Ligand-to-diimine/metal-to-diimine charge-transfer excited states of [Re(NCS)(CO)3(alpha-diimine)] (alpha-diimine = 2,2'-bipyridine, di-iPr-N,N-1,4-diazabutadiene). A spectroscopic and computational study

Two new complexes fac-[Re(NCS)(CO)3(N,N)] (N,N = 2,2'-bipyridine (bpy), di-iPr-N,N-1,4-diazabutadiene (iPr-DAB)) were synthesized and their molecular structures determined by X-ray diffraction. UV-vis absorption, resonance Raman, emission, and picosecond time-resolved IR spectra were measured experimentally and calculated with TD-DFT. A good agreement between experimental and calculated ground- and excited-state spectra is obtained, but only if the solvent (MeCN) is included into calculations and excited state structures are fully optimized at the TD-DFT level. The lowest excited states of the bpy and iPr-DAB complexes are assigned by TD-DFT as 3aA' by comparison of calculated and experimental IR spectra. Excited-state lifetimes of 23 ns and ca. 625 ps were determined for the bpy and DAB complex, respectively, in a fluid solution at room temperature. Biexponential emission decay (1.3, 2.7 micros) observed for [Re(NCS)(CO)3(bpy)] in a 77 K glass indicates the presence of two unequilibrated emissive states. Low-lying electronic transitions and excited states of both complexes have a mixed NCS --> N,N ligand-to-ligand and Re --> N,N metal-to-ligand charge-transfer character (LLCT/MLCT). It originates in mixing between Re d(pi) and NCS pi characters in high-lying occupied MOs. Experimentally, the LLCT/MLCT mixing in the lowest excited state is manifested by shifting the nu(CO) and nu(NC) IR bands to higher and lower wavenumbers, respectively, upon excitation. Resonant enhancement of both nu(CO) and nu(NC) Raman bands indicates that the same LLCT/MLCT character mixing occurs in the lowest allowed electronic transition.     

Ultrafast excited-state dynamics preceding a ligand trans-cis isomerization of fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and fac-[Re(t-4-styrylpyridine)(CO)3(2,2'-bipyridine)]+

UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)3(stpy)2] and fac-[Re(stpy)(CO)3(bpy)]+ (stpy = t-4-styrylpyridine, bpy = 2,2'-bipyridine) show that their lowest absorption bands are dominated by stpy-localized intraligand (IL) pi pi* transitions. For the latter complex a Re --> bpy transition contributes to the low-energy part of the absorption band. Optical population of the 1IL excited state of fac-[Re(Cl)(CO)3(stpy)2] is followed by an intersystem crossing (< or =0.9 ps) to an 3IL state with the original planar trans geometry of the stpy ligand. This state undergoes a approximately 90 degrees rotation around the stpy C=C bond with a 11 ps time constant. An electronically excited species with an approximately perpendicular orientation of the phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)3(bpy)]+ show the same behavior. Moreover, it was found that the planar reactive 3IL excited state is rapidly and efficiently populated after optical excitation into the Re --> bpy 1MLCT excited state. A 1MLCT --> 3MLCT intersystem crossing takes place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the ReI(CO)3(bpy) chromophore to a stpy-localized 3IL state with a 3.5 ps time constant. The fast rate ensures complete conversion. Coordination of the stpy ligand to the ReI center thus switches the ligand trans-cis isomerization mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)3(bpy)]+, opens an indirect pathway for population of the reactive 3IL excited state via MLCT states.     

Rhenium-to-benzoylpyridine and rhenium-to-bipyridine MLCT excited states of fac-[Re(Cl)(4-benzoylpyridine)(2)(CO)(3)] and fac-[Re(4-benzoylpyridine)(CO)(3)(bpy)](+): a time-resolved spectroscopic and spectroelectrochemical study

The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.     

Excited-state dynamics of fac-[ReI(L)(CO)3(phen)]+ and fac-[ReI(L)(CO)3(5-NO2-phen)]+ (L = imidazole, 4-ethylpyridine; phen = 1,10-phenanthroline) complexes

The nature and dynamics of the lowest excited states of fac-[Re(I)(L)(CO)(3)(phen)](+) and fac-[Re(I)(L)(CO)(3)(5-NO(2)-phen)](+) [L = Cl(-), 4-ethyl-pyridine (4-Etpy), imidazole (imH); phen = 1,10-phenanthroline] have been investigated by picosecond visible and IR transient absorption spectroscopy in aqueous (L = imH), acetonitrile (L = 4-Etpy, imH), and MeOH (L = imH) solutions. The phen complexes have long-lived Re(I) --> phen (3)MLCT excited states, characterized by CO stretching frequencies that are upshifted relative to their ground-state values and by widely split IR bands due to the out-of-phase A'(2) and A"nu(CO) vibrations. The lowest excited states of the 5-NO(2)-phen complexes also have (3)MLCT character; the larger upward nu(CO) shifts accord with much more extensive charge transfer from the Re(I)(CO)(3) unit to 5-NO(2)-phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO(2)-phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO(2) group in the Franck-Condon MLCT transition is manifested by the presence of an enhanced nu(NO(2)) band in the preresonance Raman spectrum of [Re(I)(4-Etpy)(CO)(3)(5-NO(2)-phen)](+). The Re(I) --> 5-NO(2)-phen (3)MLCT excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl(-), 4-Etpy, and imH, respectively, in CH(3)CN solutions. The (3)MLCT excited state of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) is even shorter-lived in MeOH (15 ps) and H(2)O (1.3 ps). In addition to (3)MLCT, excitation of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) populates a (3)LLCT (imH --> 5-NO(2)-phen) excited state. Most of the (3)LLCT population decays to the ground state (time constants of 19 (H(2)O), 50 (MeOH), and 72 ps (CH(3)CN)); in a small fraction, however, deprotonation of the imH.+ ligand occurs, producing a long-lived species, [Re(I)(im.)(CO)(3)(5-NO(2)-phen).-]+.     

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

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

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.     

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.     

Photochemical ligand substitution reactions of fac-[Re(bpy)(CO)3Cl] and derivatives

Excitation by high-energy light, such as that of 313 nm wavelength, induces a photochemical ligand substitution (PLS) reaction of fac-[Re(bpy)(CO)3Cl] (1a) to give the solvento complexes (OC-6-34)- and (OC-6-44)-[Re(bpy)(CO)2(MeCN)Cl] (2 and 3) in good yields. The disappearance quantum yield of 1a was 0.01+/-0.001 at 313 nm. The products were isolated, and X-ray crystallographic analysis was successfully performed for 2. Time-resolved IR measurements clearly indicated that the CO ligand dissociates with subpicosecond rates after excitation, leading to vibrationally hot photoproducts, which relax within 50-100 ps. Detailed studies of the reaction mechanism show that the PLS reaction of 1a does not proceed via the lowest vibrational level in the 3MLCT excited state. The PLS reaction gives 2 and (OC-6-24)-[Re(bpy)(CO)2(MeCN)Cl] (5) as primary products, and one of the products, 5, isomerizes to 3. This type of PLS reaction is more general, occurring in various fac-rhenium(I) diimine tricarbonyl complexes such as fac-[Re(X2bpy)(CO)3Cl] (X2bpy=4,4'-X2-bpy; X=MeO, NH2, CF3), fac-[Re(bpy)(CO)3(pyridine)]+, and fac-[Re(bpy)(CO)3(MeCN)]+. The stable photoproducts (OC-6-44)- and (OC-6-43)-[Re(bpy)(CO)2(MeCN)(pyridine)]+ and (OC-6-32)- and (OC-6-33)-[Re(bpy)(CO)2(MeCN)2]+ were isolated. The PLS reaction of rhenium tricarbonyl-diimine complexes is therefore applicable as a general synthetic method for novel dicarbonyls.     

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.     

Solvation-driven excited-state dynamics of [Re(4-Et-Pyridine)(CO)3(2,2'-bipyridine)]+ in imidazolium ionic liquids. A time-resolved infrared and phosphorescence study

Excited-state dynamics of [Re(Etpy)(CO)3(bpy)]+ was studied in three imidazolium ionic liquids by time-resolved IR and emission spectroscopy on the picosecond to nanosecond time scale. Low-lying excited states were characterized by TD-DFT calculations, which also provided molecular dipole moment vectors in the relevant electronic states. TRIR spectra in ionic liquids show initial populations of two excited states: predominantly bpy-localized 3IL and 3MLCT, characterized by nu(CO) bands shifted to lower and higher frequencies, respectively, relative to the ground state. Internal conversion of 3IL to the lowest triplet 3MLCT occurred on a time scale commensurate with solvent relaxation. The nu(CO) IR bands of the 3MLCT state undergo a dynamic shift to higher wavenumbers during relaxation. Its three-exponential kinetics were determined and attributed to vibrational cooling (units of picoseconds), energy dissipation to the bulk solvent (tens of picoseconds), and solvent relaxation, the lifetime of which increases with increasing viscosity: [EMIM]BF4 (330 ps) < [BMIM]BF4 (470 ps) < [BMIM]PF6 (1570 ps). Time-resolved phosphorescence spectra in [BMIM]PF6 show a approximately 2 ns drop in intensity due to the 3IL --> 3MLCT conversion and a dynamic Stokes shift to lower energies with a lifetime decreasing from 1.8 ns at 21 degrees C to 1.1 ns at 37 degrees C, due to decreasing viscosity of the ionic liquid. It is proposed that solvent relaxation predominantly involves collective translational motions of ions. It drives the 3IL --> 3MLCT conversion, increases charge reorganization in the lowest excited-state 3MLCT, and affects vibrational anharmonic coupling, which together cause the dynamic shift of excited-state IR bands. TRIR spectroscopy of carbonyl-diimine complexes emerges as a new way to investigate various aspects of solvation dynamics, while the use of slowly relaxing ionic liquids offers new insight into the photophysics of Re(I) carbonyl polypyridyls.     

Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re I(CO)3(2,2'-bipyridine)} chromophore

Two multifunctional photoactive complexes [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+)=N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy=2,2'-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans-->cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3)-->MeDpe(+ 3)MLCT (MLCT=metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximately 42 (73 %) and approximately 430 ps (27 %). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3)-->MeDpe(+) and Re(CO)(3)-->bpy (3)MLCT states, from which a MeDpe(+) localized intraligand (3)pipi* excited state ((3)IL) is populated with lifetimes of approximately 0.6 and approximately 10 ps, respectively. The (3)IL state undergoes a approximately 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe(+))(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.     

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.     

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.     

New synthetic routes to biscarbonylbipyridinerhenium(I) complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)n+ (X2bpy = 4,4'-X2-2,2'-bipyridine) via photochemical ligand substitution reactions, and their photophysical and electrochemical properties

Photochemical ligand substitution of fac-[Re(X2bpy)(CO)3(PR3)]+ (X2bpy = 4,4'-X2-2,2'-bipyridine; X = Me, H, CF3; R = OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans[Re(X2bpy)(CO)2(PR3)(MeCN)]+, coordinated with four different kinds of ligands. Similarly, other biscarbonylrhenium complexes, cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (n = 0, Y = Cl-; n = 1, Y = pyridine, PR'3), were synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me2bpy)(CO)2[P(OEt)3](PPh3)](PF6) was determined by X-ray analysis. Crystal data: C38H42N2O5F6P3Re, monoclinic, P2(1/a), a = 11.592(1) A, b = 30.953(4) A, c = 11.799(2) A, V = 4221.6(1) A3, Z = 4, 7813 reflections, R = 0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their 3MLCT state with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed in the cases Y = Cl-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y = Cl- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(MeCN)]+.     

Direct observation of competitive ultrafast CO dissociation and relaxation of an MLCT excited state: picosecond time-resolved infrared spectroscopic study of [Cr(CO)(4)(2,2'-bipyridine)

Early excited-state dynamics of [Cr(CO)(4)(bpy)] were studied in a CH(2)Cl(2) solution by picosecond time-resolved IR spectroscopy, which made it possible to characterize structurally the individual species involved and to follow separately the temporal evolution of the IR bands due to the bleached ground-state absorption, the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct, and two (3)MLCT states. It was found that the fac-[Cr(CO)(3)(Sol)(bpy)] photoproduct is formed alongside population of two (3)MLCT states during the first picosecond after excitation at 400 or 500 nm by a branched evolution of the optically populated excited state. Vibrationally relaxed (3)MLCT excited states are unreactive, decaying directly to the ground state on a picosecond time scale. The photoproduct is long-lived, persistent into the nanosecond time domain. Changing the excitation wavelength from 400 to 500 nm strongly increases the extent of the bleach recovery and decreases the yield of the photoproduct formation relative to the initial yield of the population of the unreactive (3)MLCT states. The photochemical quantum yield of CO dissociation also decreases with increasing excitation wavelength (Víchová, J.; Hartl, F.; Vlcek, A., Jr. J. Am. Chem. Soc. 1992, 114, 10903). These observations demonstrate the relationship between the early dynamics of optically populated excited states and the overall outcome of a photochemical reaction and identify the limiting role of the branching of the initial excited-state evolution between reactive and relaxation pathways as a more general principle of organometallic photochemistry.     

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.     

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

Theoretical studies of ground and excited electronic States in a series of rhenium(i) bipyridine complexes containing diarylethynyl-based structure

The photophysical properties, which vary as X is varied, of Re(I)-halide complexes (X2-bpy)ReICl(CO)3 (where X=ph, DAE, DNE, and DPE; ph = phenyl (1); DAE = di(amineoethynylbenzene) (2); DPE = di(phenylethynylbenzene) (3); DNE = di(nitroethynylbenzene) (4); bpy=2,2'bipyridine), are investigated using density functional theory (DFT). The electronic properties of the neutral molecules, in addition to the positive and negative ions, are studied using B3LYP functional. Excited singlet and triplet states are examined using time-dependent density functional theory (TDDFT). The low-lying excited-state geometries are optimized at the ab initio configuration interaction singlets level. As shown, the diarylethynyl-based structure is an integral component of the bpy pi-conjugated network, which results in a good planar structure. The occupied orbitals involved in the transitions have a significant mixture of metal Re and group Cl, and the lowest unoccupied orbital is a pi orbital, which extends from ligand bpy to diarylethynyl-based substituents. The luminescence for each complex originates from the lowest triplet excited states and is assigned to the mixing of MLCT and LLCT characters. Significant insights on the effects of these diarylethynyl conjugated structure and ending substituents (NH2, ph, and NO2) on absorption and emission spectra are observed by analysis of the results of the TDDFT method. The diarylethynyl-based pi-conjugated network makes both the absorption and emission spectra red-shifted compared with simple complex (bpy)ReICl(CO)3. Furthermore, an electron-releasing group (NH2) makes absorption and emission spectra blue-shift and an electron-withdrawing group (NO2) makes them red-shift.     

Systematic synthesis, isolation, and photophysical properties of linear-shaped Re(I) oligomers and polymers with 2-20 units

Systematic synthesis routes have been developed for the linear-shaped rhenium(I) oligomers and polymers bridged with bidentate phosphorus ligands, [Re(N--N)(CO)3-PP-{Re(N--N)(CO)2-PP-}(n)Re(N--N)(CO)3](PF6)(n+2) (N--N = diimine, PP = bidentate phosphine, n = 0-18). These were isolated by size exclusion chromatography (SEC) and identified by (1)H NMR, IR, electrospray ionization Fourier transform mass spectrometry, analytical SEC, and elemental analysis. Crystal structures of [Re(bpy)(CO)3-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)3](PF6)2, [Re(bpy)(CO)3-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)2-Ph2PC[triple bond]CPPh2-Re(bpy)(CO)3](PF6)3 and [Re(bpy)(CO)3-Ph2PC2H4PPh2-{Re(bpy)(CO)2Ph2PC2H4PPh2-}(n)Re(bpy)(CO)3](PF6)(n+2) (bpy = 2,2'-bipyridine, n = 1, 2) were obtained, showing that they have interligand pi-pi interaction between the bpy ligand and the phenyl groups on the phosphorus ligand. All of the oligomers and polymers synthesized were emissive at room temperature in solution. For the dimers, broad emission was observed with a maximum at 523-545 nm, from the (3)MLCT excited-state of the tricarbonyl complex unit, [Re(N--N)(CO)3-PP-]. Emission from the longer oligomers and polymers with > or = 3 Re(I) units was observed at wavelengths 50-60 nm longer than those of the corresponding dimers. This fact and the emission decay results clearly show that energy transfer from the edge unit to the interior unit occurs with a rate constant of (0.9 x 10(8))-(2.5 x 10(8)) s(-1). The efficient energy transfer and the smaller exclusive volume of the longer Re(I) polymers indicated intermolecular aggregation for these polymers in an MeCN solution.     

Photoinduced energy transfer in a conformationally flexible Re(I)/Ru(II) dyad probed by time-resolved infrared spectroscopy: effects of conformation and spatial localization of excited states

The dyad RuLRe contains (Re(bpy)(CO)3Cl) and (Ru(bpy)(bpyam)2)2+ termini (bpy = 2,2'-bipyridine; bpyam = 4,4'-diethylamido-2,2'-bipyridine) separated by a flexible ethylene spacer. Luminescence studies reveal the expected Re --> Ru photoinduced energy transfer, with partial quenching of Re(I)-based triplet metal-to-ligand charge-transfer (3MLCT) luminescence and consequent sensitization of the Ru(II)-based 3MLCT luminescence, which has a component with a grow-in lifetime of 0.76 (+/-0.2) ns. The presence of IR-active spectroscopic handles on both termini [CO ligands directly attached to Re(I) and amide carbonyl substituents on the bpy ligands coordinated to Ru(II)] allowed the excited-state dynamics to be studied by time-resolved IR (TRIR) spectroscopy in much more detail than allowed by luminescence methods. A combination of picosecond- and nanosecond-time-scale TRIR studies revealed the presence of at least three distinct Re --> Ru energy-transfer processes, with lifetimes of ca. 20 ps and 1 and 13 ns. This complex behavior occurs because of a combination of two different Ru-based 3MLCT states (Ru --> L and Ru --> bpyam), which are sensitized by energy transfer from the Re(I) donor at different rates; and the presence of at least two conformers of the flexible molecule RuLRe, which have different Re...Ru separations.