Photophysical and electrochemical properties of 1,3-bis(2-pyridylimino)isoindolate platinum(II) derivatives

A series of twelve platinum(II) complexes of the form (N^N^N)PtX have been synthesized and characterized where N^N^N is 1,3-bis(2-pyridylimino)isoindolate ligands (BPI) or BPI ligands whose aryl moieties are substituted with tert-butyl, nitro, alkoxy, iodo or chloro groups, and X is a chloride, fluoride, cyano, acetate, phenyl or 4-(dimethylamino)phenyl ligand. All complexes display at least one irreversible oxidation and two reversible reduction waves at potentials dependent on the position and the electron donating or withdrawing nature of both X and the substituted N^N^N ligand. Broad room temperature phosphorescence ranging in energy from 594 to 680 nm was observed from the complexes, with quantum efficiencies ranging from 0.01 to 0.05. The efficiency of emission is dictated largely by nonradiative processes since the rate constants for nonradiative deactivation [(1.1-100) × 10(5) s(-1)] show greater variation than those for radiative decay [(0.57-4.0) × 0(4) s(-1)]. Nonradiative deactivation for compounds with X = Cl follow the energy gap law, i.e. the nonradiative rate constants increase exponentially with decreasing emission energy. Deactivation of the excited state appears to be strongly influenced by a non-planar distortion of the BPI ligand.     

Photophysical and electrochemical properties of Ru(II) complexes containing tridentate bisphosphino-oligothiophene ligands

Nine Ru(II) complexes containing the conjugated oligothiophene ligands 3,3'-bis(diphenylphosphino)-2,2':5',2'-terthiophene (P(2)T(3)) and 4',3'-bis(diphenylphosphino)-3,3'-dihexyl- 2,2':5',2':5',2':5',2'-pentathiophene (P(2)T(5)) were prepared and characterized. P(2)T(3) and P(2)T(5) bond as tridentate ligands and three of the complexes (1, 2 and 5) form green five-coordinate Ru(II) complexes in solution. Cyclic voltammetry, variable temperature UV-vis spectroscopy and time-resolved transient absorption spectroscopy were used to characterize the electronic properties of the complexes. Increased conjugation in the complexes containing the P(2)T(5) ligand resulted in a lowering of the oxidation potential of the oligothiophene, but electropolymerization was not observed. The electronic spectra were dominated by π-π* transitions. All of the complexes were non-emissive both at room temperature and low temperature, indicating the excited state decays by other, non-radiative pathways. The transient absorption spectrum of complex 7 shows a species with a band at 475 nm and a lifetime of ～100 ns, assigned to a ligand-based triplet state.     

Triamine-type complexes of platinum(II) and their antitumor properties

Triamines of platinum(II) (Pt(NH₃)₂LCl]Cl with a cis-structure (L represents cytosine, cytidine, isocytosine, theobromine, theophylline) and cis- and trans-isomers with theophylline and inosine were synthesized. The coordination formulas were demonstrated by methods of conductometry and long-wave spectroscopy. The coordination of pyrimidines through the N(3) atoms and purines through the N(7) atoms was established by PMR methods. An intensification of the acidic properties of the NH group of the ligands upon coordination with platinum was demonstrated. Cis-triamines have an antitumor effect on solid tumors and leukemias, which permits them to be assigned to a new class of water-soluble cationic monofunctional antitumor complexes of platinum(II).Leningrad Chemicopharmacological Institute. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, No. 3, pp. 354–361, May–June, 1991. Original article submitted March 11, 1991.     

Photophysical and anion sensing properties of platinum(II) terpyridyl complexes with phenolic ethynyl ligands

A series of platinum(ii) terpyridyl complexes with phenolic ethynyl ligands have been synthesized and characterized. Their photophysical and sensing properties towards anions such as fluoride, acetate and dihydrogenphosphate have been investigated. These complexes show a colorimetric response and fluorescence quenching in the presence of anions including fluoride, acetate and dihydrogenphosphate, and selective sensing towards fluoride in some cases. The sensing mechanism has been investigated by spectrophotometric and (1)H NMR titration.     

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.     

Cyclometalated platinum(II) complexes bearing o-phenylenediamine derivatives: Synthesis and electrochemical behavior

New cyclometalated Pt(II) complexes bearing 1,2-phenylenediamine (pda) derivatives were synthesized and their chemical and electrochemical properties were investigated. Two Pt complexes, [Pt(bzqn) (pda)](PF₆) (bzqn = benzo[h]quinoline, [1](PF₆): pda = 1,2-phenylenediamine, [2](PF₆): pda = 4,5-dimethyl-1,2-phenylenediamine), were synthesized by the reaction of (Bu₄N)[PtCl₂(bzqn)] with corresponding pda derivatives. The Pt-pda complexes were converted to Pt-bda (bda = 1,2-benzenediamide) complexes by treatment of 2 mol equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and they showed two-step reversible redox couples in cyclic voltammetry. The cyclometalated ligand gave a strong electronic effect to the bda ligand to take place the negative shift of the bda-based redox potentials.     

Photophysical and electrochemical properties of heteroleptic tris-cyclometalated iridium(III) complexes

Mixed (difluoro)phenylpyridine/(difluoro)phenylpyrazole tris-cyclometalated iridium complexes were prepared in order to study the effect of fluorination and the pyridine/pyrazole ratio on the emission and electrochemical properties. Increasing fluorination and replacement of pyridine by pyrazole both leads to a widening of the HOMO-LUMO gap and generally leads to a blue shift in emission.     

Photophysical and electrochemical properties of heteroleptic tris-cyclometallated Ir(III) complexes

Some new heteroleptic tris-cyclometallated iridium(III) complexes have been synthesized and fully characterized. Among these iridium(III) complexes, bis(1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (3) and bis(3-methyl-1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (4) show excellent quantum yields at room temperature, the electron density being perturbed by introducing the pyridyltetrazole ligand, making k_r > k_nr. This destroys the concept of phenylpyrazole based iridium complexes.     

Cyclometalated platinum(II) diimine complexes: synthetically tuning the photophysical and electrochemical properties

Presented is the synthesis of an array of 16 heteroleptic phosphorescent diimine complexes of platinum(II) with electronically diverse ligand spheres and their full spectroscopic, photophysical, and electrochemical characterization. The complexes were found to exhibit luminescence (480-500 nm) in deaerated solutions at room temperature from a long-lived (3)LC state (τ = 2-3 μs) that exhibits significant metal character perturbed by a low-lying (1)MLCT state. Interestingly, emission from a (3)MLCT state was not observed as is the case with many other polypyridine-based d-block complexes. Electrochemical intermediates proved stable as multiple reversible reductions between -1 and -2 V vs. SCE were noted during cyclic voltammetry experiments unveiling the potential of these luminophores for use in a variety of optoelectronic and solar energy conversion applications.     

Acidities of platinum(II) mu-hydroxo complexes bearing diphosphine ligands

The pK(a) values in DMSO of the monoprotic complexes [(L(2)Pt)(2)(mu-OH)(mu-NMePh)](2+) (4) (L(2) = Ph(2)PCH(2)CH(2)PPh(2) (dppe), Ph(2)PCMe(2)PPh(2) (dppip)) are 11.9 +/- 0.1 (L(2) = dppe) and 13.5 +/- 0.2 (L(2) = dppip) as determined by (31)P NMR equilibrium titration with bases of known pK(a). Complexes 4 were prepared by treatment of [L(2)Pt(mu-OH)](2)(2+) (1) with N-methylaniline. The oxo complexes [(L(2)Pt)(2)(mu-O)(mu-NMePh)](+), formed in the equilibrium titration reactions, were independently synthesized in THF by deprotonation of [(L(2)Pt)(2)(mu-OH)(mu-NMePh)](2+) with NaN(SiMe(3))(2) and characterized as NaBF(4) adducts. Similar experiments with diprotic [L(2)Pt(mu-OH)](2)(2+) (L(2) = dppe, Ph(2)PCH(2)CH(2)CH(2)PPh(2) (dppp)) were complicated by exchange processes and were less conclusive, giving pK(a1) < 18 and pK(a2) > 18 in DMSO.     

Photophysical and electrochemical properties of chlorophyll a-cyclodextrins complexes

In this work, we have studied the interactions between two different cyclodextrins (CDs) and chlorophyll a (Chl a) in the presence of electrolyte by means of absorption, fluorescence spectroscopy, circular dichroism and cyclic voltammetry. The results obtained indicate that the presence of both CDs gives rise to an increase of Chl a solubility in water. In particular, heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin (TRIMEB) favours the dissolution of Chl a monomer in aqueous solution, whereas the presence of hydroxypropyl-beta-cyclodextrin (beta-HP-CD) promotes the pigment aggregation.     

Photophysical, electrochemical and anion sensing properties of Ru(II) bipyridine complexes with 2,2'-biimidazole-like ligand

A new anion sensor [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3) (bpy is 2, 2'-bipyridine and DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) has been developed. Its photophysical, electrochemical and anion sensing properties are compared with two previously investigated systems, [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1) and [Ru(bpy)(2)(BbimH(2))](PF(6))(2) (2) (BiimH(2) is 2,2'-biimidazole and BbimH(2) is 2,2'-bibenzimidazole). The high acidity of the N-H fragments in these complexes make them easy to be deprotonated by strong basic anions such as F(-) and OAc(-), and they form N-H···X hydrogen bonds with weak basic anions like Cl(-), Br(-), I(-), NO(3)(-), and HSO(4)(-). Complex 3 displays strong hydrogen bonding with these 5 weak basic anions, with binding constants between 17,000 and 21,000, which are larger than those observed in complex 1, with binding constants between 3300 and 5700, and in complex 2, which shows no hydrogen bonding toward Cl(-), Br(-), I(-), and NO(3)(-), and forms considerable hydrogen bonds with HSO(4)(-) with a binding constant of 11,209. These hydrogen bonding behaviours give different NMR, emission and electrochemical responses. The different anion binding affinity of these complexes may be mainly attributed to their different pK(a1) values, 7.2 for 1, 5.7 for 2, and 6.2 for 3. The additional methyl groups at the 7 and 7' positions of complex 3 may also play an important role in the enhancement of anion binding strength.     

Green photoluminescence from platinum(II) complexes bearing silylacetylide ligands

The synthesis, structural characterization, photoluminescence properties, and density functional theory analysis of three Pt(II) diimine complexes, Pt(dbbpy)(C triple bond CR)2 [dbbpy = 4,4'-di(tert-butyl-2,2'-bipyridine; R = -SiMe3, -CC-SiMe3, or -t-Bu], are presented. The Pt(dbbpy)(C triple bond C-tBu)2 complex serves as a carbon-based ligand structure for which the photophysical properties of the two silicon-bearing complexes are compared in dichloromethane. Pt(dbbpy)(C triple bond C-SiMe3)2 and Pt(dbbpy)(C triple bond C-C triple bond C-SiMe3)2 display visible absorptions with strong green emission (lambda(emmax) = 526 and 524 nm, respectively) while Pt(dbbpy)(C triple bond C-t-Bu)2 displays efficient, long-lived yellow emission (lambda(emmax) = 557 nm). Direct side by side comparisons of Pt(dbbpy)(C triple bond C-SiMe3)2 and Pt(dbbpy)(C triple bond C-t-Bu)2 suggest that the difference in excited state energy results from the relative sigma-donor strength of the acetylide ligands.     

Synthesis of pendant-type anthraquinone-bridged cofacial dinuclear platinum(II) complexes and their emission properties

Anthraquinone-bridged mononuclear and dinuclear complexes, [PtCl(AQ-amide-tpy)](PF6) (1), [Pt2Cl2(AQ-amide-tpy2)](PF6)2 (2), and [Pt2Cl2(AQ-eth-tpy2)](PF6)2 (3), were synthesized and their photochemical properties were investigated. Amide-bound mononuclear complex 1 exhibited only metal-to-ligand charge transfer (MLCT) absorption and emission, whereas dinuclear complex 2 exhibited a low-energy emission around 700 nm at room temperature. Emission lifetime analysis indicated that this emission was originated from the metal-metal-to-ligand charge transfer (MMLCT) excited state, implying the existence of an intramolecular Pt-Pt interaction at the photoexcited state. 3 with rigid ethynylene linkers showed a low-energy absorption around 520 nm (epsilon = approximately 1100 M(-1) cm(-1)) in addition to an 1MLCT absorption, which was ascribed to a 3MLCT absorption from the consideration of the Pt-Pt distance on a geometry-optimized structure. The emission of 3 appeared at 600 nm, which is higher in energy compared with the emission of 2. It is postulated that the restriction of the Pt-Pt distance flexibility in the rigid structure of 3 prevents the significant increase of the Pt-Pt interaction at the excited state.     

Photophysical properties of platinum(II)-acetylide complexes: the effect of a strongly electron-accepting diimine ligand on excited-state structure

The compounds [Pt(MesBIAN)(C[triple bond]CR)2] (R = C6H4-CN-p, 1; SiMe3, 2; C6H4-CF3-p, 3; C6H5, 4; C6H4-CH3-p 5) {MesBIAN = bis(mesitylimino)acenaphthene} have been synthesized; the X-ray crystal structure determinations of 4 and 5 and the starting material [Pt(MesBIAN)Cl2] are reported. Chemical oxidation of 4 with diiodine leads to generation of an intermediate platinum(IV) bis(acetylide) diiodide complex, which then couples and reductively eliminates the acetylide ligands as a diyne, leading to the generation of [Pt(MesBIAN)I2] 6. Compound 2 readily forms an adduct 2a with copper(I) chloride, in which the copper atom is bonded to the two acetylide triple bonds. 1-5 each undergo an irreversible oxidation, and a reversible one-electron reduction to generate a stable anion. ESR studies of 1(-)-5(-) show that the unpaired electron is localized mainly on the pi* orbital of the coordinated MesBIAN ligand, with about 10% platinum contribution to the singly occupied molecular orbital (SOMO). The compounds show a strong absorption at around 500 nm in the UV/visible spectrum, which is assigned to a "mixed metal-ligand to ligand charge transfer" (MMLL'CT) transition; this assignment is supported by time-dependent density-functional theory (TD-DFT) calculations on 5. 1-5 emit in the near-infrared region from a (3)MMLL'CT excited state, with lifetimes ranging from 8 to 36 ns. Picosecond and nanosecond time-resolved infrared (TRIR) spectroscopy has been used to probe directly the nature and dynamics of the excited state of 5. The TRIR data show a decrease of the energy of the C[triple bond]C vibration upon excitation, by about 90 cm(-1) in comparison to the ground state, and formation of a new, very intense, and very broad band at 1820 cm(-1). We propose that the excited-state structure contains some contribution from a pseudo-cumulenic form of the platinum-acetylide moiety, which is supported by TD-DFT calculations. Picosecond TRIR allowed determination of the rate of vibrational relaxation (14 ps) of the vibrationally "hot" electronic excited state of 5 formed upon initial laser excitation.     

Synthesis of lipophilic triazacycloalkanes and their platinum(II) complexes

A series of cyclic lipophilic polyamines: 1,4,7-triethyl-1,4,7-triazacyclodecane, 1,4,7-triethyl-1,4,7-triazacycloundecane, 1,4,7-trimethyl-1,4,7-triazacycloundecane, and 1,4,8-triisopropyl-1,4,8-triazacycloundecane, as well as their complexes with dimethylplatinum(II) were synthesized for the first time. Features of complex formation of triazacyclanes with platinum and their manifestation in NMR spectra were discussed.     

Photophysical and (photo)electrochemical properties of a coumarin dye

A new coumarin dye, cyano-{5,5-dimethyl-3-[2-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-1H,4H,10H-11-oxa-3a-aza-benzo[de]anthracen-9-yl)vinyl]cyclohex-2-enylidene}-acetic acid (NKX-2753), was prepared and characterized with respect to photophysical and electrochemical properties. It was employed as a dye sensitizer in dye-sensitized solar cells and showed efficient photon-to-electron conversion properties. The photocurrent action spectrum exhibited a broad feature with a maximum incident photon-to-electron conversion efficiency (IPCE) of 84% at 540 nm, which is comparable to that for the famous red dye RuL2(NCS)2 (known as N3), where L stands for 2,2'-bipyridyl-4,4'-dicarboxylic acid. The sandwich-type solar cell with NKX-2753, under illumination of full sun (AM1.5, 100 mW cm(-2)), produced 16.1 mA cm(-2) of short-circuit photocurrent, 0.60 V of open-circuit photovoltage, and 0.69 of fill factor, corresponding to 6.7% of overall energy conversion efficiency using 0.1 M LiI, 0.05 M I2, 0.1 M guanidinium thiocyanate, and 0.6 M 1,2-dimethyl-3-n-propyl-imidazolium iodide in dry acetonitrile as redox electrolyte. In comparison with its analogue NKX-2586 (Langmuir 2004, 20, 4205), NKX-2753 with an extra side ring on the alkene chain produced much higher IPCE values at the same conditions. The side ring acted as a spacer to efficiently prevent dye aggregation when adsorbed on the TiO2 surface, resulting in significant improvements of short-circuit photocurrent, open-circuit photovoltage, and fill factor compared with NKX-2586 that aggregated on the TiO2 surface.     

Phosphine and phosphonite complexes of a Ru(II) porphyrin. 2. Photophysical and electrochemical studies

The photophysical and electrochemical properties of a series of mono- and bis-phosphine complexes of a 5,15-diphenyl-substituted ruthenium porphyrin, (MeOH)Ru(II)(CO)(DPP) 1, were investigated. The ligands used were diphenyl(phenylacetenyl)phosphine (DPAP), diethyl (phenylacetenyl)phosphonite [PAP(OEt)(2)], tris(phenylacetenyl)phosphine [(PA)(3)P], and bis(diphenylphosphino)acetylene (DPPA). All complexes display two reversible one-electron oxidations at: 0.61 and 1.0 V vs SCE (1), 0.42-0.51 and 0.97-1.05 V [(PR(3))Ru(II)(CO)(DPP)], and 0.06-0.25 and 0.82-0.95 V [(PR(3))(2)Ru(II)(DPP)]. As predicted by EHMO calculations, the first oxidation is porphyrin or phosphorus centered, whereas the second one is ruthenium centered. Bulk electrolysis at the first oxidation potential yields stable monocations. Simulation of the cyclic voltammogram of (DPAP)Ru(II)(CO)(DPP) in CH(2)Cl(2) demonstrates the kinetic lability of the complex, and the association constant found (K = 1.27 x 10(6) M(-1)) is in accordance with the value determined by UV-vis titration (K = 1.2 +/- 0.3 x 10(6) M(-1)). Coordination of one phosphine ligand to Ru(II)(CO)(DPP) leads to a red shift in both the absorption and luminescence spectra. Shifts are typically 10 nm for the B- and Q-band absorptions and are not affected by the nature of the phosphorus ligand. The intense luminescence of (PR(3))Ru(II)(CO)(DPP), red-shifted by 21-28 nm compared to 1, can be attributed to originate from a (3)(pi,pi) excited state, and it exhibits lifetimes from 150 to 240 micros. In the bis-phosphine complexes (PR(3))(2)Ru(II)(DPP), the Q-band absorption is broadened and does not show any distinct peak. Judged from EHMO calculation, this could arise from a low-energy charge-transfer state involving the phosphorus ligand. The luminescence is efficiently quenched due to radiationless decay from a charge-transfer excited state, involving either the metal center or the phosphorus ligand; an unambiguous assignment could not be made.     

Photophysical and electrochemical properties of meso-substituted thien-2-yl Zn(II) porphyrins

The influence of the thiophene ring on the ground and excited state properties of the porphyrin ring is investigated, when substituted at the meso-position. A series of mono-, di-, tri-, and tetra- meso-thien-2-yl porphyrins are studied and discussed with respect to the reference compounds zinc(II)-5,10,15,20-tetra(thien-2'-yl)porphyrin ( 1a) and zinc(II)-5,10,15,20-tetraphenylporphyrin (ZnTPP). The extended conjugated system zinc(II)-5-(5'-(5'-ethynyl-2'-thiophenecarboxaldehyde)thien-2'-yl)-10,15,20-triphenylporphyrin ( 4d) is also studied and shows enhanced charge transfer character due to the presence of the terminal aldehyde accepting group. A detailed analysis of ground and excited state UV-vis absorption, steady-state and time-resolved fluorescence, laser flash photolysis, and electrochemical data all point toward substantial electronic communication between the central Zn(II) porphyrin ring and the meso-thien-2-yl substituents, which is evident from excited state charge transfer character.