COARSE TO FINE: TUNING MOLECULAR PACKING TO PINPOINT TRIPLET PAIR SEPARATION MECHANISMS AND THE ROLE OF MOLECULAR COUPLING
Open Access
- Author:
- Doucette, Grayson S
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 15, 2019
- Committee Members:
- John B Asbury, Dissertation Advisor/Co-Advisor
John B Asbury, Committee Chair/Co-Chair
Suzanne E Mohney, Committee Member
Joshua Alexander Robinson, Committee Member
Noel Christopher Giebink, Outside Member - Keywords:
- Singlet Fission
High Pressure
Intermolecular Coupling
Photophysics
Pentacene
Polymorph - Abstract:
- Research and development of renewable energy systems is a national priority. Existing technologies cannot fulfill the energy needs of an ever-progressing world. Focusing on solar energy, singlet fission (SF) is a potential avenue to improve the performance of existing solar technology. By creating two useable charges from a single photon, tandem and photon multiplier photovoltaics can make more efficient use of the solar spectrum. Currently, only a small number of molecules and polymers have been found to meet the energetic requirements for singlet fission, and among those, a broad range of SF rates and yields exists. While many factors influence intermolecular singlet fission, the role of molecular coupling in driving both the photogeneration of two triplet excitations and subsequent transport are of paramount importance. Many theoretical accounts and some experimental work make clear the need to tune coupling to be strong enough such that SF occurs rapidly and efficiently while not too strong as to prevent binding and annihilation of triplet pairs. Unfortunately, a hurdle in SF research is the means of isolating the effect of coupling without drastically altering the electronic structure of molecules and doing so in device relevant films. This study takes two steps toward continuous control of coupling to uncover mechanistic features of SF as well as to glimpse at the turning point in coupling strength. The first step employs polymorphs of the molecule 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) to form two distinct molecular packing motifs in thin film samples. These motifs alter intermolecular interactions without changing the energetics of isolated molecules. Transient absorption was used to observe the effect on the separation of correlated triplet pairs (CTP) formed from singlet fission. Mechanistic parallels were drawn by correlating the initial CTP separation rates on the femtosecond timescale with triplet transfer controlled diffusion and annihilation rates on the nanosecond timescale. Similar polymorph dependence suggested CTP separation occurs via triplet transfer as well. As a result, controlling the efficiency of SF yield requires control of both singlet and triplet mixing necessary for CTP formation and triplet orbital coupling for separation. The next experimental step honed in on tuning both singlet and triplet coupling with the use of pressure to test for a coupling strength tipping point. By focusing on a single polymorph of TIPS-Pn, pressure enabled gradual changes in intermolecular distance and slip which can have large impacts on coupling without any change to monomer energetics. Following similar experimental procedure to the polymorph work, dynamics of CTPs and separated triplets showed the onset of geminate recombination at higher pressure. Effectively, higher pressure resulted in either stronger coupling between CTPs and the ground state or produced significant triplet pair binding energy to prevent their separation. While future work is targeted at various pentacene derivatives to tease out which of these scenarios is occurring, the use of pressure in continuously tuning molecular interactions is widely applicable to a variety of photophysical processes.