Adams, Bethany Sarah
(2017)
Photophysics in conventional and supercritical fluids: excited state dynamics and sensing applications.
PhD thesis, University of Nottingham.
Abstract
Chapter 1. This Chapter gives a brief introduction to photophysics and the study of excited states. The technique of time-resolved infrared spectroscopy (TRIR), which is used throughout this Thesis to ascertain the nature of excited states, is also introduced. The utility of the techniques to examine the nature and kinetics of excited states is detailed. Subsequently, the effect of solvent medium in which the measurements take place is also discussed.
Chapter 2. The photophysical properties of rhenium dipyrido[3,2-a:2’,3’-c]phenazine complexes appended with diphenylamine (NPh2) groups have been investigated through the use of a range of techniques, including time-resolved infrared spectroscopy, cyclic voltammetry and DFT calculations. The effect of the diphenylamine donor group has been assessed with comparisons drawn between [Re(dppz-NPh2)(CO)3Cl] and [Re(dppz)(CO)3Cl]. The addition of the donor group results in the observation of three ligand centred excited states in the TRIR spectra, unlike the MLCT states observed for [Re(dppz)(CO)3Cl]. These states exhibit a complex interplay, whereby States I and III are initially formed, and the former decays to State II on the picosecond timescale. States II and III are then observed to decay to the ground state on the microsecond and nanosecond timescales respectively. A detailed assignment of these states is given, with States I and II assigned to 1ILππ* and 3ILππ* states respectively. State III was assigned to a NPh2(π)phz(π*) charge transfer state. Good evidence of the presence of three distinct states has been obtained through TRIR measurements conducted in the fingerprint region, where a marker band for State II has been observed. The molecular orbital calculations performed on the complexes indicate charge transfer character, where the highest occupied molecular orbital is observed to be based on the NPh2 ligand, unlike for [Re(dppz)(CO)3Cl] complexes where the highest energy occupied molecular orbitals are normally metal based. The lifetime of the charge transfer state is shown to be relatively independent of the number of NPh2 donors appended, where a similar lifetime is observed for [Re(dppz-PhNPh2)(CO)3Cl] and [Re(dppz-(PhNPh2)2)(CO)3Cl].
The effect of the bridge type between the NPh2 donor and phz acceptor has also been investigated. This work details the effects on all three excited states, with the charge transfer state of most interest. The effect of the bridge length has been assessed through study of phenyl-moiety bridges which possess varying lengths. It was observed that as the bridge length increased, the lifetime of the charge transfer state decreased, and this has been attributed to an increased level of insulator character. The nature of the bridge has been investigated, where phenyl, thiophene and triazole bridge moieties have been compared to an analogous complex without a bridge. The results indicated that insulator character of the bridges increases as follows, thiophene < phenyl < triazole, and this chantes the lifetimes of the charge transfer states of the resulting complexes by a factor of 100.
Chapter 3. The photophysical properties of rhenium dipyrido[3,2-a:2’,3’-c]phenazine complexes modified on the phenazine like portions of the ligands have been investigated through the use of time resolved infrared spectroscopy and DFT calculations. Firstly, substitution with a nitrogen heteroatom at the ortho (2) and meta (3) positions (the phenazine like part of the ligand), resulting in [Re(dppp2)(CO)3Cl], [Re(dppp3)(CO)3Cl] and [Re(dppp2Br)(CO)3Cl] respectively, has been investigated. The additional nitrogen atom is observed to lower the energy of the molecular orbital based on the phenazine portion of the ligand and resulting in the population of an MCLT(phz) state. A marker band for the MLCT(phz) state has been assigned in the fingerprint region, a spectral region which has proven important to ascertain the nature of these excited states. The position of the nitrogen atom was also found to be significant, where substitution at the meta-position, [Re(dppp3)(CO)3Cl], is observed to lower the phz energy to a greater extent than in the ortho-position, [Re(dppp2)(CO)3Cl]. The effect of the solvent on the rate of the MLCT(phen)/MLCT(phz) states interconversion was found to be negligible, which is unlike observations made for [Re(dppz)(CO)3Cl]. However the nature of the most stable excited state was observed to change significantly. For the most polar solvent (DMSO, εr = 46.7), the most stable excited states is best described as an equilibrium of MLCT(phz)/ILππ* states, and for the lowest polarity solvent (toluene, εr = 2.3) the lowest excited state is MLCT(phen). Additionally, the excited state lifetime is observed to increase with decreasing solvent polarity. This has been attributed to a greater MLCT(phen) contribution observed in lower polarity media. The bromine substitution also has an effect on the excited state, where [Re(dppp2Br)(CO)3Cl] exhibits a lower energy phz state than for [Re(dppp2)(CO)3Cl]. This is further investigated with the previously unreported complex [Re(dppz-I)(CO)3Cl] and compared to other [Re(dppz-X)(CO)3Cl] complexes.
Finally, the effect of amine substituents has been investigated by studying the complex [Re(dpppn-(NH2)2)(CO)3Cl] (amine substitution is at the 2nd and 4th positions of the phenazine like ligand) in a range of solvents, with dielectric constants ranging from 2.3 to 46.7 and again this was supplemented by DFT calculations. Comparisons have been made to the complexes detailed in Chapter 2, and to the [Re(dppp2)(CO)3Cl] and [Re(dppp3)(CO)3Cl] complexes. The TRIR spectral profile of the complex in the highest dielectric solvent is observed to be similar to the complexes in Chapter 2 and is assigned as a 1ILππ* state decaying to a 3ILππ* state. A marker band for the 3ILππ* state is observed in the fingerprint region and is comparable to the State II 3ILππ* marker band from Chapter 2 for the [Re(dppz-NPh2)(CO)3Cl] complexes. The nature of the excited states are observed to change with the solvent medium and these results are discussed.
Chapter 4. The photophysical properties of the nitro-appended rhenium dipyrido[3,2-a:2’,3’-c]phenazine complexes [Re(dppz-12-NO2)(CO)3Cl] and [Re(dppz-13-NO2)(CO)3Cl], have been comprehensively investigated through time resolved infrared, cyclic voltammetry and DFT calculations.
The electron withdrawing nitro-group is observed to lower the energy of the MLCT(phz) state compared to the unsubstituted complex. Furthermore, significant NO2 contribution in the excited state is observed, where the first reduction potential is considerably more positive than that observed for [Re(dppz)(CO)3Cl]. Therefore the radical anion is located on the phz like portion of the molecule with a significant contribution from the NO2 group. This is consistent with the molecular orbital calculations and the one electron reduced species absorption spectra in this Chapter. Furthermore, the 12-substituted nitro-group is shown to have a stronger electron withdrawing effect than the corresponding 13-substituted nitro-group. This has been attributed to an increase in resonance effects associated with the 12-position and is correlated to the dihedral angle between the NO2 group and the phz moiety.
Chapter 5. The photophysical properties of rhenium diimine dyes in apolar media have been investigated in Chapter 5. As a result of poor solubility, photophysical studies of [Re(dppz)(CO)3L]+ and [Re(bpy)(CO)3L]+ complexes have been limited to higher polarity media. The ability to investigate organometallic dyes in apolar media opens up a range of avenues, including CO2 reduction without the need for a co-solvent. This Chapter explores these well reported complexes in the previously unstudied apolar media. The use of the large fluorinated anion of tetrakis[3,5-bis(perfluorohexyl)phenyl]borate has been implemented to yield the complexes [Re(dppz)(CO)3(DMAP)][BArf6] and [Re(bpy)(CO)3(DMAP)][BArf6]. Their photophysical properties have been investigated in perfluoromethylcyclohexane (εr 1.8) through TRIR, absorption and emission spectroscopies, and DFT calculations. [Re(dppz)(CO)3(DMAP)][BArf6] was shown to exhibit a long lived ILππ* state in perfluoromethyl cyclohexane. This state differs significantly from the ILππ*/MLCT(phen) states observed for [Re(dppz)(CO)3(DMAP)]+ complexes in more conventional polar media.
Chapter 6. The drive for smaller technology is a foremost requirement in the fabrication of electrical devices which possess greater processing power. This Chapter discusses supercritical fluid electrodeposition as a fabrication method, along with its desirable properties which are also detailed. In order to guide the SCFED process, further understanding of fluid behaviour within the nanoporous templates employed is vital. The work within this Chapter focusses on the development and implementation of a method for high pressure luminescence spectroscopy to probe confined fluid behaviour. Luminescence spectroscopy of high pressure fluids confined within nanoporous templates has not previously been investigated. Therefore, the first section of this Chapter focusses on the development of a methodology in order to conduct luminescence measurements of supercritical fluids confined within nanopores. This includes an investigation into an appropriate dye, and the design of the high pressure apparatus required for luminescence measurements within porous materials.
Anthracene and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin have been identified as suitable luminescence probes for the measurements required. Following this, the density of CO2 under bulk conditions has been probed with both dyes. The former is observed to exhibit a solvatochromic shift in the emission, and the latter a change in the peak intensity with CO2 density. This has allowed for the calibration of the probe dye behaviour with CO2 density. The density of CO2 confined within nanoporous templates, possessing mean pore widths of 8.3, 4.5 and 3.2 nm are investigated. The results show that the density of CO2 is considerably higher in the confined environment than in the bulk. The results are consistent between the two dyes indicating that luminescence spectroscopy is a valid method to probe fluid behaviour both in bulk and confined environments.
Finally, the use of luminescence measurements to elucidate fluid polarisability has been explored. An investigation towards quantifying fluid polarisability of CO2 when confined within the porous templates have been discussed. This includes the implementation of an anthracene dye to probe the polarisability of bulk CO2 conditions. These results are then compared to the literature. Subsequently, the method was used to elucidate the fluids polarisability. The polarisability of CO2 within the confines of the porous materials was observed to increase with increasing bulk CO2 pressure.
Chapter 7. This Chapter summarises the results obtained within this work and outlines the implications that these results may have. This is followed by a detailed future outlook, including suggested avenues of study that may arise from this Thesis.
Chapter 8. The apparatus and methodology developed to carry out this work is detailed, including the methodology for the high pressure luminescence spectroscopic measurements. The spectroscopic techniques used throughout this Thesis are detailed including descriptions of the equipment, procedures and materials used.
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