated and measured absorption spectra in acetonitrile solution are graphically compared in Fig. 3. It is observed that the main spectral features of 1–5 are predicted to a good accuracy, both in position and relative intensities, by the TDDFT calculations. Also, the structural parameters of the optimized structures of 1, 3, and 5 coincide well with the experimental bond lengths and angles (see Table S2†). The calculated absorption energies associated with their oscillator strengths, their main contributions, and their assignments to the experimental results of 1–5 are given in Tables S4–S8 (see ESI†). Fig. 4 presents the molecular orbital energy level graph for the examined rhenium(I) complexes, and the selected molecular orbitals of 1–5 are shown in Fig. S4 and characterized in Tables S9–S13.†

Fig. 3 Experimental (black) and calculated (red) electronic absorption spectra of 1–5 in CH₃CN solution.

Fig. 4 Molecular orbital energy level graph of 1–5 at the DFT/B3LYP/def2-TZVPD level.

As can be seen from Fig. 4, the LUMO energy decreases as the number of chloro substituents increases, resulting in a decreased band gap in the order 5 > 4 > 3, which then leads to an increase in the λ_max of MLCT absorption (Table 2). On the other hand, electron-releasing –CH₃ groups increase the LUMO energy of 1 and 2 compared to 4, which leads to an increased HOMO–LUMO band gap. The DFT/B3LYP/def2-TZVPD calculations showed a negligible effect of the third methyl group on the energy of the HOMO and LUMO orbitals of 1 in relation to 2; the HOMO–LUMO band gap is the same for 1 and 2.

The HOMO and HOMO−1 levels of 1–5 are very close in energy, and these orbitals are predominately constituted by 5d_yz and 5d_xz rhenium orbitals in a bonding relation to the carbonyl π* orbitals and an antibonding arrangement with chlorine occupied p orbitals. The HOMO−2 orbitals of 1–5 contain significant 5d_xy rhenium (45–60%) character, along with ∼30% contributions from the CO component, and they are about ∼0.5 eV lower in energy in relation to the highest occupied molecular orbital. The LUMO and LUMO+1 levels are predominately centered on the phenanthroline ligand and particularly the π* antibonding orbital. These results are consistent with previously published studies on Re-phen carbonyl complexes.^1a,18

According to TDDFT calculations, the lowest energy absorption of 1–5 arises from one-electron excitations H−1 → L, H → L+1, and H−1 → L+1, assigned as metal-ligand-to-ligand charge transfer transitions (MLLCT). With an increasing number of chloro substituents, the calculated low-energy absorption shift to lower energy is in the order 5 > 4 > 3. On the contrary, electron-donating groups lead to blue shifts of the H−1 → L excitations for 1 and 2 in relation to 4. For complexes 1–3, transitions of MLLCT character (H−2 → L+1) were also calculated to contribute to the higher energy absorption bands, at 333 nm for 1, 327 nm for 2, and 314 nm for 3. Generally, however, the intense bands in the high energy region are largely attributed to intraligand transitions (IL) and ligand–ligand charge transfer (LLCT).

Emission spectroscopy

The luminescence properties of complexes 1–5 were investigated in solution (CHCl₃ and CH₃CN) and solid state at room temperature as well as in 4 : 1 EtOH–MeOH glass matrix at 77 K. A summary of the photophysical data for 1–5 is gathered in Table 3.

Table 3 Summary of photoluminescent properties of the complexes 1–5

Complex	Medium	λex (nm)	λem (nm)	Stokes shift (cm^−1)	τ, ns (weight)	χ^2	Φ (%)
a (4 : 1 v/v).
1	CHCl3	408	640	8885	42.80	1.054	0.69
	CH3CN	400	665	9962	15.38	1.114	0.14
	Solid state	449	594; 716	5437, 8305	130.64 (37.39%), 386.69 (62.61%), 2.34 (29.87%), 41.84 (14.69%), 237.03 (55.43%)	1.179, 0.975	0.85
	EtOH–MeOH^a (77 K)	402	540	6357	7070 (53.88%), 25 360 (46.12%)	1.030	—
	Film	390	599	8558	—	—	—
	Blend with PVK	330	386; 566	—	—
	Blend with PVK:PBD	330	384; 566	—	—	—	—
2	CHCl3	374	640	11 113	39.72	1.124	0.58
	CH3CN	375	665	11 629	13.50	1.129	0.48
	Solid state	473	618	4960	113.05	1.202	3.46
	EtOH–MeOH^a (77 K)	385	543	7558	1970 (12.80%), 6660 (59.94%), 16 380 (27.26%)	1.097	—
	Film	390	586	10 407	—	—	—
	Blend with PVK	330	387; 582		—	—	—
	Blend with PVK:PBD	330	386; 560		—	—	—
3	CHCl3	408	680	9804	38.58 (76.48%), 141.89 (23.54%)	1.145	0.57
	CH3CN	390	716	11 675	14.14 (78.38%), 57.25 (21.62%)	0.974	0.50
	Solid state	490	643	4856	119.09	1.133	4.11
	EtOH–MeOH^a (77 K)	373	569	9235	2590 (44.22%), 8720 (41.00%), 62 190 (14.78%)	1.151	—
	Film	390	610	8298	—	—	—
	Blend with PVK	330	378; 589	—	—	—	—
	Blend with PVK:PBD	330	378; 589	—	—	—	—
4	CHCl3	400	651	9639	69.53	1.198	0.89
	CH3CN	380	690	11 823	24.47	1.063	0.46
	Solid state	442	591	5704	120.00 (11.30%), 459.99 (88.70%)	1.013	7.32
	EtOH–MeOH^a (77 K)	360	567	10 141	1610 (14.95%), 5520 (75.89%), 17 770 (9.15%)	1.097	—
	Film	390	585	8351	—	—	—
	Blend with PVK	330	384; 582	—	—	—	—
	Blend with PVK:PBD	330	384; 577	—	—	—	—
5	CHCl3	397	634	9416	98.53	1.004	1.28
	CH3CN	376	650	11 211	44.81	1.123	0.48
	Solid state	463	566	3930	525.33 (26.87%), 1259 (73.13%)	1.104	11.38
	EtOH–MeOH^a (77 K)	362	548	9376	3080 (23.54%), 7750 (69.24%), 38 140 (7.22%)	1.178	—
	Film	390	561	8552	—	—	—
	Blend with PVK	330	385; 575	—	—	—	—
	Blend with PVK:PBD	330	383; 565	—	—	—	—

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Complexes 1–5 were found to be typical MLCT emitters. The irradiation of 1–5 in solution with wavelengths of 374–408 nm gave rise to emissions with maxima and Stokes' shifts in the ranges 634–716 nm and 8885–11 823 cm⁻¹, respectively. The emission bands are broad and structureless, and they show negative solvatochromism, reflected in blue-shifted emission with increasing solvent polarity. On going from CH₃CN to CHCl₃, the emissions of 1, 2, 3, 4, and 5 appeared at 25, 25, 36, 39, and 16 nm shorter wavelengths, which is attributed to a reduced (and reversed) molecular dipole in their MLCT excited states.^7a,19 As can be seen from Table 3, complexes 1–5 also exhibit shortened lifetimes and decreasing emission quantum yields upon changing the solvent from CHCl₃ to CH₃CN. In similarity to the absorption behavior, the emission maximum wavelengths of 3, 4, and 5 show red-shifts with an increasing number of chloro substituents, in the order 3 > 4 > 5. On the other hand, the emissions of 1 and 2 appear in a lower-energy region compared to 4, but there is no marked difference in the fluorescence maximum wavelengths of 1 and 2.

Another characteristic feature of these complexes (except for 1) is the large hypsochromic shift of their emission maxima on going from a fluid environment to a rigid medium, 566–643 nm in the solid state and 540–569 nm upon cooling to low temperature (77 K), described as the rigidochromic effect. This is responsible for raising the energy of the emissive ³MLLCT due to the lack of solvent reorganization following excitation.^8a,20

For 1, bimodal emission was observed, at 594 and 716 nm. A significant increase in lifetimes in the solid state and in a 77 K EtOH–MeOH matrix can be attributed to the intermolecular interactions in a more rigid environment, and decreased thermal deactivation.

The relatively long emission lifetimes, together with large absorption–emission shifts, indicate the phosphorescent nature of the emission.

The normalized emission spectra in solution, solid state as powder, and in alcohol glass at 77 K for representative compound 2 are shown in Fig. 5.

Fig. 5 Normalized emission spectra in solution, solid state, and in alcohol glass at 77 K for compound 2.

The emission spectra of the remaining compounds are included in the ESI (Fig. S2†).

Additionally, PL spectra of the complexes as thin films on glass substrates were recorded under λ_ex = 390 nm. It was found that the λ_em of films was shifted to a higher energetic region with respect to the chloroform solution, as for complexes in the form of powder. In film, they exhibited large Stokes shifts of up to 220 nm, as in solution. The large Stokes shifts are desirable for emitting materials in OLEDs. A small Stokes shift can cause self-quenching and consequently decrease the emission intensity.²¹

The emission properties of 1–5 were also examined computationally at the DFT/UB3LYP/def2-TZVPD level. The energies of phosphorescence emissions of 1–5 were calculated from the energy difference between the ground singlet and the triplet state ΔE_T1–S0, as well as being computed using TD-DFT (Table 4, Fig. S5 and Table S14†).

Table 4 Calculated phosphorescence emission energies of 1–5, compared to the experimental values recorded in acetonitrile solution

Compound	DFT		TD-DFT
	ΔET1–S0	Character	Major contribution (CI coefficient)	E [eV]	λcal [nm]	Character	λexp [nm] [eV]^−1
1	1.97/629.4	^3MLLCT	L → H (0.649)	1.75	707.08	^3MLLCT	665/1.86
2	1.98/626.2	^3MLLCT	L → H (0.663)	1.87	661.55	^3MLLCT	665/1.86
3	1.90/652.5	^3MLLCT	L → H (0.662)	1.79	691.39	^3MLLCT	716/1.73
4	1.97/629.4	^3MLLCT	L → H (0.666)	1.86	665.16	^3MLLCT	690/1.79
5	2.09/593.2	^3MLLCT	L → H (0.655)	1.97	627.63	^3MLLCT	650/1.90

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As can be seen from Table 4, the calculated emission energies are in reasonably good agreement with the experimental emission band maxima of 1–5, indicating that the luminescence has characteristics of phosphorescence and occurs from the low-lying triplet state (T₁). The nature of the transitions resulting in the emissions of 1–5 is ³MLLCT. Compared to the experimental data, the excitation energies calculated by the TD-DFT method lie closer than those of ΔE_T1–S0 for complexes 2–5, whereas ΔE_T1–S0 is more accurate than the excitation energies calculated by TDDFT for 1.

The next step of our research covers a study of the possible applications of the complexes as guest components in guest–host light-emitting diodes prepared by solution processing. Therefore, the photoluminescence ability of these complexes in the guest–host configuration as blends with poly(9-vinylcarbazole) (PVK) and in mixtures of PVK with (2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole) (PBD) (50 : 50 as weight%) were investigated. The content of the complexes in blends was 15 wt%. The layers of such compositions were used for light-emitting diodes (see the section on Electroluminescence). The emission spectra of the blends with complexes were recorded under excitation at 330 nm and 390 nm, corresponding to the λ_max values of the matrices and the complexes, respectively. Under excitation at 330 nm, both matrices, that is, PVK and the mixture of PVK:PBD, exhibited the highest intensity emission (cf. Fig. S3 in ESI†). All blends under λ_ex = 390 nm did not show emission, except of blends with complex 5. In Fig. 6, the representative photoluminescence spectra of the matrices (PVK and PVK:PBD) and blend with dispersed compound 5 are compared.

Fig. 6 (a) PL spectra of complex 5 as a thin film and blends under excitation at 330 nm and (b) absorption spectra of complex 5 in film and blend with PVK, together with emission spectra of PVK and PVK:PBD.

In the emission spectra of all complexes dispersed in matrices under excitation at 330 nm, two emission bands were observed. A band originating from the emission of both matrices was seen in a higher energy region around 400 nm, in relation to the band ascribed to the luminescence of the complexes from about 570–600 nm. In the guest–host diode configuration, two luminescence mechanisms are possible, that is, Förster energy transfer and charge trapping.²² The obtained results suggest that in PVK and PVK:PBD blends, Förster energy transfer from host to guest occurs, which is possible only if the emission spectrum of the matrix–host overlaps with the absorption spectrum of the guest molecule. As can be seen in Fig. 6b, the emission spectra of PVK and PVK:PBD overlap over a large range of the absorption spectra of the complexes. However, the emission of the matrices is still present in the PL spectra of the blends, indicating that the energy transfer is not complete. A higher intense emission was observed for complexes dispersed in a binary host matrix (PVK:PBD).

Electroluminescence

The electroluminescence (EL) of rhenium(I) complexes was investigated in simple guest–host light-emitting diodes, in which active layers were prepared by solution processing. In investigating the ability of the complexes for electroluminescence, two host matrices were selected, that is, PVK and a binary matrix PVK : PBD (50 : 50 wt%). PVK and PBD act as hole- and electron-transporting materials, respectively.

In these matrices, the complexes were dispersed with 15 wt% concentration. At the beginning of our research, devices with an emitting layer as a neat complex film with structure ITO/PEDOT:PSS:Re(I) complex/Al were prepared. Next the EL of the synthesized compounds was tested in devices with the configuration ITO/PEDOT:PSS/PVK:Re(I) complex/Al and ITO/PEDOT:PSS/PVK:PBD:Re(I) complex/Al.

For the prepared diodes, current density–voltage (J–V) characteristics were registered. Fig. 7 presents an example of the J–V characteristics, together with photos of diodes under an applied voltage, whereas the data obtained for all devices are summarized in Table 5. The thicknesses of the selected active layer and active layer surface were investigated by atomic force microscopy (AFM). The layers were characterized by similar thicknesses around 100–140 nm. The AFM images of the emitting layers show a uniform and flat surface with a surface root-mean-square roughness (RMS) in the range 1.8–4.9 nm.

Fig. 7 Current density–voltage (J–V) characteristics of the prepared devices based on complexes 3 and 5, and photos of diodes under an applied voltage.

Table 5 J–V characteristic parameters of devices based on rhenium(I) phenanthrolines

Compound	Active layer	Von [V]	Jmax [mA cm^−2]
1	Film	3.0	0.6
	PVK blend	2.8	18
	PVK:PBD blend	2.6	19
2	Film	—	—
	PVK blend	2.6	16
	PVK:PBD blend	2.6	40
3	Film	3.5	2.6
	PVK blend	2.1	35
	PVK:PBD blend	2.6	76
4	Film	2.0	6
	PVK blend	1.7	14
	PVK:PBD blend	1.2	17
5	Film	—	—
	PVK blend	2.2	65
	PVK:PBD blend	1.6	46

---

In the case of diodes with the neat complex as emission layer, a low current density (J below 5 mA cm⁻²) was obtained and no light emission under the applied voltage was seen. The devices with an active layer consisting of complexes dispersed in matrices were characterized by low values of the turn-on voltage (V_on) in the range 1.2–3.0 V, indicating efficient injection from the electrodes and transport of holes and electrons to the emission layers. The maximal achieved current density (J_max) was in the range 14–76 mA cm⁻². The highest J_max values were found for devices based on compounds 3 and 5, used as emitters. The utilization of an additional electron-transporting compound (PBD) as a component of the emitting layer resulted in an improvement in the device performance. It should be emphasized that all fabricated diodes containing complexes in the blend emitted light under the applied voltage, which was more intense in the case of devices with a binary matrix. Thus, these complexes seem to be promising for applications, and further investigations covering measurements of electroluminescence parameters, together with diode architecture modification, are necessary and will be carried out.

Conclusions

To summarize, rhenium(I) phenanthroline complexes with electron-withdrawing Cl and electron-donating CH₃ substituents were prepared and investigated, considering the effect of the number of chlorine atoms and methyl groups. It was found that the number and type of phenanthroline substituents impact the thermal behavior of the complexes. However, all of them are thermally stable enough for electronic applications. The complexes containing methyl substituents are molecular glasses, and an increase in the number of CH₃ groups results in an increase in the T_g from 118 to 127 °C. A significant drop in the T_m with a reduction in the number of chlorine atoms from three/two to one was seen. The electrochemical results revealed that the ionization potential of the investigated complexes was independent of the substituents on the phenanthroline unit. They slightly differ in electron affinity values within the range 3.50–3.80 eV, which results in similar but low energy band gaps. The lowest E_g was exhibited by the compound bearing the greatest number of electron-withdrawing chlorine atoms. Compounds with electron-donating CH₃ units undergo quasi-reversible oxidation, unlike the others without a methyl group. The complexes demonstrated photoluminescence both in solution and in the solid state, emitting light with λ_em in the ranges 574–610 nm and 566–601 nm in film and in blend, respectively. A slight bathochromic shift in λ_em from 574 to 610 nm accompanying an increase in the number of chlorine atoms is seen. In the case of compounds dispersed in PVK and PVK:PBD matrices, incomplete quenching of the emission of the matrix was observed. The application of a binary matrix yielded significantly more intensive photoluminescence of green light. The preliminary tests of utilization of the complexes in guest–host light-emitting diodes revealed their electroluminescence ability. Emission of light under an applied voltage was seen in all devices, which seems to be more intense for diodes based on the phenanthroline complex substituted with three Cl atoms, and for a device in which the complex is dispersed in a PVK:PBD matrix.

Experimental section

Materials

[Re(CO)₅Cl] was commercially available (Sigma Aldrich) and was used without further purification. The 1,10-phenanthrolines have been prepared according to a procedure described in the literature.^23–25

Poly(9-vinylcarbazole) (PVK) (M_n = 25 000–50 000) and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) were purchased from Sigma Aldrich and used without additional purification. Poly(3,4-(ethylenedioxy)thiophene):poly-(styrenesulfonate) (PEDOT:PSS) (0.1–1.0 S cm⁻¹) and substrates with pixelated ITO anodes were supplied by Ossila. All solvents for synthesis were of reagent grade and were used as received. For spectroscopy studies, HPLC grade solvents were used.

General synthesis route for rhenium(I) complexes (1–5)

[Re(CO)₅Cl] (0.10 g, 0.27 mmol) and suitable 1,10-phenanthroline derivative ligands (0.27 mmol) were dissolved in acetonitrile (80 mL) and refluxed under argon for 6 hours. The resulting reaction solution was reduced in volume to 10 mL and allowed to cool to room temperature. The resulting yellow (2 and 4), orange (1 and 5), or dark red (3) solid was collected by filtration, washed with diethyl ether, and dried. X-ray quality crystals were obtained by slow recrystallization from acetonitrile.

[ReCl(CO)₃(2,5,9-(Me)₃-4,7-Cl₂phen)] (1). Yield: 50%. IR (KBr, cm⁻¹): 2018 (vs), 1900 (vs), and 1876 (vs) ν(C≡O); 1609 (m) and 1578 (m) ν(C=N) and ν(C=C). ¹H NMR (400 MHz, DMSO-d₆): δ/ppm = δ 8.35 (d, J = 11.0 Hz, 2H), 8.18 (s, 1H), 3.20 (d, J = 4.1 Hz, 6H), 3.14 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆): δ/ppm = 197.61, 189.43, 163.24, 150.69, 148.02, 145.28, 143.69, 142.10, 135.71, 129.51, 127.71, 127.36, 126.35, 124.77. MS: m/z 620.93 g mol⁻¹ calcd for [M + Na]⁺ C₁₈H₁₂Cl₃N₂O₃ReNa m/z 619.87 g mol⁻¹. C₁₈H₁₂O₃N₂Cl₃Re (596.86 g mol⁻¹): calcd C, 36.22; H, 2.03; N, 4.69%; found: C, 36.84; H, 2.09; N, 4.71%. DSC T_m = 301 °C, T_g = 127 °C; TGA T₅ = 341 °C.

[ReCl(CO)₃(2,9-(Me)₂-4,7-Cl₂phen)] (2). Yield: 60%. IR (KBr, cm⁻¹): 2020 (vs), 1912 (vs), and 1876 (vs) ν(C≡O); 1617 (m) and 1560 (m) ν(C=N) and ν(C=C). ¹H NMR (400 MHz, DMSO-d₆): δ/ppm = 8.46 (s, 2H), 8.44 (s, 2H), 3.23 (s, 6H). ¹³C NMR (100 MHz, DMSO-d₆): δ/ppm = 197.86, 189.76, 154.64, 147.34, 145.37, 129.23, 127.61, 125.38. MS: m/z 604.92 g mol⁻¹; calcd for C₁₇H₁₀Cl₃N₂O₃ReNa [M + Na]⁺ m/z 605.84 g mol⁻¹. C₁₇H₁₀O₃N₂Cl₃Re (582.84 g mol⁻¹): calcd C, 35.03; H, 1.73; N, 4.81%; found: C, 34.95; H, 1.81; N. 4.92%; DSC T_m = 281 °C, T_g = 118 °C; TGA T₅ = 295 °C.

[ReCl(CO)₃(4,5,7-Cl₃phen)] (3). Yield: 55%. IR (KBr, cm⁻¹): 2025 (vs), 1911 (vs) and 1908 (vs) ν(C≡O); 1604 (m) and 1560 (m) ν(C=N) and ν(C=C). ¹H NMR (400 MHz, DMSO-d₆): δ/ppm = 9.41 (dd, J = 12.9, 5.6 Hz, 2H), 8.59 (s, 1H), 8.33 (d, J = 5.5 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ/ppm = 198.31, 190.65, 166.73, 155.66, 153.89, 150.04, 146.61, 144.04, 143.37, 143.28, 131.48, 131.37, 128.05, 127.63, 127.36, 122.76, 120.11, 113.48; MS: m/z 612.85 g mol⁻¹ calcd for C₁₅H₅Cl₄N₂O₃ReNa [M + Na]⁺ m/z 612.22 g mol⁻¹. C₁₅H₅O₃N₂Cl₄Re (589.23 g mol⁻¹): calcd C, 30.57; H, 0.86; N, 4.75%; found: C, 30.50; H, 0.81; N, 4.48%; DSC T_m = 391 °C; TGA T₅ = 382 °C.

[ReCl(CO)₃(4,7-Cl₂phen)] (4). Yield: 75%. IR (KBr, cm⁻¹): 2024 (vs), 1902 (vs) and 1896 (vs) ν(C≡O); 1616 (s) and 1561 (m) ν(C=N) and ν(C=C). ¹H NMR (400 MHz, DMSO-d₆): δ/ppm = 9.42 (d, J = 5.2 Hz, 2H), 8.59 (s, 2H), 8.35 (d, J = 5.3 Hz, 2H). ¹³C NMR (100 MHz, DMSO-d₆): δ/ppm = 197.39, 189.23, 164.43, 148.55, 144.96, 127.52, 127.38, 124.02. MS: m/z 576.89 g mol⁻¹; calcd for: C₁₅H₆Cl₃N₂O₃ReNa [M + Na]⁺ m/z 577.79 g mol⁻¹. C₁₅H₆O₃N₂Cl₃Re (554.78 g mol⁻¹): calcd C, 32.47; H, 1.09; N, 5.05%; found: C, 32.41; H, 1.08; N, 5.03%. DSC T_m = 390 °C; TGA T₅ = 385 °C.

[ReCl(CO)₃(5-Clphen)]_˙CH₃CN (5). Yield: 75%. IR (KBr, cm⁻¹): 2020 (vs), 1937 (vs) and 1893 (vs) ν(C≡O); 1615 (m) ν(C=N) and ν(C=C). ¹H NMR (400 MHz, DMSO-d₆): ¹H NMR (400 MHz, DMSO) δ 9.52 (d, J = 4.8 Hz, 1H), 9.43 (d, J = 4.9 Hz, 1H), 9.10 (d, J = 3.9 Hz, 1H), 8.91 (d, J = 4.3 Hz, 1H), 8.64 (s, 1H), 8.24–8.21 (m, 1H), 8.13–8.10 (m, 1H). ¹³C NMR (125 MHz, DMSO-d₆): δ/ppm = δ 198.04, 190.12, 154.85, 154.25, 147.21, 145.62, 139.23, 136.26, 130.89, 130.26, 128.96, 127.88, 127.66, 127.50. MS: m/z 484.94 g mol⁻¹; calcd for C₁₇H₁₀Cl₂N₃O₃ReNa [M + Na]⁺ m/z 483.76 g mol⁻¹. C₁₅H₇O₃N₂Cl₂Re (520.34 g mol⁻¹): calcd C, 34.62; H, 1.35; N, 5.38%; found: C, 34.46; H, 1.38; N, 5.58%. DSC T_m = 311 °C; TGA T₅ = 395 °C.

Film and blend preparation

Films and blends with 15 wt% concentration of Re(I) complex in PVK or PVK : PBD (50 : 50 in weight%) on a glass substrate were prepared by spin-coating from chloroform solution (10 mg mL⁻¹). The homogenous solutions were spin cast (1000 rpm, 60 s).

Device preparation

Devices with three different configurations: ITO:PEDOT:PSS/Re(I) complex/Al, ITO:PEDOT:PSS/PVK:Re(I) complex/Al, and ITO:PEDOT:PSS/PVK:PBD:Re(I) complex/Al with 15 wt% complex content in blend were fabricated. Devices were prepared on OSSILA substrates with pixelated ITO anodes, and cleaned sequentially with detergent, deionized water, 10% NaOH solution, water, and isopropanol in an ultrasonic bath. Substrates were covered with PEDOT:PSS thin film (40 nm) by spin coating at 5000 rpm for 60 s and annealed for 30 min at 130 °C. The active layer was spin-coated on top of the PEDOT:PSS layer from chloroform solution (10 mg ml⁻¹) at 1000 rpm for 60 s. Finally, an aluminum cathode was vacuum-deposited.

Crystal structure determination and refinement

The X-ray intensity data for 1, 3, 5, and 6 were collected on a Gemini A Ultra diffractometer equipped with an Atlas CCD detector and graphite monochromated MoKα radiation (α = 0.71073 Å) at room temperature. The unit cell determination and data integration were carried out using the CrysAlis package of Oxford Diffraction. Lorentz, polarization, and empirical absorption correction using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm were applied.²⁶ The structures were solved by the Patterson method using SHELXS97 and refined by full-matrix least-squares on F² using SHELXL97.²⁷ All the non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions refined using idealized geometries (riding model) and assigned fixed isotropic displacement parameters, d(C–H) = 0.93 Å, U_iso(H) = 1.2U_eq(C) (for aromatic); and d(C–H) = 0.96 Å, U_iso(H) = 1.5U_eq(C) (for methyl). The methyl groups were allowed to rotate about their local threefold axis. CCDC reference numbers: 1501825 (1), 1501826 (3), and 1501827 (5).†

Computational details

The calculations have been performed using the GAUSSIAN-09 program package.²⁸ The geometries of the singlet ground states (S₀) and the lowest triplet states (T₁) of 1–5 were fully optimized without any symmetry restrictions at the DFT level with the B3LYP hybrid exchange–correlation functional. The calculations were performed using the def2-TZVPD basis set for rhenium, the 6-31+G** basis set for chlorine, oxygen, and nitrogen, and 6-31G* for carbon and 6-31G for hydrogen atoms.²⁹ The starting point for geometry optimization was taken from the X-ray structure, and all the subsequent calculations were performed based on the optimized geometries. Vibrational frequencies were calculated on the basis of the optimized geometry to verify that each of the geometries is a minimum on the potential energy surface. Furthermore, on the basis of the optimized ground and excited state geometries, the absorption and emission properties in acetonitrile (CH₃CN) media were calculated by TD-DFT at the B3LYP hybrid functional level and with the polarized continuum model (PCM).³⁰ The predicted bond lengths of 1, 3, 5, and 4 are compared with the experimental data in Table S2.† The predicted bond lengths and angles for the ground state are within the range of error expected for DFT calculations of rhenium(I) complexes, and the general trends observed in the experimental data are well reproduced in the calculations, providing confidence in the reliability of the chosen method to reproduce the geometry of the studied complex (Table S2†).

Acknowledgements

The research was co-financed by the National Research and Development Center (NCBiR) under Grant ORGANOMET No. PBS2/A5/40/2014. The calculations have been carried out at the Wroclaw Centre for Networking and Supercomputing (http://www.wcss.wroc.pl). K. Laba is a scholar supported by the “DoktoRIS—scholarship program for an innovative Silesia”, co-financed by the European Union within the European Social Fund.

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Footnote

† Electronic supplementary information (ESI) available: Crystal data and structure refinement, short intra- and intermolecular contacts, comparison of experimental and theoretical bond lengths [Å] and angles [°], the energies and characters of the selected eletronic transitions with their assignment to the experimental absorption bands, percentage contributions of Re, CO, phen and Cl units in the selected occupied and unoccupied molecular orbitals in the ground state, ¹H and ¹³C NMR spectra, DSC thermograms, excitation and emission spectra, emission spectra of PVK and PVK : PBD 1 : 1: films on glass. CCDC 1501825, 1501826 and 1501827. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra23935h

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