Ligand Charge Transfer Excited State Research Articles

Aggregation effect on the luminescence properties of phenylbipyridine Pt(II) acetylide complexes. A theoretical prediction with experimental evidence.

We report a combined theoretical and experimental study of both the structural and optical properties of phosphorescent cyclometalated square-planar (phenylbipyridyl)platinum(II) acetylide complexes, namely (Pt(tBu2-ĈN̂N)(C≡C-Ph)] and (Pt(hex2-ĈN̂N)(C≡C-thienyl)] that exhibit, at high concentrations, an additional emission band at longer wavelength. The geometry optimizations of both the ground and the lowest triplet excited states of the considered monomers and different possible dimers have been performed in solution using several density functional theory (DFT) functionals corrected for dispersion effects. For the dimers, which are shown to exhibit a head-to-tail configuration, a significant shortening of the Pt···Pt distance, compared to that in the ground state, is observed in the first triplet state. Moreover, we show that trimeric species are highly improbable in solution. The UV-visible absorption spectra of the complexes are well rationalized using a vertical time-dependent DFT (TD-DFT) protocol relying on a global hybrid exchange-correlation functional. Finally, the new emission band at high concentration of the complexes can be assigned to a metal-metal to ligand charge transfer excited state ((3)MMLCT).

Influence of Ligand Substitution on Excited State Structural Dynamics in Cu(I) Bisphenanthroline Complexes

This study explores the influences of steric hindrance and excited state solvent ligation on the excited state dynamics of Cu(I) diimine complexes. Ultrafast excited state dynamics of Cu(I)bis(3,8-di(ethynyltrityl)-1,10-phenanthroline) [Cu(I)(detp)(2)](+) are measured using femtosecond transient absorption spectroscopy. The steady state electronic absorption spectra and measured lifetimes are compared to those of Cu(I)bis(1,10-phenanthroline), [Cu(I)(phen)(2)](+), and Cu(I)bis(2-9-dimethyl-1,10-phenanthroline), [Cu(I)(dmp)(2)](+), model complexes to determine the influence of different substitution patterns of the phenanthroline ligand on the structural dynamics associated with the metal to ligand charge transfer excited states. Similarities between the [Cu(I)(detp)(2)](+) and [Cu(I)(phen)(2)](+) excited state lifetimes were observed in both coordinating and noncoordinating solvents and attributed to the lack of steric hindrance from substitution at the 2- and 9-positions. The solution-phase X-ray absorption spectra of [Cu(I)(detp)(2)](+), [Cu(I)(phen)(2)](+), and [Cu(I)(dmp)(2)](+) are reported along with finite difference method calculations that are used to determine the degree of ground state dihedral angle distortion in solution and to account for the pre-edge features observed in the XANES region.

Column: Photochemical Reactions and Photophysical Processes

Recently, interest has been aroused in the synthesis and study of optical and electronic properties of π-conjugated oligomers and polymers containing transition metal complexes. The introduction of transition metal chromophores exhibiting metal to ligand charge transfer excited state (MLCT) in the π-conjugated polymer opens new possibilities in variations of electronic and optical properties. The new materials might find wide applications such as polymeric light emitting diodes (PLED), solar energy conversion devices, nonlinear optical materials (NLOs), in materials exhibiting photorefractions, in chemical sensing, electrochromism, and electrocatalysis. The reason for this increased interest is due to the fact that the electronic and optic

Preparation and photophysical properties of precursors of inorganic macromolecules. Mono and binuclear complexes of Ru(II) and terpyridine derivatized with thiophene and 4 ′-(5-bromothiophene) groups

The preparation and characterization of mono and binuclear complexes of Ru(II) with a newly synthesized derivate of the terpyridine ligand, 4 ′-(5-bromothiophene)-2,2 ′,6 ′,2″-terpyridine, are communicated. In the binuclear complex, 2,5-bis(2,2 ′,6 ′,2″-terpyridine-4 ′yl)thiophene was used as a bridge between two Ru(II) centers. The new compounds were characterized by H NMR, UV–Vis and IR spectroscopies. Bands at ∼500 nm for the Ru(II) to terpyridine charge transfer transition and absorption bands at λ<400 nm assigned to intraligand transitions, π *←π, centered in the tpy moiety were observed in the UV–Vis spectra of the complexes. Irradiation of the complexes in CH 3CN at 337 or 500 nm induced luminescence with maxima at ∼670 nm and lifetimes τ⩽10 2 ns. Time-resolved absorption spectroscopy revealed the formation of long-lived species during the decay of the metal to ligand charge transfer excited states. The intermediates were tentatively assigned as unstable products of ligand-substitution or orthometalation excited state reactions.

Preparation and photophysical properties of precursors of inorganic macromolecules. Mono and binuclear complexes of Ru (II) and terpyridine derivatized with thiophene and 4$prime;-(5-bromothiophene) groups

Mono and binuclear complexes of Ru(II) with a newly synthesized derivate of the terpyridine ligands, 4 ′ -(5-bromothiophene)-2,2 ′ ,6 ′ ,2″-terpyridine, (I), and 2,5-bis(2,2 ′ ,6 ′ ,2″-terpyridine-4 ′ yl)thiophene, (II), have been prepared and characterized via UV–Vis, NMR and IR spectroscopies. Photophysical and photochemical processes of the complexes were also investigated. The preparation and characterization of mono and binuclear complexes of Ru(II) with a newly synthesized derivate of the terpyridine ligand, 4 ′ -(5-bromothiophene)-2,2 ′ ,6 ′ ,2″-terpyridine, are communicated. In the binuclear complex, 2,5-bis(2,2 ′ ,6 ′ ,2″-terpyridine-4 ′ yl)thiophene was used as a bridge between two Ru(II) centers. The new compounds were characterized by H NMR, UV–Vis and IR spectroscopies. Bands at ∼500 nm for the Ru(II) to terpyridine charge transfer transition and absorption bands at λ <400 nm assigned to intraligand transitions, π * ←π, centered in the tpy moiety were observed in the UV–Vis spectra of the complexes. Irradiation of the complexes in CH 3 CN at 337 or 500 nm induced luminescence with maxima at ∼670 nm and lifetimes τ ⩽10 2 ns. Time-resolved absorption spectroscopy revealed the formation of long-lived species during the decay of the metal to ligand charge transfer excited states. The intermediates were tentatively assigned as unstable products of ligand-substitution or orthometalation excited state reactions.

A Tridentate-Bridged Ruthenium−Rhodium Complex as a Stereochemically Defined Light-Absorber−Electron-Acceptor Dyad

The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) (tpy = 2,2',6',2"-terpyridine and tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) has been prepared and its spectroscopic, electrochemical, and photophysical properties investigated. This complex couples a ruthenium light absorber to a rhodium electron acceptor to create the first tpp-bridged light-absorber-electron-acceptor dyad. This study illustrates the applicability of this (tpy)Ru(II)(&mgr;-tpp) chromophore in the construction of photochemical molecular devices. This system is of interest since the tpp ligand has been shown to provide stereochemically defined polymetallic complexes with reasonably long-lived metal to ligand charge transfer excited states. The complex [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) displays a Ru-->tpp CT transition centered at 516 nm that is the lowest lying electronic transition. The electrochemistry of [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) shows a Ru(II/III) couple at 1.60 V vs Ag/AgCl, an irreversible Rh(III/I) reduction at -0.23 V and, a tpp(0/)(-) couple at -0.60 V. This illustrates that although this complex has a lowest lying spin-allowed spectroscopic transition that is Ru-->tpp CT in nature, the lowest occupied molecular orbital is Rh based. Thus, following excitation of this [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) complex into the Ru-->tpp CT state, electron transfer to the rhodium is thermodyamically favorable. This electron transfer leads to a quenching of the emission normally observed for this Ru-->tpp CT excited state. Emission quenching for [(tpy)Ru(tpp)RhCl(3)](PF(6))(2) via electron transfer is 80% efficient with a k(et) of 4 x 10(7) s(-)(1). Details of these studies are presented herein.

Synthesis and Photophysical Properties of Some Conjugated Polymers Functionalized with Ruthenium Polypyridine Complexes

AbstractA series of conjugated polymers functionalized with different ruthenium polypyridine metal complexes were synthesized by the palladium catalyzed reaction. Two conjugated polymer systems have been studied: 1. poly(phenylenevinylene) with bis(2,2':6',2”-terpyridine) ruthenium (II) on the mainchain and 2. quinoxaline based polymers with tris(2,2'-bipyridine) ruthenium (II). The ruthenium polypyridine complexes exhibit a long-lived metal to ligand charge transfer excited state which can enhance the photosensitivity of the resulting polymers. Different physical properties such as the photoconductivity and charge mobility in these polymers are also studied.

The photochemistry of mixed-ligand complexes (ArS)2ZnPhen with the lowest ligand—ligand charge transfer excited states

Abstract Flash photolysis and spin-trapping experiments were performed to investigate the photochemistry of the mixed-ligand zinc complexes (4—XC 6 H 4 S) 2 Zn(Phen) (XH, MeO or Me 2 N) in liquid solutions. The lowest ligand—ligand charge transfer excited state of these complexes was shown to dissociate, yielding arylthiyl radicals and a reduced zinc-phenanthroline moiety. Back electron transfer with nearly diffusion-controlled rates was found to be a predominant process in the case of MeO- and Me 2 N-substituted arylthiolate complexes while the recombination of unsubstituted phenylthiyl radicals is also significant. The reductive dissociation product may be trapped by oxygen or methyl viologen (MV 2+ ) cations, leading to the photo-oxidation of complexes in aerated solutions or production of MV + radical cations. The latter decays via diffusion-controlled electron transfer to arylthiyl radicals. Steady state illumination produces MV + only in solutions of phenylthiolate complex where its oxidation by phenylthiyl radicals competes with recombination of the latter.

Synthesis and characterization of osmium(II) complexes incorporating polypyridyl bridging ligands

A series of osmium(II) compounds have been prepared which contain the polypyridyl bridging ligands 2,3-bis(2′-pyridyl)pyrazine (dpp), 2,3-bis(2′-pyridyl)quinoxaline (dpq) and 2,3-bis(2′-pyridyl)benzoquinoxaline (dpb). The compounds [Os(bpy) 2(dpp)](PF 6) 2, [Os(bpy) 2(dpq)](PF 6) 2 and [Os(bpy) 2(dpb)](PF 6) 2 (bpy=2,2′-bipyridine) have been synthesized and their electrochemical and spectroscopic properties have been studied and are reported herein. The complexes show absorbances throughout the visible region of the spectrum. The potential of the first electrochemical reduction as well as the energy of the lowest energy metal to ligand charge transfer excited state (MLCT) are found to be dependent upon the nature of the bridging polypyridyl ligand. This combined with the fact that the bridging ligands are more easily reduced than bipyridine support the assignment of the lowest unoccupied molecular orbital (LUMO) as arising from a bridging ligand based π * orbital.

Solvatochromism of dinuclear complexes: an alternative explanation

Abstract : Solvatochromism refers to changes in electronic absorption spectra with solvent. Although most, if not all, complexes are solvatochromic to some extent, the term is usually applied to species which show shifts in energy of at least a few hundred wavenumbers with variation in solvent. The very large solvatochromism of the metal to ligand charge transfer transitions in various, formally non-polar, ligand-bridged dinuclear metal carbonyl complexes is discussed. The similarity of this behaviour to that of related mononuclear species and the good correlations obtained with the 'polar' part of McRae's equation are used to demonstrate that dipole-dipole interactions are the main cause of the solvatochromism. This contradicts previous explanations which have attributed the solvatochromism to changes in dispersion forces. It is concluded that, in the simplest approximation, the molecules may be regarded as two polar halves, each of which interacts with the solvent. Detailed interpretation in terms of McRae's equation is not possible without knowing whether the metal to ligand charge transfer excited state is localized on one metal center or delocalized over both. The data available do not allow us to distinguish between these possibilities.

Chemiluminescence and electrochemiluminescence of coordination compounds

Abstract

Aspects of artificial photosynthesis. Photosensitized electron transfer and charge separation in cationic surfactant vesicles

Electron transfer and charge separation have been investigated in cationic dioctadecyldimethylammonium chloride (DODAC) vesicles by laser spectroscopy. h-Methylphenothiazine (MPTH) and its long-chain analogue, N-dodecylphenothiazine (DPTH), were used as electron donors, while a surfactant derivative of tris(2,2'-bipyridine)ruthenium perchlorate, RuC/sub 18/(bpy)/sub 3//sup 2 +/, acted as the photoactive electron acceptor, DODAC vesicles organized these donor and acceptor molecules. RuC/sub 18/(bpy)/sub 3//sup 2 +/ molecules were anchored onto the surface of the vesicles, while MPTH molecules were distributed among the hydrophobic bilayers of the vesicles. The metal to ligand charge transfer excited state of the ruthenium complex, RuC/sub 18/(bpy)/sub 3//sup 2 +/*, readily accepted an electron from MPTH to give MPTH/sup +/ and RuC/sub 18/(bpy)/sub 3//sup +/. First, there is a rapid geminate recombination at the very site of the generation of the cation radical. Second, some of the MPTH/sup +/. Escapes into the vesicle entrapped water pools and, owing to spatial confinement, the combination occurs at the inner surface of the vesicles. Finally, a part of MPTH/sup +/. Escapes into the bulk aqueous solution where it survives for extended periods (>>1 ms). MPTH and NaCl affect the outer surface charge density of the vesicles. The amounts of MPTH/sup +/. produced and thatmore » expelled into the bulk aqueous solution were maximized in the presence of 1.0 x 10/sup -3/ M NaCl. Under this condition there was still a sufficient electrostatic repulsion between MPTH/sup +/. and the charged surface of the vesicles to slow down considerably the undesirable charge recombination reactions. Carrying out the electron transfer using DPTH resulted only in short-lived DPTH/sup +/. in low concentrations. The long hydrocarbon chain on this molecule prevented the expulsion of this cation radical from the vesicle. Relevance of this study to light-induced photochemical energy conversion is discussed.« less