Luminescent Diimine-Pt(IV) Complexes with Axial Phenyl Selenide Ligands

: Luminescent diimine-Pt

Aiming to extend the structural motifs of photoluminescent diimine Pt(IV) complexes, we stepped over the previously reported complexes [Pt(NˆN)(Me) 2 (PhE) 2 ] (E = S or Se) containing heteroaromatic diimines like bpy and its derivatives [56][57][58][59][60]. Herein we report on the preparation of a series of Pt(IV) complexes [Pt(NˆN)(Me) 2 (PhSe) 2 ], NˆN = bpy (1b), phen (2b), and Me 2 bpy (3b), through oxidative addition reaction of diphenyl diselenide to Pt(II) precursors and the study of their UV-vis absorption and photoluminescence (PL).The synthesis and structures of complexes 1b and 2b have been previously reported; we were able to add the derivative 3b to this series.Importantly, the photophysics of the three complexes has not been reported.In parallel to the experiments, we carried out time-dependent DFT (TD-DFT) calculations on the B3LYP level of theory with LANL2DZ basis sets for Pt and Se, and 6-31G(d) basis sets for C, H, and N to probe the nature of the electronic transitions and excited states.
We optimised the geometry of the S0 ground states using DFT methods in both the gas phase (Figures 1 and S6-S8) and CH2Cl2 solution, with only marginal differences (Tables S1, S3 and S4).The DFT-calculated 1b and 2b structures agree with previously reported experimental structures from single X-ray diffraction (Tables S1 and S3) [58,59].They show that Scheme 1. Synthesis routes for complexes 1b-3b.
We optimised the geometry of the S 0 ground states using DFT methods in both the gas phase (Figures 1 and S6-S8) and CH 2 Cl 2 solution, with only marginal differences (Tables S1, S3 and S4).
We optimised the geometry of the S0 ground states using DFT methods in both th gas phase (Figures 1 and S6-S8) and CH2Cl2 solution, with only marginal differences (Ta bles S1, S3 and S4).The DFT-calculated 1b and 2b structures agree with previously reported exper mental structures from single X-ray diffraction (Tables S1 and S3) [58,59].They show tha The DFT-calculated 1b and 2b structures agree with previously reported experimental structures from single X-ray diffraction (Tables S1 and S3) [58,59].They show that both PhSe planes point towards the NˆN ligand plane (Figure 1A) but co-planarity is prevented through the Pt-Se-C angles of about 105 • and Se-Pt-Se angles of about 174 • , deviating from the ideal 180 • .The most important difference between the phen complex 2b and the bpy complexes 1b and 3b is the intramolecular stacking motif of the PhSe phenyl group with the aromatic system in the diimines (Figure 1).This corresponds with differences in the dihedral angles between the two PhSe planes, which are relatively small for the bpy and Me 2 bpy complexes (about 6-9 • , independent of the medium) but large for the phen derivative 2b with about 51 • in the gas phase calculations and about 20 • in CH 2 Cl 2 .
Based on the DFT-optimised 1b-3b structures in CH 2 Cl 2 , the energies and composition of the molecular orbitals (MOs) of the complexes were calculated (Figures 2 and S11-S13, Tables S5-S7).The gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO gaps) increase along the series 1b (3.109 eV) < 2b (3.135 eV) < 3b (3.169 eV), reflecting the increased electron density through the additional C=C group in the backbone of phen and the Me substitutes compared with bpy.The occupied MOs are distributed over the PhSe − ligands, with HOMO-HOMO-2 localised at the Se lone-pairs (~85%) with no difference for 1b-3b, while HOMO-3-HOMO-5 orbitals are cantered on the phenyl cores.Unoccupied MOs are mainly localised on the NˆN chelate ligands (92 to 99%), again with only minor differences for series 1b-3b.HOMO had small Pt contributions (~14%) with a d xy character for LUMO+1 (d z2 ), although the latter is better described as σ-Se-Pt contribution (Figure S11).For LUMO+4, a mixed Pt d x2-y2 /σ Pt-CH 3 character is observed (~70%) for 1b and 3b.For complex 2b, both contributions were also found, although the labelling was different (LUMO+2 and LUMO+5).The energies of these two orbitals are different (~1.8 eV), pointing to a distortion of the octahedral symmetry with the empty d z2 and d x2-y2 not degenerating.
both PhSe planes point towards the N^N ligand plane (Figure 1A) but co-planarity is pr vented through the Pt-Se-C angles of about 105° and Se-Pt-Se angles of about 174°, d viating from the ideal 180°.The most important difference between the phen complex 2 and the bpy complexes 1b and 3b is the intramolecular stacking motif of the PhSe phen group with the aromatic system in the diimines (Figure 1).This corresponds with diffe ences in the dihedral angles between the two PhSe planes, which are relatively small f the bpy and Me2bpy complexes (about 6-9°, independent of the medium) but large for th phen derivative 2b with about 51° in the gas phase calculations and about 20° in CH2Cl Based on the DFT-optimised 1b-3b structures in CH2Cl2, the energies and compos tion of the molecular orbitals (MOs) of the complexes were calculated (Figures 2 and S11 S13, Tables S5-S7).The gaps between the highest occupied and the lowest unoccupie molecular orbitals (HOMO-LUMO gaps) increase along the series 1b (3.109 eV) < 2b (3.13 eV) < 3b (3.169 eV), reflecting the increased electron density through the additional C= group in the backbone of phen and the Me substitutes compared with bpy.The occupie MOs are distributed over the PhSe − ligands, with HOMO-HOMO-2 localised at the S lone-pairs (~85%) with no difference for 1b-3b, while HOMO-3-HOMO-5 orbitals are ca tered on the phenyl cores.Unoccupied MOs are mainly localised on the N^N chelate li ands (92 to 99%), again with only minor differences for series 1b-3b.HOMO had small contributions (~14%) with a dxy character for LUMO+1 (dz2), although the latter is bett described as σ-Se-Pt contribution (Figure S11).For LUMO+4, a mixed Pt dx2-y2/σ Pt-CH character is observed (~70%) for 1b and 3b.For complex 2b, both contributions were als found, although the labelling was different (LUMO+2 and LUMO+5).The energies these two orbitals are different (~1.8 eV), pointing to a distortion of the octahedral sym metry with the empty dz2 and dx2-y2 not degenerating.

UV-Vis Absorption Spectra and TD-DFT Calculations
The experimental UV-vis absorption spectra of compounds 1b-3b in CH 2 Cl 2 solution are very similar (Figures 3 and S9), with two intense maxima at 240 and 320 nm.The phen complex 2b has an additional band at 274 nm.These UV bands can be provisionally assigned to π−π* transitions in the NˆN ligands.Possible admixtures can include intra-ligand IL' (PhSe) charge transfer (IL'CT), metal-to-ligand (NˆN) charge transfer (MLCT), ligand' (PhSe) -to-ligand (NˆN) charge transfer (L'LCT), and ligand'-to-metal charge transfer (L'MCT) character.Additional weak absorptions were observed in the range 350-475 nm as shoulders that are provisionally assigned to L'LCT transitions with small MLCT and L'MCT contributions in line with the ground state frontier orbitals (Figure 2).
L'MCT contributions in line with the ground state frontier orbitals (Figure 2).
For future use in biological media, we also dissolved and studied the complexes in aqueous tris(hydroxymethyl)aminomethane hydrochloride (tris-HCl) buffer with the help of small amounts of DMSO as a co-solvent.The same absorptions were found in the tris-HCl buffer solution.After standing for 30 min in the dark, the long wavelength absorptions were slightly decreased, while UV absorptions remained stable (Figure S10).This is probably due to slow hydrolysis under these conditions adding potential instability to the slow photodecomposition in the presence of light.The TD-DFT-calculated transitions match the experimental spectra in that they qualitatively reproduce the differences between the bpy complexes 1b and 3b, in contrast to the phen complex 2b, while the calculated maxima are generally red-shifted compared with the experimental maxima (for data, see Tables S8-S10).For the bpy complex 1b, the long wavelength region between 300 and 500 nm includes several excited states (S1 to S6) with very small oscillator strengths in keeping with the weak absorptions observed in this range (Figure 3, inset).They are assigned to transitions with mixed L'LCT/MLCT or L'MCT/IL'CT character, with L' representing the PhSe ligands and IL'CT describing charge transfer from Se n(Se) lone pairs (or p orbitals) to π* orbitals of the Ph group (Table S8).The HOMO→LUMO transition for 1b has mixed L'LCT (n(Se)→π*(bpy))/MLCT (d(Pt)→π*(bpy)) character in which the most important contribution form, the PhSe ligand, comes from the lone-pairs, n, on the Se atom.In contrast to this, the HOMO→L+1 transition has a mixed L'MCT/IL'CT (n(Se)→π*(Ph)) character.The S0→S7 transition (H-1→L+1, 354 nm) has the highest oscillator strength (0.9801) and mixed L'MCT/IL'CT character.Similar transitions with similar characters were calculated for the Me2bpy complex, 3b.They are all slightly blue-shifted (Table S9) in keeping with the electron-releasing character of the Me substituents on the bpy core.Despite some very small contributions to LUMO+4 in 1b and 3b, the Me ligands show overall negligible contributions.For the phen complex, 2b, the long wavelength region shows eight excited states of low energy (S1-S8), with energies slightly blue-shifted compared to 1b; both findings are in line with the additional C=C group in the phen backbone.For future use in biological media, we also dissolved and studied the complexes in aqueous tris(hydroxymethyl)aminomethane hydrochloride (tris-HCl) buffer with the help of small amounts of DMSO as a co-solvent.The same absorptions were found in the tris-HCl buffer solution.After standing for 30 min in the dark, the long wavelength absorptions were slightly decreased, while UV absorptions remained stable (Figure S10).This is probably due to slow hydrolysis under these conditions adding potential instability to the slow photodecomposition in the presence of light.
The TD-DFT-calculated transitions match the experimental spectra in that they qualitatively reproduce the differences between the bpy complexes 1b and 3b, in contrast to the phen complex 2b, while the calculated maxima are generally red-shifted compared with the experimental maxima (for data, see Tables S8-S10).For the bpy complex 1b, the long wavelength region between 300 and 500 nm includes several excited states (S 1 to S 6 ) with very small oscillator strengths in keeping with the weak absorptions observed in this range (Figure 3, inset).They are assigned to transitions with mixed L'LCT/MLCT or L'MCT/IL'CT character, with L' representing the PhSe ligands and IL'CT describing charge transfer from Se n (Se) lone pairs (or p orbitals) to π* orbitals of the Ph group (Table S8).The HOMO→LUMO transition for 1b has mixed L'LCT (n (Se) →π* (bpy) )/MLCT (d (Pt) →π* (bpy) ) character in which the most important contribution form, the PhSe ligand, comes from the lone-pairs, n, on the Se atom.In contrast to this, the HOMO→L+1 transition has a mixed L'MCT/IL'CT (n (Se) →π* (Ph) ) character.The S 0 →S 7 transition (H-1→L+1, 354 nm) has the highest oscillator strength (0.9801) and mixed L'MCT/IL'CT character.Similar transitions with similar characters were calculated for the Me 2 bpy complex, 3b.They are all slightly blue-shifted (Table S9) in keeping with the electron-releasing character of the Me substituents on the bpy core.Despite some very small contributions to LUMO+4 in 1b and 3b, the Me ligands show overall negligible contributions.For the phen complex, 2b, the long wavelength region shows eight excited states of low energy (S 1 -S 8 ), with energies slightly blue-shifted compared to 1b; both findings are in line with the additional C=C group in the phen backbone.
When comparing the results of our calculations with those previously reported for complexes [Pt(bpy)(Me) 3 (SR)] (R = H or p-CO 2 C 6 H 4 ) [50] and [Pt(bpy [52], we found a role for our PhSe − ligands that was similar to that of S-aryl ligands contributing both the S lone pairs and the aryl π* system to electronic transitions.Moreover, the σ→π* transitions involving the Pt-S and Pt-C (trans to S) in these systems are similar to the σ→π* contributions from the PhSe − ligands in our systems.On the other hand, the intramolecular stacking of the PhSe − and NˆN ligands that characterise our structures have been found for [Pt(bpy)(Me) 3

Photoluminescence Spectroscopy
Complexes 1b-3b emit yellow to orange light in different media at ambient temperatures when irradiated at 365 nm (Figures 4 and 5), in stark contrast to their corresponding deeply red-coloured Pt(II) precursors [Pt(NˆN)(Me) 2 ]), 1a-3a, which are non-emissive under the same conditions [62].Changing the excitation wavelength within the range 300-400 nm had no marked impact on the shape or maxima of the emission bands.
Similar to the Pt(IV) complexes, the intense absorptions at around 330-355 nm observed for the Re(I) thiolate complexes [Re(N^N')(CO)3(SR)] (N^N' = pyridyl-trizoles) are assigned to predominantly π-π* transitions with admixtures of LL'CT (thiolate to N^N) and MLCT (Re to N^N) character.Weak bands were observed at lower energies and DFT calculations suggest L'LCT and L'MCT (thiolate to Re) character [61].

Photoluminescence Spectroscopy
Complexes 1b-3b emit yellow to orange light in different media at ambient temperatures when irradiated at 365 nm (Figures 4 and 5), in stark contrast to their corresponding deeply red-coloured Pt(II) precursors [Pt(N^N)(Me)2]), 1a-3a, which are non-emissive under the same conditions [62].Changing the excitation wavelength within the range 300-400 nm had no marked impact on the shape or maxima of the emission bands.In the solid state, the emission spectra for 1b-3b had broad bands with maxima between 554 and 595 nm (Figure 4, Table 1).Different temperatures (298 vs. 77 K) did not make a marked difference.At 298 K, the highest photoluminescence quantum yield (Φ L ) was 0.081 for 2b, while 1b and 3b had slightly lower values of 0.069 and 0.065, respectively.At 77 K, the Φ L value for 2b increased and reached 0.14.The excitation spectra showed intense maxima in the range 300-400 nm, with the most red-shifted maximum observed for 1b.
More structured emission bands were observed in the CH 2 Cl 2 solution, with their maxima blue-shifted compared with the solids (Figure 5, Table 1).Within our series of complexes, the energy of the emission maximum increased for 2b and 3b compared with 1b.The excitation spectra showed sharp bands at around 360 nm for all three complexes.This means a Stokes Shift of about 8400 cm −1 for 1b and about 7400 cm −1 for 2b and 3b.This is consistent with a triplet character of the excited states, thus a triplet emission.The most intense excitation bands at around 360 nm are markedly red-shifted compared with the intense absorption bands observed at around 320 nm.The energy difference of about 3500 cm −1 suggests that the most intensely absorbing state is not identical to the state responsible for the emission.Unfortunately, the Φ L values are generally very low in the CH 2 Cl 2 solution (0.002-0.006) and the PL spectra for 1b differ from those for 2b and 3b.Therefore, we cannot rule out the effect of different species or impurities on PL behaviour.maxima blue-shifted compared with the solids (Figure 5, Table 1).Within our series of complexes, the energy of the emission maximum increased for 2b and 3b compared with 1b.The excitation spectra showed sharp bands at around 360 nm for all three complexes.This means a Stokes Shift of about 8400 cm −1 for 1b and about 7400 cm −1 for 2b and 3b.This is consistent with a triplet character of the excited states, thus a triplet emission.The most intense excitation bands at around 360 nm are markedly red-shifted compared with the intense absorption bands observed at around 320 nm.The energy difference of about 3500 cm −1 suggests that the most intensely absorbing state is not identical to the state responsible for the emission.Unfortunately, the ΦL values are generally very low in the CH2Cl2 solution (0.002-0.006) and the PL spectra for 1b differ from those for 2b and 3b.Therefore, we cannot rule out the effect of different species or impurities on PL behaviour.Em(Ex@365),CH 2 Cl 2 Ex(Em@485) Em(Ex@365),dmso Ex(Em@485) Em(Ex@365),Tris-HCl Ex(Em@485) Em(Ex@365),CH 2 Cl 2 Ex(Em@490) Em(Ex@365),dmso Ex(Em@490) Em(Ex@365),Tris-HCl Ex(Em@490) 2b 300 350 400 450 500 550 600 650 700 0 1 Normalized intensity Wavelength (nm) Em(Ex@365),CH 2 Cl 2 Ex(Em@490) Em(Ex@365),dmso Ex(Em@490) Em(Ex@365),Tris-HCl Ex(Em@490) 3b Figure 5. Photoluminescence (solid lines) and excitation (dashed lines) spectra for 1b, 2b, and 3b in CH2Cl2 (blue lines), DMSO (green lines), and tris-HCl (orange lines) solutions at 298 K.
To probe for the influence of solvent polarity on the PL of 1b-3b, we studied DMSO and aqueous tris-HCl buffer (pH = 7.4, 0.01 M) solutions in addition to CH2Cl2.A further background is the idea of using these complexes as probes in biological media.As in the CH2Cl2 solution, the complexes were emissive at 298 K and also showed partially structured emission bands (Figure 5).Interestingly, the solvents had almost no impact on the shape and energies of the emission band of the phen complex 2b (Figure 5), while 1b   To probe for the influence of solvent polarity on the PL of 1b-3b, we studied DMSO and aqueous tris-HCl buffer (pH = 7.4, 0.01 M) solutions in addition to CH 2 Cl 2 .A further background is the idea of using these complexes as probes in biological media.As in the CH 2 Cl 2 solution, the complexes were emissive at 298 K and also showed partially structured emission bands (Figure 5).Interestingly, the solvents had almost no impact on the shape and energies of the emission band of the phen complex 2b (Figure 5), while 1b showed a red-shifted maximum (516 nm) for the CH 2 Cl 2 solution compared with the DMSO and tris-HCl buffer solutions (484 nm), and 3b showed a blue-shifted maximum in CH 2 Cl 2 (490 nm) compared with the DMSO and tris-HCl buffer solutions (522 nm).The Φ L values are generally low (0.0017-0.0123) and are higher in each case in the polar media, for 2b and 3b, with the highest values found in aqueous tris-HCl buffer (Table 1).In the excitation spectra, the intense band observed in CH 2 Cl 2 solutions at around 360 nm is also found for 2b in tris-HCl and 3b in both polar media, but with a markedly lower intensity.Additional broad, blue-shifted bands were found for 1b in DMSO and tris-HCl and for 2b in DMSO.We assume that both Me substituents on the bpy ligand and the C=C backbone of the phen ligand render the molecules more lipophilic, thus minimising the effect of solvent polarity on the chromophoric parts of the complexes.
Unfortunately, the lifetimes were too short to measure using our equipment in the lab and we assumed that they were in the nanosecond domain.For the Pt(IV) complex [Pt(bpy)(Me) 3 (S-C 6 H 4 -R)] ( − S-C 6 H 4 -R = 4-methoxycarbonylthiophenolate), a triplet emission lifetime of 5.6 ns was reported [52], while for [Pt(Me) 3 (bpy)I], 2.8 ns was recorded [52].The lifetimes of complexes 1b-3b will be the subject of future studies.
To rationalise the emission spectra, the geometry of the T 1 triplet state for complexes 1b and 2b were DFT-optimised in the gas phase.Unfortunately, attempts to do the same for 3b were unsuccessful.Energy differences between the T 1 and S 0 states representing the theoretical emissions were calculated as 2.146 eV (578 nm) and 2.098 eV (591 nm) for complexes 1b and 2b, respectively.These are in very good agreement with the experimental values recorded for 1b and 2b at 595 nm and 564 nm in the solid at 298 K (Table 1).Furthermore, the observed red-shift in the main emission component in the CH 2 Cl 2 solution from 490 to 515 nm when going from 2b (phen) to 1b (bpy) is also qualitatively reproduced, although the total values deviate markedly.This marked deviation is due to the gas phase conditions used for the DFT calculations and is in line with the strong dependence of the emission on the medium found in the experiments.
For complexes 1b and 2b, plots of LSOMO (Lowest Singly Occupied Molecular Orbital) and HSOMO (Highest Singly Occupied Molecular Orbital) compositions (Figure 6) showed high similarities between LSOMO and HOMO compositions (Figure 2), while the compositions of HSOMO and LUMO in both complexes were different.LUMO+1 and LUMO+2 levels are stabilised when they are converted to HSOMO for 1b and 2b, respectively.LSOMO and HSOMO are mainly located on the Pt and Se atoms (Figure 6).The emissive excited states of 1b and 2b can thus be described as having mixed 3 IL'CT/ 3 L'MCT characters, with a significant contribution of the PhSe ligands.This is in agreement with the intense excitations observed at around 360 nm where the TD-DFT-calculated transitions show strong IL'CT contributions (Tables S8 and S9).This also accounts for the marked differences between absorptions with large contributions of the NˆN ligands to the transitions and the excitations populating triplet excited states with essential PhSe and Pt characters and supports our assumptions of different absorption and excitation characters.
characters, with a significant contribution of the PhSe ligands.This is in agreement with the intense excitations observed at around 360 nm where the TD-DFT-calculated transitions show strong IL'CT contributions (Tables S8 and S9).This also accounts for the marked differences between absorptions with large contributions of the N^N ligands to the transitions and the excitations populating triplet excited states with essential PhSe and Pt characters and supports our assumptions of different absorption and excitation characters.[52], although they had similar energies as our complexes.The same is true for [Pt(bpy)(Me)3I], which emits at 528 nm in EtOH at 298 K [25].Similar emissions were found for the related Re(I) thiolate complexes [Re(N^N')(CO)3(SR)] and a mixed 3 MLCT/ 3 L'LCT character was concluded from DFT calculations [61].
We found, for our previously unreported [Pt(NˆN)(Me) 2 (PhSe) 2 ] systems, a predominant PhSe ligand-character in the triplet excited states, with mixed 3 IL'CT/ 3 L'MCT contributions being responsible for the pronounced structuring and efficient emission in solution at 298 K.This contrasts with the TD-DFT-calculated absorption spectra showing appreciable contributions of the NˆN chelate ligands with mixed L'LCT/MLCT contributions in addition to the 3 IL'CT/ 3 L'MCT, and also differs from previously reported Pt(IV) systems with Pt(NˆN) or Pt(CˆN) scaffolds, for which essential contributions from the NˆN or CˆN chelate ligands (L'LCT or MLCT) to the emissive states were reported.We ascribe this to a perfect match between the axial PhSe ligands and the Pt orbitals in terms of energy and orbital overlap.

General Procedures and Materials
1 H NMR, 13 C{ 1 H} NMR, and 77 Se{ 1 H} spectra were obtained using a Bruker Avance (400 MHz) instrument at 298 K.The chemical shifts (δ) are reported in ppm relative to their external standards (SiMe 4 for 1 H and 13 C, Ph 2 Se for 77 Se), and the coupling constant J is expressed in Hz.UV-vis absorption spectra were recorded using a PerkinElmer Lambda 25 spectrophotometer.Luminescence spectra were measured at 298 and 77 K using a PerkinElmer LS45 fluorescence spectrometer.CH 2 Cl 2 , DMSO, and aqueous tris-HCl buffer solutions were Ar-purged prior to use.Due to the virtual insolubility of the complexes in the tris-HCl buffer solution, small amounts of DMSO were added.Absolute photoluminescence quantum yields in the solid state were measured using an integrating sphere.Quantum yields in solution were measured using MeCN solutions of complex [Ru(bpy) 3 ](PF 6 ) 2 as standard [63].

Computational Details
DFT calculations were carried out on the B3LYP level of theory [65][66][67] using the Gaussian09 program suite [68], LANL2DZ basis sets for Pt and Se atoms [69,70], and 6-31G(d) basis sets for C, H, and N atoms.The calculations were run either in the gas phase or CH 2 Cl 2 solution using the CPCM (conductor-like polarisable continuum model) model [71,72].The structures of the electronic S 0 and T 1 states were optimised without any symmetry constraints.The UV-vis absorption spectra were calculated using timedependent DFT (TD-DFT) methods and emission energies were calculated using DFToptimised geometries.The composition of molecular orbitals and the TD-DFT bars for electronic transitions were drawn using the "Chemcraft version 1.7" software [73].Data for the electronic transitions were extracted using the GaussSum software [74].
Upon irradiation at 365 nm, which is markedly blue-shifted to low-energy absorptions, the maximum of unstructured emission bands of the complexes in the solid ranges lies at 560 nm for 3b, 564 nm for 2b, and 595 nm for 1b, in line with the electron-donating properties of the Me substituents and the additional C=C group in the phen ligand compared with bpy.Structured bands were observed in fluid CH 2 Cl 2 solution, but the wavelength dependence of the emission maximum is the same as in the solid, with blue-shifted bands for 2b and 3b compared with 1b.In both the solid and CH 2 Cl 2 solution, intense excitation bands were found in the range 300-400 nm, with pronounced maxima at 360 nm in the CH 2 Cl 2 solution.The Stokes shift of these sharp excitations and the emission maxima are large (8400-7400 cm −1 ), which strongly supports a triplet character of the emission.However, the luminescence lifetimes are below the µs range and remain to be studied.In the very polar solvents, DMSO and aqueous tris-HCl buffer, the emission maximum for 3b was observed at 522 nm, while those for 1b and 2b were blue-shifted (~490 nm).This is remarkable, as the overall structures of the three complexes are very similar.In future studies, we will add other α-diimine ligands such as the sterically bulky 2,9-dimethylphenanthroline (neocuproin) and 6,6 -dimethyl-bipyridine as well as other bpy and phen derivatives with electron-withdrawing substituents.
Using the DFT-optimised T 1 and S 0 geometries for 1b and 2b, the T 1 →S 0 transitions were calculated as 578 nm and 591 nm, respectively, in agreement with the experimental values, 595 nm and 564 nm, recorded in the solid at 298 K.While the LSOMO (Lowest Singly Occupied Molecular Orbital) compositions showed high similarities with the HOMO compositions, the HSOMO and LUMO compositions in 1b and 2b differ.LSOMO and HSOMO are mainly located on the Pt and Se atoms and the excited states of 1b and 2b can thus be described as having essentially mixed 3 IL'CT/ 3 L'MCT character, with a significant contribution of PhSe ligands to the emissive excited states.This is in excellent agreement with the observed intense excitations at around 360 nm, for which TD-DFT-calculated transitions show strong IL'CT contributions and account for the marked differences between absorption and excitation spectra.The triplet character is in line with the observed Stokes shifts, and the structuring is in line with the strong 3 LC (= PhSe) character.
While previously reported Pt(IV) systems with Pt(NˆN) or Pt(CˆN) scaffolds show essential contributions of the NˆN or CˆN chelate ligands (L'LCT or MLCT) to the emissive states, our [Pt(NˆN)(Me) 2 (PhSe) 2 ] systems reported herein show a predominant PhSe ligand-character for the emissive triplet excited states, with mixed 3 IL'CT/ 3 L'MCT contributions.We ascribe this to a perfect match between the axial PhSe ligands and the Pt orbitals in terms of energy and orbital overlap.We will study the impact of substitutions at the axial PhSe on photophysics in the future.We will also add ps time-resolved PL spectroscopy and study the observed photo-sensibility of the compounds in solution in more detail.Finally, we will explore the potential use of the complexes as luminophores in fluorescent cell imaging.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11100387/s1. Figure S1: 1 H NMR spectrum of 1b in CDCl 3 , Figure S2: 1 H NMR spectrum of 2b in CDCl 3 , Figure S3: 1 H NMR spectrum of 3b in CDCl 3 , Figure S4: 13 C{H} NMR spectrum of 3b in CDCl 3 , Figure S5: 77 Se{H} NMR spectrum of 3b in CDCl 3 , Figure S6: View of the DFT-optimised S 0 structure of 1b in the gas phase with atom numbering, Table S1: Selected DFT-calculated metrics for the S 0 state of 1b in the gas phase and CH 2 Cl 2 compared with experimental data from single crystal XRD, Figure S7: View of the DFT-optimised S 0 structure of 2b in the gas phase with atom numbering, Table S2: Selected DFT-calculated metrics for the T 1 state of complexes 1b and 2b in the gas phase, Table S3: Selected DFT-calculated metrics for the S 0 state of 2b in the gas phase and CH 2 Cl 2 compared with experimental data from single crystal XRD, Figure S8: View of the DFT-optimised S 0 structure of 3b in the gas phase with atom numbering, Table S4: Selected DFT-calculated metrics for the S 0 state of 3b in the gas phase and CH 2 Cl 2 , Figure S9: UV-vis absorption spectra of 1b, 2b, and 3b in CH 2 Cl 2 at 298 K, Figure S10: UV-vis absorption spectra of complex 3b in tris-HCl buffer at 298 K, Figure S11: Molecular orbital plots for the DFT-optimised S 0 structure of 1b in CH 2 Cl 2 solution, Figure S12: Molecular orbital plots for the DFT-optimised S 0 structure of 2b in CH 2 Cl 2 solution, Figure S13: Molecular orbital plots for the DFT-optimised S 0 structure of 3b in CH 2 Cl 2 solution, Table S5: Composition and energies of selected molecular orbitals of 1b in CH 2 Cl 2 , Table S6: Composition and energies of selected molecular orbitals of 2b in CH 2 Cl 2 , Table S7: Composition and energies of selected molecular orbitals of 3b in CH 2 Cl 2 , Table S8: Wavelengths and the nature of transitions of complex 1b, Table S9: Wavelengths and the nature of transitions of complex 2b, Table S10: Wavelengths and the nature of transitions of complex 3b.Institutional Review Board Statement: The work and report are in full agreement with the ethical standards of all three universities and the Deutsche Forschungsgemeinschaft (DFG).

Figure 1 .
Figure 1.View of the DFT-optimised S0 structure of 3b in the gas phase, with atom numbering (A) and top views of 1b, 2b, and 3b (B).

Figure 1 .
Figure 1.View of the DFT-optimised S0 structure of 3b in the gas phase, with atom numbering (A and top views of 1b, 2b, and 3b (B).

Figure 1 .
Figure 1.View of the DFT-optimised S 0 structure of 3b in the gas phase, with atom numbering (A) and top views of 1b, 2b, and 3b (B).

Figure 2 .
Figure 2. Energy level diagrams and composition of the highest occupied molecular orbital (HOMO and the lowest unoccupied molecular orbital (LUMO) for 1b-3b.

Figure 2 .
Figure 2. Energy level diagrams and composition of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for 1b-3b.

Figure 4 .
Figure 4. Photoluminescence (solid lines) and excitation (dashed lines) spectra of 1b, 2b, and 3b in the solid state at 298 K (black lines) and 77 K (red lines).