Spectroscopic characterization of chloride and pseudohalide ruthenium(II) complexes with 4-(4-nitrobenzyl)pyridine

Chloride, isocyanate and isothiocyanate hydride carbonyl ruthenium(II) complexes of 4-(4-nitrobenzyl)pyridine were synthesized from the precursor complex [RuHCl(CO)(PPh₃)₃] and characterized by IR, NMR, UV–Vis spectroscopy and X-ray crystallography. The electronic structures of the complexes were investigated by means of DFT calculations, based on their crystal structures. The spin-allowed singlet–singlet electronic transitions of the complexes were calculated by time-dependent DFT, and the UV–Vis spectra are discussed on this basis. The emission properties of the complexes were studied at ambient temperature, and the quantum yields of fluorescence, the lifetimes and nature of the excited states are discussed. The chloride and isothiocyanate complexes are practically nonemissive, with quantum yields under 0.01 %. Interpretation of spectra, supported by TD-DFT calculations, indicates that in this energy region, the transitions have MLCT character with admixture of LLCT (chloride and isothiocyanate complexes). The dominant LLCT character was visible in the case of the most emissive (isocyanate) complex. The low values of the lifetimes and quantum yields for these complexes indicate the influence of the metal center in the emission process.     

Thermochromism of CuI Tetrakisguanidine Complexes: Reversible Activation of Metal‐to‐Ligand Charge‐Transfer Bands

Tetranuclear, intensely blue‐coloured Cu^I complexes were synthesised in which two Cu₂X₃^? units (X=Br or I) are bridged by a dicationic GFA (guanidino‐functionalised aromatic) ligand. The UV/Vis spectra show a large metal‐to‐ligand charge‐transfer (MLCT) band around 638 nm. The tetranuclear “low‐temperature” complexes are in a temperature‐dependent equilibrium with dinuclear Cu^I “high‐temperature” complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method‐dependent energy difference between the possible electronic structures, namely Cu^I and dicationic GFA ligand (closed‐shell singlet) versus Cu^II and neutral GFA ligand (triplet or broken‐symmetry state). The closed‐shell singlet state is disfavoured by hybrid‐DFT functionals, which mix in exact Hartree–Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.     

Light‐Harvesting Phthalocyanine–Diketopyrrolopyrrole Derivatives: Synthesis,Spectroscopic, Electrochemical,and Photochemical Studies

A new family of light‐harvesting zinc phthalocyanine (ZnPc)–diketopyrrolopyrrole (DPP) hybrids have been synthesized and characterized. The absorption spectral measurements showed that the major absorptions of DPP (450–600 nm) are complementary to those of zinc phthalocyanine (300–400 and 600–700 nm). Therefore, the designed hybrids absorb over a broad range in the visible region. The geometric and electronic structures of the dyads were probed by initio B3LYP/6‐311G methods. The majority of the HOMOs were found to be located on the ZnPc, while the majority of the LUMOs were on the DPP units. The DPP units serve as the antenna, which upon excitation undergo efficient singlet–singlet energy transfer to the attached ZnPc units. The formed singlet ZnPc, in turn, donates its electron to the electron‐deficient DPP forming the low‐lying radical ion pairs ZnPc^.+–DPP^.? (energy=1.44–1.56 eV as calculated from the electrochemical measurements). The excited‐state events were confirmed by using a transient absorption technique in the picosecond–microsecond time range, as well as a time‐resolved emission technique. The rates of energy transfer from the singlet DPP to ZnPc were found to be extremely fast >10¹⁰ s^?1, while the rates of electron transfer from the singlet excited state of ZnPc to DPP were found to be 3.7–6.6×10⁹ s^?1.     

Theoretical investigation on the spectroscopic properties of cyclometallated iridium (III) complexes and the deprotonation influence on them in solution

Electronic structures and spectroscopic properties of mixed‐ligand cyclometallated iridium complexes with general formula [Ir(N?C)₂(N?N)]⁺ (N?C = 2‐phenylpyridine, N?N = Hcmbpy = 4‐carboxyl‐4^′‐methyl‐2,2^′‐bipyridine, 1 ; H₂dcbpy = 4,4^′‐dicarboxyl‐2,2^′‐bipyridine, 2 ) were studied theoretically. The geometries of the complexes in ground and excited state were optimized at B3LYP and CIS levels, respectively. The absorption and emission of the complexes in CH₃CN solutions were calculated by time‐dependent density functional theory (TD‐DFT) with the PCM solvent model. The calculated absorptions and emissions of the complexes are in good agreement with the measured results. The deprotonation influence on the electronic structure and the optical properties of 2 was also investigated. The results indicate that the deprotonation which occurs on the COOH groups influences the geometries of the complexes in ground and excited state slightly but leads to significant blue‐shifts in low energy absorption and emission maximum. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

A Combined Experimental and Computational Study of Linear Ruthenium(II) Coordination Oligomers with End‐Capping Organic Redox Sites: Insight into the Light Absorption and Charge Delocalization

Two series of linear ruthenium coordination oligomers, [(Ntpy)Ru_n(tppz)_n?1(tpy)]²ⁿ⁺ (mono‐Ntpy series, n=1–3) and [(Ntpy)₂Ru_n(tppz)_n?1]²ⁿ⁺ (bis‐Ntpy series, n=1–3) have been prepared, where Ntpy is the capping ligand 4′‐di‐p‐anisylamino‐2,2′:6′,2′′‐terpyridine, tppz is tetra‐2‐pyridylpyrazine, and tpy is 2,2′:6′,2′′‐terpyridine. The electrochemical measurements evidence oxidation events from both the amine segments and the metal centers and reduction waves from tppz and the capping ligands. Both series complexes display much enhanced light absorption with respect to model complexes without terminal amine units. Density functional theory (DFT) calculations have been performed on both series and time‐dependent DFT (TD‐DFT) calculations have been performed on the bis‐Ntpy‐series compounds (n=1–4) to characterize their electronic structures and excited states and predict the electronic properties of long‐chain polymers. Upon one‐electron oxidation, the mono‐Ntpy‐series monoruthenium and diruthenium complexes display N⁺‐localized transitions and metal‐to‐nitrogen charge‐transfer (MNCT) transitions in the near‐infrared (NIR) region. DFT and TD‐DFT computations on the one‐electron‐oxidized forms of the mono‐Ntpy‐series compounds (n=1–4) provide insight into the nature of the MNCT transitions and the degree of charge delocalization.     

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc,Y, La) and the 18‐Electron Rule

We report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈^? (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20‐electron systems TM(CO)₈^? are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)₇^? in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈^? reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈^? fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.     

Theoretical study of the electronic spectrum of binuclear gold(I) complexes

The electronic structure and the spectroscopic properties of [Au₂(CS₃)₂]^?2, [Au₂(pym‐2‐S)₂] (pym = pyrimidethiolate), [Au₂(dpm)₂]⁺² (dpm = bis(diphosphino)methane) were studied using density functional theory (DFT) at the B3LYP level. The absorption spectrum of these binuclear gold(I) complexes was calculated by single excitation time‐dependent (TD) method. All complexes showed a ¹(5dσ* → 6pσ) transition associated with a metal–metal charge transfer, which is strongly interrelated with the gold–gold distance. Furthermore, we have calculated the frequency of the gold–gold vibration (ν_Au2) on the above complexes. The values obtained are theoretically in agreement with experimental range. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Molecular structures of antitumor active Pd(II) and Pt(II) complexes of N,N-donor benzimidazole methyl ester

[MLCl₂]?·?zH₂O?·?C₂H₅OH (L?=?2-[(1H-benzimidazol-2-ylmethyl)-amino]-benzoic acid methyl ester; M?=?Pd, z?=?2; M?=?Pt, z?=?0) complexes were synthesized as potential antitumor compounds and their structures were elucidated by elemental analysis and spectroscopic data. Theoretical molecular structures were investigated by the DFT/B3LYP method using the LANL2DZ basis set. The calculated molecular parameters, bond distances, and angles, revealed a square-planar geometry around the metal through pyridine-type nitrogen (N_py) of benzimidazole and the secondary amino group (NH_sec). The lone pair interaction LP(2)O48 of ethanol with anti-bonding σ*(C(16)–H(29)) is an evidence for charge transfer from ethanol to platinum. The electronic movement and assignment of electronic spectra were carried out by TD-DFT calculations. The ligand in comparison to its metal complexes was screened for antibacterial activity and cytotoxicity.     

New Cu(I) Coordination Complexes Based on Tetrathiafulvalene Substituted Triazol‐pyridine Ligand: Synthesis,Properties and Theoretical Studies

Two Cu(I) complexes based on the thioethyl‐bridged triazol‐pyridine ligand with tetrathiafulvalene unit (TTF‐TzPy, L ), [Cu(I)(Binap)(L)]BF₄ ( 5 , Binap=2,2’‐bis(diphenylphosphino)‐1,1’‐binaphthyl) and [Cu(I)(Xantphos)(L)]BF₄ ( 6 , Xantphos=9,9‐dimethyl‐4,5‐bis(diphenylphosphino)‐xanthene), have been synthesized. All new compounds are characterized by elemental analyses, ¹H NMR and mass spectroscopies. The complex 5 has been determined by X‐ray structure analyses which shows that the central copper (I) ion assumes distorted tetrahedral geometry. The photophysical, computational and electrochemical properties of L and 5 ‐ 6 have been investigated. The most representative molecular orbital energy‐level diagrams and the spin‐allowed singlet? singlet electronic transitions of the three compounds have been calculated with density functional theory (DFT) and time‐dependent DFT (TD‐DFT). The luminescence bands of Cu(I) complexes 5 ‐ 6 have been assigned as mixed intraligand and metal‐to‐ligand charge transfer ³(MLCT+π→π^*) transitions through analysis of the photophysical properties and DFT calculations. The electrochemical studies reveal that 5 ‐ 6 undergo reversible TTF/TTF^+?/TTF²⁺ redox processes and one irreversible Cu⁺→Cu²⁺ oxidation process.     

The reactions of 8-hydroxyquinoline with [RuHCl(CO)(PPh3)3]: A new ruthenium(II) carbonyl complex with a N-donor ligand

The reaction of [RuHCl(CO)(PPh₃)₃] with 8-hydroxyquinoline has been examined and a novel ruthenium(II) complex – [RuCl(CO)(PPh₃)₂(C₉H₆NO)] – has been obtained. This compound has been studied by IR, UV–Vis (absorption and emission), ¹H and ³¹P NMR spectroscopy, and X-ray crystallography. The molecular orbital diagram of the complex has been calculated with the density functional theory (DFT) method. The spin-allowed singlet–singlet electronic transitions of the complex have been calculated with the time-dependent DFT method, and the UV–Vis spectrum of the compound has been discussed on this basis.     

Excited‐State Hydrogen Bonding Strengthening and Weakening with Intramolecular Charge Transfer in Resorufin‐Water Complexes: A TD‐DFT Study

The geometric structures, infrared spectra and hydrogen bond binding energies of the various hydrogen‐bonded Res^?‐water complexes in states S0 and S1 have been calculated using the density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods, respectively. Based on the changes of the hydrogen bond lengths and binding energies as well as the spectral shifts of the vibrational mode of the hydroxyl groups, it is demonstrated that hydrogen bonds HB‐II, HB‐III and HB‐IV are strengthened while hydrogen bond HB‐I is weakened in the four singly hydrogen‐bonded Res^?‐Water complexes upon photoexcitation. When the four hydrogen bonds are formed simultaneously between one resorufin anion and four water molecules in the Res^?‐4Water complex, all the hydrogen bonds are weakened in both the ground and excited states compared with those in the corresponding singly hydrogen‐bonded Res^?‐Water complexes. Furthermore, in complex Res^?‐4Water, hydrogen bonds HB‐II and HB‐IV are strengthened while hydrogen bonds HB‐I and HB‐III are weakened after the electronic excitation. The hydrogen bond strengthening and weakening in the various hydrogen‐bonded Res^?‐water complexes should be due to the redistribution of the charges among the four heteroatoms (O_1‐3 and N₁) within the resorufin molecule upon the optical excitation.     

Emissive or Nonemissive? A Theoretical Analysis of the Phosphorescence Efficiencies of Cyclometalated Platinum(II) Complexes

We herein report a theoretical analysis based on a density functional theory/time‐dependent density functional theory (DFT/TDDFT) approach to understand the different phosphorescence efficiencies of a family of cyclometalated platinum(II) complexes: [Pt(NCN)Cl] ( 1 ; NCN=1,3‐bis(2‐pyridyl)phenyl^?), [Pt(CNN)Cl] ( 2 ; CNN=6‐phenyl‐2,2′‐bipyridyl^?), [Pt(CNC)(CNPh)] ( 3 ; CNC=2,6‐diphenylpyridyl^2?), [Pt(R‐CNN)Cl] ( 4 ; R‐CNN=3‐(6′‐(2′′‐naphthyl)‐2′‐pyridyl)isoquinolinyl^?), and [Pt(R‐CNC)(CNPh)] ( 5 ; R‐CNC=2,6‐bis(2′‐naphthyl)pyridyl^2?). By considering both the spin–orbit coupling (SOC) and the electronic structures of these complexes at their respective optimized singlet ground (S₀) and first triplet ( ) excited states, we were able to rationalize the experimental findings that 1) 1 is a strong emitter while its isomer 2 is only weakly emissive in CH₂Cl₂ solution at room temperature; 2) although the cyclometalated ligand of 3 has a higher ligand‐field strength than that of 1 , 3 is nonemissive in CH₂Cl₂ solution at 298 K; and 3) extension of π conjugation at the lateral aryl rings of the cyclometalated ligands of 2 and 3 to give 4 and 5 , respectively, leads to increased emission quantum yields under the same conditions. We found that Jahn–Teller and pseudo‐Jahn–Teller effects are operative in complexes 2 and 3 , respectively, on going from the optimized S₀ ground state to the optimized excited state, and thus lead to large excited‐state structural distortions and hence fast nonradiative decay. Furthermore, a strong‐field ligand may push the two different occupied d orbitals so far apart that the SOC effect is small and the radiative decay rate is slow. This work is an example of electronic‐structure‐driven tuning of the phosphorescence efficiency, and the DFT/TDDFT approach is demonstrated to be a versatile tool for the design of phosphorescent materials with target characteristics.     

Gas‐phase doubly charged complexes of cyclic peptides with copper in +1, +2 and +3 formal oxidation states: formation,structures and electron capture dissociation

Copper complexes with a cyclic D‐His‐β‐Ala‐L‐His‐L‐Lys and all‐L‐His‐β‐Ala‐His‐Lys peptides were generated by electrospray which were doubly charged ions that had different formal oxidation states of Cu(I), Cu(II) and Cu(III) and different protonation states of the peptide ligands. Electron capture dissociation showed no substantial differences between the D‐His and L‐His complexes. All complexes underwent peptide cross‐ring cleavages upon electron capture. The modes of ring cleavage depended on the formal oxidation state of the Cu ion and peptide protonation. Density functional theory (DFT) calculations, using the B3LYP with an effective core potential at Cu and M06‐2X functionals, identified several precursor ion structures in which the Cu ion was threecoordinated to pentacoordinated by the His and Lys side‐chain groups and the peptide amide or enolimine groups. The electronic structure of the formally Cu(III) complexes pointed to an effective Cu(I) oxidation state with the other charge residing in the peptide ligand. The relative energies of isomeric complexes of the [Cu(c‐HAHK + H)]²⁺ and [Cu(c‐HAHK ? H)]²⁺ type with closed electronic shells followed similar orders when treated by the B3LYP and M06‐2X functionals. Large differences between relative energies calculated by these methods were obtained for open‐shell complexes of the [Cu(c‐HAHK)]²⁺ type. Charge reduction resulted in lowering the coordination numbers for some Cu complexes that depended on the singlet or triplet spin state being formed. For [Cu(c‐HAHK ? H)]²⁺ complexes, solution H/D exchange involved only the N–H protons, resulting in the exchange of up to seven protons, as established by ultra‐high mass resolution measurements. Contrasting the experiments, DFT calculations found the lowest energy structures for the gas‐phase ions that were deprotonated at the peptide C_α positions. Copyright © 2012 John Wiley & Sons, Ltd.     

Dinuclear ZnII Complexes Exhibiting Thermally Activated Delayed Fluorescence and Luminescence Polymorphism

Zn^II complexes exhibiting strong emission in the solid state remain scarce, and most of them exhibit only prompt fluorescence. Herein the synthesis, structures, and photoluminescence properties of two Zn^II complexes containing new donor–acceptor ligands is reported. The new Zn^II complexes have dinuclear structures in which each metal ion adopts a distorted square-pyramidal geometry. The Zn^II complexes show strong emission in the solid state with quantum yields up to 50 %. Variable-temperature transient photoluminescence studies revealed an emission mechanism involving prompt and thermally activated delayed fluorescence (TADF). DFT calculations showed well-separated HOMO and LUMO in the ground state and small excited singlet–triplet energy splitting, accounting for the TADF. The complexes also exhibit different emission colors in the as-synthesized powder state and in single crystals, that is, they exhibit luminescence polymorphism. The single-crystal emission is responsive to mechanical grinding and was characterized by powder XRD.     

Theoretical studies of the spectroscopic properties of blue emitting iridium complexes

Electronic structures, absorptions and emissions of a series of (ppy)₂Ir(acac) derivatives (ppy = 2- phenylpyridine; acac = acetoylacetonate) with fluoro substituent on ppy ligands were investigated theoretically. The ground and excited states geometries were fully optimized at B3LYP/LANL2DZ and CIS/LANL2DZ level, respectively. The HOMO is composed of d(Ir) and π(C^∧N), while the LUMO is localized on C^∧N ligand. The absorptions and emissions in CH₂Cl₂ media were calculated under the TD–DFT level with PCM model. The lowest-lying absorption of these complexes is dominantly attributed to metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transitions and the emission of them originates from ³MLCT/³ILCT excited states. The absorption and emission of these complexes are blue-shifted by increasing the number of fluoro on phenyl, but the spectra are red-shifted by adding fluoro on pyridyl. While a single fluoro of different substituted site on phenyl results in different extent blue-shift to the spectra.     

Heat-Resistant Properties in the Phosphorescence of trans-Bis[β-(iminomethyl)aryloxy]platinum(II) Complexes: Effect of Aromaticity on d–π Conjugation Platforms

The heat-resistant properties towards thermal emission quenching of trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing 3-iminomethyl-2-naphtholato- ( 1 ), 1-iminomethyl-2-naphtholato- ( 2 ), 2-iminomethyl-1-naphtholato- ( 3 ), and 2-iminomethyl-1-phenolato ( 4 ) moieties, and a mechanistic rationale of these properties, are described in this report. Complex 1 a , with N,N′-dipentyl groups, exhibits intense red emission in 2-methyl-2,3,4,5-tetrahydrofuran (2-MeTHF) at 298 K, whereas the analogues 2 a – 4 a are less or non-emissive under the same measurement conditions. All four complexes are highly emissive at 77 K. The heat-resistant properties toward thermal emission quenching (Φ_{298 K}/Φ_{77 K}) increase in the order 1 a (0.52)> 2 a (0.09)> 3 a (0.02)>> 4 a (0.00). We investigated the emission decay and thermal-deactivation processes using density functional theory (DFT), time-dependent (TD) DFT, and double-hybrid density functional theory (DHDF) calculations of N,N′-diethyl forms 1 b – 4 b , and discuss the results with a focus on the energy levels, molecular structures, and electronic configurations in the triplet excited states. The energy differences between the triplet metal–ligand charge transfer (³MLCT) state and minimum-energy crossing point between the lowest triplet state and singlet ground state (MECP) increase in the order 1 a > 2 a , 3 a > 4 a , consistent with the experimental results for the heat-resistant properties of these complexes. The origin of the present structure dependence of the ³MLCT–MECP energy gap is ascribed to the ease or difficulty of the high-lying d_σ* orbital participating in the MECP upon thermal structural distortion. The structure dependence in energy gaps between the π* and d_σ* orbitals, which is key for facilitating the thermal deactivation process, is rationally correlated with the extent of aromaticity on the coordination platforms ( 1 b >( 2 b , 3 b )> 4 b ).     

Complexation of U(VI) with diphenyldithiophosphinic acid: spectroscopy,structure and DFT calculations

The complexation of U(VI) with diphenyldithiophosphinic acid (denoted as HL) in acetonitrile was studied by UV–Vis, FT-IR, crystallography and DFT calculations. UV–Vis absorption spectrophotometry implies that three successive complexes, UO₂L⁺, UO₂L₂, UO₂L₃^?, form in the solution. Significant ligand to metal charge transfer occurs from soft atom S to U(VI) in all the three complexes. A crystal of UO₂L₂ complex was successfully synthesized from the solution. In the crystal both the two ligands coordinate to U(VI) in bidentate form. DFT calculations confirm the formation of UO₂L₃^? complex and help illustrate the structures of all the U(VI) species in the solution.     

Optimized structures and vibration frequencies of the ether–water complex: a DFT and FTIR study

The infrared spectrum of ether was studied using Fourier transform infrared spectroscopy in conjunction with the density functional theory (DFT). The optimized structures and vibrational frequencies of the ether·(H₂O) _n (n = 1–3) complexes were obtained at B3LYP/6-31G(d) theory levels. Compared to those of free-form ether, the C–O stretching vibrational frequencies of the ether–water complexes are found to shift to red by up to 39 cm^?1 with an increase in the C–O length of 0.016 Å. Meanwhile, the frequency of the O–H stretching modes of water in the complexes appears significantly redshifted to a varying degree. The DFT calculations suggest that these shifts are caused by the hydrogen bonding between ether and water.     

Cyclic peptide nanocapsule as ion carrier for halides: a theoretical survey

In this work, the encapsulations of halide ions including F^?, Cl^?, and Br^? by cyclic peptide nanocapsule as ion carrier (F^?, Cl^?, and Br^? @(Ala4...Ala4)) were investigated using the dispersion corrected density functional theory (DFT) employing CAM-B3LYP functional and the 6–311?+?G (d, p) basis set in the gas phase. The electronic binding energy (E_bind), binding enthalpy (H_bind), and binding Gibbs free energy (G_bind) for each anion were calculated and showed that the stability order of the complexes based on their calculated E_bind is F^??>?Cl^??>?Br^? @(Ala4...Ala4). The calculated value of G_bind for F^? @(Ala4...Ala4) was ??29.77 kcal/mol showing the formation of this complex is thermodynamically favorable while the formation of Br^? @(Ala4...Ala4) is 14.35 kcal/mol which shows that the encapsulation of Br^? is not possible. The calculated value of G_bind for Cl^? @(Ala4...Ala4) was ??0.57 kcal/mol which shows that Cl^? ion can be reversibly stored inside the nanocapsule. The NBO analysis was also performed to investigate the charge transfer between two cyclic peptides in the complexes and also between the anion and the nanocapsule. The NBO analysis showed that the strongest hydrogen bonds between two cyclic peptides are in the complex.     

Structure–Property Relationships and 1O2 Photosensitisation in Sterically Encumbered Diimine PtII Acetylide Complexes

A series of sterically encumbered [Pt( L )(σ‐acetylide)₂] complexes were prepared in which L , a dendritic polyaromatic diimine ligand, was held constant ( L =1‐(2,2′‐bipyrid‐6‐yl)‐2,3,4,5‐tetrakis(4‐tert‐butylphenyl)benzene) and the cis ethynyl co‐ligands were varied. The optical properties of the complexes were tuned by changing the electronic character, extent of π conjugation and steric bulk of the ethynyl ligands. Replacing electron‐withdrawing phenyl‐CF₃ substituents ( 4 ) with electron‐donating pyrenes ( 5 ) resulted in a red shift of both the lowest‐energy absorption (ΔE=3300 cm^?1, 61 nm) and emission bands (ΔE=1930 cm^?1, 64 nm). The emission, assigned in each case as phosphorescence on the basis of the excited‐state lifetimes, switched from being ³MMLL′CT‐derived (mixed metal–ligand‐to‐ligand charge transfer) when phenyl/polyphenylene substituents ( 3 , 4 , 6 ) were present, to ligand‐centred ³ππ* when the substituents were more conjugated aromatic platforms [pyrene ( 5 ) or hexa‐peri‐hexabenzocoronene ( 7 )]. The novel Pt^II acetylide complexes 5 and 7 absorb strongly in the visible region of the electromagnetic spectrum, which along with their long triplet excited‐state lifetimes suggested they would be good candidates for use as singlet‐oxygen photosensitisers. Determined by in situ photooxidation of 1,5‐dihydroxynaphthalene (DHN), the photooxidation rate with pyrenyl‐ 5 as sensitiser (k_obs=39.3×10^?3 min^?1) was over half that of the known ¹O₂ sensitiser tetraphenylporphyrin (k_obs=78.6×10^?3 min^?1) under the same conditions. Measured ¹O₂ quantum yields of complexes 5 and 7 were half and one‐third, respectively, of that of TPP, and thus reveal an efficient triplet–triplet energy‐transfer process in both cases.