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.     

Excited-state dynamics in fac-[Re(CO)3(Me4phen)(L)]+

Excited-state dynamics in fac-[Re(CO)(3)(Me(4)phen)(cis-L)](+) (Me(4)phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, L = 4-styrylpyridine (stpy) or 1,2-bis(4-pyridyl)ethylene (bpe)) were investigated by steady-state and time-resolved techniques. A complex equilibrium among three closely lying excited states, (3)IL(cis-L), (3)MLCT(Re→Me(4)phen), and (3)IL(Me(4)phen), has been established. Under UV irradiation, cis-to-trans isomerization of coordinated cis-L is observed with a quantum yield of 0.15 in acetonitrile solutions. This photoreaction competes with radiative decay from (3)MLCT(Re→Me(4)phen) and (3)IL(Me(4)phen) excited states, leading to a decrease in the emission quantum yield relative to the nonisomerizable complex fac-[Re(CO)(3)(Me(4)phen)(bpa)](+) (bpa = 1,2-bis(4-pyridyl)ethane). From temperature-dependent time-resolved emission measurements in solution and in poly(methyl methacrylate) (PMMA) films, energy barriers (ΔE(a)) for interconversion between (3)MLCT(Re→Me(4)phen) and (3)IL(Me(4)phen) emitting states were determined. For L = cis-stpy, ΔE(a) = 11 (920 cm(-1)) and 15 kJ mol(-1) (1254 cm(-1)) in 5:4 propionitrile/butyronitrile and PMMA, respectively. For L = cis-bpe, ΔE(a) = 13 kJ mol(-1) (1087 cm(-1)) in 5:4 propionitrile/butyronitrile. These energy barriers are sufficient to decrease the rate constant for internal conversion from higher-lying (3)IL(Me(4)phen) state to (3)MLCT(Re→Me(4)phen), k(i) ? 10(6) s(-1). The decrease in rate allows for the observation of intraligand phosphorescence, even in fluid medium at room temperature. Our results provide additional insight into the role of energy gap and excited-state dynamics on the photochemical and photophysical properties of Re(I) polypyridyl complexes.     

Structure, reactivity and solution behaviour of [Re(ser)(7-MeG)(CO)(3)] and [Re(ser)(3-pic)(CO)(3)]: "nucleoside-mimicking" complexes based on the fac-[Re(CO)(3)](+) moiety

The fac-[Re(CO)(3)](+) moiety was reacted with the amino acid serine (D- and L-ser) and with 7-methylguanine (7-MeG), 3-methylpyridine (3-pic) or adenine (ade) to yield novel complexes intended as nucleoside-mimicking compounds. Reaction of [Re(H(2)O)(3)(CO)(3)](+)(1) with L-ser yields the complex [Re(L-ser)(2)(CO)(3)](L-2). X-Ray structure analysis of L-2 reveals that one of the two amino acids is bound to the metal centre in a bidentate fashion while the other amino acid is bound as a zwitterion via the carboxylate oxygen only. Reaction of L-2 and of [Re(D-ser)(2)(CO)(3)](D-2) with 7-MeG yields complexes [Re(L-ser)(7-MeG)(CO)(3)](L-3) and [Re(D-ser)(7-MeG)(CO)(3)](D-3) respectively. Complexes L-3 and D-3 are received as a mixture of diastereomers. If 3-pic is used instead of 7-MeG complex [Re(L-ser)(3-pic)(CO)(3)](L-4) is obtained in good yield, while interaction of L-2 with ade gives a mixture of five distinct species. Crystallization gave one single diastereomer for L-3 and D-3 and the two forms for 4 respectively. X-Ray structure analyses reveal that in all cases the amino acid is bound in a chelate fashion with the base occupying the sixth co-ordination site. When crystals of either 2 or 3 are dissolved in a CD(3)OD/D(2)O mixture (1:1, 293 K) rapid transformation to the diastereomeric mixture is observed. While for L-2 this reorganisation is fast on the NMR time scale even at 193 K, the rate constant for the rearrangement of L-3 and D-3 is 1.36 +/- 0.24 x 10(-2) s(-1) at 293 K.     

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.     

Photochemical synthesis of mer-[Re(bpy)(CO)3Cl

The mer isomer of Re(bpy)(CO)3Cl (bpy = 2,2'-bipyridine) has been synthesized photochemically and isolated for the first time. The mer complex is a useful starting material for the synthesis of cis,cis-[Re(bpy)(CO)2(L)Cl]-type complexes.     

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).-]+.     

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.     

Photochemistry of fac‐[Re(bpy)(CO)3Cl]

The photochemistry of fac‐[Re(bpy)(CO)₃Cl] ( 1 a ; bpy=2,2′‐bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer‐isomer 1 b . However, in the presence of a small amount of MeCN, the main product was the CO‐ligand‐substituted complex (OC‐6‐24)‐[Re(bpy)(CO)₂Cl(MeCN)] ( 2 c ; bpy=2,2′‐bipyridine). In MeCN, two isomers, 2 c and its (OC‐6‐34) form ( 2 a ), were produced. Only 2 c thermally isomerized to produce the (OC‐6‐44) form 2 b . A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV‐visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher‐energy light (e.g., 270 nm) and lower‐energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a . Irradiation of 1 a using the higher‐energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (³HES( 1 )). In THF, ³HES( 1 ) was competitively converted to both the triplet ligand field (³LF) and metal‐to‐ligand charge transfer (³M LCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)₂Cl(thf)]⁺ ( 1 c ) within 10 ps by means of a dissociative mechanism. Re‐coordination of CO to 1 c gives both 1 a and 1 b . In MeCN, irradiation of 1 a by using high‐energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c . A seven‐coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.     

Synthesis, characterization, and photophysical properties of Os(II) diimine complexes [Os(N(wedge)N)(CO)(2)I(2)] (N(wedge)N = bipyridine, phenanthroline, and pyridyl benzoxazole)

A new series of Os(II) diimine complexes with the general formula [Os(N(wedge)N)(CO)(2)I(2)], N(wedge)N = 2,2'-bipyridine (bpy) (1), 4,4'-di-tert-butyl-2,2'-bipyridine (dbubpy) (2), 4,7-diphenyl-1,10-phenanthroline (dpphen) (3), 2-(2'-pyridyl)benzoxazole (pboz) (4), and 5-tert-butyl-2-(2'-pyridyl)benzoxazole (bupboz) (5), were synthesized and characterized by spectroscopic methods and by a single-crystal X-ray diffraction study on the dpphen complex 3. The corresponding photophysical properties were studied using UV-vis and emission spectrometry. The resulting phosphorescence features both in solution and as a solid film, in combination with the MO calculation, lead us to conclude that the emissions originate from mixed halide-to-ligand (XLCT approximately 70%) and metal-to-ligand (MLCT approximately 30%) transitions instead of the typical MLCT transition. Using complexes 4 and 5 as the dopant emitters, we evaluated their potential to serve as a phosphor for organic light emitting diodes by examining their electroluminescent performances. Reddish orange electroluminescence centered around 600 nm was observed for organic light emitting diodes (OLEDs) fabricated using complex 5 as the emitter; the device efficiency was shown to be as high as 2.8% (and 5.0 cd/A or 2.7 lm/W), and the peak luminance was shown to be 5600 cd/m(2) at a driving voltage of approximately 15 V.     

Synthesis, Characterization, and Comparative Properties of [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] and [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)

The reaction of [PPN](2)[Re(6)C(CO)(19)] with Mo(CO)(6) and Ru(3)(CO)(12) under sunlamp irradiation provided the new mixed-metal clusters [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] and [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)], which were isolated in yields of 85% and 61%, respectively. The compound [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] crystallizes in the monoclinic space group P2(1)/c with a = 20.190 (7) ?, b = 16.489 (7) ?, c = 27.778 (7) ?, beta = 101.48 (2) degrees, and Z = 4 (at T = -75 degrees C). The cluster anion is composed of a Re(6)C octahedral core with a face capped by a Mo(CO)(4) fragment. There are three terminal carbonyl ligands coordinated to each rhenium atom. The four carbonyl ligands on the molybdenum center are essentially terminal, with one pair of carbonyl ligands (C72-O72 and C74-O74) subtending a relatively large angle at molybdenum (C72-Mo-C74 = 147.2(9) degrees ), whereas the remaining pair of carbonyl ligands (C71-O71 and C73-O73) subtend a much smaller angle (C71-Mo-C73 = 100.5(9) degrees ). The (13)C NMR spectrum of (13)CO-enriched [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] shows signals for four sets of carbonyl ligands at -40 degrees C, consistent with the solid state structure, but the carbonyl ligands undergo complete scrambling at ambient temperature. The (13)C NMR spectrum of (13)CO-enriched [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)] at 20 degrees C is consistent with the expected structure of an octahedral Re(6)C(CO)(18) core capped by a Ru(CO)(3) fragment. The visible spectrum of [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)] shows a broad, strong band at 670 nm (epsilon = 8100), whereas all of the absorptions of [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)] are at higher energy. An irreversible oxidation wave with E(p) at 0.34 V is observed for [PPN](2)[Re(6)C(CO)(18)Mo(CO)(4)], whereas two quasi-reversible oxidation waves with E(1/2) values of 0.21 and 0.61 V (vs Ag/AgCl) are observed for [PPN](2)[Re(6)C(CO)(18)Ru(CO)(3)]. The molybdenum cap in [Re(6)C(CO)(18)Mo(CO(4))](2-) is cleaved by heating in donor solvents, and by treatment with H(2), to give largely [H(2)Re(6)C(CO)(18)](2-). In contrast, [Re(6)C(CO)(18)Ru(CO)(3)](2-) shows no tendency to react under similar conditions.     

Darstellung,Strukturen und EPR-Spektren der Rhenium(II)-Nitrosylkomplexe [Re(NO)Cl2(PPh3)(OPPh3)(OReO3)], [Re(NO)Cl2(OPPh3)2(OReO3)] und [Re(NO)Cl2(OPPh3)3](ReO4)

Synthesis, Structures, and EPR-Spectra of the Rhenium(II) Nitrosyl Complexes [Re(NO)Cl₂(PPh₃)(OPPh₃)(OReO₃)], [Re(NO)Cl₂(OPPh₃)₂(OReO₃)], and [Re(NO)Cl₂(OPPh₃)₃](ReO₄) The paramagnetic rhenium(II) nitrosyl complexes [Re(NO)Cl₂(PPh₃)(OPPh₃)(OReO₃)], [Re(NO)Cl₂(OPPh₃)₂ · (OReO₃)], and [Re(NO)Cl₂(OPPh₃)₃](ReO₄) are formed during the reaction of [ReOCl₃(PPh₃)₂] with NO gas in CH₂Cl₂/EtOH. These and two other Re^II complexes with 5 d⁵ ”︁low-spin”︁”︁-configuration can be observed during the reaction EPR spectroscopically. Crystal structure analysis shows linear coordinated NO ligands (Re–N–O-angles between 171.9 and 177.3°). Three OPPh₃ ligands are meridionally coordinated in the final product of the reaction, [Re(NO)Cl₂(OPPh₃)₃][ReO₄] (monoclinic, P2₁/c, a = 13.47(1), b = 17.56(1), c = 24.69(2) Å, β = 95.12(4)°, Z = 4). [Re(NO)Cl₂(PPh₃)(OPPh₃)(OReO₃)] (triclinic P 1, a = 10.561(6), b = 11.770(4), c = 18.483(8) Å, α = 77.29(3), β = 73.53(3), γ = 64.70(4)°, Z = 2) and [Re(NO)Cl₂ (OPPh₃)₂(OReO₃)] (monoclinic P2₁/c, a = 10.652(1), b = 31.638(4), c = 11.886(1) Å, β = 115.59(1)°), Z = 4) can be isolated at shorter reaction times besides the complexes [Re(NO)Cl₃(Ph₃P)₂], [Re(NO)Cl₃(Ph₃P) · (Ph₃PO)], and [ReCl₄(Ph₃P)₂].     

Nanosecond CO photodissociation and excited-state character of [Ru(X)(X')(CO)2(N,N'-diisopropyl-1,4-diazabutadiene)] (X=X'=Cl or I; X=Me, X'=I; X=SnPh3, X'=Cl) studied by time-resolved infrared spectroscopy and DFT calculations

The character and dynamics of the low-lying excited states of [Ru(X)(X')(CO)2(iPr-dab)] (X=X'=Cl or I; X=Me, X'=I; X=SnPh3, X'=Cl; iPr-dab=N, N'-diisopropyl-1,4-diazabutadiene) were studied experimentally by pico- and nanosecond time-resolved IR spectroscopy (TRIR) and (for X=X'=Cl or I) computationally using density functional theory (DFT) and time-dependent DFT (TD-DFT) techniques. The lowest allowed electronic transition occurs between 390 and 460 nm and involves charge transfer from the Ru(halide)(CO) 2 unit to iPr-dab, denoted (1)MLCT/XLCT (metal-to-ligand/halide-to-ligand charge transfer). The lowest triplet state is well modeled by UKS-DFT-CPCM calculations, which quite accurately reproduce the excited-state IR spectrum in the nu(CO) region. It has a (3)MLCT/XLCT character with an intraligand (iPr-dab) (3)pipi* admixture. TRIR spectra of the lowest triplet excited state show two nu(CO) bands that are shifted to higher energies from their corresponding ground-state positions. The magnitude of this upward shift increases as a function of the ligands X and X' [(I)2 < (Sn)(Cl) < (Me)(I) < (Cl)2] and reveals increasing contribution of the Ru(CO)2-->dab MLCT character to the excited state. The lowest triplet state of [Ru(Cl)2(CO)2(iPr-dab)] undergoes a approximately 10 ps relaxation that is followed by CO dissociation, producing cis(CO,CH 3CN),trans(Cl,Cl)-[Ru(Cl)2(CH 3CN)(CO)(iPr-dab)] with a unity quantum yield and 7.2 ns lifetime and without any observable intermediate. To our knowledge, this is the first example of a "slow" CO dissociation from a thermally equilibrated triplet charge-transfer excited state.     

Photoswitches and luminescent rigidity sensors based on fac-[Re(CO)3(Me4phen)(L)]+

The fac-[Re(CO)3(Me4phen)(trans-L)]+ complexes, Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline and L = 4-styrylpyridine, stpy, or 1,2-bis(4-pyridyl)ethylene, bpe, were synthesized and characterized by their spectroscopic,photochemical, and photophysical properties. The complexes exhibit trans-to-cis isomerization upon 313, 334, 365,and 404 nm irradiation, and the true quantum yields can be efficiently determined by absorption changes combined with 1H NMR data. For fac-[Re(CO)3(Me4phen)(trans-bpe)]+ similar quantum yields were determined at all wavelengths investigated. However, a lower value (phitrue = 0.35) was determined for fac-[Re(CO)3(Me4phen)(trans-stpy)]+ at404 nm irradiation, which indicates different pathways for the photoisomerization process. The photoproducts, fac-[Re(CO)3(Me4phen)(cis-L)]+, exhibit luminescence at room temperature with two maxima ascribed to the 3ILMe4phen and 3MLCTRe-->Me4phen excited states. The luminescence properties were investigated in different media, and the behavior in glassy EPA at 77 K showed that the contribution of each emissive state is dependent on the excitation wavelength. The photochemical and photophysical behavior of the complexes were rationalized in terms of the energy gap of excited states and can be exploited in photoswitchable luminescent rigidity sensors.     

Transient mixed-valence character of Re(I)(4)(CO)(12)(4,4'-bpy)(4)Cl(4)

This study addresses, in detail, the orbital nature and the extent of metal-metal communication in the lowest emitting triplet state of Re(4)(CO)(12)(4,4'-bpy)(4)Cl(4) (where 4,4'-bpy = 4,4'-bipyridine) as well as the symmetry of the lowest (3)MLCT manifold in comparison to that of the ground state. All spectral evidence points to (1). a (3)MLCT excited manifold localized between a single Re(I) corner and an adjacent bridging ligand, (2). a transient mixed-valence state that is completely localized between a single transiently oxidized Re center and the adjacent metals, and (3). a second-order charge transfer from a localized transiently reduced bridging ligand to the adjacent Re(I) center to which it is attached, effectively lowering its oxidation state. The orbital nature of the lowest (3)MLCT manifold is fully corroborated by a molecular orbital diagram derived from quantum chemical modeling studies, while the existence of the localization, localized mixed valency, and second-order charge transfer rely on spectral evidence alone. This work makes use of low-temperature time-resolved infrared (TRIR) techniques as well as a luminescence study. Many of the nuances of the luminescence and TRIR data interpretation are extracted from statistical analysis and quantum chemical modeling studies. The relative concentrations of the dominant conformers that exist for Re(4)(CO)(12)(4,4'-bpy)(4)Cl(4) have also been estimated from Boltzmann statistics.     

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.     

Thermodynamic hydride donor abilities of [HW(CO)4L]- complexes (L = PPh3, P(OMe)3, CO) and their reactions with [C5Me5Re(PMe3)(NO)(CO)]+

The thermodynamic hydride donor abilities of [HW(CO)(5)](-) (40 kcal/mol), [HW(CO)(4)P(OMe(3))](-) (37 kcal/mol), and [HW(CO)(4)(PPh(3))](-) (36 kcal/mol) have been measured in acetonitrile by either equilibrium or calorimetric methods. The hydride donor abilities of these complexes are compared with other complexes for which similar thermodynamic measurements have been made. [HW(CO)(5)](-), [HW(CO)(4)P(OMe(3))](-), and [HW(CO)(4)(PPh(3))](-) all react rapidly with [CpRe(PMe(3))(NO)(CO)](+) to form dinuclear intermediates with bridging formyl ligands. These intermediates slowly form [CpRe(PMe(3))(NO)(CHO)] and [W(CO)(4)(L)(CH(3)CN)]. The structure of cis-[HW(CO)(4)(PPh(3))](-) has been determined and has the expected octahedral structure. The hydride ligand bends away from the CO ligand trans to PPh(3) and toward PPh(3).     

New dirhenium(III) compounds bridged by carboxylate ligands: N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] and Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2)

The solvothermal reaction of (N(C(4)H(9))(4))(2)[Re(2)Cl(8)] with trifluoroacetic acid and acetic anhydride leads to the new rhenium trifluoroacetate dimer N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] (1) and to the rhenium carbonyl dimer Re(2)(mu(2)-Cl)(2)(CO)(8) as the rhenium-reduced byproduct. The reaction of the precursor complex, N(C(4)H(9))(4)[Re(2)(OOCCF(3))Cl(6)] (1), with the organometallic carboxylic acid (CO)(6)Co(2)HCCCOOH leads to the cluster of clusters compound Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2), which has the dimer structure of Re(2)(OOCR)(4)Cl(2). Cyclic voltammetric measurements show that Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2) has one reduction centered on the dirhenium core and a reduction centered on the cobalt atoms. DFT calculations have been used to rationalize the observed displacements of the voltammetric signals in Re(2)(OOCCCHCo(2)(CO)(6))(4)Cl(2) (2) compared to the parent ligand (CO)(6)Co(2)HCCCOOH and rhenium pivalate.