Density functional theory/time-dependent DFT studies on the structures, trend in DNA-binding affinities, and spectral properties of complexes [Ru(bpy)2(p-R-pip)]2+ (R = -OH, -CH3, -H, -NO2)

Studies on the electronic structures and trend in DNA-binding affinities of a series of Ru(II) complexes [Ru(bpy)2(p-R-pip)]2+ (bpy = 2,2-bipyridine; pip = 2-phenylimidazo[4,5-f] [1,10]-phenanthroline; R = -OH, -CH3, -H, -NO2) 1-4 have been carried out, using the density functional theory (DFT) at the B3LYP/LanL2DZ level. The electronic absorption spectra of these complexes were also investigated using time-dependent DFT (TDDFT) at the B3LYP//LanL2DZ/6-31G level. The computational results show that the substituents on the parent ligand (pip) have a significant effect on the electronic structures of the complexes, in particular, on the energies of the lowest unoccupied molecular orbital (LUMO) and near some unoccupied molecular orbitals (LUMO+x, x = 1-4). With the increase in electron-withdrawing ability of the substituent in this series, the LUMO+x (x = 0-4) energies of the complexes are substantially reduced in order, for example, epsilon(LUMO)(1) approximately epsilon(LUMO)(2) > epsilon(LUMO)(3) > epsilon(LUMO)(4), whereas the pi-component populations of the LUMO+x (x = 0-4) are not substantially different. Combining the consideration of the bigger steric hindrance of complex 2, the trend in DNA-binding affinities (K(b)) of the complexes, that is, K(b)(2) < K(b)(1) < K(b)(3) < K(b)(4) can be reasonably explained. In addition, the experimental singlet metal-to-ligand charge transfer ((1)MLCT) spectra of these complexes can be well simulated and discussed by the TDDFT calculations.     

Characterization of low energy charge transfer transitions in (terpyridine)(bipyridine)ruthenium(II) complexes and their cyanide-bridged bi- and tri-metallic analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption bands found in ambient solutions of a series of [Ru(tpy)(bpy)X](m+) complexes (tpy = 2,2':3',2'-terpyridine; bpy = 2,2'-bipyridine; and X = a monodentate ancillary ligand) feature one or two partly resolved weak absorptions (bands I and/or II) on the low energy side of their absorption envelopes. Similar features are found for the related cyanide-bridged bi- and trimetallic complexes. However, the weak absorption band I of [(bpy)(2)Ru{CNRu(tpy)(bpy)}(2)](4+) is missing in its [(bpy)(2)Ru{NCRu(tpy)(bpy)}(2)](4+) linkage isomer demonstrating that this feature arises from a Ru(II)/tpy MLCT absorption. The energies of the MLCT band I components of the [Ru(tpy)(bpy)X](m+) complexes are proportional to the differences between the potentials for the first oxidation and the first reduction waves of the complexes. Time-dependent density functional theory (TD-DFT) computational modeling indicates that these band I components correspond to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered. The most intense contribution to a lowest energy MLCT absorption envelope (band III) of these complexes corresponds to the convolution of several orbitally different components, and its absorption maximum has an energy that is about 5000 cm(-1) higher than that of band I. The multimetallic complexes that contain Ru(II) centers linked by cyanide have mixed valence excited states in which more than 10% of electronic density is delocalized between the nearest neighbor ruthenium centers, and the corresponding stabilization energy contributions in the excited states are indistinguishable from those of the corresponding ground states. Single crystal X-ray structures and computational modeling indicate that the Ru-(C≡N)-Ru linkage is quite flexible and that there is not an appreciable variation in electronic structure or energy among the conformational isomers.     

Electrochemical, spectroscopic, and photophysical properties of structurally diverse polyazine-bridged Ru(II),Pt(II) and Os(II),Ru(II),Pt(II) supramolecular motifs

Five new tetrametallic supramolecules of the motif [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) and three new trimetallic light absorbers [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) (TL = bpy = 2,2'-bipyridine or phen = 1,10-phenanthroline; M = Ru(II) or Os(II); BL = dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline, or bpm = 2,2'-bipyrimidine) were synthesized and their redox, spectroscopic, and photophysical properties investigated. The tetrametallic complexes couple a Pt(II)-based reactive metal center to Ru and/or Os light absorbers through two different polyazine BL to provide structural diversity and interesting resultant properties. The redox potential of the M(II/III) couple is modulated by M variation, with the terminal Ru(II/III) occurring at 1.58-1.61 V and terminal Os(II/III) couples at 1.07-1.18 V versus Ag/AgCl. [{(TL)(2)M(dpp)}(2)Ru(BL)](PF(6))(6) display terminal M(dπ)-based highest occupied molecular orbitals (HOMOs) with the dpp(π*)-based lowest unoccupied molecular orbital (LUMO) energy relatively unaffected by the nature of BL. The coupling of Pt to the BL results in orbital inversion with localization of the LUMO on the remote BL in the tetrametallic complexes, providing a lowest energy charge separated (CS) state with an oxidized terminal Ru or Os and spatially separated reduced BL. The complexes [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) and [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) efficiently absorb light throughout the UV and visible regions with intense metal-to-ligand charge transfer (MLCT) transitions in the visible at about 540 nm (M = Ru) and 560 nm (M = Os) (ε ≈ 33,000-42,000 M(-1) cm(-1)) and direct excitation to the spin-forbidden (3)MLCT excited state in the Os complexes about 720 nm. All the trimetallic and tetrametallic Ru-based supramolecular systems emit from the terminal Ru(dπ)→dpp(π*) (3)MLCT state, λ(max)(em) ≈ 750 nm. The tetrametallic systems display complex excited state dynamics with quenching of the (3)MLCT emission at room temperature to populate the lowest-lying (3)CS state population of the emissive (3)MLCT state.     

Computational study of iron(II) systems containing ligands with nitrogen heterocyclic groups

Density functional theory (DFT) calculations show the higher energy HOMO (highest occupied molecular orbital) orbitals of four iron(II) diimine complexes are metal centered and the lower energy LUMO (lowest unoccupied molecular orbitals) are ligand centered. The energy of the orbitals correlates with electrochemical redox potentials of the complexes. Time-dependent density functional theory (TDDFT) calculations reveal ligand centered (LC) and metal-to-ligand charge transfer (MLCT) at higher energy than experimentally observed. TDDFT calculations also reveal the presence of d-d transitions which are buried under the MLCT and LC transitions. The difference in chemical and photophysical behavior of the iron complexes compared to that of their ruthenium analogues is also addressed.     

以2-(2-吡啶基)苯并噻吩为主配体的两种蓝紫光二价铂配合物的合成与性质

以2-(2-吡啶基)苯并噻吩(2-(2-pyridyl)benzothiophene,btp)作为主配体,分别以邻二氮菲[1,10]并咪唑联苯酚(2-(1Himidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol,ipap)和3-甲基-6-苯基咪唑[2,1-b]噻唑(3-Methyl-6-phenylimidazo[2,1-b]thiazole,mpmt)作为辅助配体,成功合成了2个二价铂配合物[(btp)Pt(ipap)]Cl与(btp)Pt(mpmt)Cl,并得到了配合物(btp)Pt(mpmt)Cl的晶体结构。由金属-配体电荷转移引起的2种配合物发光,具有60%左右的内量子效率,发射峰在426 nm(蓝色)与381 nm(紫色)处。HOMO/LUMO轨道能级分别为-4.69 eV/-2.55 eV与-4.80 eV/-2.21 eV。单晶衍射结果表明,较低的共轭程度导致了该类配合物的短波长发射。     

Synthesis and spectroscopic characterization of CN-substituted bipyridyl complexes of Ru(II)

A series of ruthenium complexes having the general form [Ru(bpy)(3-n)(CN-Me-bpy)(n)](PF(6))(2) (where bpy = 2,2'-bipyridine, CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, and n = 1-3 for complexes 1-3, respectively) have been synthesized and characterized using a variety of steady-state and nanosecond time-resolved spectroscopies. Electrochemical measurements indicate that the CN-Me-bpy ligand is significantly easier to reduce than the unsubstituted bipyridine (on the order of ～500 mV), implying that the lowest energy (3)MLCT (metal-to-ligand charge transfer) state will be associated with the CN-Me-bpy ligand(s) in all three compounds. Comparison of the Huang-Rhys factors derived from spectral fitting analyses of the steady state emission spectra of complexes 1-3 suggests all three compounds are characterized by excited-state geometries that are less distorted relative to their ground states as compared to [Ru(bpy)(3)](PF(6))(2); the effect of the more nested ground- and excited-state potentials is reflected in the unusually high radiative quantum yields (13% (1), 27% (2), and 40% (3)) and long (3)MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the π* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a ～40 cm(-1) bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the (3)MLCT excited state relative to the ground state. The fact that the shift is the same for complexes 1-3 is evidence that, in all three complexes, the long-lived excited state is localized on a single CN-Me-bpy ligand rather than being delocalized over multiple ligands.     

Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands

The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane ( 1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane ( 2). A homoleptic ([Ru( 2) 2] (2+)) and a heteroleptic complex ([Ru(ttpy)( 2)] (2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru( 1) 2] (2+). The excited-state lifetime of the homoleptic Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central Ru (II) ion less octahedral in [Ru( 2) 2] (2+) than in [Ru( 1) 2] (2+). Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer ( (3)MLCT) and metal-centered ( (3)MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru (II) complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from (3)MLCT states to short-lived (3)MC states.     

Characteristics and properties of metal-to-ligand charge-transfer excited states in 2,3-bis(2-pyridyl)pyrazine and 2,2'-bypyridine ruthenium complexes. Perturbation-theory-based correlations of optical absorption and emission parameters with electrochemistry and thermal kinetics and related Ab initio calculations

The absorption, emission, and infrared spectra, metal (Ru) and ligand (PP) half-wave potentials, and ab initio calculations on the ligands (PP) are compared for several [L(n)()Ru(PP)](2+) and [[L(n)Ru]dpp[RuL'(n)]](4+) complexes, where L(n) and L'(n) = (bpy)(2) or (NH(3))(4) and PP = 2,2'-bipyridine (bpy), 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3-bis(2pyridyl)benzoquinoxaline (dpb). The energy of the metal-to-ligand charge-transfer (MLCT) absorption maximum (hnu(max)) varies in nearly direct proportion to the difference between Ru(III)/Ru(II) and (PP)/(PP)(-) half-wave potentials, DeltaE(1/2), for the monometallic complexes but not for the bimetallic complexes. The MLCT spectra of [(NH(3))(4)Ru(dpp)](2+) exhibit three prominent visible-near-UV absorptions, compared to two for [(NH(3))(4)Ru(bpy)](2+), and are not easily reconciled with the MLCT spectra of [[(NH(3))(4)Ru]dpp[RuL(n)]](4+). The ab initio calculations indicate that the two lowest energy pi orbitals are not much different in energy in the PP ligands (they correlate with the degenerate pi orbitals of benzene) and that both contribute to the observed MLCT transitions. The LUMO energies calculated for the monometallic complexes correlate strongly with the observed hnu(max) (corrected for variations in metal contribution). The LUMO computed for dpp correlates with LUMO + 1 of pyrazine. This inversion of the order of the two lowest energy pi orbitals is unique to dpp in this series of ligands. Configurational mixing of the ground and MLCT excited states is treated as a small perturbation of the overall energies of the metal complexes, resulting in a contribution epsilon(s) to the ground-state energy. The fraction of charge delocalized, alpha(DA)(2), is expected to attenuate the reorganizational energy, chi(reorg), by a factor of approximately (1 - 4alpha(DA)(2) + alpha(DA)(4)), relative to the limit where there is no charge delocalization. This appears to be a substantial effect for these complexes (alpha(DA)(2) congruent with 0.1 for Ru(II)/bpy), and it leads to smaller reorganizational energies for emission than for absorption. Reorganizational energies are inferred from the bandwidths found in Gaussian analyses of the emission and/or absorption spectra. Exchange energies are estimated from the Stokes shifts combined with perturbation--theory-based relationship between the reorganizational energies for absorption and emission values. The results indicate that epsilon(s) is dominated by terms that contribute to electron delocalization between metal and PP ligand. This inference is supported by the large shifts in the N-H stretching frequency of coordinated NH(3) as the number of PP ligands is increased. The measured properties of the bpy and dpp ligands seem to be very similar, but electron delocalization appears to be slightly larger (10-40%) and the exchange energy contributions appear to be comparable (e.g., approximately 1.7 x 10(3) cm(-1) in [Ru(bpy)(2)dpp](2+) compared to approximately 1.3 x 10(3) cm(-1) in the bpy analogue).     

A multifunctional tetrametallic Ru-Pt supramolecular complex exhibiting both DNA binding and photocleavage

A new tetrametallic supramolecular complex, [{(bpy)2Ru(dpp)}2Ru(dpp)PtCl2](PF6)6 (bpy = 2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine), has been prepared and characterized. This supramolecular assembly is multifunctional, forming coordinate covalent bonds to DNA through its cis-PtCl2 moiety and photocleaving DNA through its Ru polypyridine chromophores. Electronic absorption spectroscopy shows ligand-based pi-->pi* transitions in the UV with Ru-based metal-to-ligand charge transfer transitions throughout much of the visible. The Ru-->dpp CT transitions center at 542 nm (eta = 35 000 M-1 cm-1). This complex has a HOMO localized on peripheral Ru with E(1/2)oxd = 1.58 V vs Ag/AgCl, and a LUMO based on the mu-dpp connecting Ru and Pt, E(1/2)red = -0.40 V. Gel electrophoresis analysis shows the tetrametallic coordinates to pUC18 DNA and, when excited with visible light, cleaves DNA through an oxygen-mediated pathway.     

Synthesis of panchromatic Ru(II) thienyl-dipyrrin complexes and evaluation of their light-harvesting capacity

Ru(II) complexes with 5-(3-thienyl)-4,6-dipyrrin (3-TDP), containing 2,2'-bipyridine (bpy) or 4,4'-bis(methoxycarbonyl)-2,2'-bipyridine (dcmb) as coligands, have been prepared and extensively characterized. Crystal structure determination of [Ru(bpy)(2)(3-TDP)]PF(6) (1a) and [Ru(bpy)(3-TDP)(2)] (2) reveals that the 3-thienyl substituent is rotated with respect to the plane of the dipyrrinato moiety. These complexes, as well as [Ru(dcmb)(2)(3-TDP)]PF(6) (1b), act as panchromatic light absorbers in the visible range, with two strong absorption bands observable in each case. A comparison to known Ru(II) complexes and quantum-chemical calculations at the density functional theory (DFT) level indicate that the lower-energy band is due to metal-to-ligand charge transfer (MLCT) excitation, although the frontier occupied metal-based molecular orbitals (MOs) contain significant contributions from the 3-TDP moiety. The higher energy band is assigned to the π-π* transition of the 3-TDP ligand. Each complex exhibits an easily accessible one-electron oxidation. According to DFT calculations and spectroelectrochemical experiments, the first oxidation takes place at the Ru(II) center in 1a, but is shifted to the 3-TDP ligand in 1b. An analysis of MO energy diagrams suggests that complex 1b has potential to be used for light harvesting in the dye-sensitized (Gr?tzel) solar cell.     

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.     

Electronic structures and spectroscopic properties of rhenium (I) tricarbonyl photosensitizer: [Re(4,4'-(COOEt)2-2,2'-bpy)(CO)3py]PF6

The ground state and lowest triplet-state structures of [Re(4,4'-(COOEt)(2)-2,2'-bpy)(CO)(3)py]PF(6) photosensitizer (bpy=bipyridine, py=pyridine) have been studied with density functional theory (DFT). Time-dependent density functional theory (TD-DFT) was carried out to predict the photophysical properties of the photosensitizer. The effects of the solvents were evaluated using the conductor-like polarizable continuum (CPCM) method in dichloromethane, chloroform, acetonitrile, acetone, ethanol and dimethylsulfoxide. The electronic transition energies computed with BLYP, MPWPW91, B3LYP and MPW1PW91 functionals are compared with the experimental spectra. Based on the calculated excited energies, the experimental absorption maximum is assigned as metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) mixed transition, and the luminescence originates from the lowest triplet state that is ascribed as the mixed transition of MLCT/LLCT.     

Series of new cationic iridium(III) complexes with tunable emission wavelength and excited state properties: structures, theoretical calculations, and photophysical and electrochemical properties

A series of new cationic iridium(III) complexes [Ir(piq)2(N/\N)]+PF6- (1-6) (piq =1-phenyl-isoquinoline) containing N/\N ligands with different conjugated lengths were synthesized, where the six N/\N ligands were bipyridine, phenanthroline, 2-pyridyl-quinoline, 2,2'-biquinoline, 1,1'-biisoquinoline, and 2-(2-quniolinyl)quinoxaline. Single-crystal X-ray diffraction spectra of three complexes were studied, and the iridium(III) centers were found to adopt a distorted octahedral coordination geometry with cis metalated carbons and trans nitrogen atoms. UV-vis, photoluminescence, cyclic voltammetry, and theoretical calculations were employed for studying the photophysical and electrochemical properties. And the excited-state properties were investigated in detail. The excited state of complexes is complicated and contains triplet metal-to-ligand charge transfer (3MLCT), triplet ligand-to-ligand charge transfer (3LLCT), and ligand-centered (cyclometalated) (3LC) transitions simultaneously. Importantly, the emission wavelength can be tuned significantly from 586 to 732 nm by changing the conjugated length of N/\N ligands.     

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.     

Tuning and switching MLCT phosphorescence of [Ru(bpy)3]2+ complexes with triarylboranes and anions

Four new Ru(II) complexes, [Ru(bpy)(2)(4,4'-BP2bpy)][PF(6)](2) (1), [Ru(t-Bu-bpy)(2)(4,4'-BP2bpy)][PF(6)](2) (2), [Ru(bpy)(2)(5,5'-BP2bpy)][PF(6)](2) (3), and [Ru(t-Bu-bpy)(2)(5,5'-BP2bpy)][PF(6)](2) (4) have been synthesized (where 4,4'-BP2bpy = 4,4'-bis(BMes(2)phenyl)-2,2'-bpy; 5,5'-BP2bpy = 5,5'-bis(BMes(2)phenyl)-2,2'-bpy (4,4'-BP2bpy); and t-Bu-bpy = 4,4'-bis(t-butyl)-2,2'-bipyridine). These new complexes have been fully characterized. The crystal structures of 3 and 4 were determined by single-crystal X-ray diffraction analyses. All four complexes display distinct metal-to-ligand charge transfer (MLCT) phosphorescence that has a similar quantum efficiency as that of [Ru(bpy)(3)][PF(6)](2) under air, but is at a much lower energy. The MLCT phosphorescence of these complexes has been found to be highly sensitive toward anions such as fluoride and cyanide, which switch the MLCT band to higher energy when added. The triarylboron groups in these compounds not only introduce this color switching mechanism, but also play a key role in the phosphorescence color of the complexes.     

Effect of electronic structure on the photoinduced ligand exchange of Ru(II) polypyridine complexes

The series of complexes [Ru(bpy)(2)(L)](2+), where bpy = 2,2'-bipyridine and L = 3,6-dithiaoctane (bete, 1), 1,2-bis(phenylthio)ethane (bpte, 2), ethylenediamine (en, 3), and 1,2-dianilinoethane (dae, 4), were synthesized, and their photochemistry was investigated. Photolysis experiments show that the bisthioether ligands in 1 and 2 are more easily photosubstituted by chloride ions, bpy, and H(2)O than the corresponding diammine complexes in 3 and 4 to generate the bis-substituted products. Electronic structure calculations show that bond elongation in the lowest energy triplet metal-to-ligand charge transfer ((3)MLCT) state compared to the ground state is greater for complexes containing bisthioether ligands than those with coordinated bidentate nitrogen atoms. This elongation in the excited state is attributed to Ru-S π-bonding character of the highest occupied molecular orbitals, which is not present in the diamine complexes. In the Ru→bpy (3)MLCT state, the lower electron density on the metal-centered highest occupied molecular orbital (HOMO) weakens the Ru-S bond and results in the greater photoreactivity of 1 and 2 relative to that of 3 and 4. The more efficient photoinduced ligand exchange of the complexes possessing thioether ligands results in binding of 1 and 2 to DNA upon irradiation.     

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.     

Bis(tridentate) Ruthenium-Terpyridine Complexes Featuring Microsecond Excited-State Lifetimes

A series of heteroleptic bis(tridentate) ruthenium(II) complexes, each bearing a substituted 2,2':6',2″-terpyridine (terpy) ligand, is characterized by room temperature microsecond excited-state lifetimes. This observation is a consequence of the strongly σ-donating and weakly π-accepting tridentate carbene ligand, 2',6'-bis(1-mesityl-3-methyl-1,2,3-triazol-4-yl-5-idene)pyridine (C(∧)N(∧)C), adjacent to the terpy maintaining a large separation between the ligand field and metal-to-ligand charge transfer (MLCT) states while also preserving a large (3)MLCT energy. The observed lifetimes are the highest documented lifetimes for unimolecular ruthenium(II) complexes and are four orders in magnitude higher than that associated with [Ru(terpy)(2)](2+).     

Synthesis, structural, and photophysical investigation of diimine triscarbonyl Re(I) tetrazolato complexes

The synthesis, structural, and photophysical properties of a novel family of neutral fac-[Re(N(∧)N)(CO)(3)(L)] complexes, where N(∧)N is either 2,2'-bipyridine or 1,10-phenanthroline and L is a para functionalized 5-aryltetrazolate [namely, 5-phenyltetrazolate (Tph(-)), 4-(tetrazolate-5-yl)benzaldehyde (Tbdz(-)), 5-(4-acetylphenyl)tetrazolate (Tacy(-)), and methyl 4-(tetrazolate-5-yl)benzoate (Tmeb(-))] are reported. The complexes were prepared by direct addition of the corresponding tetrazolate anion to the acetonitrile solvated fac-[Re(N(∧)N)(CO)(3)](+) precursor. NMR data demonstrate that the coordination of the metal fragment is regiospecific at the N2 atom of the tetrazolate ring. These conclusions are also supported by X-ray structural determinations. Photophysical data were obtained in diluted and deaerated dichloromethane solutions displaying broad and structureless profiles with emission maxima ranging from 566 to 578 nm. The absorption profiles indicate the presence of higher energy intraligand (IL) π-π* transitions and lower energies ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT). As the last two transitions are mixed, they are better described as a metal-ligand-to-ligand charge transfer (MLLCT), a result that is also supported by density functional theory (DFT) calculations. The complexes show excited state lifetime values ranging from 102 to 955 ns, with associated quantum yield between 0.012 and 0.099. Compared to the parent neutral chloro or bromo [Re(N(∧)N)(CO)(3)X], the complexes show a slightly improved performance because of the π accepting nature of the tetrazolato ligand. The metal-to-ligand backbonding is in fact depleting the Re center of electron density, thus widening the HOMO-LUMO gap and reducing the non-radiative decay mechanism in accordance with the energy gap law. Finally, the electron-withdrawing or donating nature of the substituent on the phenyltetrazolato ligand allows the fine-tuning of the photophysical properties.     

Low power upconversion using MLCT sensitizers

Selective low energy excitation of the metal-to-ligand charge transfer (MLCT) transition in [Ru(dmb)(3)](2+)(dmb = 4,4'-dimethyl-2,2'-bipyridine) in the presence of anthracene or 9,10-diphenylanthracene yields easily visualized upconverted singlet fluorescence resulting from triplet-triplet annihilation at low excitation power.