Photophysics and optical limiting of platinum(II) 4'-arylterpyridyl phenylacetylide complexes

A series of platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl phenylacetylide complexes (5-8) with 4'-naphthyl, 4'-phenanthryl, 4'-anthryl, and 4'-pyrenyl substituents have been synthesized and characterized. The emission properties of these complexes and their corresponding platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl chloride complexes (1-4) at room temperature and 77 K have been systematically investigated. Except for the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex that emits from an admixing state consisting of metal-to-ligand charge-transfer (3MLCT), intraligand charge-transfer (3ILCT), and 3pi,pi characters, emissions of 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl phenylacetylide complexes all originate from a 3MLCT-dominant state. The emission lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex (8) is longer than 2 mus at room temperature, and more than 300 mus at 77 K, while the other three complexes possess an emission lifetime of 200-400 ns at room temperature and tens of microseconds at 77 K. Replacing the chloride ligand in the 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl chloride complexes by a phenylacetylide ligand significantly increases the emission efficiency by an order of magnitude, and the emission lifetimes become longer. In contrast, such an alternation has no pronounced effect on the emission efficiency and lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl complexes. In the transient difference absorption (TA) spectra of 5 and 6, a moderately intense absorption band from 470 to 830 nm and a bleaching band between 400 and 470 nm were observed. For 7, the TA spectrum features a narrow, weak bleaching band at approximately 380 nm and a strong, narrow band at approximately 420 nm, as well as a broad, structureless band from 470 to 750 nm. In addition, a fourth, positive band appears above 800 nm. Complex 8 exhibits a strong, narrow bleaching band at approximately 340 nm and a broad, positive band extending from 370 to 830 nm, with the band maximum appearing at approximately 520 nm. The lifetimes obtained from the kinetic transient absorption measurement coincide with those from the kinetic emission measurement, indicating that the transient absorption originates from the same excited state that emits or, alternatively, from a state that is in equilibrium with the emitting state. All complexes exhibit optical limiting for 4.1 ns laser pulses at 532 nm, with 8 giving rise to the strongest optical limiting, presumably because of the much longer triplet excited-state lifetime and the stronger transient absorption at 532 nm.     

Photophysics and nonlinear absorption of cyclometalated 4,6-diphenyl-2,2'-bipyridyl platinum(II) complexes with different acetylide ligands

The photophysical properties of a series of 4,6-diphenyl-2,2'-bipyridyl platinum(II) complexes bearing different σ-alkynyl ancillary ligands (1a-1k) were systematically investigated. All complexes exhibit strong (1)π,π* absorption bands in the UV region; and broad, structureless charge-transfer band(s) in the visible region, which systematically red-shift(s) when the electron-donating ability of the para substituent on the phenylacetylide ligand increases. All complexes are emissive in solution at room temperature. When excited at the charge-transfer absorption band, the complexes exhibit long-lived orange emission (λ(max): 555-601 nm), which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission ((3)MLCT/(3)ILCT). Most of these complexes exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the (3)MLCT/(3)ILCT emission. The reverse saturable absorption (RSA) of these complexes were demonstrated at 532 nm using nanosecond laser pulses. The degree of RSA follows this trend: 1k ≈ 1a > 1c > 1f ≈ 1i > 1h ≈ 1b > 1e > 1d > 1g, which is mainly determined by the ratio of the triplet excited-state absorption cross section to that of the ground-state and the triplet excited-state quantum yield.     

Nonlinear Absorbing Platinum(II) Diimine Complexes: Synthesis, Photophysics, and Reverse Saturable Absorption

A series of platinum(II) diimine complexes with different substituents on fluorenyl acetylide ligands (1?a-1?e) were synthesized and characterized. The influence of the auxiliary substituent on the photophysics of these complexes has been systematically investigated spectroscopically and theoretically (using density functional theory (DFT) methods). All complexes exhibit ligand-centered (1) π,π* transitions in the UV and blue spectral region, and broad, structureless (1) MLCT/(1) LLCT (1?a, 1?b, 1?d and 1?e) or (1) MLCT/(1) LLCT/(1) π,π* (1?c) absorption bands in the visible region. All complexes are emissive in solution at room temperature, with the emitting state is tentatively assigned to mixed (3) MLCT/(3) π,π* states. The degree of (3) π,π* and (3) MLCT mixing varies with different substituents and solvent polarities. Complexes 1?a-1?e exhibit relatively strong singlet and triplet transient absorption from 450 to 800?nm, at which point reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532?nm by using nanosecond laser pulses demonstrate that 1?a-1?e are strong reverse saturable absorbers and could potentially be used as broadband nonlinear absorbing materials.     

Synthesis, photophysics, and optical limiting of platinum(II) 4'-tolylterpyridyl arylacetylide complexes

A series of 4'-tolylterpyridyl platinum(II) complexes with different arylacetylide ligands, namely, phenylacetylide, 4-bromophenylacetylide, 4-nitrophenylacetylide, 4-methoxyphenylacetylide, 4-dimethylaminophenylacetylide, 1-naphthylacetylide, and 3-quinolinylacetylide, were synthesized. Their photophysical properties, such as electronic absorption spectra, emission characteristics at room temperature and 77 K, and transient difference absorption spectra, have been investigated. All of these complexes exhibit a metal-to-ligand charge-transfer (1MLCT) transition at ca. 420-430 nm in their electronic absorption spectra. For ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, an additional solvatochromic charge-transfer band appears at ca. 460-540 nm. This band is sensitive to the para substituents on the phenylacetylide ligand and is tentatively assigned to a metal- or/and acetylide-to-terpyridyl charge-transfer transition (i.e., a 1MLCT or/and 1LLCT transition). All of the complexes exhibit room-temperature phosphorescence. The emission can be attributed to a 3MLCT state except for ttpy-C6H4NO2-4, for which the emission likely originates from an intraligand 3pi,pi* state involving the nitrophenylacetylide ligand. For ttpy-C6H4OCH3-4, ttpy-C6H4N(CH3)2-4, and ttpy-Np, there probably is more than one low-energy state in close energy proximity, resulting in multiple exponential decays. In addition, the triplet transient absorption difference spectra of ttpy-Ph, ttpy-C6H4Br-4, ttpy-C6H4NO2-4, and ttpy-Quin exhibit moderately intense, broad absorption bands in the visible region and extending into the near-IR region, which likely originate from the same excited state that emits or from a state that is in equilibrium with the emitting state. It appears that the electron-rich arylacetylide ligands, especially 4-methoxyphenylacetylide and 4-dimethylaminophenylacetylide, cause a decrease of the emission efficiency and disappearance of the transient absorption. In contrast, the complexes that exhibit positive absorption bands in the visible spectral region of the triplet transient difference absorption spectra show substantial optical limiting for nanosecond laser pulses at 532 nm.     

Synthesis, luminescence, and electrochemical studies of tris(homoleptic) ruthenium(II) and osmium(II) complexes of 6'-tolyl-2,2':4',2'-terpyridine

The synthesis and photophysical and electrochemical properties of tris(homoleptic) complexes [Ru(tpbpy)3](PF6)2 (1) and [Os(tpbpy)3](PF6)2 (2) (tpbpy = 6'-tolyl-2,2':4',2' '-terpyridine) are reported. The ligand tpbpy is formed as the side product during the synthesis of 4'-tolyl-2,2':6',2' '-terpyridine (ttpy) and characterized by single-crystal X-ray diffraction: monoclinic, P21/c. The tridentate tpbpy coordinates as a bidentate ligand. The complexes 1 and 2 exhibit two intense absorption bands in the UV region (200-350 nm) assignable to the ligand-centered (1LC) pi-pi* transitions. The ruthenium(II) complex exhibits a broad absorption band at 470 nm while the osmium(II) complex exhibits an intense absorption band at 485 nm and a weak band at 659 nm assignable to the MLCT (dpi-pi*) transitions. A red shifting of the dpi-pi* MLCT transition is observed on going from the Ru(II) to the Os(II) complex as expected from the high-lying dpi Os orbitals. These complexes exhibit ligand-sensitized emission at 732 and 736 nm, respectively, upon light excitation onto their MLCT band through excitation of higher energy LC bands at room temperature. The MLCT transitions and the emission maxima of 1 and 2 are substantially red-shifted compared to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2. The emission of both the complexes in the presence of acid is completely quenched indicating that the emission is not due to the protonation of the coordinated ligands. Our results indicate the occurrence of intramolecular energy transfer from the ligand to the metal center. Both the complexes undergo quasi-reversible metal-centered oxidation, and the E1/2 values for the M(II)/M(III) redox couples (0.94 and 0.50 V versus Ag/Ag+ for 1 and 2, respectively) are cathodically shifted with respect to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2 (E1/2 = 1.28 and 1.09 V versus Ag/Ag+, respectively). The tris(homoleptic) Ru(II) and Os(II) complexes 1 and 2 could be used to construct polynuclear complexes by using the modular synthetic approach in coordination compounds by exploiting the coordinating ability of the pyridine substituent. Furthermore, these complexes offer the possibility of studying the influence of electron-withdrawing and electron-donating substituents on the photophysical properties of Ru(II) and Os(II) polypyridine complexes.     

Ultrafast excited-state dynamics of rhenium(I) photosensitizers [Re(Cl)(CO)3(N,N)] and [Re(imidazole)(CO)3(N,N)]+: diimine effects

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)(3)(N,N)](n) (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650-285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (～100 fs)(-1) than in imidazole (～150 fs)(-1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV-vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N(?-) → Re(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy(?-)) transitions. For phen and dmp, the UV excited-state absorption occurs at ～305 nm, originating from a series of mixed ππ* and Re → CO;N,N(?-) MLCT transitions. UV-vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1-5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (～1 and 6-10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed.     

Switching between ligand-to-ligand charge-transfer, intraligand charge-transfer, and metal-to-ligand charge-transfer excited states in platinum(II) terpyridyl acetylide complexes induced by pH change and metal ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2'-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.     

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.     

Direct observation of a 1MLCT state by ultrafast transient absorption spectroscopy in Mo2(O2C-9-anthracene)4

We have observed the excited-state dynamics of Mo2(O2C-9-anthracene)4 in THF using ultrafast transient UV/vis absorption spectroscopy. Following excitation at 514.5 nm into the metal-to-ligand charge-transfer (MLCT) state, transient absorption bands of the 1MLCT state appear at 410 and 610 nm. We assign these features to the 1MLCT state, which has a lifetime of approximately 10 ps. The decay of 1MLCT is accompanied by the formation of the triplet 3MLCT state, with an absorption band peaking at 410 nm. Excitation at 347 nm populates directly the ligand-centered (LC) state. From the rise of the 1MLCT band, the lifetime of the 1LC state is estimated to be less than 1 ps.     

cis-Dicyanoosmium(II) diimine complexes bearing phosphine or sulfoxide ligands: spectroscopic and luminescent studies

A series of cis-dicyanoosmium(II) complexes [Os(PPh3)2(CN)2(N intersectionN)] (N intersectionN = Ph2phen (2a), bpy (2b), phen (2c), Ph2bpy (2d), tBu2bpy (2e)) and [Os(DMSO)2(CN)2(N intersectionN)] (3a-3e, N intersectionN = Br2phen (3f), Clphen (3g)), were synthesized and their spectroscopic and photophysical properties were examined, and [Os(PMe3)2(CN)2(phen)] (4) with axial PMe3 ligands was similarly prepared. The molecular structures of 2a, 2c, [2c.Zn(NO3)2]infinity, 2d, 2e, 3b, 3d, 3e, and 4 were determined by X-ray crystallographic analyses. The two CN ligands are cis to each other with mean Os-C bond distance of 2.0 A. The two PR3 (R = Ph, Me) or DMSO ligands are trans to each other with P/S-Os-P/S angles of approximately 177 degrees . The UV-vis absorption spectra of 2a-2e display an intense absorption band at 268-315 nm (epsilon = approximately (1.54-4.82) x 104 M-1 cm-1) that are attributed to pi --> pi*(N intersection N) and/or pi --> pi*(PPh3) transitions. The moderately intense absorption bands with lambdamax at 387-460 nm (epsilon = approximately (2.4-11.3) x 103 M(-1) cm(-1)) are attributed to a 1MLCT transition. A weak, broad absorption at 487-600 nm (epsilon = approximately 390-1900 M(-1) cm(-1)) is assigned to a 3MLCT transition. Excitation of 2a-2e in dichloromethane at 420 nm gives an emission with peak maximum at 654-703 nm and lifetime of 0.16-0.67 micros. The emission energies, lifetimes, and quantum yields show solvatochromic responses, and plots of numax, tau, and Phi, respectively, versus ET (solvent polarity parameter) show linear correlations, indicating that the emission is sensitive to the local environment. The broad structureless solid-state emission of 2a-2e at 298 (lambdamax 622-707 nm) and 77 (lambdamax 602-675 nm) K are assigned to 3MLCT excited states. The 77 K MeOH/EtOH (1:4) glassy solutions of 2a-2e also exhibit 3MLCT emissions with lambdamax = 560-585 nm. The 1MLCT absorption and 3MLCT emission of 3a-3g occur at lambdamax = 332-390 nm and 553-644 nm, respectively. In the presence of Zn(NO3)2, both the 1MLCT absorption and 3MLCT emission of 2c in acetonitrile blue-shift from 397 to 341 nm and 651 to 531 nm, respectively. The enhancement of emission intensity (I/Io) of 2e at 531 nm reached a maximum of approximately 810 upon the addition of two equivs of Zn(NO3)2. The crystallographic and spectroscopic evidence suggests that 2c undergoes binding of Zn2+ ions via the cyano moieties.     

Cyclometalated platinum(II) complex with strong and broadband nonlinear optical response

A cyclometalated platinum(II) 4,6-diphenyl-2,2'-bipyridyl pentynyl complex (1) has been synthesized and structurally characterized. Its photophysical and third-order nonlinear optical properties have been systematically investigated. This complex exhibits a metal-to-ligand charge-transfer (1MLCT) absorption band between 400 and 500 nm and a 3MLCT emission band at approximately 591 nm at room temperature with a lifetime of approximately 100 ns. At 77 K, the emission band blue shifts. Both UV-vis absorption and emission spectra show solvent dependence. Low-polarity solvents cause a bathochromic shift of the absorption and emission bands. This complex also exhibits a broad and strong transient absorption from the near-UV to the near-IR spectral region, with a triplet absorption coefficient of 4933 L mol(-1) cm(-1) at 585 nm and a quantum yield of 0.51 for the formation of the triplet excited state. Nonlinear transmission and Z-scan techniques were employed to characterize the third-order nonlinearities of this complex. A strong and broadband reverse saturable absorption was observed for nanosecond and picosecond laser pulses due to the reduced ground-state absorption in the visible spectral range. It also exhibits a self-defocusing effect at 532 nm for nanosecond laser pulses. The excited-state absorption cross section deduced from the open-aperture Z-scan increases at longer wavelengths, with an exceptionally large ratio of excited-state absorption to ground-state absorption of 160 at 570 nm for picosecond laser pulses.     

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.     

Theoretical studies of the spectroscopic properties of [Pt(trpy)C[triple bond]CR]+ (trpy = 2,2',6',2"-terpyridine; R = H, CH2OH, and C6H5)

Electronic structures and spectroscopic properties of [Pt(trpy)C[triple bond]CR](+) (trpy = 2,2', 6',2' '-terpyridine; R = H (1), CH(2)OH (2), and C(6)H(5) (3) ) are studied by ab initio and DFT methods. The ground- and excited-state structures are optimized by the MP2 and CIS methods, respectively. The absorption and emission spectra in the dichloromethane solution are obtained by using TD-DFT (B3LYP) method associated with the PCM model. The calculations indicate that, for 1-3, the variation of the substituents on the acetylide ligand only slightly changes their structures in ground and excited states but leads to a sizable difference in the electronic structures. In both cases of absorption and emission, the energy levels of HOMOs for 1-3 are sensitive to the substituents on acetylide ligand and increase obviously with the introduction of the electron-donating groups; however, those of trpy-based LUMOs vary slightly. The lowest-energy emissions are attributed to triplet acetylide/Pt --> trpy charge transfer ((3)LLCT/(3)MLCT) transitions and the lowest-energy absorptions and emissions for 1-3 are red-shifted on the order of 1 < 2 < 3 when the electron-donating groups are introduced into the acetylide ligand. By comparison of the results obtained by using different functionals in TD-DFT method, the calculations indicate that the exchange-correlation functionals (B3LYP, B3P86 and B3PW91) involving Becke three parameter hybrid functionals are appropriate for the terpyridyl platinum(II) acetylide complexes to get the relatively satisfactory results for the absorption spectra. The underestimated excitation energies of lowest-lying absorption bands are probably due to insufficient flexibility in TD-DFT method to describe states with large charge transfer.     

Ultrafast photochemical dissociation of an equatorial CO ligand from trans(X,X)-[Ru(X)2(CO)2(bpy)] (X = Cl, Br, I): a picosecond time-resolved infrared spectroscopic and DFT computational study

Ultrafast photochemistry of the complexes trans(X,X)-[Ru(X)(2)(CO)(2)(bpy)] (X = Cl, Br, I) was studied in order to understand excited-state reactivity of equatorial CO ligands, coordinated trans to the 2,2'-bipyridine ligand (bpy). TD-DFT calculations have identified the lowest electronic transitions and singlet excited states as mixed X -->bpy/Ru --> bpy ligand to ligand/metal to ligand charge transfer (LLCT/MLCT). Picosecond time-resolved IR spectroscopy in the region of nu(CO) vibrations has revealed that, for X = Cl and Br, subpicosecond CO dissociation is accompanied by bending of the X-Ru-X moiety, producing a pentacoordinated intermediate trans(X,X)-[Ru(X)(2)(CO)(bpy)]. Final movement of an axial halide ligand to the vacant equatorial position and solvent (CH(3)CN) coordination follows with a time constant of 13-15 ps, forming the photoproduct cis(X,X)-[Ru(X)(2)(CO)(CH(3)CN)(bpy)]. For X = I, the optically populated (1)LLCT/MLCT excited state undergoes a simultaneous subpicosecond CO dissociation and relaxation to a triplet IRuI-localized excited state which involves population of an orbital that is sigma-antibonding with respect to the axial I-Ru-I bonds. Vibrationally relaxed photoproduct cis(I,I)-[Ru(I)(2)(CO)(CH(3)CN)(bpy)] is formed with a time constant of ca. 55 ps. The triplet excited state is unreactive, decaying to the ground state with a 155 ps lifetime. The experimentally observed photochemical intermediates and excited states were assigned by comparing calculated (DFT) and experimental IR spectra. The different behavior of the chloro and bromo complexes from that of the iodo complex is caused by different characters of the lowest triplet excited states.     

Structures, spectroscopic properties and redox potentials of quaterpyridyl Ru(II) photosensitizer and its derivatives for solar energy cell: a density functional study

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru((R)L)(NCS)(2)] ((R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-2,2' : 6',2' : 6',2'-quaterpyridine, R = H (1), Me (2), (t)Bu (3) and COOH (4); (R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-cycloquaterpyridine, R = COOH (5)). The geometries of the molecular ground, univalent cationic and triplet excited states of 1-5 were optimized. In complexes 1-4, the quaterpyridine ligand retains its planarity in the molecular, cationic and excited states, although the C≡N-Ru angle representing the SCN → Ru coordination approaches 180° in the univalent cationic and triplet excited states. The theoretically designed complex 5 displays a curved cycloquaterpyridine ligand with significantly distorted SCN → Ru coordination. The electron spin density distributions reveal that one electron is removed from the Ru/NCS moieties upon oxidation and the triplet excited state is due to the Ru/NCS → polypyridine charge transfer (MLCT/L'LCT). The experimental absorption spectra were well reproduced by the time-dependent DFT calculations. In the visible region, two MLCT/L'LCT absorption bands were calculated to be at 652 and 506 nm for 3, agreeing with experimental values of 637 and 515 nm, respectively. The replacement of the R- group with -COOH stabilizes the lower-energy unoccupied orbitals of π* character in the quaterpyridine ligand in 4. This results in a large red shift for these two MLCT/L'LCT bands. In contrast, the lower-energy MLCT/L'LCT peak of 5 nearly disappears due to the introduction of cycloquaterpyridine ligand. The higher energy bands in 5 however become broader and more intense. As far as absorption in the visible region is concerned, the theoretically designed 5 may be a very promising sensitizer for DSSC. In addition, the redox potentials of 1-5 were calculated and discussed, in conjunction with photosensitizers such as cis-[Ru(L(1))(2)(X)(2)] (L(1) = 4,4'-bis(carboxylic acid)-2,2'-bipyridine; X = NCS(-) (6), Cl(-) (7) and CN(-) (8)), cis-[Ru(L(1)')(2)(NCS)(2)] (L(1)' = 4,7-bis(carboxylic acid)-1,10-phenanthroline, 9), [NH(4)][Ru(L(2))(NCS)(3)] (L(2) = 4,4',4'-tris(carboxylic acid)-2,2' : 6',2'-terpyridine, 10) and [Ru(L(2))(NCS)(3)](-) (11).     

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.     

Self-assembled light-harvesting systems: Ru(II) complexes assembled about Rh-Rh cores

Ru(II) polypyridine species have been assembled about dirhodium(II, II) tetracarboxylate cores. The complexes prepared have general formulas [{(terpy)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (a-type compounds: terpy = 2,2':6',2' '-terpyridine; La = 4'-(p-carboxyphenyl)-2,2':6',2' '-terpyridine; n = 1, 1a; n = 2, cis-2a and trans-2a-cis and trans refer to the arrangement of the Ru(II) species around the dirhodium core; n = 3, 3a), [{(Lb)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (b-type compounds: Lb = 6-phenyl-2,4-di(2-pyridyl)-s-triazine; n = 1, 1b; n = 2, an inseparable mixture of cis-2b and trans-2b; n = 3, 3b; n = 4, 4b), and [{(terpy)Ru(Lc)}{Rh2(CH3COO)3(CH3CN)2}]2+ (1c; Lc = 6-(p-carboxyphenyl)-2,4-di(2-pyridyl)-s-triazine). As model species, also the mononuclear [(terpy)Ru(La)]2+ (5a), [(La)Ru(Lb)]2+ (5b), and [(terpy)Ru(Lc)]2+ (5c) have been prepared. All of the complexes have been characterized by several techniques, including NMR and mass spectra, and the stability of the various species is discussed. The absorption spectra of all of the compounds are dominated by the Ru(II) polypyridine moieties, showing intense ligand-centered (LC) bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible. The compounds exhibit several metal-centered oxidation and ligand-centered reduction processes, which have been assigned to specific subunits. Both absorption and redox data indicate a supramolecular nature of the assembled systems. Efficient energy transfer from the MLCT triplet state of the Ru-based components to the lowest-energy excited state of the dirhodium core takes place for the a-type compounds at 298 K in acetonitrile solution, whereas such a process is inefficient for the b-type and c-type species, which exhibit the typical MLCT emission. At 77 K in butyronitrile matrix, Ru-to-Rh2 energy transfer is partly efficient for both the a-type and the b-type compounds and is inefficient for 1c. The reasons for such behavior are discussed by taking into account arguments concerning the driving force and reorganization energy of the complexes.     

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.     

Excited states of nitro-polypyridine metal complexes and their ultrafast decay. Time-resolved IR Absorption, spectroelectrochemistry, and TD-DFT calculations of fac-[Re(Cl)(CO)3(5-nitro-1,10-phenanthroline)

The lowest absorption band of fac-[Re(Cl)(CO)3(5-NO2-phen)] encompasses two close-lying MLCT transitions. The lower one is directed to LUMO, which is heavily localized on the NO2 group. The UV-vis absorption spectrum is well accounted for by TD-DFT (G03/PBEPBE1/CPCM), provided that the solvent, MeCN, is included in the calculations. Near-UV excitation of fac-[Re(Cl)(CO)3(5-NO2-phen)] populates a triplet metal to ligand charge-transfer excited state, 3MLCT, that was characterized by picosecond time-resolved IR spectroscopy. Large positive shifts of the nu(CO) bands upon excitation (+70 cm(-1) for the A'1 band) signify a very large charge separation between the Re(Cl)(CO)3 unit and the 5-NO2-phen ligand. Details of the excited-state character are revealed by TD-DFT calculated changes of electron density distribution. Experimental excited-state nu(CO) wavenumbers agree well with those calculated by DFT. The 3MLCT state decays with a ca. 10 ps lifetime (in MeCN) into another transient species, that was identified by TRIR and TD-DFT calculations as an intraligand 3npi excited state, whereby the electron density is excited from the NO2 oxygen lone pairs to the pi system of 5-NO2-phen. This state is short-lived, decaying to the ground state with a approximately 30 ps lifetime. The presence of an npi state seems to be the main factor responsible for the lack of emission and the very short lifetimes of 3MLCT states seen in all d6-metal complexes of nitro-polypyridyl ligands. Localization of the excited electron density in the lowest 3MLCT states parallels localization of the extra electron in the reduced state that is characterized by a very small negative shift of the nu(CO) IR bands (-6 cm(-1) for A'1) but a large downward shift of the nu(s)(NO2) IR band. The Re-Cl bond is unusually stable toward reduction, whereas the Cl ligand is readily substituted upon oxidation.     

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.