Photophyical properties of ligand localized excited state in ruthenium(II) polypyridyl complexes: a combined effect of electron donor-acceptor ligand

We have synthesized ruthenium(II) polypyridyl complexes (1) Ru(II)(bpy)<SUB>2</SUB>(L<SUB>1</SUB>), (2) Ru(II)(bpy)<SUB>2</SUB>(L<SUB>2</SUB>) and (3) Ru(II)(bpy)(L<SUB>1</SUB>)(L<SUB>2</SUB>), where bpy = 2,2'-bipyridyl, L<SUB>1</SUB> = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L<SUB>2</SUB> = 4-(N,N-dimethylamino-phenyl)-(2,2'-bipyridine) and investigated the intra-ligand charge transfer (ILCT) and ligand-ligand charge transfer (LLCT) states by optical absorption and emission studies. Our studies show that the presence of electron donating -NMe<SUB>2</SUB> functionality in L<SUB>2</SUB> and electron withdrawing catechol fragment in L<SUB>1</SUB> ligands of complex 3 introduces low energy LLCT excited states to aboriginal MLCT states. The superimposed LLCT and MLCT state produces redshift and broadening in the optical absorption spectra of complex 3 in comparison to complexes 1 and 2. The emission quantum yield of complex 3 is observed to be extremely low in comparison to that of complex 1 and 2 at room temperature. This is attributed to quenching of the <SUP>3</SUP>MLCT state by the low-emissive <SUP>3</SUP>LLCT state. The emission due to ligand localized CT state (ILCT and LLCT) of complexes 2 and 3 is revealed at 77 K in the form of a new luminescence band which appeared in the 670-760 nm region. The LLCT excited state of complex 3 is populated either via direct photoexcitation in the LLCT absorption band (350-700 nm) or through internal conversion from the photoexcited <SUP>3</SUP>MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the <SUP>3</SUP>MLCT state in a time resolved emission study. The internal conversion to LLCT and ILCT excited states are observed to be as fast as ~200 ps and ~700 ps for complexes 3 and 2, respectively. The present study illustrates the photophysical property of the ligand localized excited state of newly synthesized heteroleptic ruthenium(II) polypyridyl complexes

Influence of urea N-H acidity on receptor-anionic and neutral analyte binding in a ruthenium(II)-polypyridyl-based colorimetric sensor

A new ruthenium(II)-polypyridyl complex {[Ru(bpy)2L]2+; bpy = 2,2'-bipyridyl, L = 1-(6-nitro-1,10-phenanthrolin-5-yl)-3-phenylurea} having a urea functionality as a receptor fragment for anionic analytes was synthesized. Its binding affinity towards various oxy anions and halides was studied. This complex was found to act as a selective colorimetric sensor for F- among halides and CH3COO-/H2PO4- among oxy anions. The relative binding affinity of different anions towards this receptor was examined by using quantum chemical calculations. This complex was also found to act as a colorimetric sensor for neutral molecules like DMSO and DMF, though the binding affinity was weaker than that of the three anions mentioned above. The relative acidity of two HNurea atoms was compared with that of one from the related complex by using pKa calculations, and its influence on binding affinities towards different analytes is discussed. Results of the time-resolved fluorescence studies reveal that two nonequilibrated excited states exist involving two different 3MLCT transitions, namely RudΠ → bpyΠ* and RudΠ → LΠ*

Newly designed resorcinolate binding for Ru(II)– and Re(I)–polypyridyl complexes on oleic acid capped TiO₂ in nonaqueous solvent: prolonged charge separation and substantial thermalized ³MLCT injection

Femtosecond pump–probe spectroscopic studies on a series of newly synthesized resorcinol-based Ru(II) and Re(I) complexes on oleic acid capped TiO2 nanoparticles have been carried out in chloroform, and the results are compared with those of the catechol analogues. The ruthenium complex shows biexponential injection; the second component arises due to injection from the thermally equilibrated 3MLCT states as a result of a weaker strength of the resorcinolate binding. Also, in comparison with catechol binding, as a result of a greater diffusion of the injected electrons into TiO2 , the back electron transfer (BET) is slowed down significantly for the ruthenium complex. These are distinctive observations for any mononuclear ruthenium–polypyridyl–enediol complex reported thus far. However, the rhenium complex educes single exponential ultrafast injection (&#60;120 fs) because of apparent injection in a high density of states and shows the most prominent results with ∼50% slowdown in the charge recombination rate as compared to the analogous catechol bound system. These results exemplify the probable development of highly capable sensitizer dyes with resorcinol as the anchoring group for the development of efficient dye-sensitized solar cells

Synthesis, steady-state, and femtosecond transient absorption studies of resorcinol bound ruthenium(II)- and osmium(II)-polypyridyl complexes on nano-TiO₂ surface in water

The synthesis of two new ruthenium(II)- and osmium(II)-polypyridyl complexes 3 and 4, respectively, with resorcinol as the enediol anchoring moiety, is described. Steady-state photochemical and electrochemical studies of the two sensitizer dyes confirm strong binding of the dyes to TiO₂ in water. Femtosecond transient absorption studies have been carried out on the dye–TiO2 systems in water to reveal &#60;120 fs and 1.5 ps electron injection times along with 30% slower back electron transfer time for the ruthenium complex 3. However, the corresponding osmium complex 4 shows strikingly different behavior for which only a &#60;120 fs ultrafast injection is observed. Most remarkably, the back electron transfer is faster as compared to the corresponding catechol analogue of the dye. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This Article on the electron transfer dynamics of the aforesaid systems reinforces the possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar cells

Synthesis, Steady-State, and Femtosecond Transient Absorption Studies of Resorcinol Bound Ruthenium(II)- and Osmium(II)-polypyridyl Complexes on Nano-TiO₂ Surface in Water

The synthesis of two new ruthenium­(II)- and osmium­(II)-polypyridyl complexes <b>3</b> and <b>4</b>, respectively, with resorcinol as the enediol anchoring moiety, is described. Steady-state photochemical and electrochemical studies of the two sensitizer dyes confirm strong binding of the dyes to TiO₂ in water. Femtosecond transient absorption studies have been carried out on the dye–TiO₂ systems in water to reveal <120 fs and 1.5 ps electron injection times along with 30% slower back electron transfer time for the ruthenium complex <b>3</b>. However, the corresponding osmium complex <b>4</b> shows strikingly different behavior for which only a <120 fs ultrafast injection is observed. Most remarkably, the back electron transfer is faster as compared to the corresponding catechol analogue of the dye. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This Article on the electron transfer dynamics of the aforesaid systems reinforces the possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar cells

Sensitization of nanocrystalline TiO<SUB>2</SUB> anchored with pendant catechol functionality using a new tetracyanato ruthenium (II) polypyridyl complex

We have synthesized a new photoactive ruthenium(II) complex having a pendant catechol functionality (K<SUB>2</SUB>[Ru(CN)<SUB>4</SUB>(L)] (1) (L is 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO<SUB>2</SUB> and the photoexcited states of this Ru(II) complex using femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies revealed that the complex 1 showed a strong solvatochromic behavior in solvents or solvent mixtures of varying polarity. Our steady-state absorption studies further revealed that 1 is bound to TiO<SUB>2</SUB> surfaces through the catechol functionality, though 1 has two different types of functionalities (catecholate and cyanato) for binding to TiO<SUB>2</SUB> surfaces. The longer wavelength absorption band tail for 1, bound to TiO<SUB>2</SUB> through the proposed catecholate functionality, could also be explained on the basis of the DFT calculations. Dynamics of the interfacial electron transfer between 1 and TiO<SUB>2</SUB> nanoparticles was investigated by studying kinetics at various wavelengths in the visible and near-infrared region. Electron injection to the conduction band of the nanoparticulate TiO<SUB>2</SUB> was confirmed by detection of the conduction band electron in TiO<SUB>2</SUB> ([e<SUP>-</SUP>]<SUB>TiO</SUB><SUB>2</SUB><SUP>CB</SUP>) and cation radical of the adsorbed dye (1<SUP>·+</SUP>) in real time as monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (&lt;100 fs) electron injection was observed. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1<SUP>·+</SUP> and [e<SUP>-</SUP>]<SUB>TiO<SUB>2</SUB></SUB><SUP>CB</SUP>. This is the first report on ultrafast ET dynamics on TiO<SUB>2</SUB> nanoparticle surface using a solvatochromic sensitizer molecul

New Ru(II)/Os(II)-polypyridyl complexes for coupling to TiO₂ surfaces through acetylacetone functionality and studies on interfacial electron-transfer dynamics

New Ru(II)- and Os(II)-polypyridyl complexes have been synthesized with pendant acetylacetone (acac) functionality for anchoring on nanoparticulate TiO₂ surfaces with a goal of developing an alternate sensitizer that could be utilized for designing an efficient dye-sensitized solar cell (DSSC). Time-resolved transient absorption spectroscopic studies in the femtosecond time domain have been carried out. The charge recombination rates are observed to be very slow, compared with those for strongly coupled dye molecules having catechol as the anchoring functionality. The results of such studies reveal that electron-injection rates from the metal complex-based LUMO to the conduction band of TiO₂ are faster than one would expect for an analogous complex in which the chromophoric core and the anchoring moiety are separated with multiple saturated C–C linkages. Such an observation is rationalized based on computational studies, and a relatively smaller spatial distance between the dye LUMO and the TiO₂ surface accounted for this. Results of this study are compared with those for analogous complexes having a gem-dicarboxy group as the anchoring functionality for covalent binding to the TiO₂ surface to compare the role of binding functionalities on electron-transfer dynamics

Superior grafting and state-of-the-art interfacial electron transfer rates for newly designed geminal dicarboxylate bound ruthenium(II)– and osmium(II)–polypyridyl dyes on TiO₂ nanosurface

Two new Ru(II)–/Os(II)–polypyridyl based sensitizer dyes with geminal dicarboxylic acid group as the binding unit for superior grafting of the dye to TiO₂ have been designed and synthesized. Steady-state photochemical studies of the two sensitizer dyes in presence of TiO₂ in water confirm strong binding of the dyes to TiO₂. Femtosecond transient absorption studies of these newly synthesized dyes on TiO₂ nanosurface have been carried out in water and the results have been compared with those for the corresponding 4,4′-dicarboxy-2,2′-bipyridine analogues of the dyes. While the charge recombination rates are considerably slower, interestingly, the electron injection rates are very fast for multiple saturated C–C linkages present between the chromophoric core and the anchoring moiety. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This is the first report on the ultrafast transient absorption studies of dyes with geminal dicarboxylic acid binding groups, which we believe will add significantly to the present research efforts toward the development of robust and efficient dyes for use in dye solar applications