Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2′:6′,2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2′:6′,2″-terpyridine-based ligands (R-terpy)—encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2′:6′,2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X—mono-negative anion, L—neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure–property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Regarding photophysical behavior, [ReCl(CO) 3 (terpy-κ 2 N)] was initially found to be non-emissive in solution at RT [32], which led to a noticeable decline in scientific interest in this class of compounds compared to diimine Re(I) tricarbonyl compounds.The striking difference in the emission properties between [ReCl(CO) 3 (terpy-κ 2 N)] and its analog [ReCl(CO) 3 (bipy)] (bipy-2,2 ′ -bipyridine) was rationalized by the thermal coupling of 3 MLCT and 3 IL excited states, diminished in the bipy-based Re(I) carbonyl due to the larger energy separation between 3 MLCT and 3 IL [39].However, the repetition of spectroscopic investigation of [ReCl(CO) 3 (terpy-κ 2 N)] by Amoroso et al. showed that the complex is weakly emissive, both in solution and the solid state [39].Since 2013, there has been renewed interest in the photophysics of terpy-based Re(I) carbonyl complexes.It was assumed that variations in the terpy core and ancillary ligand (X/L) might lead to a significant enhancement of photoluminescence and improve the photocatalytic performance of [Re(X/L)(CO) 3 (R-terpy-κ 2 N)] 0/+ systems, offering a chance to develop new functional materials for modern technologies and expand the fundamental knowledge and understanding in optimizing the photophysical properties of transition metal complexes.The great advantage of 2,2 ′ :6 ′ ,2 ′′ -terpyridines is their efficient synthesis method (Kröhnke condensation), allowing for the incorporation of a wide range of electron-withdrawing or donating groups into the terpy core.Furthermore, compared to diimine derivatives, 2,2 ′ :6 ′ ,2 ′′ -terpyridine-based ligands provide the possibility of additional modification of the photophysical properties of Re(I) carbonyl systems through coordination to the Re(I) center.
(a) (b) A different coordination mode of terpy induces noticeable variations in the bond lengths and angles in [ReX(CO)3(terpy-κ 2 N)] and [ReX(CO)2(terpy-κ 3 N)] (Table 1).The Re-N distances, especially those between the metal ion and the central pyridine ring of terpy, undergo shortening after conversion from the κ 2 N to κ 3 N coordination mode, indicating a stronger Re-terpy interaction in the complex bearing a meridionally-coordinated terpy ligand.In turn, the Re-CO bonds become elongated upon changing the terpy coordination mode from bidentate to tridentate, and elongation of the Re-CO distances in [ReX(CO)2(terpy-κ 3   1).
As widely reported in [33,35,[41][42][43][44], the terpy bidentate coordination mode of [ReX(CO)3(terpy-κ 2 N)] is also maintained in solution, with the terpy ligand exhibiting fluxionality.It switches its metal coordination sites between pendant pyridyl groups via an associative mechanism implying a seven-coordinate intermediate (Scheme 1).A different coordination mode of terpy induces noticeable variations in the bond lengths and angles in [ReX(CO) 3 (terpy-κ 2 N)] and [ReX(CO) 2 (terpy-κ 3 N)] (Table 1).The Re-N distances, especially those between the metal ion and the central pyridine ring of terpy, undergo shortening after conversion from the κ 2 N to κ 3 N coordination mode, indicating a stronger Re-terpy interaction in the complex bearing a meridionally-coordinated terpy ligand.In turn, the Re-CO bonds become elongated upon changing the terpy co-   1).

Photophysical
Regarding the photoluminescence properties of the Re(I) complex bearing the terpyκ 2 N ligand, there is a significant discrepancy in the literature data (Table 2).The complex [ReCl(CO) 3 (terpy-κ 2 N)] was initially reported as non-luminescent in DMF solution at RT, displaying emission only at 77 K in a 9:1 DMF-CH 2 Cl 2 glass [32].Subsequent studies of [ReCl(CO) 3 (terpy-κ 2 N)] [39] revealed weak emission in its acetonitrile solution at 506 nm following the photoexcitation at 360 nm, but no photoluminescence quantum yield and lifetimes were recorded.The authors of [49] demonstrated that [ReCl(CO) 3 (terpy-κ 2 N)] in CH 2 Cl 2 , excited at 442 nm, displayed the emission band at 509 nm, with photoluminescence quantum yield and decay times recorded as 0.3 % and 2.02 µs, respectively.Worthy of note, the emission spectrum of [ReCl(CO) 3 (terpy-κ 2 N)], presented in [49], comprised two emission bands (at 509 and ~630 nm), but only the higher energy one was discussed by the authors.Concerning time-resolved measurements, the accuracy cannot be estimated based on the decay curves included in the ESI materials [49].The latest studies conducted by our research group [46] revealed somewhat different photophysics of [ReCl(CO) 3 (terpyκ 2 N)].The emission of this complex was found to occur above 600 nm, namely at 638 nm in CHCl 3 and 656 nm in MeCN, with excited-state lifetimes falling in the nanosecond range (Figure 2 and Table 2).These emission maxima and lifetimes correlate well with the results for the related bromide complex [ReBr(CO) 3 (terpy-κ 2 N)] in acetonitrile, recently reported in [47].According to the findings of the latest research [46,47], the Re(I) complexes with the bidentate terpy ligand exhibit typical features of 3 MLCT emitters, similar to their structural analogs [ReX(CO) 3 (bipy)].Typically of 3 MLCT, the emission bands of [ReCl(CO) 3 (terpy-κ 2 N)] remain broad and structureless in both solution and rigid-glass matrix (77 K).The solvent polarity induces bathochromic shift of the emission upon going from chloroform to acetonitrile, and the frozen-state emissions are significantly blue-shifted and show prolonged lifetimes due to the rigidochromic effect [51,52].Consistent with a larger conjugation of terpy relative to bipy, the solution emission of [ReCl(CO) 3 (terpyκ 2 N)] appears at slightly longer wavelengths compared to that for [ReCl(CO) 3 (bipy)] (Figure 2).In turn, the frozen-state emission of [ReCl(CO) 3 (terpy-κ 2 N)] is of higher energy relative to that for [ReCl(CO) 3 (bipy)] and better overlaps with the terpy phosphorescence, indicating a larger competition between 3 MLCT and 3 IL in the case of the terpy Re(I) complex, as suggested in [39].A shortening of the emitting triplet-state lifetime (τ = 3.0 ns in CHCl 3 ) of [ReCl(CO) 3 (terpy-κ 2 N)] relative to [ReCl(CO) 3 (bipy)] (τ = 51.0 ns in CHCl 3 ) was rationalized by the presence of a dangling (non-coordinated) pyridine ring in [ReCl(CO) 3 (terpy-κ 2 N)], resulting in greater complex flexibility [50].] and [ReCl(CO)3(bipy)], performed using the spectral data reported in our previous works [46,50].
A more comprehensive understanding of excited-state processes in [ReX(CO) 3 (terpyκ 2 N)] was achieved through femtosecond transient absorption (fs-TA) spectroscopy [48,50].By analogy to [ReX(CO) 3 (bipy)] [53,54], the TA spectra of [ReX(CO) 3 (terpy-κ 2 N)] were characterized by two positive signals attributed to excited-state absorptions (ESA): a sharp band below 400 nm assigned to the absorption of the bipy/terpy anion radical and a broad absorption in the visible region corresponding to X/L •− →Re (Ligand-to-Metal-Charge-Transfer, LMCT) transitions.Based on global lifetime analysis, it was revealed that the optically populated 1 MLCT state of [ReCl(CO) 3 (terpy-κ 2 N)] underwent femtosecond intersystem crossing (ISC), populating an interligand-localized excited state ( 3 IL) and vibrationally hot 3 MLCT excited states.The former one is converted into 3 MLCT on a picosecond timescale, and the relaxed 3 MLCT state decays via minor radiative and major non-radiative pathways to the ground state [50].
As reported in [36,47,55], the conversion from the terpy-κ 2 N to terpy-κ 3 N coordination mode results in dramatic changes in the absorption profile, attributed to the significant increase in conjugation due to the planarization of the terpy ligand, and the destabilization of the HOMO level of [ReX(CO) 2 (terpy-κ 3 N)] in relation to that of [ReX(CO) 3 (terpy-κ 2 N)], owing to the replacement of a strongly π-accepting CO group by weakly π-accepting pyridine of the terpy ligand.Regarding the LUMO level, almost no energy changes are observed after the conversion from κ 2 N to the κ 3 N coordination mode.Consistent with the reduced HOMO-LUMO energy gap, the longest wavelength absorption band of [ReX(CO) 2 (terpyκ 3 N)] exhibits a significant bathochromic shift relative to that of [ReX(CO) 3 (terpy-κ 2 N)].
The complexes [ReX(CO) 2 (terpy-κ 3 N)] are rare examples of dyes that display panchromatic absorption, occurring across the entire visible range of 400-800 nm (Table 3 and Figure 3).Based on the solvent sensitivity of the absorption bands, comparative analysis with the absorption features of the free ligand and theoretical calculations, it was found that intense absorptions in the range of 200-300 nm are best represented by IL transitions, while three broad bands in the visible part of the spectrum of [ReX(CO) 2 (terpy-κ 3 N)] correspond to MLCT transitions.As reported in [36,47,55], the conversion from the terpy-κ 2 N to terpy-κ 3 N coordination mode results in dramatic changes in the absorption profile, attributed to the significant increase in conjugation due to the planarization of the terpy ligand, and the destabilization of the HOMO level of [ReX(CO)2(terpy-κ 3 N)] in relation to that of [ReX(CO)3(terpy-κ 2 N)], owing to the replacement of a strongly π-accepting CO group by weakly π-accepting pyridine of the terpy ligand.Regarding the LUMO level, almost no energy changes are observed after the conversion from κ 2 N to the κ 3 N coordination mode.Consistent with the reduced HOMO-LUMO energy gap, the longest wavelength absorption band of [ReX(CO)2(terpy-κ 3 N)] exhibits a significant bathochromic shift relative to that of [ReX(CO)3(terpy-κ 2 N)].The complexes [ReX(CO)2(terpy-κ 3 N)] are rare examples of dyes that display panchromatic absorption, occurring across the entire visible range of 400-800 nm (Table 3 and Figure 3).Based on the solvent sensitivity of the absorption bands, comparative analysis with the absorption features of the free ligand and theoretical calculations, it was found that intense absorptions in the range of 200-300 nm are best represented by IL transitions, while three broad bands in the visible part of the spectrum of [ReX(CO)2(terpy-κ 3 N)] correspond to MLCT transitions.Conversely to [ReX(CO) 3 (terpy-κ 2 N)], the emission spectrum of [ReX(CO) 2 (terpyκ 3 N)] was only recorded at 77 K in a 4:1 ethanol-methanol glass.Typically of Re-based 3 MLCT emitters, the frozen emission band was structureless [36].In solution at RT, [ReX(CO) 2 (terpy-κ 3 N)] appeared to be non-emissive up to 800 nm [36,47].According to density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations, the emission of Re(I) complexes with meridionally-coordinated terpy is predicted at wavelengths longer than 900 nm [47], but experimentally has not been evidenced so far.

Phenyl and More π-Conjugated Hydrocarbon Groups
The relationships between the π-conjugation and linking mode of the aryl hydrocarbon groups attached to the terpy core and the photophysics of resulting complexes [ReCl(CO) 3 (R-terpy-κ 2 N)] were extensively investigated by our research group for the series of chloride complexes 1-7 [59][60][61][62].
Molecules 2024, 29, x FOR PEER REVIEW 8 of 28 ly-coordinated terpy is predicted at wavelengths longer than 900 nm [47], but experimentally has not been evidenced so far.

Phenyl and More π-Conjugated Hydrocarbon Groups
The relationships between the π-conjugation and linking mode of the aryl hydrocarbon groups attached to the terpy core and the photophysics of resulting complexes [ReCl(CO)3(R-terpy-κ 2 N)] were extensively investigated by our research group for the series of chloride complexes 1-7 [59][60][61][62].Based on UV-Vis absorption spectra, emission wavelengths, and lifetimes, we demonstrated that the complexes [ReCl(CO)3(R-terpy-κ 2 N)] with phenyl, naphtyl, and phenanthrenyl pendant groups preserve MLCT character, and the aryl-localized triplet excited state is not accessed upon photoexcitation in these systems.Relative to the model chromophore [ReCl(CO)3(terpy-κ 2 N)], the changes in the absorption and solution RT emission maxima are rather negligible; likewise, the lifetimes of 1 Cl -4 Cl fall in the nanosecond range.The presence of the π-conjugated naphtyl and phenanthrenyl substituents is generally manifested in a slight intensity increase of the visible light absorption and the appearance of vibrational progression in the frozen emission band (Figure 4 and Table S5).Consistent with the predominant 3 MLCT character of the lowest triplet state of 1 Cl -4 Cl and its destabilization due to an increase in medium rigidity upon cooling [51,52], the Based on UV-Vis absorption spectra, emission wavelengths, and lifetimes, we demonstrated that the complexes [ReCl(CO) 3 (R-terpy-κ 2 N)] with phenyl, naphtyl, and phenanthrenyl pendant groups preserve MLCT character, and the aryl-localized triplet excited state is not accessed upon photoexcitation in these systems.Relative to the model chromophore [ReCl(CO) 3 (terpy-κ 2 N)], the changes in the absorption and solution RT emission maxima are rather negligible; likewise, the lifetimes of 1 Cl -4 Cl fall in the nanosecond range.The presence of the π-conjugated naphtyl and phenanthrenyl substituents is generally manifested in a slight intensity increase of the visible light absorption and the appearance of vibrational progression in the frozen emission band (Figure 4 and Table S5).Consistent with the predominant 3 MLCT character of the lowest triplet state of 1 Cl -4 Cl and its destabilization due to an increase in medium rigidity upon cooling [51,52], the frozen-state emission of 1 Cl -4 Cl appears in a noticeably higher energy region and shows a significantly prolonged luminescence lifetime in relation to the RT emission in solution.
Molecules 2024, 29, x FOR PEER REVIEW 9 of 28 frozen-state emission of 1 Cl -4 Cl appears in a noticeably higher energy region and shows a significantly prolonged luminescence lifetime in relation to the RT emission in solution.Further evidence for the formation of the 3 MLCT state in these systems was provided using fs-TA spectroscopy, carried out for the representative complexes [ReX(CO)3(R-terpy-κ 2 N)] with phenyl (1 Cl , 1 Br ) and 1-naphtyl substituent (2 Cl ) [48,59,61], as well as time-resolved infrared spectroscopy performed for 1 Cl [63].All these studies confirmed the formation of the 3   S5) were attributed to the halide contribution to the excited state [47].
Distinctly from the Re(I) complexes with phenyl, naphtyl, and phenanthrenyl pendant groups, the complexes [ReCl(CO) 3 (R-terpy-κ 2 N)] with 9-anthryl (5 Cl ), 2-anthryl (6 Cl ), and 1-pyrenyl (7 Cl ) groups are non-emissive in the solid state, and their solution photophysical properties are strongly affected by the aryl substituent [61,62].As supported by DFT calculations, the HOMO of these systems is localized on the aryl substituent and is effectively destabilized (~0.4 eV) compared to the model chromophore.The LUMO largely resides on the terpy core, and its energy is hardly perturbed by the anthryl and pyrenyl substituents in relation to that for [ReCl(CO) 3 (terpy-κ 2 N)].A slight stabilization of the LUMO can be noticed upon the replacement of 9-anthryl by 2-anthryl, which was rationalized by a stronger coupling between the 2-anthryl group and the terpy unit.Consistent with the reduced HOMO-LUMO gaps, the lowest energy absorptions of 5 Cl -7 Cl have a noticeable red-shift relative to 1 Cl .For 6 Cl and 7 Cl , a bathochromic shift of the longest wavelength absorption is accompanied by a significant increase in its visible absorptivity due to the overlapping of 1 MLCT and 1 ILCT/ 1 IL transitions.A large dihedral angle between the appended 9-anthryl and central pyridine ring of terpy results in the separation of 1 MLCT and 1 IL states in 5 Cl .
The complex [ReCl(CO) 3 (R-terpy-κ 2 N)] with the pendant pyrenyl group (7 Cl ) was demonstrated to exhibit "ping-pong" energy transfer.Its excitation leads to a predominant population of the 1 ILCT state, which undergoes energy transfer to the 1 MLCT* state via the Förster resonance energy transfer (FRET) mechanism.In the next step, the 1 MLCT is converted to the 3 MLCT* by femtosecond intersystem crossing (ISC), and the formed 3 MLCT is further relaxed to the lower energy triplet excited state localized on the pyrenylterpy ligand.Also, the time-resolved emission spectra at 77 K and ns-TA spectroscopy confirmed that the T 1 state of 7 Cl is localized on the pyrenyl moiety.Due to the energetic proximity of the 3 MLCT and 3 IL/ 3 ILCT excited states, and the establishment of the tripletstate equilibrium between them, the complex 7 Cl shows a prolonged triplet excited-state lifetime at RT (4.4 µs) [61].The pyrene chromophore in 7 Cl acts as an energy reservoir for 3 MLCT [65][66][67][68][69][70].
Also, complexes 5 Cl and 6 Cl were found to exhibit a substantial enhancement of RT lifetimes in DMSO solution as a result of accessing the low-lying 3 IL state of the anthracene chromophore.Using steady-state and time-resolved optical techniques, our group demonstrated the impact of the different relative orientations of anthracene and {ReCl(CO) 3 (terpyκ 2 N)} chromophores on the photophysical behavior of the resulting complexes, 5 Cl and 6 Cl .A more planar geometry of 2-anthryl-terpy, and thus stronger overlapping orbitals of 2-anthryl and terpy moieties, was evidenced to facilitate the population of the anthracene triplet excited state, leading to the prolongation of its lifetime.The phosphorescence lifetimes of 5 Cl and 6 Cl were determined as 14.28 µs for 5 Cl and 22.71 µs for 6 Cl .It should be noted that transition metal complexes with extended emission lifetimes are strongly desirable for applications involving intermolecular photoinduced energy triplet state transfer, such as photodynamic therapy (PDT), time-resolved bioimaging, or triplet-triplet annihilation up-conversion (TTA UC) [19,69,[71][72][73][74][75][76][77].The suitability of 5 Cl and 6 Cl to transfer the excited triplet state energy to molecular oxygen was confirmed in our studies [62], demonstrating a slightly enhanced singlet oxygen sensitizing ability of 6 Cl (Φ ∆O 2 = 0.45) in relation to 5 Cl (Φ ∆O 2 = 0.42).Importantly, the complexes [ReCl(CO) 3 (R-terpy-κ 2 N)] with 9-anthryl (5 Cl ) and 2-anthryl (6 Cl ) were demonstrated to be rare examples that show both 3 MLCT and 3 anthracene emission.Consequently, their DMSO solution and electroluminescence spectra cover a broad range from 500 nm to the near-infrared region of 700-900 nm.The addition of a component with an emission from 400 to 500 nm might yield a diode, which emits white light [62].

Methoxy-Decorated Phenyl and Naphthyl Groups
The photophysical properties of [ReX(CO) 3 (R-terpy-κ 2 N)] with methoxy-decorated phenyl and naphthyl groups (class B) were the subject of the research reported in [23,[78][79][80].The attachment of one or more methoxy groups was found to induce only subtle variations in the solution emission properties of 8 Cl -13 Cl relative to those for corresponding model chromophores 1 Cl -3 Cl , implying that the emitting state is of 3 MLCT nature in all these systems.As shown in Table S6, the wavelength maxima of the broad and structureless emission bands of 8 Cl -13 Cl in acetonitrile and chloroform fall in a narrow range of 645-675 nm, lifetimes are in the nanosecond domain, and emission quantum yields are below 1.5%, similarly to features of model chromophores 1 Cl -3 Cl .
Noticeable differences were also observed in the solid-state emission lifetimes of 8 Cl -13 Cl , which varied from nano-to microseconds.Relative to the model chromophores 1 Cl -3 Cl , a substantial prolongation of the solid-state emission lifetime was evidenced for complexes 8 Cl , 10 Cl , 11 Cl , and 13 Cl .The triplet excited-state lifetimes were extended to 586 ns for 8 Cl , 446 ns for 10 Cl , 8.62 µs for 11 Cl , and 49.62 µs for 13 Cl , compared to 52 ns for 1 Cl , 162 ns for 2 Cl , and 102 ns for 3 Cl .Except for 12 Cl , the solid-state emission lifetimes of the Re(I) complexes belonging to class B underwent extension with an increase in π-conjugation of the pendant aryl group.Lowering the temperature down to 77 K resulted in a further increase in their excited-state lifetimes and a blue-shift of the emission maxima.As supported theoretically [78,79,82], methoxy-decoration of naphthyl groups resulted in an increased contribution of the organic ligand in the ground and excited states of resulting [ReX(CO)3(R-terpy-κ 2 N)], which may explain their prolonged solid-state emission lifetimes.To obtain triplet emitters with long excited states, both radiative and non-radiative decay rate constants must be small, meaning that the triplet excited state has a predominant ligand character [83].
Having good thermal properties and suitable energy levels, IP, and EA, the compounds 8 Cl -13 Cl were employed as active layers in light-emitting diodes The attachment of one or more methoxy groups was found to induce only subtle variations in the solution emission properties of 8 Cl -13 Cl relative to those for corresponding model chromophores 1 Cl -3 Cl , implying that the emitting state is of 3 MLCT nature in all these systems.As shown in Table S6, the wavelength maxima of the broad and structureless emission bands of 8 Cl -13 Cl in acetonitrile and chloroform fall in a narrow range of 645-675 nm, lifetimes are in the nanosecond domain, and emission quantum yields are below 1.5%, similarly to features of model chromophores 1 Cl -3 Cl .
Noticeable differences were also observed in the solid-state emission lifetimes of 8 Cl -13 Cl , which varied from nano-to microseconds.Relative to the model chromophores 1 Cl -3 Cl , a substantial prolongation of the solid-state emission lifetime was evidenced for complexes 8 Cl , 10 Cl , 11 Cl , and 13 Cl .The triplet excited-state lifetimes were extended to 586 ns for 8 Cl , 446 ns for 10 Cl , 8.62 µs for 11 Cl , and 49.62 µs for 13 Cl , compared to 52 ns for 1 Cl , 162 ns for 2 Cl , and 102 ns for 3 Cl .Except for 12 Cl , the solid-state emission lifetimes of the Re(I) complexes belonging to class B underwent extension with an increase in πconjugation of the pendant aryl group.Lowering the temperature down to 77 K resulted in a further increase in their excited-state lifetimes and a blue-shift of the emission maxima.As supported theoretically [78,79,82], methoxy-decoration of naphthyl groups resulted in an increased contribution of the organic ligand in the ground and excited states of resulting [ReX(CO) 3 (R-terpy-κ 2 N)], which may explain their prolonged solid-state emission lifetimes.To obtain triplet emitters with long excited states, both radiative and non-radiative decay rate constants must be small, meaning that the triplet excited state has a predominant ligand character [83].
Having good thermal properties and suitable energy levels, IP, and EA, the compounds 8 Cl -13 Cl were employed as active layers in light-emitting diodes ITO/PEDOT:PSS/compound/ Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al, fabricated in the laboratory.Most of the obtained devices with the Re(I) complexes exhibited orange or red emission under external voltage [78,79,82].The authors of [82] demonstrated the additional possibility of electroluminescence enhancement by incorporating silver nanowires (AgNWs) into the PEDOT:PSS layer.

Heterocyclic or Strong Electron-Releasing Groups Directly Attached to the Terpy Core at 4′-Position
Among the compounds 14 Cl -24 Cl [37,47,50,84,85], outstanding photophysical behavior was revealed for the chloride Re(I) complex with the strong electron-donating bithiophene substituent (17 Cl ).Its emission profile was found to be markedly distinct in comparison to other compounds of this class.As depicted in Figure 5, the emission spectrum of 17 Cl in CHCl3 is primarily dominated by the 1 ILCT fluorescence band.The lower energy phosphorescence band of 17 Cl exhibits a weak vibronic structure, and it is nearly overlapping with the solid-state and frozen ligand-centered emission.The triplet-emitting state of 17 Cl is of 3 ILCT nature, likely originating from optically populated 1 ILCT.Both fluorescence and phosphorescence are discernible in the solution and frozen matrix emission spectra.Compared to 14 Cl -16 Cl and 18 Cl -24 Cl , the absorption and phosphorescence bands of 17 Cl appear at significantly lower energies (Figure 5 and Table S7).In the solid state and EtOH-MeOH (4:1) glass matrix, the triplet excited-state lifetimes of 17 Cl are extended to 24.4 µs and 178 µs, respectively.Its emission profile was found to be markedly distinct in comparison to other compounds of this class.As depicted in Figure 5, the emission spectrum of 17 Cl in CHCl 3 is primarily dominated by the 1 ILCT fluorescence band.The lower energy phosphorescence band of 17 Cl exhibits a weak vibronic structure, and it is nearly overlapping with the solidstate and frozen ligand-centered emission.The triplet-emitting state of 17 Cl is of 3 ILCT nature, likely originating from optically populated 1 ILCT.Both fluorescence and phosphorescence are discernible in the solution and frozen matrix emission spectra.Compared to 14 Cl -16 Cl and 18 Cl -24 Cl , the absorption and phosphorescence bands of 17 Cl appear at significantly lower energies (Figure 5 and Table S7).In the solid state and EtOH-MeOH (4:1) glass matrix, the triplet excited-state lifetimes of 17 Cl are extended to 24.4 µs and 178 µs, respectively.
Using static and time-resolved emission spectroscopy, ultrafast transient absorption measurements, and DFT/TD-DFT calculations [37,47,50,84,85], the emitting excited states of 14 Cl -16 Cl and 18 Cl -24 Cl were demonstrated to have a predominant 3 MLCT character.A significantly noticeable effect on the position of the emission band in solution was observed for complexes bearing electron-donating groups, namely 14 Cl in both MeCN and CHCl 3 , 18 Cl in MeCN, and 19 Cl in MeCN (Table S7), compared to the model chromophore [ReCl(CO) 3 (terpy-κ 2 N)].In the research work [85], the variations in the emission position of 20 Cl -22 Cl were correlated with Hammett σ parameters, demonstrating a decrease in the emission energy in accordance with the increased electron-withdrawing properties of the pendant n-pyridyl groups attached to terpy.For all complexes 14 Cl -16 Cl and 18 Cl -24 Cl , the emission lifetimes in solution are in the nanosecond domain, and quantum yields are below 1%.Furthermore, typically of the emission from a 3 MLCT state, the frozen and solid-state emission of 14 Cl -16 Cl and 18 Cl -24 Cl occurs in a higher energy region, showing prolonged lifetimes with reference to the solution (Table S7).
Analogously to the compounds of class B, the solid-state photo-characteristics of 14 Cl -24 Cl were noticeably affected by substituents at the 4 ′ -position of terpy.The solid emission wavelengths varied with the terpy substituents from 543 nm for 14 Cl to 636 nm for 21 Cl , confirming the key role of the donor-acceptor abilities of the pendant substituent.A substantial prolongation of the solid-state emission lifetimes was confirmed for 14 Cl (11.8 µs), 18 Cl (5.2 µs), and 19 Cl (17.0 µs).As supported theoretically [37], the appended N-methyl-pyrrole, benzothiophene, and ethylenedioxythiophene electron-donating groups induce an enhanced contribution of the organic ligand in the ground and excited states of the resulting [ReX(CO) 3 (R-terpy-κ 2 N)].Enhanced emission efficiency in the solid state (above 10%) was found for compounds 19 Cl and 24 Cl .Using static and time-resolved emission spectroscopy, ultrafast transient absorption measurements, and DFT/TD-DFT calculations [37,47,50,84,85], the emitting excited states of 14 Cl -16 Cl and 18 Cl -24 Cl were demonstrated to have a predominant 3 MLCT character.A significantly noticeable effect on the position of the emission band in solution was observed for complexes bearing electron-donating groups, namely 14 Cl in both MeCN and CHCl3, 18 Cl in MeCN, and 19 Cl in MeCN (Table S7), compared to the model chromophore [ReCl(CO)3(terpy-κ 2 N)].In the research work [85], the variations in the emission position of 20 Cl -22 Cl were correlated with Hammett σ parameters, demonstrating a decrease in the emission energy in accordance with the increased electron-withdrawing properties of the pendant n-pyridyl groups attached to terpy.For all complexes 14 Cl -16 Cl and 18 Cl -24 Cl , the emission lifetimes in solution are in the nanosecond domain, and quantum yields are below 1%.Furthermore, typically of the emission from a 3 MLCT state, the frozen and solid-state emission of 14 Cl -16 Cl and 18 Cl -24 Cl occurs in a higher energy region, showing prolonged lifetimes with reference to the solution (Table S7).
Analogously to the compounds of class B, the solid-state photo-characteristics of 14 Cl -24 Cl were noticeably affected by substituents at the 4′-position of terpy.The solid emission wavelengths varied with the terpy substituents from 543 nm for 14 Cl to 636 nm for 21 Cl , confirming the key role of the donor-acceptor abilities of the pendant substituent.A substantial prolongation of the solid-state emission lifetimes was confirmed for 14 Cl (11.8 µs), 18 Cl (5.2 µs), and 19 Cl (17.0 µs).As supported theoretically [37], the appended N-methyl-pyrrole, benzothiophene, and ethylenedioxythiophene electron-donating groups induce an enhanced contribution of the organic ligand in the ground and excited states of the resulting [ReX(CO)3(R-terpy-κ 2 N)].Enhanced emission efficiency in the solid state (above 10%) was found for compounds 19 Cl and 24 Cl .
The capability of 14 Cl -24 Cl for the emission of light under voltage was examined in [50,84,85], and the fabricated diodes ITO/PEDOT:PSS/PVK:PBD:complex/Al were found The capability of 14 Cl -24 Cl for the emission of light under voltage was examined in [50,84,85], and the fabricated diodes ITO/PEDOT:PSS/PVK:PBD:complex/Al were found to emit light under the applied voltage, with maximum electroluminescence falling in the light range from yellow to red.
The photophysical properties of 25 Cl -29 Cl were the subject of experimental studies reported in [49].It was evidenced that the hole-transporting carbazole and diphenylamine moieties, directly attached to the central pyridine of the terpy core, favor the energy transfer process from the substituent to the terpy core, leading to an enhancement of the visible absorptivity and luminescence performance of the resulting [ReX(CO) 3 (R-terpyκ 2 N)] complexes.The emission performances of 26 Cl -28 Cl largely exceeded that of the model chromophore [ReCl(CO) 3 (terpy-κ 2 N)] (Table S7).The phosphorescence maximum of 26 Cl -28 Cl appears in the range of 578-601 nm in CH 2 Cl 2 solution and 533-611 nm in the solid state.In solution, the emission lifetime decays are in the microsecond domain, and emission quantum yields vary from 0.3% to 1.3%.
The photophysical properties of 25 Cl -29 Cl were also investigated theoretically, and computed ground-and excited-state properties were analyzed in terms of the potential utility of these systems for OLED technology [86].The visible absorption of 25 Cl -29 Cl was assigned to electronic transitions of mixed 1 MLCT/ 1 LLCT/ 1 ILCT character, while the phosphorescence was associated with 3 MLCT/ 3 LLCT/ 3 ILCT triplet states.As supported by the reorganization energy calculations, the carbazole and diphenylamine moieties noticeably improve the electron transport performance of the resulting Re(I) complexes, and 25 Cl -29 Cl can be regarded as suitable candidates for OLED materials.
utility of these systems for OLED technology [86].The visible absorption of 25 Cl -29 Cl was assigned to electronic transitions of mixed 1 MLCT/ 1 LLCT/ 1 ILCT character, while the phosphorescence was associated with 3 MLCT/ 3 LLCT/ 3 ILCT triplet states.As supported by the reorganization energy calculations, the carbazole and diphenylamine moieties noticeably improve the electron transport performance of the resulting Re(I) complexes, and 25 Cl -29 Cl can be regarded as suitable candidates for OLED materials.

[ReX(CO)3(R-C6H4-terpy-κ 2 N)] with Remote Substituents Attached via a Phenylene Bridge to the Central Pyridine Ring of Terpy
The remote substituent impact in [ReX(CO)3(R-C6H4-terpy-κ 2 N)] was explored by the Fernández-Terán [63,64], Hanan [47], and Machura [46,59,60,78,82,87,88] research groups (Table S8).The authors of [63] conducted a comprehensive investigation of a series of complexes 1 Cl , 8 Cl , 31 Cl , 32 Cl , 34 Cl , 35 Cl , and provided definitive experimental evidence for a change in the excited-state character from 3 MLCT (1 Cl , 8 Cl , 31 Cl , 32 Cl and 34 Cl ) to 3 ILCT in [ReX(CO)3(R-C6H5-terpy-κ 2 N)] with the strongest electron-releasing substituent, -NMe2 (35 Cl ).The optical properties of 1 Cl , 8 Cl , 31 Cl , 32 Cl , 34 Cl were found to be systematically varied with electron-donating substituent abilities, as demonstrated by a hypsochromic shift of the 1 MLCT absorption and 3 MLCT emission bands with the increased electrondonating character of the substituent, linear correlations between the 3 MLCT lifetimes and Hammett σ p substituent constants, and linear correlations of ∆G S-T values obtained from a linear fit of the high-energy side of the low-temperature emission spectra with the Hammett σ p substituent constants.At RT, all these systems exhibit broad and unstructured emission, with lifetimes varying from 0.58 to 2.3 ns between the CN-and OMe-substituted complexes.Typically of 3 MLCT emitters, their photoluminescence spectra show significant hypsochromic shifts in solid states and at cryogenic temperatures (77 K).
Markedly different absorption and emission properties were demonstrated for 35 Cl .Most importantly, the change in the singlet and triplet excited-state character from MLCT to ILCT was manifested in the appearance of a very strong visible absorption band, red-shifted by ~100 nm relative to 1 Cl , dramatic enhancement of the excited-state lifetime in solution (380 ns in DMF), and a bathochromic shift of the emission upon cooling, accompanied by the appearance of a vibronic structure.Clear differences between 3 MLCT and 3 ILCT Re(I) emitters belonging to this group were also evidenced in photocatalytic hydrogen evolution experiments, performed for 8 Cl and 35 Cl as representative photosensitizers [63].Contrary to 8 Cl , which induced stable hydrogen evolution with a turnover number (TON) for the photosensitizer of 580 ± 40, the use of 35 Cl with 10-fold-smaller photosensitizer concentrations resulted in very fast hydrogen evolution, with TONs of over 2100.The complex 35 Cl was also demonstrated to possess superior capability for 1 O 2 generation and release of CO under ultrasound irradiation.Its excellent sonocytotoxicities towards both normoxic and hypoxic cancer cells were confirmed in in vitro and in vivo experiments.Notably, the complex 35 Cl had significant advantages in sonocytotoxicity relative to its analog with electron-withdrawing 36 Cl , characterized by considerably lower luminescence intensity, shorter lifetime, and smaller 1 O 2 quantum yield [89].
In the paper [64], Fernández-Terán and co-workers presented a theoretical and experimental comparative analysis of the one-photon and two-photon absorption properties of 8 Cl and 35 Cl .Their studies revealed that the nonlinear behavior of [ReX(CO) 3 (R-C 6 H 4terpy-κ 2 N)] is predominately governed by the conjugation size of the aromatic system, while the increased charge-transfer character of the excited states plays a minor role in the two-photon absorption behavior, contrary to the one-photon properties of these systems.
The effect of the conjugation degree and electron-donating ability of remote groups in controlling ground-and excited-state properties of [ReX(CO) 3 (R-C 6 H 4 -terpy-κ 2 N)] was demonstrated for compounds 42 Cl and 43 Cl in [59,60].In contrast to five-membered pyrrolidine, the N-carbazolyl is also able to accept electron density due to the presence of two benzene rings fused on either side of the heterocyclic amine ring.Based on transient absorption studies in nano-and femtosecond domains, it was demonstrated that the attachment of 9-carbazole via the phenylene bridge to the terpy core did not lead to a switch in the excited-state character of [ReX(CO) 3 (R-C 6 H 4 -terpy-κ 2 N)] from 3 MLCT to 3 ILCT.The emission of 43 Cl was largely superimposed with the band of 1 Cl in solution and rigid-glass matrix at 77 K.In turn, the introduction of the electron-donating pyrrolidine group resulted in different emission behavior of 42 Cl in acetonitrile and chloroform solutions, showing a red-and blue-shift in relation to the reference chromophore 1 Cl , respectively.Upon cooling to 77 K, the complex 42 Cl showed a clear bathochromic shift relative to the model chromophore.Similar trends were also observed for 38 Cl , 39 Cl , 43 Cl , and 44 Cl , indicating the presence of 3 MLCT to 3 ILCT in energy proximity in [ReX(CO) 3 (R-C 6 H 4 -terpy-κ 2 N)] with electron-donating substituents (Table S8).
The authors of [88] conducted comprehensive studies of photoinduced processes in 37 Cl and 40 Cl depending on the polarity of the environment, confirming the change in the nature of the triplet excited state from 3 MLCT to 3 ILCT in polar solvents.A bathochromic shift in the emission position of these systems in polar solvents was accompanied by the enhancement of the excited-state lifetime relative to the unsubstituted chromophore 1 Cl (Table S8).
By analogy to the model chromophores [ReX(CO) 2 (terpy-κ 3 N)] (see Section 3), all Re(I) complexes with meridionally-coordinated R-terpys absorb in the entire visible region, exhibiting three strong bands at approximately 720 nm, 480 nm, and 410 nm, assigned exper-imentally and theoretically to 1 MLCT transitions.For complexes 46 Br , 47 Br , and 48 Br , only a slight red-shift is observed relative to [ReBr(CO) 2 (terpy-κ 3 N)], attributed to the extended conjugation following the introduction of the aromatic substituent [47].As reported by the Fernández-Terán group [80], the complexes 46 Cl , 49 Cl -53 Cl exhibit small hypsochromic shifts in the visible absorption maxima with an increased electron-donating character of the substituent, accompanied by a noticeable increase in the extinction coefficients for [ReX(CO) 3 (R-C 6 H 4 -terpy-κ 2 N)] with the strongest electron-releasing substituent -NMe 2 (53 Cl ).In contrast to 35 Cl , however, the visible absorptions of 53 Cl are not contributed by ILCT transitions.
Based on the time-resolved infrared (TRIR) spectra, Fernández-Terán and co-workers [80] evidenced that the photophysical properties of 46 Cl , 49 Cl -53 Cl are governed by the 3 MLCT excited state, independent of the substituent on the terpy core.The 3 MLCT excited state evolves from the optically populated 1 MLCT state via intersystem crossing, and complexes 46 Cl , 49 Cl -53 Cl show linear correlations between the 3 MLCT lifetimes and Hammett σ p substituent constants.The studies demonstrated clearly that the presence of the electron-rich {Re(CO) 2 } + moiety in [ReX(CO) 2 (R-terpy-κ 3 N)] systems hinders access to ILCT excited states.
As shown in Table S12, the Re-based multicomponent systems exhibit photophysical properties typical of the corresponding mononuclear compounds [ReCl(CO) 3 (R-terpyκ 2 N)] (76-80) and [ReCl(CO) 2 (R-terpy-κ 3 N)] ( 81).The complex 81 with meridionallycoordinated terpy absorbs across the entire UV-Vis range, up to 800 nm, and emits in the NIR region, with a maximum of 980 nm [91].The homonuclear systems with R-terpy-κ 2 N (76-80) absorb energy in a much narrower range of wavelengths, and their emission maxima fall in the range of 460-650.By analogy to the mononuclear chromophores, the absorptions of 76-81 occurring above 350 nm are predominantly of a MLCT nature, while higher energy bands in UV-Vis spectra correspond to ligand-centered transitions [91,103,108,109].The emission of 76, 78, 79, 80, and 81 was found to be consistent with 3 MLCT phosphorescence [91,103,109].For complex 77, it was evidenced as an unusual excitation-dependent variation of the emission wavelength, assigned to the presence of different molecular species in solution due to the rapid exchange between the coordinated and free terminal pyridines [109].The incorporation of the silver ion into the structure 79 shifts the 3 MLCT emission maximum from 557 nm to 566 nm, as well as decreasing the emission quantum yield and photostability of the macrocyclic system 82.While the macrocyclic system 79 shows no evidence of any decomposition upon irradiation for 24 h at 405 nm, the silvercontaining form easily loses the silver ion upon standing in sunlight [108].The potential of the rhenium trimer system 79 as a metal ion transport vector was reported in [110].
Typically of d 6 metal transition complexes, the absorption features of 83 and 84 are governed by 1 IL and 1 MLCT bands occurring in energy ranges of 200-340 nm and 340-500 nm, respectively.MLCT transitions attributed to Ru II → π* L transitions occur at lower energies (400-500 nm) compared to Re I -based 1 MLCT (340-400 nm), but some mixing of Ru II → π* L and Re I → π* L is also possible [104,105].For complex 83, the electronic communication between Ru-and Re-based units was evidenced by the remarkable increase in nonlinear response, confirming its potential applications in optical signal processing [104,105].Conversely to 83 which shows no noticeable changes in the solution color and the longest wavelength absorption band as the pendant pyridine converts to its pyridinium form upon acid titration, compound 84 undergoes two successive protonation-deprotonation processes upon increasing the pH from 0.40 to 10 due to the proton dissociation from the protonated imidazole group and uncoordinated pyridyl of the terpy moiety [104,105].Excitation into any of the absorption bands of 84 resulted in a broad emission at 608 nm, attributed to the emission of the Ru-based chromophore.The attachment of {ReCl(CO) 3 (Rterpy-κ 2 N)} unit was found to induce some quenching of the emission quantum yield and lifetime, without any changes in the emission position compared to the model Ru-based chromophore.Importantly, complex 84 was demonstrated to act as a sensitive pH-induced "off-on-off" luminescence switching molecule, an efficient "turn on" emission sensor for H 2 PO 4 − , and a "turn off" emission sensor for F − and OAc − .In addition, it was found to be better for cell imaging than the Ru-based chromophore [105].The photoluminescence properties of 83 were not investigated [104].The Re-Zn dyads (85 and 86) were reported in [106,107], but comprehensive photophysical studies were performed only for compound 86.The absorption properties of the so-called hetero-Pacman compound (86), built as a result of the {ReCl(CO) 3 (R-terpy-κ 2 N)} fragment being covalently linked through the xanthene backbone to the porphyrin unit with the encapsulated Zn 2+ ion, were found to be a superposition of the individual units.These include a strong (Soret) absorption centered at 423 nm and moderately intense Q-band absorption bands centered at 547 nm of the tetramesitylporphyrinato zinc unit, as well as 1 IL (200-350) and 1 MLCT (350-400 nm) bands of {ReCl(CO) 3 (R-terpy-κ 2 N)}.Regardless of the excitation wavelength, dyad 86 shows luminescence assigned to the fluorescence of the zinc porphyrin unit.A modest quenching effect of the attached {ReCl(CO) 3 (R-terpy-κ 2 N)} moiety on the emission quantum yield and lifetime of 86 may indicate the photoinduced electron transfer from the porphyrin S 1 state to form a charge-separated state involving the [ReCl(CO) 3 (R-terpy-κ 2 N)].However, no definitive spectroscopic evidence for the formation of a long-lived charge-separated state in a Re-based fragment could be found, even with the use of TA spectroscopy on the pico-and nanosecond timescales.Importantly, the Re-Zn dyad 86 exhibits enhanced photocatalytic activity in CO 2 -to-CO reduction upon excitation >450 nm relative to the corresponding Zn-and Re-based mononuclear chromophores and their 1:1 mixture.
The ferrocenyl-appended Re(I) compound (87) was also designed as an electrocatalyst for CO 2 reduction, and the ferrocenyl group was introduced to extend the visible absorptivity of the dyad.The lowest energy band at 516 nm, tailing up to 650 nm, was assigned to d-d transitions in the ferrocene moiety, while the second visible absorption band was characterized as a combination of ILCT and MLCT transitions.The presence of two triplet charge-transfer excited states in energetic proximity was evidenced using TA spectroscopy [48].

Conclusions and Future Directions
Within this review, we demonstrated variations in the structural and photophysical properties of rhenium(I) carbonyl complexes with terpy-based ligands, induced by the conversion of terpy coordination mode from terpy-κ 2 N to terpy-κ 3 N, structural modifications of the terpy framework, and changes of the ancillary ligands.The Re(I) carbonyls with meridionally-coordinated terpys, which show the most reduced HOMO-LUMO gaps due to a significant destabilization of the HOMO level as a result of the replacement of a strongly π-accepting CO group by the weakly π-accepting pyridine of the terpy ligand, are examples of coordination compounds that show panchromatic absorption.Independent of structural modifications of the terpy framework and type of ancillary ligand, the groundand excited-state properties of [Re(X/L)(CO) 2 (R-terpy-κ 3 N)] 0/+ are governed by electronic transitions of a pure MLCT nature.While the emission of the halide Re(I) complexes with meridionally-coordinated terpys, predicted theoretically at wavelengths longer than 900 nm, has not been evidenced experimentally, the replacement of the halide ion of [ReX(CO) 2 (Rterpy-κ 3 N)] by the neutral N-and P-donor ligand induces a blue-shift of the lowest energy absorption and made it possible to obtain [ReL(CO) 2 (R-terpy-κ 3 N)] 0/+ systems showing emission in the NIR range, where cells and tissues show negligible absorption and autofluorescence.Conversely to [Re(X/L)(CO) 2 (R-terpy-κ 3 N)] 0 / + , the excited-state character of [Re(X/L)(CO) 3 (R-terpy-κ 2 N)] 0 / + may be switched from 3 MLCT to 3 IL or 3 ILCT, which results in the formation of Re-based emitters with significantly prolonged triplet lifetimes.As demonstrated, Re(I) carbonyl complexes with a triplet excited state based on the organic ligand may be obtained by the introduction of extended π-conjugated polyaromatic hydrocarbons or strong electron-donating groups into the central pyridine of terpy.These systems were evidenced to be promising for applications involving intermolecular photoinduced energy triplet state transfer.Their usefulness as triplet photosensitizers was demonstrated in experiments concerning 1 O 2 generation, photocatalytic hydrogen evolution, and CO 2 -to-CO reduction.The complex [ReCl(CO) 3 (R-C 6 H 4 -terpy-κ 2 N)] with the strong electron-releasing substituent -NMe 2 was found to be suitable for simultaneous production of CO and 1 O 2 in anti-tumor treatment under ultrasound irradiation, showing excellent sonocytotoxicities towards both normoxic and hypoxic cancer cells.
Regarding the promising application potential of Re(I) carbonyl complexes as luminophores, photosensitizers, and photocatalysts, the presented structure-property relationships are of high significance for better understanding and controlling the excited-state nature in these systems, and making further progress in the development of more efficient phosphorescent materials for innovative technologies, such as photodynamic therapy, time-resolved bioimaging, photocatalysis, and triplet-triplet annihilation up-conversion.Principally, triplet emitters with strong absorption in the entire visible range and sufficiently long excited-state lifetimes still face challenges.As we demonstrated in this review, such systems can be obtained via the attachment of the properly designed terpy-based ligand to the {Re(CO) 3 } and {Re(CO) 2 )} units, accompanied by suitable ancillary ligands.

Figure 5 .
Figure 5.The normalized emission spectra of 14 Cl -24 Cl in CHCl 3 , solid state and frozen matrices at 77 K.The spectra are readapted from our previous works [50,84,85].

Scheme 5 . 5 .
Scheme 5. Molecular structures of [ReX(CO)3(R-terpy-κ 2 N)] complexes discussed in Section 4.4 (Class D).The optical properties of 1 Cl , 8 Cl , 31 Cl , 32 Cl , 34 Cl were found to be systematically varied with electron-donating substituent abilities, as demonstrated by a hypsochromic shift of the 1 MLCT absorption and 3 MLCT emission bands with the increased electron-donating character of the substituent, linear correlations between the 3 MLCT lifetimes and Hammett σp substituent constants, and linear correlations of ΔGS-T values obtained from a linear fit of the high-energy side of the low-temperature emission spectra with the Hammett σp substituent constants.At RT, all these systems exhibit broad and unstructured emission, with lifetimes varying from 0.58 to 2.3 ns between the CN-and OMe-substituted complexes.Typically of 3 MLCT emitters, their photoluminescence Scheme 5. Molecular structures of [ReX(CO) 3 (R-terpy-κ 2 N)] complexes discussed in Section 4.4 (Class D).

Figure 7 .
Figure 7.The HOMO and LUMO energy levels of [Re(X/L)(CO) 3 (R-terpy-κ 2 N)] 0/+ and [Re(X/L)(CO) 2 (R-terpy-κ 3 N)] 0/+ , estimated on the basis of potentials of the oxidation and reduction couples with regard to the energy level of the ferrocene reference: HOMO energy levels estimated from equation IP= −5.1 − E ox ; LUMO energy levels estimated from equation EA= −5.1 − E red .

Author Contributions:
Conceptualization, B.M., writing-original draft preparation, B.M., J.P.-G., K.C., A.M.M. and E.M.; writing-review and editing, B.M., J.P.-G., K.C., A.M.M. and E.M.; visualization, J.P.-G.and K.C.; supervision, B.M.; funding acquisition, B.M. and A.M.M.All authors have read and agreed to the published version of the manuscript.Funding: This work was funded by the National Science Centre of Poland, SONATA grant no.2020/39/D/ST4/00286 (AM), and co-financed by the funds granted under the Research Excellence Initiative of the University of Silesia in Katowice.Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.

Table 2 .
The