Reinvestigating 2,5-di(pyridin-2-yl)pyrazine ruthenium complexes: selective deuteration and Raman spectroscopy as tools to probe ground and excited-state electronic structure in homo- and heterobimetallic complexes

The mono- (1) and dinuclear (2) ruthenium(II) bis(2,2'-bipyridine) complexes of 2,5-di(pyridin-2-yl)pyrazine (2,5-dpp), for which the UV/Vis absorption and emission as well as electrochemical properties have been described earlier, are reinvestigated here by resonance, surface enhanced and transient resonance Raman spectroscopy together with selective deuteration to determine the location of the lowest lying excited metal to ligand charge transfer ((3)MLCT) states. The ground state absorption spectrum of both the mono- and dinuclear complexes are characterised by resonance Raman spectroscopy. The effect of deuteration on emission lifetimes together with the absence of characteristic bipy anion radical modes in the transient Raman spectra for both the mono- and dinuclear complexes bridged by the 2,5-dpp ligand confirms that the excited state is 2,5-dpp based; however DFT calculations and the effect of deuteration on emission lifetimes indicate that the bipy based MLCT states contribute to excited state deactivation. Resonance Raman and surface enhanced Raman spectroscopic (SERS) data for 1 and 2 are compared with that of the heterobimetallic complexes [Ru(bipy)(2)(2,5-dpp)PdCl(2)](2+)3 and [Ru(bipy)(2)(2,5-dpp)PtCl(2)](2+)4. The SERS data for 1 indicates that a heterobimetallic Ru-Au complex forms in situ upon addition of 1 to a gold colloid.     

Influence of the halogen ligand on the near-UV-visible spectrum of [Ru(X)(Me)(CO)2(alpha-diimine)] (X = Cl, I; alpha-diimine = Me-DAB, iPr-DAB; DAB = 1,4-diaza-1,3-butadiene): an ab initio and TD-DFT analysis

The near-UV-vis electronic spectroscopy of [Ru(X)(Me)(CO)(2)(iPr-DAB)] (X = Cl or I; iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) is investigated through CASSCF/CASPT2 and TD-DFT calculations on the model complexes [Ru(X)(Me)(CO)(2)(Me-DAB)] (X = Cl or I). Convergence of the calculated transition energies for the low-lying metal-to-ligand charge-transfer (MLCT), X-to-ligand charge-transfer (XLCT, X halide ligand), or sigma-bond-to-ligand charge-transfer (SBLCT) to experimental values is analyzed for both methods. On the basis of these accurate calculations, it is shown that whereas the lowest singlet state can be assigned to a nearly pure XLCT state in [Ru(I)(Me)(CO)(2)(Me-DAB)], its character is mainly MLCT in [Ru(Cl)(Me)(CO)(2)(Me-DAB)]. These results are in agreement with time-resolved emission/IR and resonance Raman experimental data. The experimental UV-vis bands are well reproduced by the CASSCF/CASPT2 calculations. The TD-DFT transition energies to the long-range charge transfer states are dramatically affected by the nature of the functional, with lowering leading to meaningless values in the case of nonhybrid functionals. Both methods reproduce well the red shift of the absorption bands on going from the chloride to the iodide complex as well as the shift of the strongly absorbing higher MLCT transition from the visible to the UV domain of energy.     

几种(C^N)PtIIQ型配合物的电子结构和紫外-可见吸收光谱

在B3LYP/LANL2DZ水平上优化了三种(C^N)PtIIQ 型配合物基态的几何结构, 进行了频率计算, 并采用含时密度泛函(TD-DFT)方法结合极化连续体模型(PCM)计算了目标配合物在CH2Cl2溶液中的电子结构和紫外-可见吸收光谱. 计算值与文献报道值相似. 计算结果表明这三种(C^N)PtIIQ型配合物在可见光区都有强度较大而且宽的吸收峰, 它们的最低能量吸收峰的跃迁具有ILCT(配体内部电荷转移)和部分MLCT(金属向配体的电荷转移)的特征, 不同于PtIIQ2型配合物在多数情况下表现出的ILCT的跃迁性质.     

Bidirectional "ping-pong" energy transfer and 3000-fold lifetime enhancement in a Re(I) charge transfer complex

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Fo?rster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.     

Photophysics and Acid‐Base Chemistry of Re(CH3bpyCOOH)(CO)3X (X=Cl−, and Imidazole) Complexes

Rheniumtricarbonyl(4′‐methyl‐2,2′‐bipyridine‐4‐carboxylic acid)X (where X is Cl^? and imidazole) complexes have been prepared. These two complexes exhibit similar spectroscopic properties. The metal‐ to‐ligand charge‐transfer (MLCT) absorption and the corresponding emission are observed. This charge transfer band is highly solvent dependent. Due to the stronger electron‐withdrawing ability of the ‐COOH (in comparison to the ‐COO^? group), the MLCT band has red‐shifted during protonation. Emission quantum yields are dramatically reduced while life time remains similar upon protonation. These behaviors are typical in static quenching mechanism by protons. The ground state pK_a of Re(CO)₃ (CH₃bpyCOOH)Cl obtained from the pH titration curve of the complex absorption at 409 nm was 2.5.     

New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.     

DFT/TDDFT studies on the electronic structures and spectral properties of rhenium(I) pyridinybenzoimidazole complexes

The electronic structures and spectral properties of three Re(I) complexes [Re(CO)3XL] (X = Br, Cl; L = 1-(4-5'-phenyl-1,3,4-oxadiazolylbenzyl)-2-pyridinylbenzoimidazole (1), 1-(4-carbazolylbutyl)-2-pyridinylbenzoimidazole (2), and 2-(1-ethylbenzimidazol-2-yl)pyridine (3)) were investigated theoretically. The ground and the lowest lying triplet excited states were fully optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. TDDFT/PCM calculations have been employed to predict the absorption and emission spectra starting from the ground and excited state geometries, respectively. The lowest lying absorptions were calculated to be at 481, 493, and 486 nm for 1-3, respectively, and all have the transition configuration of HOMO-->LUMO. The lowest lying transitions can be assigned as metal/ligand-to-ligand charge transfer (MLCT/LLCT) character for 1, ligand-to-ligand charge transfer (LLCT) character for 2, and mixed MLCT/LLCT and intraligand pi-->pi* charge transfer (ILCT) character for 3. The emission of 1 at 551 nm has the MLCT/(3)LLCT character, 2 has the (3)MLCT/(3)LLCT character at 675 nm, and the 651 nm transition of 3 has the character of (3)MLCT/(3)LLCT/(3)ILCT. Ionization potentials (IP) and electron affinities (EA) calculations show that the comparable EA and smaller IP values and the relatively balanceable charges transfer ability of 2 with respect to 1 and 3 result in the higher efficiency of OLEDs. The calculated results show that the absorption and emission transition character and device's efficiency can be changed by altering the ancillary ligands.     

Photophysical 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)(2)(L(1)), (2) Ru(II)(bpy)(2)(L(2)) and (3) Ru(II)(bpy)(L(1))(L(2)), where bpy = 2,2'-bipyridyl, L(1) = 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) and L(2) = 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(2) functionality in L(2) and electron withdrawing catechol fragment in L(1) 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 (3)MLCT state by the low-emissive (3)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 (3)MLCT (400-600 nm) states. The internal conversion rate is determined by quenching of the (3)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.     

Ground and excited state resonance Raman spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: characterization of excited states, determination of structure and bonding, and observation of metal cation release from the azacrown

A [(bpy)Re(CO)3L+] complex (bpy = 2,2'-bipyridine) in which L contains a phenyl-azacrown ether that is attached to Re via an amidopyridyl linking group has been studied by steady state and nanosecond time-resolved resonance Raman spectroscopy. Vibrational band assignments have been aided by studies of model complexes in which a similar electron-donating dimethylamino group replaces the azacrown or in which an electron-donor group is absent, and by density functional theory calculations. The ground state resonance Raman spectra show nu(bpy) and nu(CO) bands of the (bpy)Re(CO)3 group when excitation is exclusively in resonance with the Re --> bpy metal-to-ligand charge-transfer (MLCT) transition, whereas L ligand bands are dominant when it is in resonance with the strong intra-ligand charge-transfer (ILCT) transition present for L ligands with electron-donor groups. Transient resonance Raman (RR) spectra obtained on single color (385 nm) pulsed excitation of the complexes in which an electron-donor group is absent show bpy*- bands of the MLCT excited state, whereas those of the complexes with electron-donor groups show both bpy*- bands and a down-shifted nu(CO) band that together are characteristic of an L-to-bpy ligand-to-ligand charge-transfer (LLCT) excited state. Samples in which a metal cation (Li+, Na+, Ca2+, Ba2+) is bound to the azacrown in the ground state show bands from both excited states, consistent with a mechanism in which the LLCT state forms after metal cation release from the MLCT state. Nanosecond time-resolved RR spectra from two-color (355 nm pump, 500 nm probe) experiments on the electron-donor systems show L-ligand bands characteristic of the LLCT state; the same bands are observed from samples in which a metal cation is bound to the azacrown in the ground state, and their time dependence is consistent with the proposed mechanism in which the rate constant for ion release in the MLCT state depends on the identity of the metal cation.     

Mechanism of ligand photodissociation in photoactivable [Ru(bpy)2L2]2+ complexes: a density functional theory study

A series of four photodissociable Ru polypyridyl complexes of general formula [Ru(bpy)2L2](2+), where bpy = 2,2'-bipyridine and L = 4-aminopyridine (1), pyridine (2), butylamine (3), and gamma-aminobutyric acid (4), was studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). DFT calculations (B3LYP/LanL2DZ) were able to predict and elucidate singlet and triplet excited-state properties of 1-4 and describe the photodissociation mechanism of one monodentate ligand. All derivatives display a Ru --> bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet (3)MLCT state (Ru --> bpy). 1-4 have three singlet metal-centered (MC) states 0.4 eV above the major (1)MLCT states. The energy gap between the MC states and lower-energy MLCT states is significantly diminished by intersystem crossing and consequent triplet formation. Relaxed potential energy surface scans along the Ru-L stretching coordinate were performed on singlet and triplet excited states for all derivatives employing DFT and TDDFT. Excited-state evolution along the reaction coordinate allowed identification and characterization of the triplet state responsible for the photodissociation process in 1-4; moreover, calculation showed that no singlet state is able to cause dissociation of monodentate ligands. Two antibonding MC orbitals contribute to the (3)MC state responsible for the release of one of the two monodentate ligands in each complex. Comparison of theoretical triplet excited-state energy diagrams from TDDFT and unrestricted Kohn-Sham data reveals the experimental photodissociation yields as well as other structural and spectroscopic features.     

Ground- and excited-state infrared spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: structure and bonding in ground and excited states and effects of Ba2+ binding

Ground- and excited-state infrared spectra are reported for a [(bpy)ReI(CO)3L]+ complex (bpy = 2,2'-bipyridine) in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band assignments are made with the aid of spectra from model complexes in which a similar electron-donating dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the nu(CO) region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy*-)ReII(CO)3L]+, from the complex in which an electron-donor group is absent, whereas those from the azacrown complex show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT) excited state, [(bpy*-)ReI(CO)3(L*+)]+, formed upon intramolecular electron transfer. Picosecond TRIR spectra of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms. Picosecond TRIR spectra obtained when Ba2+ was complexed to the azacrown show bands of only an MLCT state at all times up to 2 ns, consistent with the presence of Ba2+ inhibiting electron transfer from the azacrown N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence for the proposal that charge redistribution within the L ligand induces Ba2+ release from the azacrown in the MLCT state.     

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP⁻ (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.     

一种新型联吡啶钌配合物的合成及电致化学发光性能研究

本文合成了一种多联吡啶钌配合物(bpy)2Ru(phenCl4)(PF6)2,bpy为2,2′-联吡啶, phenCl4为3,4,7,8-四氯-1,10-邻菲罗啉,并用元素分析、红外光谱、核磁共振谱对其结构进行了表征。此化合物在紫外和可见光区都有吸收,在可见光区的最大吸收波长是440nm,这是典型的金属到配体（MLCT）的跃迁,其光致发光性能也显示出MLCT迁移特征,并且随溶剂不同,其最大发射波长从630nm变化到649nm。值得注意的是,此配合物的电致化学发光性能受pH影响不大,这与传统的随pH增大电致化学发光强度增大的联吡啶钌不同,尤其在强碱条件下,其背景电致化学发光很小。     

Photochemical reactions of fac-[Mn(CO)3(phen)imidazole]+: evidence for long-lived radical species intermediates

The electronic absorption spectrum of fac-[Mn(CO)(3)(phen)imH](+), fac-1 in CH(2)Cl(2) is characterized by a strong absorption band at 378 nm (epsilon(max) = 3200 mol(-1) L cm(-1)). On the basis of quantum mechanical calculations, the visible absorption band has been assigned to ligand-to-ligand charge-transfer (LLCT, im-->phen) and metal-to-ligand charge-transfer (MLCT, Mn-->phen) charge transfer transition. When fac-1 in CH(2)Cl(2) is irradiated with 350 nm continuous light, the absorption features are gradually shifted to represent those of the meridional complex mer-[Mn(CO)(3)(phen)imH](+), mer-1 (lambda(max) = 556 nm). The net photoreaction under these conditions is a photoisomerization, although, the presence of the long-lived radical species was also detected by (1)H NMR and FTIR spectroscopy. 355 nm continuous photolysis of fac-1 in CH(3)CN solution also gives the long-lived intermediate which is readily trapped by metylviologen (MV(2+)) giving rise to the formation of the one-electron reduced methyl viologen (MV(*+)). The UV-vis spectra monitored during the slow (45 min) thermal back reaction exhibited isosbestic conversion at 426 nm. On the basis of spectroscopic techniques and quantum mechanical calculations, the role of the radicals produced is analyzed.     

Dual emission from 3MLCT and 3ILCT excited states in a new Ru(II) diimine complex

The unique behavior of a new Ru(II) diimine complex, Ru(bpy)(2)(L)(2+) (where L is 4-methyl-4'-[p-(dimethyl- amino)-alpha-styryl]-2,2'-bipyridine, bpy is 2,2'-bipyridine), was studied in detail. Due to the strong electron donating property of the amino group, an ILCT (intraligand charge transfer) state is involved either in the absorption spectra or in the time-resolved emission spectra. Dual emission based on (3)MLCT and (3)ILCT states was observed at room temperature for the first time via a time-resolved technique in Ru(II) diimine complexes.     

Excited-state metal-to-ligand charge transfer dynamics of a ruthenium(II) dye in solution and adsorbed on TiO2 nanoparticles from resonance Raman spectroscopy

The dynamics of metal-to-ligand charge transfer (MLCT) in a cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II) dye (N3) are compared for the free dye in solution and the dye adsorbed on the surface of the TiO(2) nanoparticles from resonance Raman spectroscopy. The 544-nm MLCT absorption band of N3 adsorbed on TiO(2) is slightly blue-shifted from that of the free N3, indicating a weak electronic coupling between N3 and TiO(2). The resonance Raman spectra of N3 and the N3|TiO(2) complex obtained upon excitation within the lowest-lying MLCT singlet state of the dye are similar except for slight shifts in band positions. Resonance Raman cross sections have been obtained for the vibrational modes of both N3 and N3|TiO(2) with excitation frequencies spanning the 544-nm MLCT band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism over two electronic states yields mode-specific vibrational and solvent reorganization energies. Despite the weak electronic coupling between N3 and TiO(2) in N3|TiO(2), adsorption strongly affects the reorganization energies of N3 in the intramolecular MLCT state. Adsorption of N3 onto TiO(2) increases the absolute Raman cross section of each mode by a factor of ca. 1.6 and decreases the vibrational and solvent reorganization energies by factors of 2 and 6, respectively. The excited-state dynamics of N3 adsorbed on the surface of TiO(2) nanoparticles were observed to be independent of the number of N3 molecules adsorbed per TiO(2) nanoparticle. The effect of TiO(2) on the dynamics of the adsorbed N3 is primarily due to both mode-specific vibrational and electronic pure dephasing, with the dominant contribution from the latter process.     

Strongly Absorbing π–π* States in Heteroleptic Dipyrrin/2,2′‐Bipyridine Ruthenium Complexes: Excited‐State Dynamics from Resonance Raman Spectroscopy

A recently reported new class of ruthenium complexes containing 2,2′‐bipyridine and a dipyrrin ligand in the coordination sphere exhibit both strong metal‐to‐ligand charge‐transfer (MLCT) and π–π* transitions. Quantitative analysis of the resonance Raman scattering intensities and absorption spectra reveals only weak electronic interactions between these states despite direct coordination of the bipyridyl and dipyrrin ligands to the central ruthenium atom. On the basis of DFT calculations and time‐dependent DFT (TD‐DFT), we propose that the electronic excited states closely resemble “pure” MLCT and π–π* states. Resonance Raman intensity analysis demonstrates that a large amplitude transannular torsional motion provides a mechanism for relaxation on the π–π* excited‐state surface. We assert that this result is generally applicable to a range of dipyrrin complexes such as boron–dipyrrin and metallodipyrrin systems. Despite the large torsional distortion between the phenyl ring and the dipyrromethene plane, π–π* excitation extends out onto the phenyl ring which may have important consequences in solar‐energy‐conversion applications of ruthenium–dipyrrin complexes.     

Theoretical characterization of the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium dication complex

We present a theoretical study of the ground and the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium complex [Ru(tap)3]2+. Density functional theory (DFT) was used to obtain the relaxed geometries and emission energies (Delta-SCF), whereas time-dependent DFT (TD-DFT) was used to compute the absorption spectrum. Our calculations have revealed the presence of three low-lying excited-state minima, which may be relevant in the photophysical/photochemical properties of this complex. Two minima with similar energies correspond to the MLCT 3A2 and MLCT 3B metal-to-ligand charge-transfer states, the first one corresponding to a D3 structure, whereas the second is a slightly localized C2 species. The third and lowest one corresponds to the metal-centered MC 3A state and displays a pronounced C2 distortion. We have examined for the first time the localized character of the excitation in the computed MLCT states. In particular, we have evaluated the pseudorotation barrier between the Jahn-Teller C2 MLCT 3B minima in the moat around the D3 conical intersection. We have shown that the complex should be viewed as a delocalized [Ru3+(tap(-1/3))3]2+ complex in the lowest MLCT states, in agreement with subpicosecond interligand electron transfer observed by femtosecond transient absorption anisotropy study. Upper-bound estimates of the MLCT-->MC (3 kcal/mol) and MC-->MLCT (10 kcal/mol) activation energy barriers obtained from potential energy profiles in vacuum corroborate the high photoinstability of the MLCT states of the [Ru(tap)3]2+complex.     

Theoretical studies on effective metal-to-ligand charge transfer characteristics of novel ruthenium dyes for dye sensitized solar cells

The development of ruthenium dye-sensitizers with highly effective metal-to-ligand charge transfer (MLCT) characteristics and narrowed transition energy gaps are essential for the new generation of dye-sensitized solar cells. Here, we designed a novel anchoring ligand by inserting the cyanovinyl-branches inside the anchoring ligands of selected highly efficient dye-sensitizers and studied their intrinsic optical properties using theoretical methods. Our calculated results show that the designed ruthenium dyes provide good performances as sensitizers compared to the selected efficient dyes, because of their red-shift in the UV–visible absorption spectra with an increase in the absorption intensity, smaller energy gaps and thereby enhancing MLCT transitions. We found that, the designed anchoring ligand acts as an efficient “electron-acceptor” which boosts electron-transfer from a –NCS ligand to this ligand via a Ru-bridge, thus providing a way to lower the transition energy gap and enhance the MLCT transitions.     

Electron transfer across modular oligo-p-phenylene bridges in Ru(bpy)2(bpy-ph(n)-DQ)4+ (n = 1-5) dyads. Unusual effects of bridge elongation

A series of dyads of general formula Ru(bpy)(2)(bpy-ph(n)-DQ)(4+) (n = 1-5), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1-5 modular units, and a cyclo-diquaternarized 2,2'-bipyridine (DQ(2+)) as electron acceptor unit, have been synthesized. Their spectroscopic and photophysical properties have been investigated in CH(3)CN and CH(2)Cl(2) by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The experimental study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addition to those characteristic of the donor, bridge, and acceptor fragments. DFT calculations suggest the assignment of such bands as bridge-to-acceptor (π ph(n)) → (π* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochemical behavior of the new bands. For all the dyads at room temperature in fluid solution, the typical (3)MLCT luminescence of the Ru(II) polypyridine unit is strongly (>90%) quenched, supporting the occurrence of an efficient intramolecular photoinduced electron transfer. The study has revealed, however, that the photophysical mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been observed, followed by slower interligand hopping and charge recombination. For n = 2-5, MLCT excited-state quenching takes place without transient detection of charge-separated product, indicating that charge recombination is faster than charge separation. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1-5 does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1-3, the rate constants become almost insensitive to bridge length for n = 3-5. The rationale of this uncommon behavior, as suggested by DFT calculations, lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge.