Long‐Lived Room‐Temperature Deep‐Red‐Emissive Intraligand Triplet Excited State of Naphthalimide in Cyclometalated IrIII Complexes and its Application in Triplet‐Triplet Annihilation‐Based Upconversion

Cyclometalated Ir^III complexes with acetylide ppy and bpy ligands were prepared (ppy=2‐phenylpyridine, bpy=2,2′‐bipyridine) in which naphthal ( Ir‐2 ) and naphthalimide (NI) were attached onto the ppy ( Ir‐3 ) and bpy ligands ( Ir‐4 ) through acetylide bonds. [Ir(ppy)₃] ( Ir‐1 ) was also prepared as a model complex. Room‐temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir‐3 and Ir‐4 showed strong absorption in the visible range (ε=39600 M ^?1 cm^?1 at 402 nm and ε=25100 M ^?1 cm^?1 at 404 nm, respectively), long‐lived triplet excited states (τ_T=9.30 μs and 16.45 μs) and room‐temperature red emission (λ_em=640 nm, Φ_p=1.4 % and λ_em=627 nm, Φ_p=0.3 %; cf. Ir‐1 : ε=16600 M ^?1 cm^?1 at 382 nm, τ_em=1.16 μs, Φ_p=72.6 %). Ir‐3 was strongly phosphorescent in non‐polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir‐4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non‐polar solvents. Emission of Ir‐1 and Ir‐2 was not solvent‐polarity‐dependent. The T₁ excited states of Ir‐2 , Ir‐3 , and Ir‐4 were identified as mainly intraligand triplet excited states (³IL) by their small thermally induced Stokes shifts (ΔE_s), nanosecond time‐resolved transient difference absorption spectroscopy, and spin‐density analysis. The complexes were used as triplet photosensitizers for triplet‐triplet annihilation (TTA) upconversion and quantum yields of 7.1 % and 14.4 % were observed for Ir‐2 and Ir‐3 , respectively, whereas the upconversion was negligible for Ir‐1 and Ir‐4 . These results will be useful for designing visible‐light‐harvesting transition‐metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.     

Phosphine-Manipulated p-π and π-π Synergy Enables Efficient Ultralong Organic Room-Temperature Phosphorescence

Organic room temperature phosphorescence (RTP) attracts extensive attentions, but still faces the challenge of achieving both high RTP efficiencies (η_RTP) and long lifetimes (τ_RTP), due to the intrinsic contradiction between triplet radiation and stabilization. In this work, we developed three carbazole-triphenylphosphine hybrids named xCzTPP, in which phosphine groups provide nonbonding electrons and steric hindrance to modulate intermolecular p-π and π-π interactions. With the rational orientations and spatial positions of functional groups, para-substituted pCzTPP achieves high η_RTP over 10 % and more than twofold increased τ_RTP (>600 ms), compared to ortho- and meta- isomers. Theoretical simulation and photophysical investigation indicate that the strongest intermolecular p-π and π-π electronic interplays of pCzTPP harmonize high transition probability of ³pπ state and triplet stability of ³ππ state, reflecting the p-π and π-π synergy in RTP process.     

Synthesis of main chain polymeric benzophenone photoinitiator via thiol‐ene click chemistry and Its use in free radical polymerization

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using “Thiol‐ene Click Chemistry” and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absorption properties (ε₂₉₄ = 28,300 mol^?1L^?1cm^?1) compared to the molecular initiator BP (ε₂₅₂ = 16,600 mol^?1L^?1cm^?1). The triplet energy of PBP was obtained from the phosphorescence measurement in 2‐methyl tetrahydrofurane at 77 K as 298.3 kJ/mol and according to phosphorescence lifetime, the lowest triplet state of PBP has an n‐π* nature. Triplet–triplet absorption spectrum of PBP at 550 nm following laser excitation (355 nm) were recorded and triplet lifetime of PBP was found as 250 ns. The photoinitiation efficiency of PBP was determined for the polymerization of Hexanedioldiacrylate (HDDA) with PBP and BP in the presence of a coinitiator namely, N‐methyldiethanolamine (MDEA) by Photo‐DSC. The initiation efficiency of PBP for polymerization of HDDA is much higher than for the formulation consisting of BP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

A Study of Intramolecular Energy Transfer in Conformationally Rigid molecules with stereoisomerically oriented donor and acceptor groups

The indanono naphthaleno compounds 1 , 3 , 5 and 7 exhibit, on both irradiation in the ¹L_b band (315 nm) and the n → π* transition (> 340 nm) in EPA at 77° K, neither fluorescence from naphthalene nor phosphorescence from indanone, but exclusively phosphorescence from naphthalene, and quenching in liquid solution with 1,3-pentadiene results in triplet energy transfer from the naphthaleno group only. The naphthalene phosphorescence exhibited by the ketones ( 1 , 3 , 5 , 7 ) has an enhanced quantum efficiency with respect to that on direct excitation of the corresponding hydrocarbons ( 2 , 4 , 6 , 8 ) and more strongly in the exo than in the endo orientation. In order to account for the energy wasting in the intramolecular triplet energy transfer in the endo compounds, a transfer route via a weak triplet donor-acceptor exciplex, specific to through-space interaction in the endo configuration (providing for additional radiationless T → S energy dissipation through vibrational coupling), competing with an efficient through-σ-bond exchange transfer mechanism operative in both configurations is proposed.     

Thiazole Boron Difluoride Dyes with Large Stokes Shift,Solid State Emission and Room-Temperature Phosphorescence

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Δλ, 77–101 nm) with quantum yields (ϕ_FL) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (ϕ_PL) and lifetime (τ_p up to 251 μs). The presence of intramolecular H-bonds and negligible π-π stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, π-π and C−H^…π interactions could be responsible for the observed RTP of iodine containing phosphors.     

EFFECT OF THE BROMINE ATOM UPON THE DISSOCIATION CONSTANTS OF BROMOPYRIDINES,BROMOQUINOLINES AND 4-BROMOISOQUINOLINE IN THEIR LOWEST EXCITED SINGLET AND TRIPLET STATES*

Abstract— The lowest excited singlet-state dissociation constants (pK^S_a) of bromosubstituted pyridines, quinolines, and isoquinolines were determined from the pH-dependent shifts in their electronic absorption spectra. The lowest excited triplet-state dissociation constants (pK^T_a) of bromosubstituted quinolines and 4-bromoisoquinoline were obtained from the shifts of the 0–0 phosphorescence bands measured in rigid aqueous solution at 77 K. The pK^S_a values indicate that the basicity of these brominated nitrogen heterocycles is increased in the lowest excited singlet state by 2 to 10 orders of magnitude as compared with the ground state. The pK^T_a values are found to be significantly different from the corresponding ground-state pK_a values, indicating that the basicity of bromoquinolines is increased in the lowest excited triplet state by 1.7 to 3.0 pK units. The enhancement of the excited singlet-and triplet-state basicity of brominated nitrogen heterocycle derivatives as compared with the unsuhstituted parent compounds is attributed to the increased electron-donor conjugative interactions of the bromine atom pπ orbitals with π orbitals in the lowest excited singlet and triplet state.     

Long‐Lived Room‐Temperature Near‐IR Phosphorescence of BODIPY in a Visible‐Light‐Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore

Room‐temperature long‐lived near‐IR phosphorescence of boron‐dipyrromethene (BODIPY) was observed (λ_em=770 nm, Φ_P=3.5 %, τ_P=128.4 μs). Our molecular‐design strategy is to attach Pt^II coordination centers directly onto the BODIPY π‐core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy‐atom effect of Pt^II. In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T₁ excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε=53800 M ^?1 cm^?1 at 574 nm), which is rare for Pt^II–acetylide complexes. The complex is dual emissive with ³M LCT emission at 660 nm and the ³IL emission at 770 nm. The T₁ excited state of the complex is mainly localized on the BODIPY moiety (i.e. ³IL state, as determined by steady‐state and time‐resolved spectroscopy, 77 K emission spectra, and spin‐density analysis). The strong visible‐light‐harvesting ability and long‐lived T₁ excite state of the complex were used for triplet‐triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η=ε×Φ_UC) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition‐metal complexes that show strong absorption of visible light and long‐lived ³IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.     

Polyether Natural Product Inspired Cascade Cyclizations: Autocatalysis on π-Acidic Aromatic Surfaces

Anion-π catalysis functions by stabilizing anionic transition states on aromatic π surfaces, thus providing a new approach to molecular transformation. The delocalized nature of anion–π interactions suggests that they serve best in stabilizing long-distance charge displacements. Aiming therefore for an anionic cascade reaction that is as charismatic as the steroid cyclization is for conventional cation-π biocatalysis, reported here is the anion-π-catalyzed epoxide-opening ether cyclizations of oligomers. Only on π-acidic aromatic surfaces having a positive quadrupole moment, such as hexafluorobenzene to naphthalenediimides, do these polyether cascade cyclizations proceed with exceptionally high autocatalysis (rate enhancements k_auto/k_cat >10⁴ m ⁻¹). This distinctive characteristic adds complexity to reaction mechanisms (Goldilocks-type substrate concentration dependence, entropy-centered substrate destabilization) and opens intriguing perspectives for future developments.     

Photoexcited States of UV Absorbers,Benzophenone Derivatives

The UV absorption, phosphorescence and phosphorescence‐excitation spectra of benzophenone (BP) derivatives used as organic UV absorbers have been observed in rigid solutions at 77 K. The triplet–triplet absorption spectra have been observed in acetonitrile at room temperature. The BP derivatives studied are 2,2′,4,4′‐tetrahydroxybenzophenone (BP‐2), 2‐hydroxy‐4‐methoxybenzophenone (BP‐3), 2,2′‐dihydroxy‐4,4′‐dimethoxybenzophenone (BP‐6), 5‐chloro‐2‐hydroxybenzophenone (BP‐7) and 2‐hydroxy‐4‐n‐octyloxybenzophenone (BP‐12). The energy levels and lifetimes of the lowest excited triplet (T₁) states of these BP derivatives were determined from the first peak of phosphorescence. The time‐resolved near‐IR emission spectrum of singlet oxygen generated by photosensitization with BP‐7 was observed in acetonitrile at room temperature. BP‐2, BP‐3, BP‐6 and BP‐12 show photoinduced phosphorescence enhancement in ethanol at 77 K. The possible mechanism of the observed phosphorescence enhancement is discussed. The T₁ states of 2‐hydroxy‐5‐methylbenzophenone, 4‐methoxybenzophenone and 2,4′‐dimethoxybenzophenone have been studied for comparison.     

A thioxanthone‐based visible photoinitiator

Thioxanthone‐based 9‐(2‐Morpholine‐4yl‐acetyl)‐5‐thia‐napthasen‐12‐one (TX‐MPM) was synthesized and characterized as a one‐component novel visible photoinitiator. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA) was examined in photoreactor and also daylight. Photophysical properties: fluorescence and phosphorescence emission spectra and fluorescence quantum yield of TX‐MPM (?_f = 0.29) were determined. The phosphorescence lifetime was found 131 ms for TX‐MPM and 110 ms for initiator‐attached polymer (PMMA) at 77 K, indicated a π→π* nature of the lowest triplet state. A model compound, morpholino acetonapthone was used as quencher for the triplet states of TX‐MPM and the quenching rate constant was determined (k_q = 1.26 × 10⁹ M^?1s^?1). According to laser flash photolysis studies, intermolecular hydrogen abstraction process was more dominant path to the formation of the initiating radicals. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Phosphorescence at Low Temperature by External Heavy-Atom Effect in Zinc(II) Clusters

Luminescent Zn^II clusters [Zn₄L₄(μ₃-OMe)₂X₂] (X=SCN ( 1 ), Cl ( 2 ), Br ( 3 )) and [Zn₇L₆(μ₃-OMe)₂(μ₃-OH)₄]Y₂ (Y=I⁻ ( 4 ), ClO₄⁻ ( 5 )), HL=methyl-3-methoxysalicylate, exhibiting blue fluorescence at room temperature (λ_max=416≈429 nm, Φ_em=0.09–0.36) have been synthesised and investigated in detail. In one case the external heavy-atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (λ_max=520 nm, τ=95.3 ms) at 77 K. Single-crystal X-ray structural analysis and time-dependent density-functional theory (TD-DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand-centred (¹LC) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH−I interactions.     

The phosphorescence of benzene obtained byab initio and semi-empirical calculations

Summary Radiative decay and phosphorescence of triplet stare benzene is doubly -orbital and spin- forbidden and is only activated through vibronic coupling among the manifold of triplet states. For this reason the determination of lifetime and transition moments for the decay of triplet benzene has posed a considerable challenge to both theory and experiment. In the present work we have addressed the triplet benzene problem at several levels of theory; by truncated perturbation theory and semiempirical, CNDO/S-CI, calculations; by complete sum-over-state calculations as implemented in recentab initio multiconfiguration quadratic response (MCQR) theory; and by direct MCQR calculations of vibronic phosphorescence. The vibronic coupling is in the two former cases treated by the Herzberg-Teller (H-T) perturbation theory, involving four main mechanisms for the phosphorescent decay of triplet benzene. The results and interpretations given by these approaches as well as their merits and limitations are presented and discussed in some detail. Our calculations indicate that the phosphorescent decay of the³ B _1u state takes place predominantly through vibronic coupling along thee _2g mode. We obtain a phosphorescence that is almost completely out-of-plane polarized, which is in line with more recent measurements by the microwave-induced delayed phosphorescence technique, and could reproduce quite well the intensity ratios for different vibronic bands obtained in that experiment. The final triplet state lifetime is the result of a delicate sum of contributions from several vibronic degenerate and non-degenerate modes. The direct vibronic phosphorescence calculations predict a long lifetime, about one minute — 68 seconds for the best wavefunction — and seem to focus on a doubling of the assumed, albeit not established, best experimental value for the radiative lifetime of triplet benzene; 30 seconds.Dedicated to Inga Fischer-Hjalmars on her 75th birthday     

The strength and selectivity of perfluorinated nano-hoops and buckybowls for anion binding and the nature of anion-π interactions

Perfluorinated cycloparaphenylenes (F-[n]CPP, n = 5–8), boron nitride nanohoop (F-[5]BNNH), and buckybowls (F-BBs) were proposed as anion receptors via anion-π interactions with halide anions (Cl⁻, Br⁻ and I⁻), and remarkable binding strengths up to −294.8 kJ/mol were computationally verified. The energy decomposition approach based on the block-localized wavefunction method, which combines the computational efficiency of molecular orbital theory and the chemical intuition of ab initio valence bond theory, was applied to the above anion-π complexes, in order to elucidate the nature and selectivity of these interactions. The overall attraction is mainly governed by the frozen energy component, in which the electrostatic interaction is included. Remarkable binding strengths with F-[n]CPPs can be attributed to the accumulated anion-π interactions between the anion and each conjugated ring on the hoop, while for F-BBs, additional stability results from the curved frameworks, which distribute electron densities unequally on π-faces. Interestingly, the strongest host was proved to be the F-[5]BNNH, which exhibits the most significant anisotropy of the electrostatic potential surface due to the difference in the electronegativities of nitrogen and boron. The selectivity of each host for anions was explored and the importance of the often-overlooked Pauli exchange repulsion was illustrated. Chloride anion turns out to be the most favorable anion for all receptors, due to the smallest ionic radius and the weakest destabilizing Pauli exchange repulsion.     

PHOSPHORESCENCE QUENCHING OF POLY rA IN GLUCOSE-SODIUM ACETATE SOLUTION AT 77 K*

Abstract. The phosphorescence of poly rA at 77 K in an aqueous medium containing glucose is half-quenched by bound ethidium bromide at a fractional concentration, r_1/2, of 0.005, and by bound proflavine at r_1/2= 0.002: r_1/2= 0.024 for Co²⁺ and r_1/2= 0.039 for Mn²⁺. The decay of the dye-quenched poly rA phosphorescence is markedly nonexponential and decays more rapidly with increasing dye concentration, while the decay of the metal-ion quenched poly rA phosphorescence parallels that of the unquenched poly rA, independent of metal-ion concentration. Förster overlap integrals and critical distances for transfer of both poly rA singlet and triplet excitation to dye singlet states are calculated, and used to explain a consistent interpretation of the experimental results in terms of one-step direct excitation transfer from base to dye or metal ion in a highly folded polymer conformation.     

Anion-(π)n-π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)_n-π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)_n-π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)_n-π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.     

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H₂P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S₀) and low-lying triplet (T₁) excited states of ππ* type. The S₀–T₁ transition probability and radiative lifetime of phosphorescence (τ_p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence (τ_p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ_p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H₂P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b_2g mode at 430 cm⁻¹, while the 679 and 715 cm⁻¹ vibronic bands of b_3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S₁ → T₁ intersystem crossing. Among the in-plane vibrations the a_g mode at 1614 cm⁻¹ is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.     

Efficient structural modification of electron-withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF₃, and M1–CN) with electron-withdrawing substituents (–F, –CF₃, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time-dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the ³MLCT/π → π* to triplet metal-centered ³MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (k_nr) decreased in the order M1 > M1–CF₃ > M1–F > M1–CN. The <T₁|H_SOC|S_m> and kr values of M1-F are similar with those of M1, however the Knr rate ofM1-F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′-N,N′-dicarbazole-biphenyl to these complexes. In view of the electroluminescent applications in organic light-emitting diodes, M1–F can serve as efficient deep-red guest materials with improved electron injection and transport ability.     

Pyrene sensitized phosphorescence enhanced by the heavey atom effect in the water-heptane-sodium dodecyl sulfate-pentanol microemulsion

The influence of the external TI⁺ “heavy” ion on the singlet and triplet excited states of the pyrene and Trypaflavine molecules solubilized in the water-heptane-sodium dodecyl sulfate-pentanol microemulsion was studied. The triplet-triplet (T-T) energy transfer in the system Trypaflavine (donor)-pyrene (acceptor) and the sensitized phosphorescence of pyrene were first observed in the microemulsion containg Tl⁺ ions. The quenching rate constants of the excited states of the luminophores by Tl⁺ ions were calculated. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1529–1532, September, 2000.     

Stable Expanded Porphycene‐Based Diradicaloid and Tetraradicaloid

The synthesis of a bithiophene‐bridged 34π conjugated aromatic expanded porphycene 1 and a cyclopentabithiophene bridged 32π conjugated anti‐aromatic expanded porphycene 2 by a McMurry coupling strategy is presented. Magnetic measurements and theoretical calculations reveal that both 1 and 2 exhibit an open‐shell singlet ground state with significant radical character (y₀=0.63 for 1 ; y₀=0.68, y₁=0.18 for 2 ; y₀: diradical character, y₁: tetraradical character) and a small singlet–triplet energy gap (ΔE_S‐T=?3.25 kcal mol^?1 for 1 and ΔE_S‐T=?0.92 kcal mol^?1 for 2 ). Despite the open‐shell radical character, both compounds display exceptional stability under ambient air and light conditions owing to effective delocalization of unpaired electrons in the extended cyclic π‐conjugation pathway.     

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.