The Role of Intraligand Charge Transfer Processes in Iridium(III) Complexes with Morpholine-Decorated 4′-Phenyl-2,2′:6′,2″-terpyridine

To investigate the impact of the electron-donating morpholinyl (morph) group on the ground- and excited-state properties of two different types of Ir(III) complexes, [IrCl3(R-C6H4-terpy-κ3N)] and [Ir(R-C6H4-terpy-κ3N)2](PF6)3, the compounds [IrCl3(morph-C6H4-terpy-κ3N)] (1A), 4[Ir(morph-C6H4-terpy-κ3N)2](PF6)3 (2A), [IrCl3(Ph-terpy-κ3N)] (1B) and [Ir(Ph-terpy-κ3N)2](PF6)3 (2B) were obtained. Their photophysical properties were comprehensively investigated with the aid of static and time-resolved spectroscopic methods accompanied by theoretical DFT/TD-DFT calculations. In the case of bis-terpyridyl iridium(III) complexes, the attachment of the morpholinyl group induced dramatic changes in the absorption and emission characteristics, manifested by the appearance of a new, very strong visible absorption tailing up to 600 nm, and a significant bathochromic shift in the emission of 2A relative to the model chromophore. The emission features of 2A and 2B were found to originate from the triplet excited states of different natures: intraligand charge transfer (3ILCT) for 2A and intraligand with a small admixture of metal-to-ligand charge transfer (3IL–3MLCT) for 2B. The optical properties of the mono-terpyridyl iridium(III) complexes were less significantly impacted by the morpholinyl substituent. Based on UV–Vis absorption spectra, emission wavelengths and lifetimes in different environments, transient absorption studies, and theoretical calculations, it was demonstrated that the visible absorption and emission features of 1A are governed by singlet and triplet excited states of a mixed MLLCT-ILCT nature, with a dominant contribution of the first component, that is, metal-ligand-to-ligand charge transfer (MLLCT). The involvement of ILCT transitions was reflected by an enhancement of the molar extinction coefficients of the absorption bands of 1A in the range of 350–550 nm, and a small red shift in its emission relative to the model chromophore.

cussed in [37,38].We demonstrated that the morpholinyl remote group has a significantly larger impact on the photophysical behavior of the bis-terpyridyl Ir(III) complex.
As demonstrated in Figure 1, the proton signals of the central pyridine ring in the terpyridine backbone (represented by the singlet) were noticeably shifted upfield in the morpholinyl-substituted Ir(III) systems, indicating that the appended electron-donating morpholine group affects the electron density in the distal part of the ligand molecule.The difference in chemical shifts, from 9.11 ppm for 1B to 9.04 ppm for 1A and from 9.62 ppm for 2B to 9.49 for 2A, shows that this effect was stronger for the cationic Ir(III) complexes.The FT-IR spectra of all the investigated Ir(III) complexes displayed characteristic bands in the region 1615-1525 cm −1 , attributable to ν(C=N)terpy and ν(C=C)terpy stretches.The intense bands occurring at ∼840 cm −1 and ∼555 cm −1 in the FT-IR spectra of 2A and 2B are indicative of PF6 − ions (Figure S5) [42].
Additionally, the molecular structures of 1A, 1B, and 2B were unambiguously determined by X-ray analysis.The full structural data of these systems are provided in the Supplementary Materials (Tables S1-S4 and Figure S6).Perspective views of the molecular structures of 1A, 1B, and 2B, with atom numbering, are depicted in Figure 2, while the most relevant bond lengths and angles are summarized in Table 1.As demonstrated in Figure 1, the proton signals of the central pyridine ring in the terpyridine backbone (represented by the singlet) were noticeably shifted upfield in the morpholinyl-substituted Ir(III) systems, indicating that the appended electron-donating morpholine group affects the electron density in the distal part of the ligand molecule.The difference in chemical shifts, from 9.11 ppm for 1B to 9.04 ppm for 1A and from 9.62 ppm for 2B to 9.49 for 2A, shows that this effect was stronger for the cationic Ir(III) complexes.
The FT-IR spectra of all the investigated Ir(III) complexes displayed characteristic bands in the region 1615-1525 cm −1 , attributable to ν(C=N) terpy and ν(C=C) terpy stretches.The intense bands occurring at ~840 cm −1 and ~555 cm −1 in the FT-IR spectra of 2A and 2B are indicative of PF 6 − ions (Figure S5) [42].Additionally, the molecular structures of 1A, 1B, and 2B were unambiguously determined by X-ray analysis.The full structural data of these systems are provided in the Supplementary Materials (Tables S1-S4 and Figure S6).Perspective views of the molecular structures of 1A, 1B, and 2B, with atom numbering, are depicted in Figure 2, while the most relevant bond lengths and angles are summarized in Table 1.In all reported complexes, the Ir(III) ion was located in a distorted octahedral environment.The coordination sphere of 1A and 1B was defined by three chloride ions and three nitrogen atoms of the tridendate-coordinated R-C6H4-terpy (κ 3 N) ligand.Due to the geometrical constraints imposed by the planar terpy framework, both nitrogen and chlorine donor atoms adopted meridional arrangements.The crystal structure of 2B consisted  In all reported complexes, the Ir(III) ion was located in a distorted octahedral environment.The coordination sphere of 1A and 1B was defined by three chloride ions and three nitrogen atoms of the tridendate-coordinated R-C 6 H 4 -terpy (κ 3 N) ligand.Due to the geometrical constraints imposed by the planar terpy framework, both nitrogen and chlorine donor atoms adopted meridional arrangements.The crystal structure of 2B consisted of the complex cations [Ir(Ph-terpy-κ 3 N) 2 ] 3+ , PF 6 − counteranions in a 1:3 molar ratio, and solvent (MeCN) molecules.The metal center of [Ir(Ph-terpy-κ 3 N) 2 ] 3+ was octahedrally surrounded by six nitrogen atoms from two Ph-terpy-κ 3 N ligands, coordinated in a mer-fashion.
Typical of the terpy-κ 3 N coordination mode [43], the Ir-N bond lengths involving peripheral pyridine rings in structures 1A, 1B, and 2B were longer than the Ir-N central pyridine distances (Table 1).The shortening of the Ir-N central pyridine bond length was rationalized by a more efficient overlap of the t 2g metal orbitals with the π* orbitals of the central pyridine relative to the peripheral pyridyl groups.Consequently, the chloride ligand trans-located to the central pyridine ring of R-C 6 H 4 -terpy in molecules 1A and 1B exhibited an elongated bond distance in relation to the other Ir-Cl ones (Table 1).
In all the studied Ir(III) complexes, a considerable angular distortion from the octahedral geometry was reflected in the N-Ir-N bite angles due to the formation of fivemembered metallocycles upon the chelating coordination of R-C 6 H 4 -terpy.The N-Ir-N bite angles varied from 79.84 (12) to 80.99 (16) • (Table 1).The terpy framework in structures 1A, 1B, and 2B showed good planarity, with the dihedral angles between the mean planes of the central pyridine and terminal aromatic rings ranging from 1.  S2 and S4).The shortest Ir•••F distances were 5.542 Å and 5.940 Å.  S2 and S4).The shortest Ir•••F distances were 5.542 Å and 5.940 Å.   S2-S4).
Molecules 2024, 29, x FOR PEER REVIEW prior to the discussion of the optical properties of 1A-2A and 1B-2B.DFT calcul were performed using Gaussian-16 software [44] at the TD-DFT/PCM/PBE0/SDD TZVP level [45] (Figure 4, Tables S5-S7, Figure S7).As expected, the morpholine substituent also altered the nature of the HOMO o Unlike the HOMO of 1B, which predominantly comprised the orbitals of the Ir(III) (61.8 %) and chloride ions (27.4 %), the HOMO of 1A was a combination of morp (32.1 %), phenylene (48.1%), and terpy (9.5%) orbitals.Compared to 1B, the metal c ter of the HOMO of 1A decreased from 61.8 % to 8.5 %.The highest occupied mol orbitals of 2A and 2B resided almost exclusively on the R-C6H4-terpy ligand.For pholinyl-substituted complex (2A), it comprised the morpholine group (35.9 %) and nylene linker (46.5%).The metal contributions in the HOMO of 2A and 2B were 2.1 12.5%, respectively.The lowest unoccupied molecular orbital of all the studied Ir(III plexes was predominately constituted by π* orbitals of the terpy backbone.As expected, the morpholine substituent also altered the nature of the HOMO orbital.Unlike the HOMO of 1B, which predominantly comprised the orbitals of the Ir(III) center (61.8 %) and chloride ions (27.4 %), the HOMO of 1A was a combination of morpholine (32.1%), phenylene (48.1%), and terpy (9.5%) orbitals.Compared to 1B, the metal character of the HOMO of 1A decreased from 61.8 % to 8.5 %.The highest occupied molecular orbitals of 2A and 2B resided almost exclusively on the R-C 6 H 4 -terpy ligand.For morpholinylsubstituted complex (2A), it comprised the morpholine group (35.9 %) and phenylene linker (46.5%).The metal contributions in the HOMO of 2A and 2B were 2.1% and 12.5%, respectively.The lowest unoccupied molecular orbital of all the studied Ir(III) complexes was predominately constituted by π* orbitals of the terpy backbone.
As demonstrated in Figure 5, the model complex 1B exhibited intense high-energy absorption bands in the range of 225-350 nm, which were attributed to π-π* transitions of the Ph-terpy ligand, and noticeably weaker absorptions with maxima above 350 nm, which were tentatively assigned to charge-transfer transitions 1 MLLCT with a possible admixture of spin-forbidden singlet-triplet 3 MLLCT due to the high spin-orbit coupling constant of iridium [46][47][48].In contrast, the complex 2B principally absorbed wavelengths below 400 nm.As previously established for related systems [37,41,[49][50][51][52], these UV bands of 2B were predominantly attributed to 1 IL transitions within the coordinated Phterpy ligands.The very low magnitudes of molar extinction coefficients for the absorptions of 2B in the visible region (Table S8 and Figure S8) are consistent with spin-forbidden singlet-triplet 3 MLCT transitions [49,53].Within the series [IrCl3(R-C6H4-terpy-κ 3 N)], the attachment of the electron-donating morpholinyl group to the phenyl ring of 2,2′:6′,2″-terpyridine evoked a minor effect on the absorption energies but led to a significant enhancement of the molar extinction coefficients of absorptions bands in the range of 350-550 nm (Table S8 and Figure 5).In contrast, dramatic changes in the absorption characteristics were observed for 2A in relation to 2B.The morpholinyl-substituted complex (2A) was deeply red and displayed a very strong absorption tailing up to 600 nm, which was absent in the UV-Vis spectrum of its parent As demonstrated in Figure 5, the model complex 1B exhibited intense high-energy absorption bands in the range of 225-350 nm, which were attributed to π-π* transitions of the Ph-terpy ligand, and noticeably weaker absorptions with maxima above 350 nm, which were tentatively assigned to charge-transfer transitions 1 MLLCT with a possible admixture of spin-forbidden singlet-triplet 3 MLLCT due to the high spin-orbit coupling constant of iridium [46][47][48].In contrast, the complex 2B principally absorbed wavelengths below 400 nm.As previously established for related systems [37,41,[49][50][51][52], these UV bands of 2B were predominantly attributed to 1 IL transitions within the coordinated Ph-terpy ligands.The very low magnitudes of molar extinction coefficients for the absorptions of 2B in the visible region (Table S8 and Figure S8) are consistent with spin-forbidden singlet-triplet 3 MLCT transitions [49,53].
Within the series [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)], the attachment of the electron-donating morpholinyl group to the phenyl ring of 2,2 ′ :6 ′ ,2 ′′ -terpyridine evoked a minor effect on the absorption energies but led to a significant enhancement of the molar extinction coefficients of absorptions bands in the range of 350-550 nm (Table S8 and Figure 5).In contrast, dramatic changes in the absorption characteristics were observed for 2A in relation to 2B.The morpholinyl-substituted complex (2A) was deeply red and displayed a very strong absorption tailing up to 600 nm, which was absent in the UV-Vis spectrum of its parent chromophore 2B, which was pale yellow in solution.The molar extinction coefficient of this band exceeded 4 × 10 4 M −1 •cm −1 in solution.
The character of electronic transitions underlying the absorption features of [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)] and [Ir(R-C 6 H 4 -terpy-κ 3 N) 2 ](PF 6 ) 3 was also investigated theoretically at the DFT/PCM/PBE0/SDD/def2-TZVP level (Figure 6).The calculations confirmed that the changes in the absorption characteristics of the morpholinyl-substituted Ir(III) complexes (1A and 2A) relative to the parent chromophores (1B and 2B) were due to the involvement of 1 ILCT transitions, occurring from the electron-donating morpholinyl group to the πacceptor terpy unit.The low-energy absorptions (above 350 nm) of 1A mainly originated from the excitations HOMO→LUMO, HOMO→L + 1, H-1→LUMO, H-1→L + 1, H-2→LUMO, and H-2→L + 1.Based on the percentage composition of the molecular orbitals, the transitions HOMO→LUMO and HOMO→L + 1 can be designated as 1 ILCT/ 1 IL, while the other ones correspond to the 1 MLLCT excitations.The low-energy absorptions of 1B only comprised 1 MLLCT transitions (Table S6).Some discrepancies between the experimental and theoretical electronic spectra of 1A and 1B in the low energy part may be rationalized by the possible involvement of spin-forbidden singlet-triplet transitions, which are evidenced by the TD-DFT calculations [46].The strong visible absorption of 2A was dominated by the transitions HOMO→LUMO and HOMO-1→LUMO + 1 of prevailing ILCT nature.Both HOMO and HOMO-1 comprised the morpholine group and phenylene linker, while LUMO and LUMO + 1 resided predominately on the terpy backbone.The character of electronic transitions underlying the absorption features of [IrCl3(R-C6H4-terpy-κ 3 N)] and [Ir(R-C6H4-terpy-κ 3 N)2](PF6)3 was also investigated theoretically at the DFT/PCM/PBE0/SDD/def2-TZVP level (Figure 6).The calculations confirmed that the changes in the absorption characteristics of the morpholinyl-substituted Ir(III) complexes (1A and 2A) relative to the parent chromophores (1B and 2B) were due to the involvement of 1 ILCT transitions, occurring from the electron-donating morpholinyl group to the πacceptor terpy unit.The low-energy absorptions (above 350 nm) of 1A mainly originated from the excitations HOMO→LUMO, HOMO→L + 1, H-1→LUMO, H-1→L + 1, H-2→LUMO, and H-2→L + 1.Based on the percentage composition of the molecular orbitals, the transitions HOMO→LUMO and HOMO→L + 1 can be designated as 1 ILCT/ 1 IL, while the other ones correspond to the 1 MLLCT excitations.The low-energy absorptions of 1B only comprised 1 MLLCT transitions (Table S6).Some discrepancies between the experimental and theoretical electronic spectra of 1A and 1B in the low energy part may be rationalized by the possible involvement of spin-forbidden singlet-triplet transitions, which are evidenced by the TD-DFT calculations [46].The strong visible absorption of 2A was dominated by the transitions HOMO→LUMO and HOMO-1→LUMO + 1 of prevailing ILCT nature.Both HOMO and HOMO-1 comprised the morpholine group and phenylene linker, while LUMO and LUMO + 1 resided predominately on the terpy backbone.Prior to the investigations of the excited-state properties of 1-2, their stability and photostability in solution were confirmed by UV-Vis spectroscopy, as demonstrated in Figures S9 and S10.The photoluminescence properties of the Ir(III) complexes were explored in MeCN and DMSO solutions at room temperature (RT), in a solid state, and in a frozen matrix of EtOH/MeOH (4:1 v/v) at 77 K, as presented in Figure 7 and Table 2. Some additional photophysical data of 1-2 are provided in the Supplementary Materials (Figures S11-S18).In contrast to 1B, which showed excitation-independent emission, the energy of the emission of 1A was affected by excitation wavelengths, becoming noticeably red-shifted upon wavelength excitations in the range of 400-525 (Figure S13).A difference between 1A and 1B was also noticed when the frozen-state emissions were considered.Upon cooling to 77 K, the emission band of 1A remained structureless and appeared almost in the same range as the RT emission band, while the frozen-state emission band of 1B exhibited well-resolved vibronic progressions and appeared at a higher energy in relation to that in solution, as shown in Figure 7.All these findings indicate that the complex 1B emits from a triplet excited state of 3 MLLCT character with a minor contribution of 3 π→π*terpy transitions ( 3 IL), while the emissive triplet excited state of the morpholinyl-substituted Ir(III) analog (1A) has a mixed nature 3 MLLCT-3 ILCT.By analogy to previous findings [14,[29][30][31][32][33][34][35][36], the contribution of 3 ILCT in the triplet excited state of 1A manifested in a bathochromic shift of the emission in solution at RT and frozen matrix EtOH/MeOH relative to the parent chromophore.Transition metal complexes functionalized with electron-rich groups, which emit from the 3 MLLCT excited state, are expected to show a hypsochromic shift compared to the unsubstituted model compounds [14,29].
For the bis-terpyridyl iridium(III) complexes, the morpholinyl group induced dramatic changes in the emission characteristics (Figures 7 and S14-S17).In solution at RT, the model chromophore 2B displayed a green emission.Its emission band exhibited a weak vibronic structure, which became well resolved upon cooling to 77 K.The frozenstate emission band of 2B appeared almost in the same range as that in MeCN at RT upon excitations of ≤375 nm, but it was slightly blue-shifted in relation to its emission in DMSO (Figures 7 and S14).The differences in the emission energies and spectral profiles of 2B in MeCN and DMSO can be rationalized by the fact that the emission of [Ir(R-terpyκ 3 N)2](PF6)3 systems in solution may occur from the triplet excited state of the cation [Ir(R-  For all the studied complexes, their lifetimes were in the microsecond or sub-microsecond domain (Table 2), and the photoluminescence intensities and lifetimes were sensitive to oxygen quenching, supporting the idea that the observed emission originates from a triplet excited state (Figures S11 and S12).
The complexes 1A and 1B in deaerated MeCN and DMSO solutions showed unstructured emission bands.Compared to the model chromophore 1B, the phosphorescence band of 1A was noticeably broader and appeared at a lower energy, with the emission maximum red-shifted by 50 nm for the DMSO solution and 10 nm for MeCN upon excitation at 520 nm.A bathochromic shift in the emission maximum of 1A was accompanied by a slight decrease in the excited-state lifetime relative to 1B (Table 2).
In contrast to 1B, which showed excitation-independent emission, the energy of the emission of 1A was affected by excitation wavelengths, becoming noticeably red-shifted upon wavelength excitations in the range of 400-525 (Figure S13).A difference between 1A and 1B was also noticed when the frozen-state emissions were considered.Upon cooling to 77 K, the emission band of 1A remained structureless and appeared almost in the same range as the RT emission band, while the frozen-state emission band of 1B exhibited well-resolved vibronic progressions and appeared at a higher energy in relation to that in solution, as shown in Figure 7.All these findings indicate that the complex 1B emits from a triplet excited state of 3 MLLCT character with a minor contribution of 3 π→π* terpy transitions ( 3 IL), while the emissive triplet excited state of the morpholinyl-substituted Ir(III) analog (1A) has a mixed nature 3 MLLCT-3 ILCT.By analogy to previous findings [14,[29][30][31][32][33][34][35][36], the contribution of 3 ILCT in the triplet excited state of 1A manifested in a bathochromic shift of the emission in solution at RT and frozen matrix EtOH/MeOH relative to the parent chromophore.Transition metal complexes functionalized with electron-rich groups, which emit from the 3 MLLCT excited state, are expected to show a hypsochromic shift compared to the unsubstituted model compounds [14,29].
For the bis-terpyridyl iridium(III) complexes, the morpholinyl group induced dramatic changes in the emission characteristics (Figures 7 and S14-S17).In solution at RT, the model chromophore 2B displayed a green emission.Its emission band exhibited a weak vibronic structure, which became well resolved upon cooling to 77 K.The frozen-state emission band of 2B appeared almost in the same range as that in MeCN at RT upon excitations of ≤375 nm, but it was slightly blue-shifted in relation to its emission in DMSO (Figures 7 and S14).The differences in the emission energies and spectral profiles of 2B in MeCN and DMSO can be rationalized by the fact that the emission of [Ir(R-terpy-κ 3 N) 2 ](PF 6 ) 3 systems in solution may occur from the triplet excited state of the cation [Ir(R-terpy-κ 3 N) 2 ] 3+ and ion-pair adduct [Ir(R-terpy-κ 41,50,[54][55][56][57].While the emission of 2B in MeCN upon excitations of ≤375 nm can be attributed to the phosphorescence of [Ir(R-terpy-κ 3 N) 2 ] 3+ , the ion-pair formation quenches the phosphorescence of [Ir(R-terpy-κ 3 N) 2 ] 3+ in DMSO.The observed emission of 2B in DMSO originated mainly from the ion-pair adduct [Ir(R-terpy- − .In acetonitrile solution, the emission from the ion-pair adduct occurred using longer excitation wavelengths (Figure S14), as previously reported for the related complex [Ir(terpy-κ 3 N) 2 ](PF 6 ) 3 [57].It is worth noting that there were also two alternative explanations of the photoluminescence behavior of [Ir(R-terpy-κ 3 N) 2 ] 3+ cations in previous reports.The emission properties of such systems are attributed to the triplet ligand-centered ( 3 IL) or mixed 3 IL-3 MLCT excited states [39].Regarding the first assignment, the lack of the strong vibronic progression of the emission band at RT, typical of 3 IL emission, is rationalized by the thermal distribution of conformers with different torsion angles between the terpy core and aryl pendant group, leading to a less well-resolved emission band [39].The transient absorption findings, discussed in the next section, indicate that the emission of 2B is not of pure of 3 IL phosphorescence, but rather occurs from the triplet excited state of the mixed 3 IL-3 MLCT character.
The emission spectrum of the morpholinyl-substituted complex (2A) was dominated by the structureless band in the NIR region, with the maximum at 760 nm in MeCN and 750 nm in DMSO, red-shifted by ~240 and 180 nm relative to the model complex 2B, A bathochromic shift in the emission of 2A was accompanied by a noticeable decrease in the excited-state lifetime relative to 2B (Table 2).The shoulder/band at higher energies of 2A can be assigned the reabsorption or emission from the 3 MLLCT excited state (Figure S14).The frozen-state emission of 2A remained non-structured and occurred in a higher energy region (630 nm) compared to its room-temperature emission.The photoluminescence characteristics of 2A are typical of the emission occurring from 3 ILCT excited state [58][59][60][61].It is worth noting that NIR emitters are of high significance due to their potential applications in biomedical imaging [62][63][64].
The photoluminescence properties of 1-2 were also investigated in the solid state.As demonstrated in Figure 8, the solid-state triplet emission band of 2B occurred in a higher energy region (orange-yellow) and was characterized by a noticeably prolonged lifetime (~10 µs) compared to other studied complexes, showing emission in the red wavelength range, with lifetimes in the sub-microsecond domain (Table 2).
The emission spectrum of the morpholinyl-substituted complex (2A) was domi by the structureless band in the NIR region, with the maximum at 760 nm in MeCN 750 nm in DMSO, red-shifted by ∼240 and 180 nm relative to the model complex 2 spectively.A bathochromic shift in the emission of 2A was accompanied by a notic decrease in the excited-state lifetime relative to 2B (Table 2).The shoulder/band at h energies of 2A can be assigned the reabsorption or emission from the 3 MLLCT ex state (Figure S14).The frozen-state emission of 2A remained non-structured and occ in a higher energy region (630 nm) compared to its room-temperature emission.The toluminescence characteristics of 2A are typical of the emission occurring from 3 ILC cited state [58][59][60][61].It is worth noting that NIR emitters are of high significance due to potential applications in biomedical imaging [62][63][64].
The photoluminescence properties of 1-2 were also investigated in the solid sta demonstrated in Figure 8, the solid-state triplet emission band of 2B occurred in a h energy region (orange-yellow) and was characterized by a noticeably prolonged lif (∼10 µs) compared to other studied complexes, showing emission in the red wavele range, with lifetimes in the sub-microsecond domain (Table 2).To theoretically investigate the excited-state properties of complexes 1-2, the ometries were optimized in triplet states (T1) and triplet energies were calculated fro energy difference between the ground singlet and triplet excited states To theoretically investigate the excited-state properties of complexes 1-2, their geometries were optimized in triplet states (T 1 ) and triplet energies were calculated from the energy difference between the ground singlet and triplet excited states ∆E T 1 −S 0 .The theoretically determined triplet emissions, 694 nm for 1A, 590 nm for 1B, 832 nm for 2A, and 540 nm for 2B, are in satisfactory agreement with the experimental values (Tables 2 and S7).Regarding the spin density surface plots (Figures 9 and S7), the emission in the complexes 1A, 1B, 2A, and 2B occurred due to the triplet excited state of an MLCT-ILCT, MLLCT, ILCT, and IL-MLCT nature, respectively.

Femtosecond Transient Absorption Spectra
To further understand the triplet excited state characteristics of the studied complexes, femtosecond transient absorption (fs-TA) measurements were recorded in deaerated DMSO (1A and 1B) and MeCN (2A and 2B) solutions upon excitation at 355 nm.TA solutions were prepared at concentrations of 50-250 µM to provide an absorbance of ∼0.5 in 2 mm path length quartz cells at the excitation wavelength.The results of fs-TA measurements and global fitting analyses are summarized in Figures 10 and S19-S23.The photostability of the Ir(III) complexes during the TA experiments was verified by comparing their UV-Vis spectra recorded before and after irradiation (Figure S24).

Femtosecond Transient Absorption Spectra
To further understand the triplet excited state characteristics of the studied complexes, femtosecond transient absorption (fs-TA) measurements were recorded in deaerated DMSO (1A and 1B) and MeCN (2A and 2B) solutions upon excitation at 355 nm.TA solutions were prepared at concentrations of 50-250 µM to provide an absorbance of ~0.5 in 2 mm path length quartz cells at the excitation wavelength.The results of the fs-TA measurements and global fitting analyses are summarized in Figure 10 and Figures S19-S23.The photostability of the Ir(III) complexes during the TA experiments was verified by comparing their UV-Vis spectra recorded before and after irradiation (Figure S24).
The complexes 1A and 2A exhibited distinct transient absorption (TA) features compared to their parent chromophores (1B and 2B), indicating that the introduction of the electron-donating morpholinyl group significantly affected the nature of the lowest triplet excited state of [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)] and [Ir(R-C 6 H 4 -terpy-κ 3 N) 2 ](PF 6 ) 3 .Specifically, 1A and 2A displayed ground-state bleaching (GSB) in wavelength regions corresponding to their visible charge-transfer absorptions, along with excited-state absorption bands (ESA) in the ranges of 375-415 nm and 522-670 nm for 1A, and 375-425 nm and 540-670 nm for 2A.In contrast, the TA spectra of the parent chromophores displayed only positive bands across the UV and visible regions, indicating that 1B and 2B have a stronger triplet excited-state absorption than the ground-state absorption [65][66][67][68].For all the studied Ir(III) complexes, the TA signals appeared promptly after photoexcitation at 355 nm.After vibrational cooling, solvation, and geometrical relaxation within the triplet manifold, these signals persisted up to the end of the delay stage (7 ns).Ultrafast intersystem crossing occurs on a time scale shorter than the instrument response and is not detected with our experimental setup [48].
The comparison of the TA spectral profiles of 2A and 2B (Figure S23) indicated the involvement of different triplet excited states in their transient absorptions.For complex 2A, the TA spectra comprised two ESA bands separated by a strong GSB signal, with two well-defined isosbestic points at 425 and 540 nm.The decay of both ESA bands and the recovery of the GSB occurred on the same time scale.The ESA band in the UV range was attributed to the terpyridyl anion radical [69,70], associated with the 3 ILCT excited state, represented by the ESA with a maximum at 645 nm.The triplet excited state was of a predominantly 3 ILCT nature.The complexes 1A and 2A exhibited distinct transient absorption (TA) features compared to their parent chromophores (1B and 2B), indicating that the introduction of the morpholinyl group significantly affected the nature of the lowest triplet excited state of [IrCl3(R-C6H4-terpy-κ 3 N)] and [Ir(R-C6H4-terpy-κ 3 N)2](PF6)3.Specifically, 1A and 2A displayed ground-state bleaching (GSB) in wavelength regions corresponding to their visible charge-transfer absorptions, along with excited-state absorption bands (ESA) in the ranges of 375-415 nm and 522-670 nm for 1A, and 375-425 nm and 540-670 nm for 2A.In contrast, the TA spectra of the parent chromophores displayed only positive bands across the UV and visible regions, indicating that 1B and 2B have a stronger triplet excited-state absorption than the ground-state absorption [65][66][67][68].For all the studied Ir(III) complexes, the TA signals appeared promptly after photoexcitation at 355 nm.After vibrational cooling, solvation, and geometrical relaxation within the triplet manifold, these signals persisted up to the end of the delay stage (7 ns).Ultrafast intersystem crossing occurs on a time scale shorter than the instrument response and is not detected with our experimental setup [48].
The comparison of the TA spectral profiles of 2A and 2B (Figure S23) indicated the involvement of different triplet excited states in their transient absorptions.For complex 2A, the TA spectra comprised two ESA bands separated by a strong GSB signal, with two well-defined isosbestic points at 425 and 540 nm.The decay of both ESA bands and the recovery of the GSB occurred on the same time scale.The ESA band in the UV range was attributed to the terpyridyl anion radical [69,70], associated with the 3 ILCT excited state, The fs-TA data of 2B indicated a mixed 3 IL-3 MLCT nature.With reference to the TA findings for [Ir(terpy-κ 3 N) 2 ](PF 6 ) 3 [51], the higher energy TA signal suggests a 3 IL terpy triplet excited state, while the lower energy one can be assigned to the 3 MLCT transient absorption.The presence of two different transient species undergoing interconversion was supported by the isosbestic point at 515 nm in the TA spectrum of 2B.Additionally, the lower energy ESA band of 2A was similar to the low energy ESA band of the model chromophore 1B, characterized by the lowest triplet excited state of a 3 MLLCT nature (Figure S23).
The comparison of the TA spectral profiles of 1A and 1B (Figure S23) demonstrated that the morpholinyl group induces smaller variations in the TA spectral features of [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)].The presence of a GSB band, the broadening of the visible ESA band, and different decay kinetics relative to 1B suggest that the TA spectral features of 1A correspond to a triplet excited state of a mixed 3 MLLCT-3 ILCT nature, with a dominant contribution of the first component.

Conclusions
The work presents comprehensive studies of [IrCl 3 (morph-C 6 H 4 -terpy-κ 3 N)] (1A), [Ir(morph-C 6 H 4 -terpy-κ 3 N) 2 ](PF 6 ) 3 (2A), [IrCl 3 (Ph-terpy-κ 3 N)] (1B), and [Ir(Ph-terpyκ 3 N) 2 ](PF 6 ) 3 (2B), which were designed to explore the impact of the electron-donating morpholinyl (morph) group on the ground-and excited-state properties of two different types of Ir(III) complexes: [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)] and [Ir(R-C 6 H 4 -terpy-κ 3 N) 2 ](PF 6 ) 3 .We demonstrated that the attachment of morpholine leads to a change in the nature of the singlet and triplet excited states of bis-terpyridyl iridium(III) complexes, switching from intraligand with a small admixture of metal-to-ligand charge transfer (IL-MLCT) for 2B to intraligand charge transfer (ILCT) in 2A.Consequently, the compounds 2A and 2B exhibited completely different absorption and emission features.The deeply red complex 2A displayed very strong absorption tailing up to 600 nm, which was absent in the UV-Vis spectrum of its parent chromophore 2B, which was pale yellow in solution and predominately absorbed wavelengths below 400 nm.The room-temperature emission of 2A was red-shifted ~200 nm and showed a decreased lifetime relative to the model complex 2B.The optical properties of the mono-terpyridyl iridium(III) complexes were less impacted by the morpholinyl substituent.Only an absorption enhancement in the range of 350-550 nm and a small red shift in the emission were observed for 1A compared to the model chromophore 1B.Bases on the static and time-resolved spectroscopic findings, accompanied with theoretical DFT/TD-DFT calculations, the character of the singlet and triplet excited states of 1A was assigned as a mixed MLLCT-ILCT.The structure-property relationships discussed herein are of high importance for controlling the photophysical characteristics of [IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)] and [Ir(R-C 6 H 4 -terpy-κ 3 N) 2 ](PF 6 ) 3 , and making further progress in the development of Ir-based luminophores.

Materials and Methods
Commercially available iridium(III) chloride salt hydrate, ammonium hexafluorophosphate, 2-acetylpyridine, 4-(4-morpholinyl)benzaldehyde and benzaldehyde were used without further purification.The solvents for the syntheses and spectroscopic measurements were reagent and HPLC grade, respectively.Ligands morph-C 6 H 4 -terpy and Ph-terpy were prepared through base-mediated Kröhnke condensation from 2-acetylpyridine and two equivalents of the appropriate aldehyde (4-(4-morpholinyl)benzaldehyde for morph-C 6 H 4terpy and benzaldehyde for Ph-terpy, as described previously [31,71].Elemental analyses were performed using a Vario EL Cube (Elementar) for the C, H, and N content.NMR spectra were recorded on a Bruker Avance 500 NMR spectrometer in DMSO-d 6 .IR spectra were acquired using a Nicolet iS5 FTIR spectrophotometer (4000-400 cm −1 ) using the KBr pellet method.X-ray diffraction data were collected at room temperature using a Gemini A Ultra diffractometer (Oxford Diffraction) with MoKα radiation (λ = 0.71073 Å), and crystallographic data for 1A, 1B, and 2B were deposited with the Cambridge Crystallographic Data Center (CCDC 2359006-2359008).The UV-Vis absorption spectra were recorded on an Evolution 220 (ThermoScientific) UV-Vis spectrometer.The emission and excitation spectra along with the time-resolved TCSPC measurements at room temperature and in 77 K frozen matrix (4:1 v/v) were measured on an FLS-980 fluorescence spectrophotometer (Edinburgh Instruments).The fs TA spectra were acquired using a pump-probe transient absorption spectroscopy system (Ultrafast Systems, Helios).Theoretical calculations were performed using the GAUSSIAN-16 (Rev.C.01) program package [44] at the DFT or TD-DFT level with the PBE0 [72][73][74] functional.The basis sets used were Stuttgart Relativistic Small Core ECP with the corresponding pseudopotentials [45,75] for iridium (obtained from Basis Set Exchange Database [76]) and def2-TZVP for other elements [45,77].A more extended experimental description is provided in the Supplementary Materials.

Preparation of Ir(III) Complexes
[IrCl 3 (R-C 6 H 4 -terpy-κ 3 N)]: A mixture of IrCl 3 •xH 2 O (0.30 g; 1 mmol) and the appropriate R-terpy ligand (1 mmol) in degassed methoxyethanol was placed in a 25 mL Teflon-lined hydrothermal synthesis autoclave reactor and heated at 120 • C for 24 h.After that, the autoclave was gradually cooled to room temperature (24 h).The resulting reddish-brown crystalline precipitate was collected by filtration; washed with acetonitrile, chloroform, and diethyl ether; and dried in air.

Figure 4 .
Figure 4.The partial molecular orbital energy level diagram with the plots of electron densi tributions in the HOMO and LUMO of 1A-2A and 1B-2B.As shown in Figure4, the attachment of the morpholinyl group to Ph-terpy led destabilization of both the HOMO and LUMO orbitals, with more pronounced e variations in the case of the HOMO level.In the pairs 1A-1B and 2A-2B, the HOM ergy increased by ∼0.60 eV and ∼1.47 eV upon going from the unsubstituted to the pholinyl-decorated system, respectively.The destabilization of the LUMO level due morpholine incorporation was ∼0.08 eV and ∼0.14 eV for pairs 1A-1B and 2A-2B.C quently, the Ir(III) complexes with the morph-C6H4-terpy ligand (1A and 2A) exh significantly reduced HOMO-LUMO gaps relative to their reference complexes (1 2B).The HOMO-LUMO energy gap decreased in the order of 2B (4.20 eV) > 1B (3.> 1A (3.23 eV) > 2A (2.87 eV).Notably, the replacement of three electron-donating halide ions in [IrCl3(Rterpy-κ 3 N)] by the π-accepting R-C6H4-terpyκ 3 N ligand in [Ir(R-C6H4-terpy-κ 3 N sulted in the stabilization of the HOMO and LUMO levels of the bis-terpyridyl sy In the pairs 1A-2A and 1B-2B, the HOMO energy decreased by ∼0.22 eV and ∼1 respectively.In turn, the LUMO energy of 1A and 1B dropped by ∼0.58 eV and ∼0 compared to 2A and 2B, respectively.As expected, the morpholine substituent also altered the nature of the HOMO o Unlike the HOMO of 1B, which predominantly comprised the orbitals of the Ir(III) (61.8 %) and chloride ions (27.4 %), the HOMO of 1A was a combination of morp (32.1 %), phenylene (48.1%), and terpy (9.5%) orbitals.Compared to 1B, the metal c ter of the HOMO of 1A decreased from 61.8 % to 8.5 %.The highest occupied mol orbitals of 2A and 2B resided almost exclusively on the R-C6H4-terpy ligand.For pholinyl-substituted complex (2A), it comprised the morpholine group (35.9 %) and nylene linker (46.5%).The metal contributions in the HOMO of 2A and 2B were 2.1 12.5%, respectively.The lowest unoccupied molecular orbital of all the studied Ir(III plexes was predominately constituted by π* orbitals of the terpy backbone.

Figure 4 .
Figure 4.The partial molecular orbital energy level diagram with the plots of electron density distributions in the HOMO and LUMO of 1A-2A and 1B-2B.As shown in Figure4, the attachment of the morpholinyl group to Ph-terpy led to the destabilization of both the HOMO and LUMO orbitals, with more pronounced energy variations in the case of the HOMO level.In the pairs 1A-1B and 2A-2B, the HOMO energy increased by ~0.60 eV and ~1.47 eV upon going from the unsubstituted to the morpholinyl-decorated system, respectively.The destabilization of the LUMO level due to the morpholine incorporation was ~0.08 eV and ~0.14 eV for pairs 1A-1B and 2A-2B.Consequently, the Ir(III) complexes with the morph-C 6 H 4 -terpy ligand (1A and 2A) exhibited significantly reduced HOMO-LUMO gaps relative to their reference complexes (1B and 2B).The HOMO-LUMO energy gap decreased in the order of 2B (4.20 eV) > 1B (3.74 eV) > 1A (3.23 eV) > 2A (2.87 eV).Notably, the replacement of three electron-donating halide ions in [IrCl 3 (R-C 6 H 4 -terpyκ 3 N)] by the π-accepting R-C 6 H 4 -terpyκ 3 N ligand in [Ir(R-C 6 H 4 -terpy-κ 3 N) 2 ] 3+ resulted in the stabilization of the HOMO and LUMO levels of the bis-terpyridyl systems.In the pairs 1A-2A and 1B-2B, the HOMO energy decreased by ~0.22 eV and ~1.1 eV, respectively.In turn, the LUMO energy of 1A and 1B dropped by ~0.58 eV and ~0.64 eV compared to 2A and 2B, respectively.As expected, the morpholine substituent also altered the nature of the HOMO orbital.Unlike the HOMO of 1B, which predominantly comprised the orbitals of the Ir(III) center (61.8 %) and chloride ions (27.4 %), the HOMO of 1A was a combination of morpholine (32.1%), phenylene (48.1%), and terpy (9.5%) orbitals.Compared to 1B, the metal character of the HOMO of 1A decreased from 61.8 % to 8.5 %.The highest occupied molecular orbitals of 2A and 2B resided almost exclusively on the R-C 6 H 4 -terpy ligand.For morpholinylsubstituted complex (2A), it comprised the morpholine group (35.9 %) and phenylene linker (46.5%).The metal contributions in the HOMO of 2A and 2B were 2.1% and 12.5%, respectively.The lowest unoccupied molecular orbital of all the studied Ir(III) complexes was predominately constituted by π* orbitals of the terpy backbone.

Figure 5 .
Figure 5. UV-Vis spectra of Ir(III) complexes recorded in MeCN and DMSO solutions.

Figure 5 .
Figure 5. UV-Vis spectra of Ir(III) complexes recorded in MeCN and DMSO solutions.

Molecules 2024 ,
29,  x FOR PEER REVIEW 9 of 21 chromophore 2B, which was pale yellow in solution.The molar extinction coefficient of this band exceeded 4 × 10 4 M −1 •cm −1 in solution.

Figure 6 .
Figure 6.Experimental absorption spectra (black line) of 1A and 2A alongside vertical lines presenting singlet-singlet transitions with corresponding oscillator strengths, computed at the TD-DFT/PCM/PBE0/SDD/def2-TZVP level with the use of the PCM model at polarities corresponding to MeCN (a); electron density distributions plots of frontier low energy transitions (b).The data for parent compounds 1B and 2B are provided inTable S6 in the Supplementary Materials.

Figure 6 .
Figure 6.Experimental absorption spectra (black line) of 1A and 2A alongside vertical lines presenting singlet-singlet transitions with corresponding oscillator strengths, computed at the TD-DFT/PCM/PBE0/SDD/def2-TZVP level with the use of the PCM model at polarities corresponding to MeCN (a); electron density distributions plots of frontier low energy transitions (b).The data for parent compounds 1B and 2B are provided inTable S6 in the Supplementary Materials.

Figure 8 .
Figure 8. Normalized emission spectra of pairs 1A-1B and 2A-2B in the solid state alongside the CIE chromaticity diagram.

sFigure 9 .
Figure 9. Spin density surface plots for 1A and 2A.For parent chromophores, spin density surface plots are provided in the Supplementary Materials.

Figure 9 .
Figure 9. Spin density surface plots for 1A and 2A.For parent chromophores, spin density surface plots are provided in the Supplementary Materials.

Figure 10 .
Figure 10.The results of fs-TA measurements and global fitting analyses: (a) 2D time-wavelength plots; (b) fs-TA spectra at selected time delays (ps); (c) decay-associated spectra DASi with appropriate time constant ti (see also Figures S19-S22).

Figure 10 .
Figure 10.The results of fs-TA measurements and global fitting analyses: (a) 2D time-wavelength plots; (b) fs-TA spectra at selected time delays (ps); (c) decay-associated spectra DAS i with appropriate time constant t i (see also Figures S19-S22).

Table 2 .
Summary of luminescence properties of