In-Depth Studies of Ground- and Excited-State Properties of Re(I) Carbonyl Complexes Bearing 2,2':6',2″-Terpyridine and 2,6-Bis(pyrazin-2-yl)pyridine Coupled with π-Conjugated Aryl Chromophores.

In the current work, comprehensive photophysical and electrochemical studies were performed for eight rhenium(I) complexes incorporating 2,2':6',2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) with appended 1-naphthyl-, 2-naphthyl-, 9-phenanthrenyl, and 1-pyrenyl groups. Naphthyl and phenanthrenyl substituents marginally affected the energy of the MLCT absorption and emission bands, signaling a weak electronic coupling of the appended aryl group with the Re(I) center. The triplet MLCT state in these complexes is so low lying relative to the triplet 3ILaryl that the thermal population of the triplet excited state delocalized on the organic chromophore is ineffective. The attachment of the electron-rich pyrenyl group resulted in a noticeable red shift and a significant increase in molar absorption coefficients of the lowest energy absorption of the resulting Re(I) complexes due to the contribution of intraligand charge-transfer (ILCT) transitions occurring from the pyrenyl substituent to the terpy/dppy core. At 77 K, the excited states of [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-functionalized ligands were found to have predominant 3ILpyrene/3ILCTpyrene→terpy character. The 3IL/3ILCT nature of the lowest energy excited state of [ReCl(CO)3(4'-(1-pyrenyl)-terpy-κ2N)] was also evidenced by nanosecond transient absorption and time-resolved emission spectroscopy. Enhanced room-temperature emission lifetimes of the complexes [ReCl(CO)3(Ln-κ2N)] with 1-pyrenyl-substituted ligands are indicative of the thermal activation between 3MLCT and 3IL/3ILCT excited states. Deactivation pathways occurring upon light excitation in [ReCl(CO)3(4'-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4'-(1-pyrenyl)-terpy-κ2N)] were determined by femtosecond transient absorption studies.

Introduction

Transition-metal complexes with 2,2′:6′,2″-terpyridines and their structural analogues have been receiving widespread attention from scientists due to their optical, electrochemical, catalytic, and medicinal properties, making these compounds appealing for potential applications in biological imaging,[1,2] catalysis,[3−7] and organic light-emitting devices.[8−10]

Most of the Re(I) carbonyl complexes [ReCl(CO)3(L-κ2N)] bearing substituted terpy-like ligands coordinated to the metal center in a bidentate way that have been developed so far[11−20] emit from the triplet excited state of metal to ligand charge-transfer character (3MLCT). Generally, they are weakly emissive at room temperature and have short excited-state lifetimes. Exploring remote substituent effects in [ReCl(CO)3(L-κ2N)] with 4′-(4-substituted phenyl)terpyridine ligands, Fernández-Terán and Sévery demonstrated that the introduction of the strongly electron donating −NMe2 group leads to the switching from 3MLCT to 3ILCT (intraligand charge transfer), which is accompanied by significant lengthening of the excited-state lifetime (380 vs 1.5 ns). The obtained [ReCl(CO)3(L-κ2N)] with 4′-(4-NMe2-phenyl)-2,2′:6′,2″-terpyridine has been successfully used as photosensitizers for hydrogen production, reaching TONRe values of over 2100.[20] Prolonged lifetimes have also been confirmed for some Re(I) diamine carbonyls [ReX(CO)3(phen-TPA)] (X = Cl, Br; TPA = triphenylamine) with the emitting state of 3ILCT nature,[21] and they have been supported for some other terpyridine Re(I) complexes [ReCl(CO)3(L-κ2N)] incorporating strong electron-donating substituents by the Wang group[22] and our group.[14,16,23]

To extend room-temperature triplet excited state lifetimes and improve the photophysical properties of transition-metal complexes, many other strategies have been reported.[24−40] Among them, there is the bichromophoric approach,[24−31,41−47] based on the attachment of an organic chromophore with a nonemissive triplet state close in energy to an emissive 3MLCT state. Between the 3MLCT and 3IL states sharing a similar energy, an excited-state equilibrium may be established. In such a case, the organic chromophore repopulates the luminescent 3MLCT excited state, playing the role of an energy “reservoir” or excited-state storage element.[24−31,41−47] Extending luminescence lifetimes via the excited-state equilibration strategy, however, is not accompanied by a quantum yield increase.[28] The most popular chromophores used for this method are π-conjugated aryl groups: e.g., anthryl and pyrenyl.[24−31,41−47]

In the present work, the ground- and excited-state properties of new Re(I) carbonyl complexes bearing 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) substituted with π-conjugated aryl groups (Scheme ) were explored by cyclic voltammetry, absorption and emission spectroscopy, and transient absorption, and they were elucidated with the use of density functional theory (DFT) and time-dependent DFT. The attached naphthyl, phenanthrenyl, and pyrenyl substituents differ among themselves in the number of fused rings, and the naphthyl group is attached to the central pyridine ring of the terpy/dppy core via its 1- and 2-positions in order to investigate the effect of the torsional strain due to the inter-ring H···H and hydrogen−π-ring repulsive interactions.

Scheme 1

Structures of Rhenium(I) Carbonyl Complexes Studied in the Present Work

The main emphasis in these studies was placed on the examination of the effect of the π-conjugated aryl substituent and triimine core on the electrochemical and luminescence properties of the resulting [ReCl(CO)3(L-κ2N)].

Results and Discussion

Synthesis and Characterization

All of the complexes [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1A–4A) and [ReCl(CO)3(4-Ar-dppy-κ2N)] (1B–4B) were obtained by refluxing 1:1 mixtures of [Re(CO)5Cl] with the corresponding 4′-Ar-terpy or 4-Ar-dppy ligand (see details in the Supporting Information). The molecular structures of 1A–4A and 1B–4B were confirmed by 1H and 13C NMR spectroscopy (Figures S1–S8), the FT-IR technique (Figures S9–S16), elemental analysis, and HR-MS spectrometry (Figures S17–S24). Due to the bidentate coordination mode of 4′-Ar-terpy/4-Ar-dppy, peripheral ring protons become magnetically distinct and show separate signals in 1H NMR spectra, with an integration corresponding to one proton for each peak. For [ReCl(CO)3(4′-Ar-terpy-κ2N)] with naphthyl-substituted 2,2′:6′,2″-terpyridines (1A and 2A), the complete signal assignment in the 1H and 13C NMR spectra was achieved with the aid of the two-dimensional techniques 1H–1H COSY, 1H–13C HMQC, and 1H–13C HMBC (Figures S1 and S2). It is worth noting that the signals of the central pyridine protons of 2A show significant downfield shifts relative to those recorded for complex 1A, bearing a more sterically hindered 1-naphthyl unit. The FT-IR spectra of [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1A–4A) and [ReCl(CO)3(4-Ar-dppy-κ2N)] (1B–4B) exhibit a sharp, intense C≡O stretching band (2025–2019 cm–1) and two poorly resolved bands in a lower energy range (1936–1875 cm–1), which is consistent with a facial geometry of CO ligands in the moiety {Re(CO)3}+. The increase in the average value of CO stretching frequencies of [ReCl(CO)3(4-Ar-dppy-κ2N)] (1957 cm–1 for 1B, 1948 cm–1 for 2B, 1943 cm–1 for 3B, and 1950 cm–1 for 4B) in relation to that for [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1943 cm–1 for 1A, 1944 cm–1 for 2A, 1937 cm–1 for 3A, and 1938 cm–1 for 4A) is indicative of a weaker donor capacity of the 4-Ar-dppy ligand in comparison to the corresponding 4′-Ar-terpy ligand.[48]

The molecular structure of 1A has been additionally determined by an X-ray analysis. The coordination sphere of Re(I) in 1A is best described as a highly distorted octahedron (Figure S25), and it has a small bite angle N(2)–Re(1)–N(1) of 74.38(10)°, an increase in the C(2)–Re(1)–N(2) angle to above 90° (100.80(17)°), and elongation of Re–Ncentral pyridine (2.218(3) Å) relative to Re–Nperipheral pyridine (2.164(3) Å). With reference to the related systems,[10−19] these structural features are mainly induced by the κ2N coordination of 4′-Ar-terpy and strong steric repulsion between the noncoordinated pyridine and C(2)–O(2) group. The noncoordinated pyridyl ring is inclined to the central pyridine plane at 48.17°, while the dihedral angle between the 1-naphthyl and central pyridine planes is 38.53°. Additional structural data of 1A, along with the thermal properties of [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1A–4A) and [ReCl(CO)3(4-Ar-dppy-κ2N)] (1B–4B) are available in Tables S2–S6 in the Supporting Information.

Electronic Structure Calculations

To get a better understanding of the effect of aromatic groups (Ar) on the electronic structures of [ReCl(CO)3(4′-Ar-terpy-κ2N)] and [ReCl(CO)3(4-Ar-dppy-κ2N)], calculations at the DFT/PBE0/def2-TZVPD/def2-TZVP level were performed for all molecules 1A–4A and 1B–4B (see Table S7 and Figures S27–S30 in the Supporting Information).

The partial molecular orbital energy-level diagrams for 1A–4A and 1B–4B, together with the plots of the frontier molecular orbitals of [ReCl(CO)3(4′-Ar-terpy-κ2N)] and [ReCl(CO)3(4-Ar-dppy-κ2N)] are shown in Figure .

Figure 1

Partial molecular orbital energy level diagrams for 1A–4A and 1B–4B.

For both series 1A–4A and 1B–4B, the LUMO is negligibly affected by the aryl group introduced into terpy and dppy cores. Its energy varies from −2.64 eV to −2.60 eV for [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1A–4A) and from −2.95 eV to −2.94 eV for [ReCl(CO)3(4-Ar-dppy-κ2N)] (1B–4B). The replacement of peripheral pyridyl rings in 4′-Ar-terpy by pyrazinyl rings in 4-Ar-dppy results in the stabilization of the LUMO energy levels of [ReCl(CO)3(4-Ar-dppy-κ2N)] relative to [ReCl(CO)3(4′-Ar-terpy-κ2N)] by ∼0.3 eV. For all of the molecules 1A–4A and 1B–4B, the LUMO resides predominately on the coordinated rings of terpy/dppy cores.

The HOMO of 1A–3A and 1B–3B is largely located on the {Re(CO)3Cl} unit, and its energy is almost independent of the attached aryl group. The replacement of the peripheral pyridyl rings in 4′-Ar-terpy by pyrazinyl ones in 4-Ar-dppy results in a negligible stabilization of the HOMO energy levels of [ReCl(CO)3(4-Ar-dppy-κ2N)] relative to [ReCl(CO)3(4′-Ar-terpy-κ2N)], by ∼0.05 eV. The HOMO–LUMO gap across the series 1A–3A varies marginally from 3.79 eV (2A) to 3.84 eV (1A), and it is comparable to the values reported previously for [ReCl(CO)3(terpy-κ2N)] (3.89 eV) and [ReCl(CO)3(4′-Ph-terpy-κ2N)] (3.82) eV[14,23] (Figure S29). Similarly to the terpyridine Re(I) complexes 1A–3A, the HOMO–LUMO gap of 1B–3B is insensitive to the attached aryl group (3.54 eV for 1A and 2A and 3.52 eV for 3A). In relation to [ReCl(CO)3(4′-Ar-terpy-κ2N)], however, it becomes noticeably smaller as a result of the stabilization of the LUMO energy level in [ReCl(CO)3(4-Ar-dppy-κ2N)].

In contrast to 1A–3A and 1B–3B, the HOMO of 4A and 4B is principally localized on the pyrenyl group attached to the terpy/dppy core, and it is effectively destabilized relative to the HOMO levels of 1A–3A and 1B–3B, respectively. In both series, an increase in the HOMO energy level of 4A and 4B is ∼0.4 eV. This leads to a noticeable decrease in the HOMO–LUMO gap in the case of 4A (3.41 eV) and 4B (3.13 eV) in comparison to that of other complexes [ReCl(CO)3(4-Ar-dppy-κ2N)] and [ReCl(CO)3(4′-Ar-terpy-κ2N)] investigated within this work.

The calculations also indicate that the attachment of the pyrenyl to the terpy/dppy core leads to a noticeable decrease in ionization potentials (IPs) of 4A and 4B in relation to 1A–3A and 1B–3B, respectively. Across the series 1A–3A and 1B–3B, calculated IP values vary marginally depending on the aryl substituent. In turn, the triimine core was found to affect electronic affinities (EAs), and the replacement of the peripheral pyridyl rings in 4′-Ar-terpy by pyrazinyl rings in 4-Ar-dppy results in an increase in EA values (Table S8).

Electrochemistry

The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed to experimentally estimate the HOMO and LUMO energy levels of [ReCl(CO)3(4′-Ar-terpy-κ2N)] and [ReCl(CO)3(4-Ar-dppy-κ2N)] complexes (1A–4B and 1B–4B). The relevant electrochemical data of 1A–4A and 1B–4B are gathered in Table and Table S9, while Figure S31 shows the CVs and DPVs for 1A–4A and 1B–4B. The ferrocene/ferrocenium couple was used as the reference redox couple. The values of IP and EA, which are closely related to HOMO and LUMO levels, were estimated from the first oxidation and reduction waves, respectively.[49]

Table 1

Electrochemical Data for Compounds 1A–4A and 1B–4B

compound	E1redonset (V)	E1oxonset (V)	IPa (CV)	EAb (CV)	Eg(CV)c (eV)	Eg(OPT)d (eV)
1A	–1.67	0.73	–5.83	–3.43	2.40	2.62
2A	–1.63	0.67	–5.77	–3.47	2.30	2.62
3A	–1.67	0.66	–5.76	–3.43	2.33	2.65
4A	–1.65	0.71	–5.81	–3.45	2.36	2.67
1B	–1.32	0.96	–6.06	–3.78	2.28	2.42
2B	–1.46	0.90	–6.00	–3.64	2.36	2.40
3B	–1.35	0.83	–5.93	–3.75	2.18	2.42
4B	–1.29	0.92	–6.02	–3.81	2.21	2.47

IP = −5.1 – Eox.

EA = −5.1 – Ered.

Eg(CV) = Eoxonset – Eredonset.

Eg(OPT) = 1240/λonset.

As shown in Table , the differences in the values of E1oxonset for [ReCl(CO)3(4′-Ar-terpy-κ2N)] and [ReCl(CO)3(4-Ar-dppy-κ2N)] were less than 0.07 and 0.13 V, respectively. Also, E1redonset values fall in a narrow range, from −1.63 to −1.67 V for 1A–4A and from −1.29 to −1.46 V for 1B–4B. Upon the replacement of the pyridyl by pyrazinyl, the first reduction wave moves into a more positive region. The complexes 1B–4B become easier to reduce but more difficult to oxidize relative to 1A–4A. It can be safely assumed that the reduction in 1A–4A and 1B–4B occurs in the triimine core. As supported by DFT studies, the LUMO of these systems is largely contributed by π* orbitals of the terpy/dppy core, and additional N atoms in the peripheral rings (dppy) result in lowering of the LUMO energy level of Re(I) complexes bearing 2,6-bis(pyrazin-2-yl)pyridine derivatives, which is manifested in the shift of the first reduction peaks of 1B–4B toward less negative potentials. The first oxidation waves of 1A–3A and 1B–3B most probably correspond to the Re(I)-based oxidation process Re(I)/Re(II). As reported in refs (50 and 51), the first oxidation of the free Ar1–3-terpy and Ar1–3-dppy was found to occur at significantly higher potentials in relation to the corresponding Re(I) complexes 1A–3A and 1B–3B. The values of E1oxonset of 4A and 4B are comparable with those for 1A–3A and 1B–3B, and they are also consistent with those reported for the free terpy and dppy functionalized with 1-pyrenyl.[50,51] On the basis of the DFT results, showing that the HOMO of 4A and 4B is principally localized on the pyrenyl group attached to the terpy/dppy core, it can be suggested that the oxidation in 4A and 4B occurs in the pyrene core.

Absorption Spectroscopy and TD-DFT Calculations

The electronic absorption spectra of 1A–4A and 1B–4B were recorded in acetonitrile and chloroform and in thin films on a glass substrate (Figure and Figures S32–S35). The spectroscopic data are summarized in Table S10. All of the complexes [ReCl(CO)3(4′-Ar-terpy-κ2N)] (1A–4A) and [ReCl(CO)3(4-Ar-dppy-κ2N)] (1B–4B) exhibited absorption properties of typical [ReX(CO)3(diimine)]0/+ chromophores, with intense bands below 350 nm due to the organic ligand π → π* transitions and moderate absorption in the range 350–480 nm attributed to the electronic transitions of charge-transfer (CT) character (Figure ).

Figure 2

UV–vis absorption spectra of 1A–4A (a) and 1B–4B (b) in acetonitrile.

From 1A–4A to 1B–4B, the low-energy band experiences a bathochromic shift (Figure S34), which can be rationalized by stronger electron-withdrawing abilities of Ar-dppy due to the presence of additional nitrogen atoms in the peripheral rings, leading to a decrease in the LUMO energy level of [ReCl(CO)3(4-Ar-dppy-κ2N)] in relation to [ReCl(CO)3(4′-Ar-terpy-κ2N)]. In both 1A–4A and 1B–4B series, the attachment of the pyrenyl group to the central pyridine ring of the terpy or dppy core leads to a noticeable red shift of the lowest energy absorption of the resulting Re(I) complexes, while the effect of naphthyl and phenanthrenyl units on the position of the lowest-energy absorption of [ReCl(CO)3(4-Ar-dppy-κ2N)] and [ReCl(CO)3(4′-Ar-terpy-κ2N)] is rather marginal. In comparison to [ReCl(CO)3(terpy-κ2N)] and [ReCl(CO)3(4′-Ph-terpy-κ2N)]/[ReCl(CO)3(4-Ph-dppy-κ2N)], there is a noticeable intensity increase of the visible light absorption for 1A–4A and 1B–4B, which can be attributed to the introduction of extended aryl groups into the terpy or dppy core[14,23] (Figure S32). With an increase in the solvent polarity, the CT absorption of 1A–4A to 1B–4B experiences a hypsochromic shift, typical of rhenium(I) tricarbonyl diimine complexes with a metal to ligand charge-transfer (MLCT) absorption band[52] (Table S10 and Figure S34).

Given these observations, that is, a blue shift of the low-energy band of 1A–4A to 1B–4B with increasing solvent polarity and higher light absorption in the case of 4A to 4B, the low-energy absorption of 1A–3A and 1B–3B is most likely of MLCT character, while the visible part of the electronic spectra of 4A and 4B is most probably a combination of ILCT and MLCT transitions. Such an assignment is in agreement with the TD-DFT calculations presented in Table S11 and Figure S36.

For 1A–3A to 1B–3B, the dominant calculated transition for the lowest energy absorption band is the S0 → S2 excitation, which corresponds to charge transfer from the {Re(CO)3Cl} moiety to the π* orbital of the 4′-Ar-terpy or 4-Ar-dppy ligand. The same MLCT character can be assigned to the excitations S0 → S1 appearing at the red end of the visible absorption band of 1A–3A to 1B–3B and S0 → S3 (2A, 2B) and S0 → S4 (1A, 3A, 1B, 3B) making a contribution to the blue edge of the lowest-lying absorption band. The excitations S0 → S3 (1A, 3A, 1B, 3B) and S0 → S4 (2A, 2B) are IL (intraligand) with an admixture of MLCT in nature, and they contribute to the blue edge of the lowest-lying absorption band. In comparison to the S0 → S2 transition, all they have much lower oscillator strengths.

For complexes 4A and 4B, the electronic excitations S0 → S2, S0 → S3, and S0 → S4 (4A) and S0 → S3 and S0 → S4 (4B) conserve the MLCT character. In contrast, the transitions S0 → S1 and S0 → S5 (4A) and S0 → S1, S0 → S2, and S0 → S5 (4B) are intraligand and MLCT in nature. The significant increase in their oscillator strengths can be assigned to the contribution of intraligand charge-transfer (ILCT) transitions originating from charge delocalization from the pyrenyl group to the terpy/dppy acceptor core.

Luminescence Studies

The photoluminescence (PL) properties of the synthesized Re(I) complexes were investigated in two solvents of different polarities (CHCl3, ε = 4.8; CH3CN, ε = 37.5), in a rigid matrix at 77 K (BuCN), and in the solid state as powders and thin films. The emission spectral data of 1A–4A and 1B–4B are gathered in Table . The normalized emission spectra of the synthesized Re(I) complexes upon photoexcitation at the lowest absorption band are shown in Figures –9 and Figures S37–S58 in the Supporting Information.

Table 2

Summary of Photoluminescence Properties of Complexes 1A–4A and 1B–4Ba

	CH3CN				CHCl3				solid				BuCN (77 K)
	λexc (nm)	λem (nm)	τav (ns)	φ (%)	λexc (nm)	λem (nm)	τav (ns)	φ (%)	λexc (nm)	λem (nm)	τav (ns)	φ (%)	λexc (nm)	λem (nm)	τav (μs)
1A	410	654	4.5	1.9	420	660	6.5	7.7	420	628	162.8	4.8	420	522, 558	249.2
2A	405	663	4.0	1.1	450	645	6.1	9.8	400	578	102.2	1.6	420	518, 556	95.6
3A	410	641	3.1	0.6	440	665	4.2	6.8	370	619	13.0	0.6	420	522 sh, 558	104.6
4A	420	620	3.3	6.1	450	500	4.65	<0.1	nd	nd			440	627, 680, 756	5738.7
						650, 696 sh	4.404	7.6
1B	400	750	3.4	2.4	480	730	4.5	6.9	440	651	55.9	9.7	400	597	2.1
2B	385	680	3.5	1.8	480	730	4.9	10.4	440	666	63.1	10.7	440	600	2.3
3B	440	737	3.3	5.0	480	736	4.4	6.4	435	642	78.9	9.8	410	595	2.2
4B	425	520	4.0	0.3	475	532	8.2	0.5	nd	nd			450	632, 694, 770	1933.9
		708	122.9	4.4		730	130.9	10.9

τav denotes the average lifetime. Experimental errors for emission lifetimes are given in Figure S37 in the Supporting Information.

Figure 3

(a) Normalized emission spectra of 1A–3A in CH3CN in comparison to the emission spectra of [ReCl(CO)3(terpy-κ2N)] and [ReCl(CO)3(4′-Ph-terpy-κ2N)] in CH3CN. Reproduced from refs (14 and 23). Copyright John Wiley and Sons 2018 and Royal Society of Chemistry 2020, respectively. (b) Normalized emission spectra of 1A–3A in CHCl3 and a rigid matrix at 77 K.

Figure 9

(a) fsTA spectra of 1A, 4A, and L measured using 420 and 355 nm pump light: energies per pulse equal to 0.18 μJ (1A), 0.20 μJ (4A, 420 nm), 0.20 (4A, 355 nm), 0.26 μJ (L); delay time window 0.5–5000 ps. (b) nsTA spectra of 4A. (c) nsTA spectra of L.

In solution, the maximum emission energies of 1A–3A are similar to each other and resemble those for [ReCl(CO)3(terpy-κ2N)] and [ReCl(CO)3(4′-Ph-terpy-κ2N)],[14,23] implying that the emitting state in these complexes is of the same origin, and it is only slightly affected by the aryl group attached to the terpy core (Figure a and Table ). All of these complexes exhibit broad and structureless steady-state emission spectra with maxima in the narrow range 641–663 nm for CH3CN and 645–665 nm for CHCl3, with lifetimes in the nanosecond domain and rather low emission quantum yields. From the solution at room temperature to the solid state and matrix at 77 K, the emission of 1A–3A exhibits a hypsochromic shift (Figure b). Such behavior is typical for 3MLCT excited states. As the rigidity of the medium increases and solvent reorganization decreases, the 3MLCT excited state is destabilized and consequently the excited- to ground-state energy gap is increased, which is reflected in a blue shift of the emission and leads to a significant decrease in the nonradiative decay rate constant.[53−55]

The photoluminescence spectra of 1B–3B also originated from 3MLCT states at room temperature. All of these compounds exhibit broad and nonstructured emission bands with maxima in the range 680–750 nm in solution, 651–666 nm in the solid state, and 595–600 nm at 77 K, short lifetimes in solution, and longer lifetimes in the solid state and a rigid matrix at 77 K. In comparison to 1A–3A, the emission energies of 1B–3B are significantly red shifted, consistent with the LUMO energy stabilization and decrease in HOMO–LUMO energy gap upon replacing the terpy core by the dppy core (Figure ).

Figure 4

Normalized emission spectra of 1B–3B compared to the emission spectra of 1A–3A in CH3CN.

Another striking difference between 1A–3A and 1B–3B concerns spectral profiles and excited-state lifetimes of the emission at 77 K. The frozen-state emission bands of 1B–3B remain broad and nonstructured, clearly indicating that they still originate from 3MLCT states. As shown in Figure , the frozen-state emissions of 1B–3B occur at significantly lower energies relative to the phosphorescence of the free ligands. For 1A–3A at 77 K, the emission band shows a vibrionic structure, and the triplet emission of 1A–3A largely overlaps with the phosphorescence of the free ligands (Figure ). A larger contribution of 3IL in Re(I) complexes with naphthyl- and phenanthrenyl-substituted terpyridine ligands is also manifested in a noticeable increase of emission lifetimes of 1A–3A relative to 1B–3B (Table ). The replacement of the terpy core by the dppy core results in an increase in the energy gap between 3MLCT and 3IL in 1B–3B.

Figure 5

(a) Phosphorescence spectra of 1A–3A versus the phosphorescence spectra of the free ligands at 77 K. (b) Phosphorescence spectra of 1B–3B versus the phosphorescence spectra of the free ligands at 77 K. The triplet ligand emissions were induced by addition of 10% ethyl iodide and recorded at 77 K in BuCN.

The 3MLCT emission nature of 1A–3A was additionally confirmed by theoretical results. The energies of the theoretical phosphorescence emission, corresponding to the difference ΔET, reproduce the experimental data well, differing from the experimental values by 0.09–0.13 eV.

The spin density surface plots generated from the T1 states of complexes 1A–3A (Table S12) depict spin density to be localized on the {Re(CO)3Cl} unit and π* orbitals of the pyridine rings coordinated to the Re(I) center, supporting that the lowest energy triplet state in these complexes is 3MLCT.

The introduction of the pyrene ring into the terpy/dppy core substantially affects the photophysical properties of the Re(I) complexes (4A and 4B). Low-temperature (77 K) emission spectra of 4A and 4B resemble each other. They appear at lower energy in relation to those for 1A–3A to 1B–3B and show a well-resolved vibronic structure (Figure ). As the frozen-state emission of 4A and 4B falls in the range of the phosphorescence of the free ligands and pyrene, and it is significantly red shifted relative to the emission of the parent Re(I) complexes, we can assume that their emission at 77 K occurs predominately from the excited state of 3ILpyrene with an admixture of 3ILCTpyrene→terpy character (Figures S50 and S51). Such an assignment is supported by lifetimes in milliseconds, 1–3 orders of magnitude longer than those for 1A–3A to 1B–3B chromophores at 77 K (Table ). Also, spin density surface plots generated from the T1 states of complexes 4A and 4B illustrate that the spin density is localized on the pyrene and central pyridine of terpy/dppy (Table S13).

Figure 6

Comparison of low-temperature emission spectra of 4A and 4B with low-temperature emission spectra of 1A–3A to 1B–3B.

To further examine the nature of the lowest triplet state of [ReCl(CO)3(L-κ2N)] with pyrene-substituted ligands, time-resolved emission spectra of 4A and 4B at 77 K were recorded (Figure and Figures S61–S64). For both complexes, the presence of two components was revealed. The higher energy emission in the range below 600 nm closely resembles the 3MLCT emission spectrum at low temperature (77 K) for 1A and 1B. The longer lifetime of the structured component with a maximum at 620 nm is typical of the organic chromophore phosphorescence, and it can be assigned to 3ILpyrene with a small admixture of 3ILCTpyrene→terpy. These findings allow us to conclude that an intramolecular energy transfer from the 3MLCT excited state to the 3ILpyrene/3ILCTpyrene→trimine state occurs in 4A and 4B.

Figure 7

Time-resolved emission spectrum of 4A and 1A at 77 K excited at 395 nm.

The room-temperature photoluminescence spectra of 4A and 4B in solution are shown in Figure . The emission spectra of deaerated solutions of 4A in CHCl3 and 4B in CH3CN and CHCl3 are composed of two well-separated bands, as reported previously for other bichromophoric systems.[47,56] The emission peak at longer wavelengths is quenched upon exposure to air (Figure S58), demonstrating that it arises from a triplet state. The possibility that the emission peak at shorter wavelength is due to the free ligand that has dissociated from the complex can be excluded. The examined Re(I) complexes show stability and photostability in the media used (Figure S59 and S60), and the high-energy band of 4A in CHCl3 and 4B in CH3CN and CHCl3 is bathochromically shifted relative to the emission of the appropriate free ligand (Figure ). As shown in Figures S52–S55, the singlet ligand-centered excited state is promoted by higher energy excitation, and the ratio of the fluorescence and phosphorescence is dependent on the excitation wavelength. In comparison to the free ligand, however, the complex fluorescence is clearly quenched, suggesting the occurrence of energy transfer from 1IL/1ILCT to 1MLCT via a Förster resonance energy transfer (FRET) mechanism (Figures S56 and S57).

Figure 8

Normalized room-temperature emission spectra of the free ligands and complexes 4A and 4B in CH3CN and in CHCl3. Excitation wavelengths are given in parentheses. Reproduced from refs (50 and 51). Copyright Elsevier 2020 and 2021.

In the case of the deaerated solution 4A in CH3CN, the prevailing emission originates from the singlet excited state. To better understand the differences between 4A in CH3CN and 4A in CHCl3, the efficiency of the energy transfer from 1IL/1ILCT to 1MLCT occurring through the FRET mechanism was calculated using the equation , where QY is the quantum yield of the fluorescence emanating from the L ligand in the complex 4A, while QYL is the quantum yield of the free ligand.[47] Noticeably less efficient Förster energy transfer was found for 4A in CH3CN (87%) in comparison to 4A in CHCl3 (99%). According to ref (47), this residual fluorescence in the acetonitrile solution of 4A is sufficient to conceal the triplet emission (see also Figures S58 and S64). The overlap of the phosphorescence with much stronger fluorescence prevented us from determining the phosphorescence lifetime of 4A in CH3CN (Table ). For 4A in CHCl3, due to the greater FRET efficiency (99%), both fluorescence and phosphorescence were observed.[47]

Most remarkably, the room-temperature phosphorescence lifetimes of 4A in CHCl3 and 4B in CH3CN/CHCl3 are 2–3 orders of magnitude longer than those for 1A–3A to 1B–3B chromophores. Such a significant prolongation of excited-state lifetimes indicates the thermal activation between closely lying 3MLCT and 3IL/3ILCT. The pyrene chromophore repopulates the luminescent 3MLCT excited state and acts as an excited-state storage element, resulting in an extension of the 3MLCT emission. The longer lifetime of 4A in relation to 4B seems to be a result of the greater energy separation between the triplet states 3MLCT and 3ILpyrene/3ILCTpyrene→trimine in the case of 4A (Figure ) and thus the larger contribution of the lowest excited state 3ILpyrene/3ILCTpyrene→trimine with a much longer lifetime.[47] However, it should be noted that the phosphorescence lifetimes of 4B in CH3CN/CHCl3 may be affected to some extent by the presence of the greater residual fluorescence in the case of 4B in comparison to 4A in CHCl3 (Figure and Figures S61–S64).

The solid-state emission is changed upon the replacement of naphthyl and phenanthrenyl substituents by the pyrenyl substituent. In contrast to 1A–3A and 1B–3B, the solid-state emission of 4A and 4B is completely quenched, which can be assigned to aggregation-caused quenching (Table ). Preliminary investigations of the PL ability of Re(I) complexes in film blends are given in Table S14 and Figures S65–S68 in the Supporting Information.

Femto- and Nanosecond Transient Absorption

The excited-state dynamics and the nature of the lowest triplet state of 4A were investigated using transient absorption spectroscopy in femtosecond (fsTA) and nanosecond (nsTA) regimes, and the results for 4A were compared to those for the free ligand L and 1A.

The fsTA spectra of 1A display only positive features across the UV and visible region (Figure ). According to refs (57−59), they are indicative of 3MLCT excited-state absorptions. The intense band at 376 nm is typical of the absorption of the bipyridine anion radical bipy•–, whereas the excited-state absorption (ESA) in the visible part is attributed to Cl/L•– → Re (ligand to metal charge transfer, LMCT) transitions. Due to the high similarity between 1A and previously reported [ReCl(CO)3(terpy-κ2N)][18] and [ReCl(CO)3(bipy)],[57−59] we can safely state that the excitation of 1A produces the 1MLCT state, which undergoes femtosecond intersystem crossing (ISC) and simultaneously populates an intermediate π → π* intraligand state (3IL) and a vibrationally hot 3MLCT state. The conversion of the 3IL excited state into the 3CT state occurs on a picosecond time scale. Such an assignment is supported by a global lifetime analysis, given in Figure S69 in the Supporting Information. Three components, revealed by the global lifetime analysis and characterized by time constants t, are attributed to the conversion of the intermediate state 3IL to 3MLCT, vibrational relaxation of the lowest triplet state 3CT, and ground-state recovery times, respectively (Figure ). The ultrafast intersystem crossing occurs in a time range (∼140 fs) shorter than the internal response function (IRF = 175 fs).

Figure 10

Decay-associated spectra of 1A and 4A.

The fsTA spectra of 4A collected upon excitation at 355 nm, populating predominately the 1ILCT state, are characterized by a clear ground-state bleaching (GSB) at the wavelength ∼400 nm and excited-state absorption (ESA) with two discernible maxima at 517 and 594 nm. The GSB is in accordance with 1MLCT/1ILCT of 4A, and it covers the range of the fluorescence of the L (due to their overlapping) (see Figure and Figures S70–S72). The high similarity between the line shapes of the relaxed excited state of 4A in the fsTA regime and spectra in nsTA allows us to safely assume that no additional excited states occur between the femtosecond and nanosecond time scales. The spectral similarities in nsTA spectra between the complex 4A and L indicate that the lowest triplet state of 4A is of 3ILpyrene/3ILCTpyrene→terpy character, while a strong resemblance of the temporal evolution of the transient absorption spectra of 4A excited at 355 nm and those measured for 4A upon excitation of 420 nm allows us to assume that 4A follows the same photophysical pathway on excitation at 355 and 420 nm (Figure ).

The proposed energy level diagram of 4A with the deactivation pathway, demonstrated in Figure , is well supported by the global lifetime analysis of 4A (Figure ).

Figure 11

Schematic energy level diagram with the deactivation pathway proposed for 4A.

A global lifetime analysis confirmed that the fsTA data of 4A are best described in terms of four components, each characterized by a time constant t (Figure and Figure S70 and S71). Upon excitation at 355 nm, the excited state 1L is formed (DAS1), and the molecules undergo energy transfer through the FRET mechanism to the 1MLCT* state, which promptly undergoes intersystem crossing (ISC) to the 3MLCT* (t1 = 0.41 ps). The DAS2 is negative across the whole visible spectral region, reflecting the increase in the 400–700 nm excited state, which corresponds to the relaxed 3MLCT state. The t2 relaxation is due to reorganization processes occurring within a supramolecular moiety comprising the Re(I) chromophore and local solvent molecules, also including rotation about the pyrene–terpy bond. The relaxed 3MLCT state undergoes triplet–triplet energy transfer into 3IL/3ILCT localized on L (t3 = 136 ps). DAS4 corresponds to the absorption spectrum of the fully relaxed lowest triplet state 3IL/3ILCT.

Conclusions

A series of eight rhenium(I) complexes with 2,2′:6′,2″-terpyridine (terpy) and 2,6-bis(pyrazin-2-yl)pyridine (dppy) substituted with 1-naphthyl, 2-naphthyl, 9-phenanthrenyl, and 1-pyrenyl groups was synthesized, and the effect of the tris-heterocyclic core and aryl substituent on selected properties of the obtained [ReCl(CO)3(L-κ2N)] was studied in detail. All complexes exhibited high melting temperatures above 200 °C, and they can form amorphous materials with very high Tg values. The ligands functionalized with naphthyl and phenanthrenyl units were found to have a rather marginal effect on the electrochemical and optical properties of the resulting Re(I) complexes. In contrast, the introduction of the pyrenyl group to the central pyridine ring of the terpy or dppy core resulted in an increase in the molar absorption coefficients and led to a noticeable bathochromic shift of the lowest energy absorption of [ReCl(CO)3(L-κ2N)] due to the large involvement of 1ILCT. With regard to the emission properties, the complexes [ReCl(CO)3(L-κ2N)] with 1-pyrenyl-subsituted ligands exhibited greatly enhanced room-temperature photoluminescence lifetimes, consistent with the formation of an equilibrium between the 3MLCT and 3IL/3ILCT excited states. The deactivation pathway occurring upon the light excitation in [ReCl(CO)3(4′-(1-naphthyl)-terpy-κ2N)] and [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)] was determined by femtosecond transient absorption studies. The excitation of the first complex produces the 1MLCT state, which undergoes femtosecond intersystem crossing (ISC) and simultaneously populates an intermediate π → π* intraligand state (3IL) and a vibrationally hot 3MLCT state. The conversion of the 3IL excited state into the 3CT state occurs on a picosecond time scale. In the case of [ReCl(CO)3(4′-(1-pyrenyl)-terpy-κ2N)], upon excitation at 355 an 420 nm, the singlet excited state pyrenyl-terpy* is formed, which undergoes an energy transfer through the FRET mechanism to the 1MLCT* state, being further transformed to the 3MLCT* by ISC. The relaxed 3MLCT state undergoes triplet–triplet energy transfer into 3IL/3ILCT localized on the pyrenyl-terpy ligand.

Experimental Section

The synthesis and characterization of the Re(I) carbonyl complexes are provided in the Supporting Information. Elemental analysis was recorded on a Vario EL Cube apparatus. NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer in DMSO-d6 solution. IR spectra were measured using a Nicolet iS5 FTIR spectrophotometer (KBr). High-resolution mass spectrometry analyses were performed on a Waters Xevo G2 Q-TOF mass spectrometer (Waters Corporation) equipped with an ESI source operating in positive-ion mode. Single-crystal X-ray diffraction data were collected on a Gemini A Ultra diffractometer (Mo Kα),[18] and crystallographic data for 1A were deposited with the Cambridge Crystallographic Data Center (CCDC 2094604). Differential scanning calorimetry (DSC) studies were carried out with the use of a TA-DSC 2010 apparatus under nitrogen atmosphere, with a heating rate of 20 °C/min. Electrochemical measurements were performed using an Eco ChemieAutolab PGSTAT128n potentiostat.[18] The electronic spectra were registered on a ThermoScientific Evolution 220 UV/vis spectrometer. The photoluminescence was obtained on a Hitachi F-2500 spectrometer or an FLS-980 fluorescence spectrophotometer.[18] Femtosecond transient absorption spectra were measured using a Helios pump–probe transient absorption spectrometer (Ultrafast Systems).[18] Nanosecond transient absorption spectra were recorded according to the procedure given previously.[18] The calculations were performed using the GAUSSIAN-16 program package.[60] More experimental details are given in the Supporting Information.