Insulated and Communicating Polypyridyl Ligands and Their Rhenium Complexes: A Computational and Spectroscopic Study

Abstract

Rhenium tricarbonyl chloride complexes of a number of polypyridyl ligands were investigated using a range of spectroscopic and computational chemistry techniques. The amount of electronic communication differed between the various ligands with some showing a great deal while others were effectively insulated. Both vibrational, including Raman, resonance Raman, infrared and time-resolved infrared, and electronic, including electronic absorption, emission, transient emission and transient absorption spectroscopies were employed. These were considered alongside calculated properties which included, ground-state geometries, the nature and intensities of vibrational modes and electronic properties such as molecular orbitals and electronic transitions.

Complexes with ligands involving a hexa-peri-hexabenzocoronene (HBC) moiety appended to a bpy are reported. The HBC unit is functionalised with either tBu or C12H25 in order to influence the aggregating properties of the molecule. The electronic absorption spectra are dominated by pi-pi* transitions (at between 250 and 400 nm) based on the HBC plane which are blue-shifted ~550 cm-1 upon complexation. An additional weak metal-to-ligand charge transfer (MLCT) transition from the rhenium and HBC to the bpy is also observed for the complex. An electron transition density and natural transition orbital analysis of the time-dependent density functional theory (TD-DFT) results confirmed the transitions. This was in addition to resonance Raman spectra of the P and β bands which revealed enhancement of HBC based modes when probing the β band and bpy modes when probing the P band. Two emissions are observed, a highly structured one based on the HBC pi-pi* state (~500 nm) and a broad one from the MLCT excited-state (~660 nm). The MLCT emission increases in intensity and red-shifts upon increasing aggregation relative to the HBC emission. From transient emission spectroscopy two lifetimes are obtained, around 40 ns for the pi-pi* state and less than 10 ns for the MLCT state. Transient absorption spectroscopy also indicates two lifetimes for the complexes but one for the ligands, which are affected by solvent and concentration. Time-resolved infrared spectroscopy confirms the MLCT and two pi-pi* excited states for both complexes. Structural calculations using the M06 functional predicted a decrease in energy of around 170 kJ mol-1 for a dimer of the system compared to two free monomers, due to pi-stacking energy. Three different types of triazole ligands are studied; a `regular' type bidentate triazole and bi- and tri- dentate `inverse' triazoles. The degree of electronic communication is strongly influenced by the presence or absence of a methylene bridging group either in the R group or in the ligand itself. Intense pi-pi* bands (~280 nm) dominate the electronic absorption spectra of all systems which also include a weak MLCT band at 330 nm for the regular ligand and 300 nm for the inverse ones. TD-DFT calculations predict the lack of involvement of the R group in the MLCT transition of the insulated systems, which is consistent with the lack of enhancement of vibrational modes based on the R-group in the resonance Raman spectrum (where obtainable). The emission spectral profiles are generally unaffected by the nature of R, however the lifetimes are affected modestly. Additionally, the emission quantum yields of the ligand types differ by an order of magnitude, decreasing for the insulated-type ligands. Supramolecular cage architectures with triazole functionalisation are studied. These systems involve a limited amount of electronic communication, for example the electronic absorption spectra are identical (bands at 280 and 330 nm) except for an extra band in cases where the R group includes its own inherent transition. TD-DFT predicts this insulation from the functional moiety, that the electronic transitions are concentrated on the tripyridyl backbone and that synthesis of the cage results in little change to the underlying electronic properties of the system. Enhancement of ligand based vibrational modes during resonance Raman spectroscopy of the UV absorption bands is consistent with TD-DFT as is the enhancement of R based modes for the extra bands observed in some systems in the visible region. The ligand is non-emissive however some R groups impart their own emission behaviour on the system consistent with the properties of the R group alone. A ferrocene based actuator where a bpy moiety is attached to one or both cyclopentadiene (Cp-) rings both directly and through an alkyne linker is reported. When both Cp- rings include the bpy substituent the arms tend to pi-stack, forming a closed structure, complexation of the bpy with a Cu diphenyl bipyridine forces the arms to open through steric hindrance. DFT calculations confirm the energy stabilisation of the eclipsed forms and the open form of the complex. Electronic absorption bands observed in the UV are predicted by TD-DFT (and confirmed by resonance Raman) to be Fc to bpy charge transfers while an extra band at 475 nm in the complex systems is MLCT from Cu to bpy.