Synthesis and spectroscopic studies on chromium(III) complex containing mixed-valence chrysenesemiquinone-chrysenecatecholate ligands and 2,2'-bipyridine coligand

The synthesis and spectroscopic properties of Cr(bpy)(chrySQ)(chryCat), a complex containing chromium(III) metal ion and chrysenequinone ligand in its partially reduced (chrySQ) and fully reduced (chryCat) forms, are described. The complex has been prepared by two different routes from Cr(CO)6 and Cr(chrySQ)3. Variable temperature magnetic susceptibility measurements indicated a strong antiferromagnetic coupling between Cr(III) (S=3/2) and chrysenesemiquinone radical (S=1/2), giving a magnetic coupling constant J=-342 cm(-1). Ligand-based redox couples were observed in the electrochemical studies that consist of quasi-reversible chrySQ/chryCat and bpy/bpy*- reductions and chryCat/chrySQ oxidation at negative potentials and irreversible chrySQ/chryBQ oxidation at positive potential. However, the metal was inert in the studied potential range. The electronic spectra of the complex revealed interesting properties. In addition to interaligand pi-pi* and n-pi* transitions, other bands corresponding to Cr(t(2g))-->chrySQ(pi*) and Cr(t(2g))-->bpy(pi*) metal-to-ligand charge-transfer MLCT transitions were observed. The infrared spectral analysis was informative in assigning the vibrations due to SQ and Cat ligands. Also, it was a useful tool in confirming the coordination of bpy ligand to chromium metal ion.     

Lowest electronic excited states of platinum(II) diimine complexes

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.     

Ytterbocene charge-transfer molecular wire complexes

A systematic study of the novel charge-transfer [(f)14-(pi)0-(f)14 --> (f)13-(pi)2-(f)13] electronic state found in 2:1 metal-to-ligand adducts of the type [(Cp)2Yb](BL)[Yb(Cp)2] [BL = tetra(2-pyridyl)pyrazine (tppz) (1), 6',6' '-bis(2-pyridyl)-2,2':4',4':2',2'-quaterpyridine (qtp) (2), 1,4-di(terpyridyl)-benzene (dtb) (3), Cp = (C5Me5)] has been conducted with the aim of determining the effects of increased Yb-Yb separation on the magnetic and electronic properties of these materials. The neutral [(f)13-(pi)2-(f)13], cationic [(f)13-(pi)1-(f)13] and dicationic [(f)13-(pi)0-(f)13] states of these complexes were studied by cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, NMR, X-ray crystallography, and magnetic susceptibility measurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistent with an [(f)13(pi)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donates one electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. The voltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1-3, as manifested in the separation between successive metal reduction waves, is large compared to analogous transition metal systems. Electronic spectra for the neutral and monocationic bimetallic species are dominated by pi-pi and pi-pi transitions, masking the f-f bands that are expected to best reflect the electronic metal-metal interactions. However, these metal-localized transitions are observed when the electrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, and they suggest very little electronic interaction between metal centers in the absence of pi electrons on the bridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagnetic coupling of the type Yb(alpha)(alphabeta)Yb(beta) at approximately 13 K.     

Replacement of 2,2'-bipyridine by 1,4-diazabutadiene acceptor ligands: why the bathochromic shift for [(N empty set N)IrCl(C5Me5)]+ complexes but the hypsochromic shift for (N empty set N)Ir(C5Me5)?

Replacement of 2,2'-bipyridine (bpy) by substituted 1,4-diazabutadiene (R-DAB) alpha-diimine ligands N empty set N leads to a substantial hypsochromic shift of about 0.8 eV for the long-wavelength absorption band in compounds (N empty set N)Ir(C(5)Me(5)) but to a bathochromic absorption shift of about 0.4 eV for the complex ions [(N empty set N)IrCl(C(5)Me(5))](+). DFT calculations on model complexes based on experimental (R-DAB compounds) and geometry-optimized structures (bpy systems) reveal that the low-energy transitions of the cationic chloro complexes are largely of ligand-to-ligand charge-transfer character L'LCT (L = alpha-diimine, L' = Cl) whereas the neutral compounds exhibit pi --> pi transitions between the considerably mixed metal d(pi) and alpha-diimine pi orbitals. The much more pronounced metal-ligand orbital interaction for the R-DAB complexes causes the qualitatively different shifts on replacing the stronger basic bpy by the better pi-acceptors R-DAB. Only the LUMO of the neutral compounds is destabilized on replacement of bpy by R-DAB whereas the LUMO of [(N empty set N)IrCl(C(5)R'(5))](+) and both HOMOs are stabilized through this change.     

Ligand substituent effect observed for ytterbocene 4'-cyano-2,2':6',2' '-terpyridine

A new N-heterocyclic complex of ytterbocene (Cp(2)Yb(II), Cp = C(5)Me(5)) has been prepared by the addition of 4'-cyano-2,2':6',2' '-terpyridine (tpyCN) to Cp(2)Yb(II)(OEt(2)) in toluene to give a dark blue species designated as Cp(2)Yb(tpyCN). The effect of the electron-withdrawing group (-CN) on the redox potentials of the charge-transfer form of this complex [in which an electron is transferred from the f(14) metal center to the lowest unoccupied (pi) molecular orbital of the tpyCN ligand to give a 4f(13)-pi(1) electronic configuration] has been quantified by cyclic voltammetry. The tpyCN ligand stabilizes this configuration by 60 mV more than that in the unsubstituted tpy ligand complex and by 110 mV more than that in the unsubstituted bpy ligand complex. Magnetic susceptibility measurements corroborate the enhanced stabilization of the 4f(13)-pi(1) configuration by the substituted terpyridyl ligand complex. Furthermore, the temperature dependence of the magnetic data is most consistent with a thermally induced valence tautomeric equilibrium between this paramagnetic 4f(13)-pi(1) form that dominates near room temperature and the diamagnetic 4f(14)-pi(0) form that dominates at low temperature. Differing coordination modes for the tpyCN ligand to the ytterbocene center have also been confirmed by isolation and X-ray crystallographic characterization of complexes binding through either the cyano nitrogen of tpyCN or the three terpyridyl nitrogen atoms of tpyCN.     

Photophysics of diimine platinum(II) bis-acetylide complexes

A comprehensive photophysical investigation has been carried out on a series of eight complexes of the type (diimine)Pt(-C=C-Ar)(2), where diimine is a series of 2,2'-bipyridine (bpy) ligands and -C=C-Ar is a series of substituted aryl acetylide ligands. In one series of complexes, the energy of the Pt --> bpy metal-to-ligand charge transfer (MLCT) excited state is varied by changing the substituents on the 4,4'- and/or the 5,5'-positions of the bpy ligand. In a second series of complexes the electronic demand of the aryl acetylide ligand is varied by changing the para substituent (X) on the aryl ring (X = -CF(3), -CH(3), -OCH(3), and -N(CH(3))(2)). The effect of variation of the substituents on the excited states of the complexes has been assessed by examining their UV-visible absorption, variable-temperature photoluminescence, transient absorption, and time-resolved infrared spectroscopy. In addition, the nonradiative decay rates of the series of complexes are subjected to a quantitative energy gap law analysis. The results of this study reveal that in most cases the photophysics of the complexes is dominated by the energetically low lying Pt --> bpy (3)MLCT state. Some of the complexes also feature a low-lying intraligand (IL) (3)pi,pi excited state that is derived from transitions between pi- and pi-type orbitals localized largely on the aryl acetylide ligands. The involvement of the IL (3)pi,pi state in the photophysics of some of the complexes is signaled by unusual features in the transient absorption, time-resolved infrared, and photoluminescence spectra and in the excited-state decay kinetics. The time-resolved infrared difference spectroscopy indicates that Pt --> bpy MLCT excitation induces a +25 to + 35 cm(-)(1) shift in the frequency of the C=C stretching band. This is the first study to report the effect of MLCT excitation on the vibrational frequency of an acetylide ligand.     

Ruthenium(II)-polyazine light absorbers bridged to reactive cis-dichlororhodium(III) centers in a bimetallic molecular architecture

Bimetallic complexes of the form [(bpy)(2)Ru(BL)RhCl(2)(phen)](PF(6))(3), where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and BL = 2,3-bis(2-pyridyl)pyrazine (dpp) or 2,2'-bipyrimidine (bpm), were synthesized, characterized, and compared to the [{(bpy)(2)Ru(BL)}(2)RhCl(2)](PF(6))(5) trimetallic analogues. The new complexes were synthesized via the building block method, exploiting the known coordination chemistry of Rh(III) polyazine complexes. In contrast to [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) and [{(bpy)(2)Ru(bpm)}(2)RhCl(2)](PF(6))(5), [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) have a single visible light absorber subunit coupled to the cis-Rh(III)Cl(2) moiety, an unexplored molecular architecture. The electrochemistry of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) showed a reversible oxidation at 1.61 V (vs Ag/AgCl) (Ru(III/II)), quasi-reversible reductions at -0.39 V, -0.74, and -0.98 V. The first two reductive couples corresponded to two electrons, consistent with Rh reduction. The electrochemistry of [(bpy)(2)Ru(bpm)RhCl(2)(phen)](PF(6))(3) exhibited a reversible oxidation at 1.76 V (Ru(III/II)). A reversible reduction at -0.14 V (bpm(0/-)), and quasi-reversible reductions at -0.77 and -0.91 V each corresponded to a one electron process, bpm(0/-), Rh(III/II), and Rh(II/I). The dpp bridged bimetallic and trimetallic display Ru(dpi)-->dpp(pi*) metal-to-ligand charge transfer (MLCT) transitions at 509 nm (14,700 M(-1) cm(-1)) and 518 nm (26,100 M(-1) cm(-1)), respectively. The bpm bridged bimetallic and trimetallic display Ru(dpi)-->bpm(pi*) charge transfer (CT) transitions at 581 nm (4,000 M(-1) cm(-1)) and 594 nm (9,900 M(-1) cm(-1)), respectively. The heteronuclear complexes [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) and [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) had (3)MLCT emissions that are Ru(dpi)-->dpp(pi*) CT in nature but were red-shifted and lower intensity than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4). The lifetimes of the (3)MLCT state of [(bpy)(2)Ru(dpp)RhCl(2)(phen)](PF(6))(3) at room temperature (30 ns) was shorter than [(bpy)(2)Ru(dpp)Ru(bpy)(2)](PF(6))(4), consistent with favorable electron transfer to Rh(III) to generate a metal-to-metal charge-transfer ((3)MMCT) state. The reported synthetic methods provide means to a new molecular architecture coupling a single Ru light absorber to the Rh(III) center while retaining the interesting cis-Rh(III)Cl(2) moiety.     

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.     

Structural,photophysical, and electrophosphorescent properties of platinum(II) complexes supported by tetradentate N2O2 chelates

We present an examination of the structural and photophysical characteristics of [Pt(N(2)O(2))] complexes bearing bis(phenoxy)diimine auxiliaries (diimine=4,7-Ph(2)phen (1) and 4,4'-tBu(2)bpy (2)) that are tetradentate relatives of the quinolinolato (q) ligand. These neutral derivatives display high thermal stability (>400 degrees C in N(2)). While the crystal lattice in 1 consists of (head-to-tail)-interacting dimers, molecules of 2 are arranged into infinitely stacked planar sheets with possible pi-pi interactions but no close Pt.Pt contacts. Complexes 1 and 2 exhibit moderately intense low-energy UV/Vis absorptions around lambda=400-500 nm that undergo negative solvatochromic shifts. Both derivatives are highly luminescent in solution at 298 K with emission lifetimes in the micros range, and mixed (3)[l-->pi*(diimine)] (l=lone pair/phenoxide) and (3)[Pt(d)-->pi*(diimine)] charge-transfer states are tentatively assigned. The excited-state properties of 2 are also investigated by time-resolved absorption spectroscopy and by quenching experiments with pyridinium acceptors to estimate the excited-state redox potential. These emitters have been employed as electrophosphorescent dopants in multilayer OLEDs. Differences between the brightness, color, and overall performance of devices incorporating 1 and 2 are attributed to the influence of the diimine substituents.     

Synthesis, characterization, and crystal structure of the Pd(phen)(bdt) complex. A DFT and TDDFT study of its ground electronic and excited states compared to those of analogous complexes

The synthesis and characterization of Pd(phen)(bdt) (1) (phen = 1,10-phenanthroline, bdt = 1,2-benzenedithiolate) is presented. 1 crystallizes in the monoclinic space group P2(1)/c, alpha = 11.281(4) A, b = 20.498(8) A, c = 8.374(3) A, beta = 90.234(8), V = 1936.5(13) A(3), Z = 4, and is isostructural with its previously reported related complexes. The ground and low lying excited electronic states in 1 and in the related complexes Pd(bpy)(bdt) (2), Pt(bpy)(bdt) (3), Pt(bpy)(mnt) (4), and Pt(bpy)(edt) (5) [where bpy = 2,2'-bipyridine, edt = ethylene-1,2-dithiolate, and mnt = maleonitriledithiolate] are studied using density functional theory techniques. The electronic properties of 1-5 are studied using the B3LYP functional. Optimized geometries are compared to experimentally observed structures. Time dependent density functional theory (TDDFT) is employed to investigate the excited singlet and triplet states. The calculated energies of the lowest singlet state and the lowest triplet state in all five complexes are in considerable agreement with experimental data. It is shown that variation of both metal and dithiolate-ligand going from 1 and 2 to 3, 4, and 5 has a substantial impact on the spectroscopic and excited-state properties, indicating at the same time the mixed metal/dithiolate character of the HOMO orbital. All the low-lying transitions are categorized as MMLL'CT transitions. The emissive state of all complexes is assigned as a triplet dithiolate/metal to diimine charge transfer with differences in the structures of the emissions resulting from differences in the pi dithiolate orbital of the mnt, bdt, and edt as well as from differences in metal.     

Probing the solvent dependent photophysics of fac-[Re(CO)3(dppz-X2)Cl] (dppz-X2 = 11,12-X2-dipyrido[3,2-a:2',3'-c]phenazine); X = CH3, H, F, Cl, CF3)

The results of electrochemical measurements, density-functional theory calculations, emission and time-resolved IR (TRIR) spectroscopic studies for fac-[ReCl(CO)3(dppz-X2)], (dppz = dipyrido[3,2-a:2',3'-c]phenazine; X = CH3, H, F, Cl, CF3) are reported. For all complexes the calculations show that the lowest unoccupied molecular orbital (LUMO) is a phenazine based orbital localized on the dppz ligand. We observe that three different excited states, IL pi pi*, metal-to-ligand charge-transfer (MLCT) (phen), and MLCT (phz), are formed depending upon the substituent on the dppz ligand and on the nature of the solvent. This means that both the energy and the nature of the photophysically active state(s) can be tuned by both chemical modification of dppz ligand and solvent properties. The excited-state dynamics in these systems is directly related to the mechanism of the "light switch effect", and ps-TRIR has allowed a deeper insight into this mechanism by being able to directly monitor the change in the population of the higher lying emissive phen-type (3)MLCT and IL pi pi* states and the dark (3)MLCT (phz) state depending on the different environmental factors.     

Photophysical behavior of open-shell first-row transition-metal octabutoxynaphthalocyanines: CoNc(OBu)8 and CuNc(OBu)8 as case studies

Ultrafast photodynamics and density functional theory/time-dependent density functional theory (DFT/TDDFT) results for complexes of Co and Cu with 5,9,14,18,23,27,32,36-octabutoxynaphthalocyanine [CoNc(OBu)8 and CuNc(OBu)8] are reported. As a basis for this work, details concerning the syntheses of these complexes and the corresponding Zn complex (used as a reference) are given. Transient absorption spectrometry with femtosecond time resolution combined with a detailed DFT/TDDFT analysis has been employed to construct a complete picture of the excited-state dynamics after Q-band excitation of the Co and Cu complexes and to gain an understanding of the relationship between the nature of the metal center and the excited-state lifetime. The Co complex was shown to return to its ground state via two competing channels: a (2)T1(pi, pi*) state that decayed with a lifetime of 1 ps and a low-lying (2)(d, d) state that repopulated the ground-state surface with a lifetime of 15 ps. CuNc(OBu)8 showed ground-state repopulation from the (2)T1(pi, pi*) state via a lower-lying ligand-to-metal charge-transfer (LMCT) state that was completed within a few nanoseconds. The photophysical behavior of the cobalt and copper complexes was compared to that previously reported for the nickel analog in an effort to highlight the effect of the central metal on the nature and rates of the deactivation pathways. The results described in this work provide basic knowledge that is relevant to the use of these compounds as photothermal sensitizers in cancer therapy.     

Electronic structure and excited states of rhenium(I) amido and phosphido carbonyl-bipyridine complexes studied by picosecond time-resolved IR spectroscopy and DFT calculations

UV-vis absorption and picosecond time-resolved IR (TRIR) spectra of amido and phosphido complexes fac-[Re(ER2)(CO)3(bpy)] (ER2 = NHPh, NTol2, PPh2, bpy = 2,2'-bipyridine, Tol = 4-methylphenyl) were investigated in conjunction with DFT and TD-DFT calculations in order to understand their ground-state electronic structure, low-lying electronic transitions and excited-state character and dynamics. The HOMO is localized at the amido/phosphido ligand. Amide and phosphide ligands are sigma-bonded to Re, the pi interaction being negligible. Absorption spectra show a weak band at low energies (1.7-2.1 eV) that arises from essentially pure ER(2) --> bpy ligand-to-ligand charge transfer (LLCT). The lowest excited state is the corresponding triplet, (3)LLCT. Low triplet energies and large distortions diminish the excited-state lifetimes to 85 and 270 ps for NHPh and NTol(2), respectively, and to ca. 30 ps for PPh2. nu(CO) vibrations undergo only very small ( bpy MLCT character, is a unique feature of the amido/phoshido complexes, whose lowest excited state can be viewed as containing a highly unusual aminyl/phosphinyl radical-cationic ligand. For comparison, the amino and phosphino complexes fac-[Re(NHPh(2))(CO)3(bpy)]+ and fac-[Re(PPh3)(CO)3(bpy)]+ are shown to have the usual Re --> bpy (3)MLCT lowest excited states, characterized by upshifted nu(CO) bands.     

On the viability of cyclometalated Ru(II) complexes as dyes in DSSC regulated by COOH group, a DFT study

The Ru(II) complexes [Ru(bpp)(dcbpy)Cl](+) (1), [Ru(tcbpp)(bpy)Cl](+) (2), and [Ru(tc'bpp)(bpy)Cl](+) (3) (bpp = 2,6-bis(N-pyrazolyl)pyridine, dcbpy = 4,4'-dicarboxyl-bipyridine, bpy = bipyridine, tcbpp = 4-carboxyl-2,6-bis(2-carboxyl-N-pyrazolyl)pyridine, tc'bpp = 4-carboxyl-2,6-bis(4-carboxyl-N-pyrazolyl)pyridine) are studied theoretically using density functional theory (DFT) techniques to explore their properties as dye in a solar cell. The calculated geometry structure and absorption spectrum of 1 are consistent with its experimental results. The calculation results indicate which sites the COOH groups attach to can significantly influence the electronic structure of the complex. By migrating the COOH groups from the bpy ligand in 1 to bpp ligand in 2 and 3, the nature of LUMO changes from bpy-localized to bpp dominated. The calculated low-lying absorptions at λ > 370 nm of the three complexes are categorized as metal-to-ligand charge-transfer (MLCT) transitions and the transition terminates at the orbital populated by the COOH appended ligand. The atomic spin density analysis also indicates that the ligand which is modified by the COOH groups is the ideal spot for the captured electron to situate. It can be predicted that the performance of 2 and 3 in the dye-sensitized solar cell can be enhanced as compared with 1.     

Charge transfer in the Cl-CO cluster induced by core ionization

Ab initio calculations of core-ionization spectra of the anion-molecule Cl-CO cluster are performed. Particular attention is paid to the investigation of charge-transfer screening processes accompanying core ionization of the CO molecule in the cluster. The charge-transfer processes are very efficient and favored by the presence of a low-lying unoccupied pi* orbital in CO capable of accepting an electron from Cl-. The O1s(-1) and C1s(-1) core-ionization spectra are calculated and compared. Both reveal a breakdown of the quasiparticle picture of core ionization caused by the charge-transfer processes. Remarkable differences between these two spectra are found which manifest themselves in distinct intensity distributions in the prominent low-energy spectral bands. The underlying reason for these differences is elucidated and linked with the preference of the pi* orbital to localize mainly on carbon. Core-ionization spectra of anion-molecule clusters are very sensitive to the type of the molecule involved as the comparative analysis of the O1s(-1) core-ionization spectra of the Cl-CO and Cl-H(2)O clusters show.     

Electroswitchable photoluminescence activity: synthesis, spectroscopy, electrochemistry, photophysics, and X-ray crystal and electronic structures of [Re(bpy)(CO)3(C[triple bond]C[bond]C6H4[bond]C[triple bond]C)Fe(C5Me5)(dppe)][PF6](n) (n = 0, 1)

A novel heterobimetallic alkynyl-bridged complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)], 1, and its oxidized species, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 2, have been synthesized and their X-ray crystal structures determined. A related vinylidene complex, [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond](H)C[double bond]C)Fe(C(5)Me(5))(dppe)][PF(6)], 3, has also been synthesized and characterized. The cyclic voltammogram of 1 shows a quasireversible reduction couple at -1.49 V (vs SCE), a fully reversible oxidation at -0.19 V, and a quasireversible oxidation at +0.88 V. In accord with the electrochemical results, density-functional theory calculations on the hydrogen-substituted model complex Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C)Fe(C(5)H(5))(dHpe) (Cp = C(5)H(5), dHpe = H(2)P[bond](CH(2))(2)[bond]PH(2)) (1-H) show that the LUMO is mainly bipyridine ligand pi* in character while the HOMO is largely iron(II) d orbital in character. The electronic absorption spectrum of 1 shows low-energy absorption at 390 nm with a 420 nm shoulder in CH(2)Cl(2), while that of 2 exhibits less intense low-energy bands at 432 and 474 nm and additional low-energy bands in the NIR at ca. 830, 1389, and 1773 nm. Unlike the related luminescent rhenium(I)-alkynyl complex [Re(bpy)(CO)(3)(C[triple bond]C[bond]C(6)H(4)[bond]C[triple bond]C[bond]H)], 4, complex 1 is found to be nonemissive, and such a phenomenon is attributed to an intramolecular quenching of the emissive d pi(Re) --> pi*(bpy) (3)MLCT state by the low-lying MLCT and LF excited states of the iron moiety. Interestingly, switching on of the luminescence property derived from the d pi(Re) --> pi*(bpy) (3)MLCT state can be demonstrated in the oxidized species 2 and the related vinylidene analogue 3 due to the absence of the quenching pathway.     

Luminescent charge-transfer platinum(II) metallacycle

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan-2,2'-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine] (1), are compared with two reference compounds, Pt(dbbpy)(C[triple bond]CPh)(2) (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane] (3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1, whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet manifold. In CH(2)Cl(2), 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77 K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for 3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as (3)tda in nature, implying that the charge-transfer states are raised in energy, relative to the (3)tda levels in 1 in the low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined for 1-3 in CH(2)Cl(2) and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed at room temperature. As 3 does not possess any charge-transfer character, excitation into the pipi* transitions of the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence are responsible for the absorption transients. ESR spectroscopy of the anions 1- and 2- and reductive spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the pi* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH(2)Cl(2) continuum revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions in these molecules. A DeltaSCF-method-derived shift applied to the calculated transition energies in the solvent continuum yielded good agreement between theory and experiment for each molecule in this study.     

trans-cis Photoisomerization of the styrylpyridine Ligand in [Re(CO)3(2,2'-bipyridine)(t-4-styrylpyridine)]+: role of the metal-to-ligand charge-transfer excited states

The trans-cis isomerization of the styrylpyridine carbon-carbon double bond induced by visible light irradiation in fac-[Re(CO)(3)(bpy)(stpy)](+) (bpy = 2,2'-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)(3)(bpy)(stpy)](+) have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol(-1) each other. The energy difference between the cis and trans isomers is 27.0 kJ mol(-1). The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d(Re)-->pi*(bpy)) transitions calculated at about 25,000 cm(-1) and by a strong intraligand (1)IL (pi(stpy)-->pi*(stpy)) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the C=C bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the (1)IL (S(1)) and electronic ground (S(0)) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the (3)IL state. From the (3)IL(t) (t = trans) the system evolves to the perpendicular intermediate (3)IL(p) (p = perpendicular) following a 90 degrees rotation around the styrylpyridine C=C bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the (3)IL and electronic ground state PEC determines the quantum yield of the isomerization process.     

Synthesis and study of Ru,Rh,Ru triads: modulation of orbital energies in a supramolecular architecture

Supramolecular trimetallic complexes [((tpy)RuCl(BL))(2)RhCl(2)](3+) where tpy = 2,2':6',2' '-terpyridine and BL = dpp or bpm [dpp = 2,3-bis(2-pyridyl)pyrazine and bpm = 2,2'-bipyrimidine] have been synthesized and characterized. The mixed-metal complexes couple a reactive rhodium(III) center to two ruthenium(II) light absorbers to form a light absorber-electron collector-light absorber triad. The variation of the bridging (dpp and bpm) and terminal (tpy in lieu of bpy) ligands has some profound effects on the properties of these complexes, and they are remarkably different from the previously reported [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+) system. The electrochemical data for both title trimetallics consist of overlapping Ru(III/II) couples for both terminal metals at 1.12 V versus the Ag/AgCl reference electrode. Cathodically an irreversible Rh(III/I) reduction followed by bridging ligand reductions is seen. This is indicative of highest occupied molecular orbitals (HOMO) localized on the terminal ruthenium metal centers and a lowest unoccupied molecular orbital (LUMO) residing on the rhodium. This rhodium-based LUMO is in contrast to the bpy analogue [((bpy)(2)Ru(bpm))(2)RhCl(2)](5+), which has a bpm(pi) localized LUMO. This orbital inversion by terminal ligand variation illustrates the similar energy of these Rh(dsigma) and bpm(pi) orbitals within this structural motif. Both title trimetallics possess broad, low-energy Ru --> BL charge transfer absorbances at 540 nm (dpp) and 656 nm (bpm). A comparison of the spectroscopic, electrochemical, and spectroelectrochemical properties of these trimetallic complexes is presented.     

Density functional theory calculations of selected Ru(II) two ring diimine complex dications

Geometry optimization for a series of ten, two-ring diimine Ru(II) complexes was effected using the Gaussian 98 protocol at density functional theory (DFT) B3LYP level with basis sets 3-21G*and 3-21G**. HOMO-LUMO energy difference values compared favorably to the experimental data from electrochemistry [Delta E(1/2) = (E(1/2ox) - E(1/2red))] and the lowest energy absorption maxima, which for these complexes correspond to the metal-to-ligand charge transfer (MLCT) band. The HOMO and LUMO distributions from DFT support the idea that the lowest energy transitions are metal-to-ligand charge transfer and that the lowest energy LUMO for the mixed ligand complexes is located on 2,2'-bipyrazine (bpz), followed by 2,2'-bipyrimidine (bpm) and then 2,2'-bipyridine (bpy).