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Mono- and Bimetallic Polypyridyl Systems for Solar Energy Applications: Tuning and Identification of Excited States through Ultrafast Spectroscopy.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Mono- and Bimetallic Polypyridyl Systems for Solar Energy Applications: Tuning and Identification of Excited States through Ultrafast Spectroscopy./
Author:	Whittemore, Tyler Joseph .
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2018,
Description:	237 p.
Notes:	Source: Dissertations Abstracts International, Volume: 81-05, Section: B.
Contained By:	Dissertations Abstracts International81-05B.
Subject:	Chemistry. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=27539086
ISBN:	9781687936752

Mono- and Bimetallic Polypyridyl Systems for Solar Energy Applications: Tuning and Identification of Excited States through Ultrafast Spectroscopy.
Whittemore, Tyler Joseph .

Mono- and Bimetallic Polypyridyl Systems for Solar Energy Applications: Tuning and Identification of Excited States through Ultrafast Spectroscopy. - Ann Arbor : ProQuest Dissertations & Theses, 2018 - 237 p.

Source: Dissertations Abstracts International, Volume: 81-05, Section: B.

Thesis (Ph.D.)--The Ohio State University, 2018.

This item must not be sold to any third party vendors.

Photon energy fuels life on earth; efforts to better use sunlight for the creation of energy through solar photovoltaics and solar fuels have been of great interest across scientific disciplines. A key factor for using the energy stored in photons is the ability to characterize, to change, and ultimately, to tune the excited states of molecules to absorb wavelengths of light to better harness the solar spectrum while also designing molecules with have excited state energies that are favorable for electron transfer and photocatalytic reactions. This work demonstrates, using Ru(II) and Rh2(II,II) transition metal complexes, the rational synthetic modification of inorganic complexes to better harness low energy wavelengths of the solar spectrum and the tuning of those states to do useful chemical transformations.This is first demonstrated using excited state using Ru(II) complexes, which were explored by small modifications to the ligand geometry in fused donor-acceptor systems using quinone-based electron accepting groups. In this molecule, a side-on geometry was determined to more fully delocalize the electron density on the quinone-containing ligand. This delocalization was determined to both red-shift the absorption to the red (λmax = 546 nm) and increase the excited state lifetime from 0.35 ns to 19 ns. This excited state is capable of performing excited state electron transfer reactions to oxidize phenothiazine, lending the complex to p-type semiconductor applications. These findings outline how small changes to the ligand coordination environment impact the light absorption profile and the excited state dynamics.The effect of synthetic modifications on the excited state properties of transition metal complexes was further explored using dirhodium(II,II) formamidinate complexes. Ultrafast UV-Vis and infrared time-resolved spectroscopy demonstrated the relatively long singlet lifetimes (τS = 7 ps) and 25 ns triplet lifetimes of these complexes, which were then used to perform electron transfer reactions. The electron transfers only observed when the Rh2(II,II) axial sites are capped with carboxylate groups, which increase the excited state lifetime over an order of magnitude while extending the absorption profile of the molecule into the red. This complex is ideal for charge injection to n-type semiconductors for solar energy applications. Similarly, neutral pyridine-based capping groups were used, demonstrating that the versatility of the synthetic modifications to extend the absorption profile into the near infrared and to tune the relative HOMO and LUMO energies in predictable ways. These latter systems are able to perform bimolecular oxidation of the reversible electron donor p-phenylene diamine with red to near infrared light, extending this class of complexes use to p-type dye-sensitized solar cell applications.Decreasing the Rh-Rh bond distance was also used as a means to increase the excited state lifetime by raising the energy of the deactivating metal- centered states. Dirhodium formamidinate with pyridazine, pthalazine, and benzo[c]cinnoline as a diimine ligand synthesized and characterized. This complex uses a 4-coordinate metallocycle, which was shown crystallographically to shorten the Rh-Rh bond. This complex exhibits a number of improvements over the 5-coordinate napthyridine-based ligands, most notably, that due to greater HOMO-LUMO Franck Condon overlap, these complexes are emissive at 77 K, a phenomenon not previously observed for Rh2 d7-d7 systems. In addition, it was observed that 3 is capable of reducing protons in solution to hydrogen gas at low overpotentials (η = 0.37 V) with appreciable activity TOF = 1.7x105 s-1. Importantly, 3 also catalyzes the reduction of protons to dihydrogen photochemically in the presence of a sacrificial donor when irradiated with red to near infrared light. This is an important finding, as typical solar fuel generation involves the separation of the light absorbing and catalytic moieties. While sensitizer-less homogeneous catalysts have been reported, they are relatively uncommon and have not been reported with spectral coverage demonstrated in the present work.This work demonstrates how minor synthetic variations to the ligand environment around a metal can have profound consequences on the identity and lifetime of the excited state, the thermodynamic driving force for charge transfer, and the kinetics of excited state reactions. The careful design of a series of dirhodium(II,II) paddlewheel complexes allowed understanding of the interplay between the charge transfer states and deactivating metal-centered states, such that they could be used as photochemical catalysis with red to near infrared light. This work illustrates how careful system design and understanding using a variety of spectroscopic techniques can inform synthetic choices to create useful photosensitizers and photocatalysts.

ISBN: 9781687936752Subjects--Topical Terms:

516420
Chemistry.
Subjects--Index Terms:

Spectroscopy

Mono- and Bimetallic Polypyridyl Systems for Solar Energy Applications: Tuning and Identification of Excited States through Ultrafast Spectroscopy.
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506 $a This item must not be sold to any third party vendors.
520 $a Photon energy fuels life on earth; efforts to better use sunlight for the creation of energy through solar photovoltaics and solar fuels have been of great interest across scientific disciplines. A key factor for using the energy stored in photons is the ability to characterize, to change, and ultimately, to tune the excited states of molecules to absorb wavelengths of light to better harness the solar spectrum while also designing molecules with have excited state energies that are favorable for electron transfer and photocatalytic reactions. This work demonstrates, using Ru(II) and Rh2(II,II) transition metal complexes, the rational synthetic modification of inorganic complexes to better harness low energy wavelengths of the solar spectrum and the tuning of those states to do useful chemical transformations.This is first demonstrated using excited state using Ru(II) complexes, which were explored by small modifications to the ligand geometry in fused donor-acceptor systems using quinone-based electron accepting groups. In this molecule, a side-on geometry was determined to more fully delocalize the electron density on the quinone-containing ligand. This delocalization was determined to both red-shift the absorption to the red (λmax = 546 nm) and increase the excited state lifetime from 0.35 ns to 19 ns. This excited state is capable of performing excited state electron transfer reactions to oxidize phenothiazine, lending the complex to p-type semiconductor applications. These findings outline how small changes to the ligand coordination environment impact the light absorption profile and the excited state dynamics.The effect of synthetic modifications on the excited state properties of transition metal complexes was further explored using dirhodium(II,II) formamidinate complexes. Ultrafast UV-Vis and infrared time-resolved spectroscopy demonstrated the relatively long singlet lifetimes (τS = 7 ps) and 25 ns triplet lifetimes of these complexes, which were then used to perform electron transfer reactions. The electron transfers only observed when the Rh2(II,II) axial sites are capped with carboxylate groups, which increase the excited state lifetime over an order of magnitude while extending the absorption profile of the molecule into the red. This complex is ideal for charge injection to n-type semiconductors for solar energy applications. Similarly, neutral pyridine-based capping groups were used, demonstrating that the versatility of the synthetic modifications to extend the absorption profile into the near infrared and to tune the relative HOMO and LUMO energies in predictable ways. These latter systems are able to perform bimolecular oxidation of the reversible electron donor p-phenylene diamine with red to near infrared light, extending this class of complexes use to p-type dye-sensitized solar cell applications.Decreasing the Rh-Rh bond distance was also used as a means to increase the excited state lifetime by raising the energy of the deactivating metal- centered states. Dirhodium formamidinate with pyridazine, pthalazine, and benzo[c]cinnoline as a diimine ligand synthesized and characterized. This complex uses a 4-coordinate metallocycle, which was shown crystallographically to shorten the Rh-Rh bond. This complex exhibits a number of improvements over the 5-coordinate napthyridine-based ligands, most notably, that due to greater HOMO-LUMO Franck Condon overlap, these complexes are emissive at 77 K, a phenomenon not previously observed for Rh2 d7-d7 systems. In addition, it was observed that 3 is capable of reducing protons in solution to hydrogen gas at low overpotentials (η = 0.37 V) with appreciable activity TOF = 1.7x105 s-1. Importantly, 3 also catalyzes the reduction of protons to dihydrogen photochemically in the presence of a sacrificial donor when irradiated with red to near infrared light. This is an important finding, as typical solar fuel generation involves the separation of the light absorbing and catalytic moieties. While sensitizer-less homogeneous catalysts have been reported, they are relatively uncommon and have not been reported with spectral coverage demonstrated in the present work.This work demonstrates how minor synthetic variations to the ligand environment around a metal can have profound consequences on the identity and lifetime of the excited state, the thermodynamic driving force for charge transfer, and the kinetics of excited state reactions. The careful design of a series of dirhodium(II,II) paddlewheel complexes allowed understanding of the interplay between the charge transfer states and deactivating metal-centered states, such that they could be used as photochemical catalysis with red to near infrared light. This work illustrates how careful system design and understanding using a variety of spectroscopic techniques can inform synthetic choices to create useful photosensitizers and photocatalysts.
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