Synthetic porphyrin assemblies have gained attraction recently for their potential utility as light-harvesting materials. The water-soluble nanotube aggregates of tetra(sulfonatophenyl)porphyrin (TSPP) are particularly intriguing as their chiral properties and tubular structure resemble that of green sulfur bacteria chlorosomes. Considerable research has been conducted on this particular light-harvesting aggregate (LHA) revealing its optical properties, the influence of solvent and environmental effects on its electronic properties, as well as the helical nature of the associated molecular excitons. However, a precise description of the internal structure of the aggregate, i.e. how the porphyrin monomers arrange themselves within the aggregate, is still debated among investigators. Thus far, high resolution microscopy efforts have not been able to unveil this internal structure and thus a combination of imaging and spectroscopic studies have been used to devise a variety of possible structures. Recently, our group discovered that heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy is capable of measuring both monomeric and excitonic properties of these aggregates by keeping track of the “phase acrobatics” associated with various vibrational and electronic resonances. In VSFG spectroscopy, a mid-infrared (IR) pulse and a visible pulse are overlapped on a sample in space and time to produce a signal emission whose frequency is the sum of the frequencies of the two input pulses. For heterodyne detection, a local oscillator interferes with the VSFG emission to produce a spectral interferogram which provides for phase-sensitive information. We found that when the VSFG signal frequency is resonant with the J<sub>Q </sub>exciton electronic transition and the IR frequency is resonant with vibrational modes associated with macrocycle C=C vibrations, PPP-polarized HD-VSFG spectra exhibit doubly-resonant conditions reflective of excitonic properties of the aggregate and SSP-polarized HD-VSFG spectra exhibit singly-resonant conditions and are reflective of properties of the constituent monomers. We utilize an excitonic polaron model, to fit ultraviolet-visible (UV-vis) absorption, HD-VSFG, and resonance Raman spectra and convey the organization of the porphyrin within these aggregates and its excitonic properties. We propose a model which describes a 26-start helical nanotube that best represents the structure of TSPP aggregates.

Unveiling the internal structure of light-harvesting porphyrin nanoaggregates using phase-sensitive vibrational sum frequency generation spectroscopy

254th National Meeting (2017)

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