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

Spectroscopy of Donor-Acceptor Compounds

A number of donor-acceptor compounds have been investigated using a range of spectroscopic and computational techniques. Donor-acceptor compounds are widely used in molecular electronics applications, and it is of interest to investigate how their properties can be manipulated and probed. Ground state properties are characterised with Fourier Transform (FT) - Raman spectroscopy, which is also used to verify density functional theory calculations. The initially formed excited state (Franck-Condon state) is characterised with electronic absorption and resonance Raman spectroscopy. Transient absorption spectroscopy tends to probe the longest-lived excited state, while time-resolved infrared spectroscopy can probe intermediate states and kinetic processes as the time between pump and probe is varied. Experimental techniques are complemented by the use of density functional theory (DFT) calculations. A series of complexes containing dimethyl or diphenyl amine- substituted dipyrido[3,2-a:2',3'-c]phenazine (dppz) in which the ancillary ligand at the metal centre was varied were studied. The properties of these ligands and complexes were found to be dominated by a strong intra-ligand charge transfer (ILCT) transition from the amine to dppz, with little contribution from typical low-energy metal-to-ligand charge transfer (MLCT) transitions for complexes. Protonation shifted this ILCT transition to the near-IR; this was characterised with resonance Raman spectroscopy and time-dependent DFT (TD-DFT) calculations. Based on this study, amine-substituted dppz systems were altered in various ways in order to manipulate this ILCT transition. The effect of changing the distance between the amine donor and the dppz acceptor was investigated; this was found to influence the energy of the ILCT transition, the relative intensity of vibrational modes associated with different parts of the molecule, and the excited state lifetime, but the ILCT character appeared to be retained. The effect of the angle between the donor and acceptor units for dppz was altered in various ways. Increasing the donor-acceptor angle was found to increase the energy and decrease the intensity of the ILCT transition, and increase the intensity of an MLCT transition for Re(I) complexes. Experimental and calculated non-resonant Raman cross sections also decreased as donor-acceptor angle was increased. The effect of changing the bridge type between donor and acceptor from conducting thiophene to more insulating triazole was investigated. This increased the energy of the lowest electronic transition, and reduced the degree of ILCT in this transition, with behaviour tending towards more typical dppz as the linker became more insulating. The properties of the ligand 5,6,11,12,17,18 - hexaazatrinaphthalene (HATN) substituted with electron-donating sulfur groups were investigated. The mono-, bi- and tri-Re(CO)3Cl complexes of this ligand showed broad and intense visible absorption, which TD-DFT calculations suggested involved mixed MLCT/ILCT transitions. Resonance Raman spectroscopy was consistent with this, and time-resolved infrared (TRIR) spectroscopy provided evidence for a mixed MLCT/ILCT excited state. Finally, a series of donor-acceptor compounds that are used in dye-sensitised solar cells are discussed, in order to try and understand what makes some more efficient than others. The first series were zinc porphyrin -based with carbazole -thiophene chains, which increased their visible absorption. The second series were organic dyes, also using a carbazole donor and thiophene chain, but for these compounds the reason for differences in solar cell performance could not be established with the techniques used

Spectroscopy of Donor-Acceptor Compounds

A number of donor-acceptor compounds have been investigated using a range of spectroscopic and computational techniques. Donor-acceptor compounds are widely used in molecular electronics applications, and it is of interest to investigate how their properties can be manipulated and probed. Ground state properties are characterised with Fourier Transform (FT) - Raman spectroscopy, which is also used to verify density functional theory calculations. The initially formed excited state (Franck-Condon state) is characterised with electronic absorption and resonance Raman spectroscopy. Transient absorption spectroscopy tends to probe the longest-lived excited state, while time-resolved infrared spectroscopy can probe intermediate states and kinetic processes as the time between pump and probe is varied. Experimental techniques are complemented by the use of density functional theory (DFT) calculations. A series of complexes containing dimethyl or diphenyl amine- substituted dipyrido[3,2-a:2',3'-c]phenazine (dppz) in which the ancillary ligand at the metal centre was varied were studied. The properties of these ligands and complexes were found to be dominated by a strong intra-ligand charge transfer (ILCT) transition from the amine to dppz, with little contribution from typical low-energy metal-to-ligand charge transfer (MLCT) transitions for complexes. Protonation shifted this ILCT transition to the near-IR; this was characterised with resonance Raman spectroscopy and time-dependent DFT (TD-DFT) calculations. Based on this study, amine-substituted dppz systems were altered in various ways in order to manipulate this ILCT transition. The effect of changing the distance between the amine donor and the dppz acceptor was investigated; this was found to influence the energy of the ILCT transition, the relative intensity of vibrational modes associated with different parts of the molecule, and the excited state lifetime, but the ILCT character appeared to be retained. The effect of the angle between the donor and acceptor units for dppz was altered in various ways. Increasing the donor-acceptor angle was found to increase the energy and decrease the intensity of the ILCT transition, and increase the intensity of an MLCT transition for Re(I) complexes. Experimental and calculated non-resonant Raman cross sections also decreased as donor-acceptor angle was increased. The effect of changing the bridge type between donor and acceptor from conducting thiophene to more insulating triazole was investigated. This increased the energy of the lowest electronic transition, and reduced the degree of ILCT in this transition, with behaviour tending towards more typical dppz as the linker became more insulating. The properties of the ligand 5,6,11,12,17,18 - hexaazatrinaphthalene (HATN) substituted with electron-donating sulfur groups were investigated. The mono-, bi- and tri-Re(CO)3Cl complexes of this ligand showed broad and intense visible absorption, which TD-DFT calculations suggested involved mixed MLCT/ILCT transitions. Resonance Raman spectroscopy was consistent with this, and time-resolved infrared (TRIR) spectroscopy provided evidence for a mixed MLCT/ILCT excited state. Finally, a series of donor-acceptor compounds that are used in dye-sensitised solar cells are discussed, in order to try and understand what makes some more efficient than others. The first series were zinc porphyrin -based with carbazole -thiophene chains, which increased their visible absorption. The second series were organic dyes, also using a carbazole donor and thiophene chain, but for these compounds the reason for differences in solar cell performance could not be established with the techniques used

Synthesis and Light-Harvesting Potential of Cyanovinyl β-Substituted Porphyrins and Dyads

Knoevenagel condensation has been utilized as an alternative way to synthesize a series of β-vinyl-substituted porphyrins and porphyrin dyads with good to excellent yields. The condensation of β-formyl porphyrins and phenylacetonitriles allows control of the substitution pattern and metal centres in the porphyrin dyads, allowing the use of metallated synthons. While the optical and electronic properties of the resulting porphyrin dyes are perturbed by the presence of the cyano substituent, this does not significantly affect their use. For example, Raman spectroscopy, in agreement with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, show porphyrin electronic transitions with delocalization of frontier molecular orbital electron density onto the β substituent. A comparison of the photovoltaic performance of a carboxylated cyanostyryl condensation product and the unsubstituted analogue in dye-sensitized solar cells (DSSCs) was made. Although the devices showed similar efficiency, the device containing the cyano-substituted dye showed an extended incident photon-to-current conversion efficiency (IPCE) due to a slight red-shift in absorption and an increase in photovoltage as a result of a longer electron lifetime. This minimal change in light-harvesting performance highlights the potential of this Knoevenagel synthetic methodology for producing light-harvesting porphyrin dyes

The electronic characterization of conjugated aryl-substituted 2,5-bis(2-thien-2-ylethenyl) thiophene-based oligomers

A series of 2,5-bis(2-thien-2-ylethenyl) thiophene-based oligomers with a para-R-arylethenyl substituent have been subjected to electrochemical (cyclic voltammetry and electronic absorption spectroscopy) and density functional theory characterization. The primary aim of this investigation is to characterize the behavior of these oligomers in the oxidized state. Oligomers without methyl ‘end-caps’ undergo facile r-dimerization; however there is no evidence for the formation of higher oligomers. The oxidized r-dimers exist in both cationic and dicationic form. Oligomers with methyl ‘end-caps’ do not show any evidence of r-dimerization. The inductive capacity of the para-R substituent has a significant bearing on the electronic properties of the oligomer, in particular, oligomers with more electron-withdrawing substituents have charge transfer character associated with the dominant electronic excitations

Flexible tuning of unsaturated β-substituents on Zn porphyrins: A synthetic, spectroscopic and computational study

A series of zinc porphyrins substituted at adjacent β-positions with a CN group and para-substituted ethenyl/ethynyl-phenyl group have been studied using electronic absorption spectroscopy, resonance Raman spectroscopy and DFT calculations. The oxidative nucleophilic substitution of hydrogen was utilized for the introduction of a cyano substituent on the porphyrin ring. This modification has a remarkable electronic effect on the ring. The resulting porphyrin cyanoaldehyde was further modified in Wittig condensations to give series of arylalkene- and arylalkyne-substituted derivatives. This substitution pattern caused significant redshifting and broadening of the B band, tuning from 433-446nm. Additionally the Q/B band intensity ratios show much higher values than observed for the parent porphyrin ZnTPP (0.20 vs. 0.03). Careful analysis of the electronic transitions using DFT and resonance Raman spectroscopy reveal that the substituent does not significantly perturb the electronic structure of the porphyrin core, which is still well described by Gouterman\u27s four-orbital model. However, the substituents do play a role in elongating the conjugation length and this results in the observed spectral changes

Entomologiceskie i parazitologiceskie issledovanija v povolz'e : sbornik nauciych trudov

Electron lifetimes in dye-sensitized solar cells employing a porphyrin dye, an organic dye, a 1:1 mixture of the two dyes, and a dichromophoric dye design consisting of the two dyes using a nonconjugated linker were measured, suggesting that the dispersion force of the organic dyes has a significant detrimental effect on the electron lifetime and that the dichromophoric design can be utilized to control the effect of the dispersion force

Probing donor-acceptor interactions in meso-substituted Zn(II) porphyrins using resonance raman spectroscopy and computational chemistry

A series of Zn(II) porphyrins which have asymmetrically substituted meso groups have been studied with UV-vis, resonance Raman, emission spectroscopies, and density functional theory (DFT) calculations. Dye-sensitized solar cells (DSSCs) of these materials have also been fabricated and their performance parameters measured. DFT calculations show perturbation of frontier molecular orbitals, and redox-active substituents cause greater perturbation than nonredox active substituents. All substituents cause a broadening of the B band, as is common for substituted porphyrins. TD-DFT calculations and resonance Raman spectroscopy suggest the donor and acceptor substituents play a small role in transitions of the B band. The meso donor substituent is electronically isolated and does not significantly perturb the molecular orbitals (MOs), while the meso cyanoacrylic acid TiO2 binding group has a much larger effect on the eg MO in particular. However, in the oxidized porphyrin species, the hole is located on the meso substituent, localizing it away from the semiconductor surface, which should reduce recombination and also improve performance. They show modest efficiency when incorporated into solar cells; however, the pattern of behavior is consistent with localization of charge at the meso unit