High Pressure Raman Investigations On Porphyrin J Aggregaties

This thesis described the effects of solvents, substitution and counterion onporphyrin Jaggregations by electronic absorptionand Raman scattering spectroscopy,and a model of orientation change of porphyrin in aggregate was proposed. Usingdiamond anvil cell (DAC) microscopic Raman technique, we reported the in situRaman measurements of Tpp J aggregates under high pressure up to 13 GPa. Andaccording to the spectral phenomena, Tpp J aggregates can be treated as a candidatefor the potential application of pressure-driven molecular spring washers in parallelcombinations. Additionally, the stiffness of molecular washers can be tunable byproperly choice of meso-substitution.This dissertation can be divided into three following parts:(1) UV-Visible absorption spectra of four Tpp aggregates which were formedfromdichloromethane solution were obtained, all the spectra show red-shifted B band,which are the characterof Jaggregates. But the red shift in the absorption spectra ofTpp (H2SO4)-agg is two times larger than that of the other three Tpp aggregates. Atthe same time, Raman spectra of fourporphyrin aggregates were obtained. Comparedto the single band at around 1080cm-1 in otherthree aggregates, double bands can beseen in the Raman spectrum of Tpp (H2SO4)-agg, and then two parallel experimentsincluding the effects of solvent and substitution on the Raman spectra of Tpp (H2SO4)-agg were given. The results showed that the structure of porphyrinaggregates can be affected by nature of titrating acid and solvent and peripheralsubstitution. At last, a stack card model was built and shown in figure 1. Vibronicspectroscopic technique has potential applications in elucidating the orientation ofporphyrins inorganized molecularassemblies.(2) As aprimary parameterof thermodynamic, pressure has relationship with thetotal energy of the compressed substance, and the phase structure and reaction waymay be controlled by changing pressure because the interatomic distance can be reduced during compression. Porphyrin is the pigment of life, and then high pressureinvestigations on porphyrin aggregates can provide more information about themechanismof structure andphase changes.Figure 2 shows the Raman spectra of Tpp J aggregates at ambient and selectedhigh pressures carried outup to 13GPa.From the frequency-pressure relationship, there is a monotonic increase in thefrequency of all Raman modes throughout the compression process, and we have notobserved changes in the spectra including splitting of modes, appearance of newmodes, or sudden changes in the slope of frequency-pressure curve, so no obviousevidence of phase transition can be concluded during the entire compressionprocesses, on the whole, the slopes (dω/dP) at high-(1010-1060 cm-1) and low-frequency (230-330 cm-1) are greater than that in the middle-frequency (380-1000cm-1) region, suggesting that the chemical bond of greater compressibility, which isassociated with the high and low frequency bands, is more sensitive to compression and makes a majorcontribution to the reduction of the volume.It is considered that the coplanarity of the phenyl groups and the mean porphyrinplane could enhance the coupling between porphyrin and phenylπ-systems,whichthen induce some of the phenyl Raman modes of porphyrin systems get stronger. Inour high pressure Raman experiments, as shown in Figure 2, another interestingobservation is the continuing intensity enhancement of phenylψ3 C-C stretchingmode. Unlike other molecular spring driven by chemical interactions, Tpp Jaggregates can be treated as a candidate for the potential application ofpressure-driven molecularwashers inparallel combinations (Figure 3). And due to thestructure of Tpp, each spring washer has four waves equal to the number of phenylsubstitutes. The dihedral angle between porphyrin plane and phenyl groups decreasedwith increasing pressure, and this transformation is completely reversible duringdecompression. Additionally, this method could be used as a suitable tool forinvestigating the continual variation of dihedral angle between the porphyrin ring andmeso-aryl substituents inporphyrin systems.(3) The physicochemical properties of porphyrin compounds can be affect by βand meso-substitution, in this reports, high pressure Raman investigations on fourmeso-substitution porphyrin Jaggregates have been measured and shown in figure 4.Figure 4 High pressure Raman spectra of porphyrin aggregates, A:Tspp, B: Tnpp, C: Tmpp, and D:Tpyp.According to the high pressure spectral phenomena of porphyrin aggregates, wegot the following conclusion:3.1 There is a monotonic increase in the frequency of all Raman modesthroughout the compression process, and we have not observed changes in the spectraincluding splitting of modes, appearance of new modes, or sudden changes in theslope of frequency-pressure curve, so no obvious evidence of phase transition forallthe porphyrin aggregates canbe concluded during the entire compressionprocesses.3.2 Forall the porphyrin aggregates, the slopes (dω/dP) athigh-(1010-1060 cm-1)and low- frequency (230-330 cm-1) are greater than that in the middle-frequency (380-1000 cm-1) region, suggesting that the chemical bond of greatercompressibility,which is associated with the high and low frequency bands, is more sensitive tocompression and makes a majorcontribution to the reduction of the volume.3.3 The intensity ratio of phenylψ3 C-C in-plane stretching andυ2 Cb-Cbstretching mode (which is located at 1575 and 1540 cm-1 under ambient pressurerespectively) exhibitanearlinearly increase during the pressure rise up below 13GPa.And the over all pressure range 0-13GPa can be divided into two domains (0-6 and6-13Gpa) in which linear relationship have been assumed. At different pressureregions, the slope is different, providing more information about the mechanic of theintensity enhancement. Pressure has a strong impact both on the dihedral anglebetween the porphyrin ring and meso-aryl substitute and on theπelectiondelocalization, and both of them can induce the intensity enhancement of aromaticψ3C-C stretching vibration. Theπelectron delocalization is normally occurred underhigh pressure, so under the pressure range from ambient to 6GPa, below 6Gpa,decrease of dihedral angle between the porphyrin ring and meso-aryl substitute playan important role in the intensity enhancement, while above 6GPa,πelectrondelocalization domains. As we have investigated before, the phenomena of thisdihedral angle decrease make Tpp J aggregates a good candidate for the potentialapplication in molecular spring washer, within the first pressure range, the slope ofthe intensity ratio of aromatcψ3 C-C in-plane stretching toυ2 Cb-Cb stretching modeforTspp, Tnpp, Tpp, Tmpp, and Tpypp self-assembles are 0.151, 0.128, 0.081, 0.053and 0.042 respectively, indicating that the stiffness of molecular spring washers canbe tunable by properly choice of meso-substitution.