The alteration of intra-ligand donor-acceptor interactions through torsional connectivity in substituted Re-dppz complexes

The ground and excited properties of a series of [ReCl(CO)3(dppz)] complexes with substituted donor groups have been investigated. Alteration of donor-acceptor communication through modulation of torsional angle and the number and nature of the donor substituent allowed the effects on the photophysical properties to be characterized though both computational and spectroscopic techniques, including TD-DFT, resonance Raman and time resolved infrared. The ground state optical properties show significant variation as a result of donor group modulation, with increased angle between the donor and acceptor blue-shifting and depleting the intensity of the lowest energy transition, which was consistently ILCT in nature. However, across all complexes studied there was minimal perturbation to the excited state properties and dynamics. Three excited states on the picosecond, nanosecond and microsecond time scales were observed in all cases, corresponding to 1ILCT, ππ* and 3ILCT respectively

Nature of Excited States of Ruthenium-Based Solar Cell Dyes in Solution: A Comprehensive Spectroscopic Study

The photophysical properties of a number of ruthenium complexes of the general structure [Ru­(L1)­(L2)­(NCS)₂], related to the prominent solar cell dye [Ru­(dcb)₂(NCS)₂] (dcb = 4,4′-dicarboxylato-2,2′-bipyridine) are investigated. For L1 = dcb and dmb (dmb = 4,4′-dimethyl-2,2′-bipyridine), several variations of L2 show very little difference in the lowest energy absorption peak. Resonance Raman and density functional theory calculations have been used to assign the corresponding transitions as {Ru­(NCS)₂} → dcb with significant contributions of the NCS ligands. Transient absorption, time-resolved infrared, and transient resonance Raman spectroscopic techniques were used to probe the photophysics of the complexes and relatively short-lived {Ru­(NCS)₂} → dcb/dpb (dpb = 4,4′-diphenylethenyl-2,2′-bipyridine) excited states were observed with the exception of [Ru­(dcb)­(dab)­(NCS)₂] (dab = 4,4′-dianthracenethenyl-2,2′-bipyridine), which showed a long-lived excited state assigned as ligand centered charge separated

Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids

The synthesis of two bipyridine-hexa-<i>peri</i>-hexabenzocoronene (bpy-HBC) ligands functionalized with either ^<i>t</i>Bu or C₁₂H₂₅ and their Re­(I) tricarbonyl chloride complexes are reported and their electronic properties investigated using spectroscopic and computational methods. The metal complexes show unusual properties, and we observed the formation of a long-lived excited state using time-resolved infrared spectroscopy. Depending on the solvent, this appears to be of the form Rebpy^•HBC^•+ or a bpy-centered π,π* state. TD-DFT calculations support the donor–acceptor charge transfer character of these systems, in which HBC is the donor and bpy is the acceptor. The ground state optical properties are dominated by the HBC chromophore with additional distinct transitions of the complexes, one associated with MLCT 450 nm (ε > 17 000 L mol^–1 cm^–1) and another with a HBC/metal to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000 L mol^–1 cm^–1). These assignments are also supported by resonance Raman spectroscopy

Long-Lived Charge Transfer Excited States in HBC-Polypyridyl Complex Hybrids

The synthesis of two bipyridine-hexa-<i>peri</i>-hexabenzocoronene (bpy-HBC) ligands functionalized with either ^<i>t</i>Bu or C₁₂H₂₅ and their Re­(I) tricarbonyl chloride complexes are reported and their electronic properties investigated using spectroscopic and computational methods. The metal complexes show unusual properties, and we observed the formation of a long-lived excited state using time-resolved infrared spectroscopy. Depending on the solvent, this appears to be of the form Rebpy^•HBC^•+ or a bpy-centered π,π* state. TD-DFT calculations support the donor–acceptor charge transfer character of these systems, in which HBC is the donor and bpy is the acceptor. The ground state optical properties are dominated by the HBC chromophore with additional distinct transitions of the complexes, one associated with MLCT 450 nm (ε > 17 000 L mol^–1 cm^–1) and another with a HBC/metal to bpy charge transfer, termed the MLLCT band (373 nm, ε = 66 000 L mol^–1 cm^–1). These assignments are also supported by resonance Raman spectroscopy

Re(I) Complexes of Substituted dppz: A Computational and Spectroscopic Study

A series of dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine (dppz)-based ligands with electron-withdrawing substituents and their [Re­(CO)₃(L)­Cl] and [Re­(CO)₃(L)­(py)]­PF₆ complexes have been studied using Raman, resonance Raman, and transient resonance Raman (TR²) and time-resolved infrared (TRIR) spectroscopic techinques in conjunction with computational chemistry as well as electrochemical studies, emission, and absorption of ground and excited states. DFT (B3LYP) frequency calculations show good agreement with nonresonant Raman spectra, which allowed these to be used to identify phenanthroline, phenazine, and delocalized modes. These band assignments were used to establish the nature of chromophores active in resonance Raman spectra, probed with wavelengths between 350.7 and 457.9 nm. X-ray crystallography of [Re­(CO)₃(dppzBr₂)­Cl] and [Re­(CO)₃(dppzBr)­(py)]­PF₆ showed these crystallize in space groups triclinic <i>P</i>1 and monoclinic <i>P</i>2_1/<i>n</i>, respectively. Electrochemical studies showed that substituents have a strong effect on the phenazine MO, changing the reduction potential by 200 mV. Transient absorption studies showed that generally the [Re­(CO)₃(L)­(py)]­PF₆ complexes had longer lifetimes than the corresponding [Re­(CO)₃(L)­Cl] complexes; the probed state is likely to be ³π → π* (phz) in nature. TR² spectra of the ligands provided a marker for the triplet π → π* state, and the TR² spectra of the complexes suggest an intraligand (IL) π,π* state for [Re­(CO)₃(L)­(py)]⁺ complexes, and a potentially mixed IL/MLCT state for [Re­(CO)₃(L)­Cl] complexes. TRIR spectroscopy is more definitive with THEXI state assignments, and analysis of the metal–carbonyl region (1800–2100 cm^–1) on the picosecond and nanosecond time scales indicates the formation of MLCT­(phen/phz) states for all [Re­(CO)₃(L)­Cl] complexes, and IL π → π* (phen) states for all [Re­(CO)₃(L)­(py)]⁺ complexes, with all but [Re­(CO)₃(dppzBr­(CF₃))­(py)]⁺ showing some contribution from an MLCT­(phen) state also

Competing Pathways in the Photochemistry of Ru(H)₂(CO)(PPh₃)₃

The photochemistry of Ru­(H)₂(CO)­(PPh₃)₃ (<b>1</b>) has been reinvestigated employing laser and conventional light sources in conjunction with NMR spectroscopy and IR spectroscopy. The sensitivity of NMR experiments was enhanced by use of <i>p</i>-H₂-induced polarization (PHIP), and a series of unexpected reactions were observed. The photoinduced reductive elimination of H₂ was demonstrated (a) via NMR spectroscopy by the observation of hyperpolarized <b>1</b> on pulsed laser photolysis in the presence of <i>p</i>-H₂ and (b) via nanosecond time-resolved infrared (TRIR) spectroscopy studies of the transient [Ru­(CO)­(PPh₃)₃]. Elimination of H₂ competes with photoinduced loss of PPh₃, as demonstrated by formation of dihydrogen, triphenylarsine, and pyridine substitution products which are detected by NMR spectroscopy. The corresponding coordinatively unsaturated 16-electron intermediate [Ru­(H)₂(CO)­(PPh₃)₂] exists in two isomeric forms according to TRIR spectroscopy that react with H₂ and with pyridine on a nanosecond time scale. These two pathways, reductive elimination of H₂ and PPh₃ loss, are shown to occur with approximately equal quantum yields upon 355 nm irradiation. Low-temperature photolysis in the presence of H₂ reveals the formation of the dihydrogen complex Ru­(H)₂(η²-H₂)­(CO)­(PPh₃)₂, which is detected by NMR and IR spectroscopy. This complex reacts further within seconds at room temperature, and its behavior provides a rationale to explain the PHIP results. Furthermore, photolysis in the presence of AsPh₃ and H₂ generates Ru­(H)₂(AsPh₃)­(CO)­(PPh₃)₂. Two isomers of Ru­(H)₂(CO)­(PPh₃)₂(pyridine) are formed according to NMR spectroscopy on initial photolysis of <b>1</b> in the presence of pyridine under H₂. Two further isomers are formed as minor products; the configuration of each isomer was identified by NMR spectroscopy. Laser pump-NMR probe spectroscopy was used to observe coherent oscillations in the magnetization of one of the isomers of the pyridine complex; the oscillation frequency corresponds to the difference in chemical shift between the hydride resonances. Pyridine substitution products were also detected by TRIR spectroscopy

Red-Absorbing Cationic Acceptor Dyes for Photocathodes in Tandem Solar Cells

A pair of new donor−π–acceptor dyes that absorb toward the red region of the visible spectrum (CAD 1 and CAD 2) utilizing indolium cationic acceptor units have been synthesized for use in p-type dye-sensitized solar cells (p-DSC). Their optical and electrochemical properties were determined experimentally, including application of ultrafast transient absorption and time-resolved infrared spectroscopies. Our results are supported by computational modeling. NiO-based p-DSCs with CAD 1 and CAD 2 gave short-circuit photocurrent densities of 3.6 and 3.3 mA cm^–2, respectively, which are substantially higher than that of any previous red-absorbing p-DSC. These results are a step toward tandem dye-sensitized solar cells that absorb higher-energy photons at the TiO₂ anode and lower-energy photons at the NiO cathode. Routes to further improve the efficiency of NiO DSCS are also discussed

Dual Charge-Transfer in Rhenium(I) Thioether Substituted Hexaazanaphthalene Complexes

The ligand 2,3,8,9,14,15-hexa­(octyl-thioether)-5,6,11,12,17,18-hexaazatri­naphthalene (HATN-(SOct)₆) and its mono-, bi-, and trinuclear Re­(CO)₃Cl complexes are reported. These are characterized by ¹H NMR spectroscopy and electrochemistry, and show broad, intense absorption across the visible wavelength region. Using time-dependent density functional theory (TD-DFT) calculations and resonance Raman spectroscopy these absorption bands are shown to be π → π*, MLCT, ILCT­(sulfur → HATN), or mixed MLCT/ILCT in nature. Time-resolved infrared spectroscopy is used to probe structural changes and dynamics on short time scales and supports the assignment of a mixed MLCT/ILCT state in which both sulfur groups and one metal center act as electron donors to the HATN core

Intraligand Charge-Transfer Excited States in Re(I) Complexes with Donor-Substituted Dipyridophenazine Ligands

The donor–acceptor ligands 11-(4-diphenylaminophenyl)­dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine (dppz-PhNPh₂) and 11-(4-dimethylaminophenyl)­dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine (dppz-PhNMe₂), and their rhenium complexes, [Re­(CO)₃X] (X = Cl^–, py, 4-dimethylaminopyridine (dmap)), are reported. Crystal structures of the two ligands were obtained. The optical properties of the ligands and complexes are dominated by intraligand charge transfer (ILCT) transitions from the amine to the dppz moieties with λ_abs = 463 nm (ε = 13 100 M^–1 cm^–1) for dppz-PhNMe₂ and with λ_abs = 457 nm (ε = 16 900 M^–1 cm^–1) for dppz-PhNPh₂. This assignment is supported by CAM-B3LYP TD-DFT calculations. These ligands are strongly emissive in organic solvents and, consistent with the ILCT character, show strong solvatochromic behavior. Lippert–Mataga plots of the data are linear and yield Δμ values of 22 D for dppz-PhNPh₂ and 20 D for dppz-PhNMe₂. The rhenium­(I) complexes are less emissive, and it is possible to measure resonance Raman spectra. These data show relative band intensities that are virtually unchanged from λ_exc = 351 to 532 nm, consistent with a single dominant transition in the visible region. Resonance Raman excitation profiles are solvent sensitive; these data are modeled using wavepacket theory yielding reorganization energies ranging from 1800 cm^–1 in toluene to 6900 cm^–1 in CH₃CN. The excited state electronic absorption and infrared spectroscopy reveal the presence of dark excited states with nanosecond to microsecond lifetimes that are sensitive to the ancillary ligand on the rhenium. These dark states were assigned as phenazine-based ³ILCT states by time-resolved infrared spectroscopy. Time-resolved infrared spectroscopy shows transient features in which Δν­(CO) is approximately −7 cm^–1, consistent with a ligand-centered excited state. Evidence for two such states is seen in mid-infrared transient spectra

Intraligand Charge-Transfer Excited States in Re(I) Complexes with Donor-Substituted Dipyridophenazine Ligands

The donor–acceptor ligands 11-(4-diphenylaminophenyl)­dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine (dppz-PhNPh₂) and 11-(4-dimethylaminophenyl)­dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine (dppz-PhNMe₂), and their rhenium complexes, [Re­(CO)₃X] (X = Cl^–, py, 4-dimethylaminopyridine (dmap)), are reported. Crystal structures of the two ligands were obtained. The optical properties of the ligands and complexes are dominated by intraligand charge transfer (ILCT) transitions from the amine to the dppz moieties with λ_abs = 463 nm (ε = 13 100 M^–1 cm^–1) for dppz-PhNMe₂ and with λ_abs = 457 nm (ε = 16 900 M^–1 cm^–1) for dppz-PhNPh₂. This assignment is supported by CAM-B3LYP TD-DFT calculations. These ligands are strongly emissive in organic solvents and, consistent with the ILCT character, show strong solvatochromic behavior. Lippert–Mataga plots of the data are linear and yield Δμ values of 22 D for dppz-PhNPh₂ and 20 D for dppz-PhNMe₂. The rhenium­(I) complexes are less emissive, and it is possible to measure resonance Raman spectra. These data show relative band intensities that are virtually unchanged from λ_exc = 351 to 532 nm, consistent with a single dominant transition in the visible region. Resonance Raman excitation profiles are solvent sensitive; these data are modeled using wavepacket theory yielding reorganization energies ranging from 1800 cm^–1 in toluene to 6900 cm^–1 in CH₃CN. The excited state electronic absorption and infrared spectroscopy reveal the presence of dark excited states with nanosecond to microsecond lifetimes that are sensitive to the ancillary ligand on the rhenium. These dark states were assigned as phenazine-based ³ILCT states by time-resolved infrared spectroscopy. Time-resolved infrared spectroscopy shows transient features in which Δν­(CO) is approximately −7 cm^–1, consistent with a ligand-centered excited state. Evidence for two such states is seen in mid-infrared transient spectra