Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
Justin C. Johnson
Singlet fission (SF) is a multichromophore charge multiplication process in organic systems in which a singlet exciton shares its energy with a neighboring chromophore, thus generating two triplet excitons from one photon. SF chromophores can boost photocurrent in solar cells, raising the maximum theoretical power conversion efficiency of a single-junction solar cell from ~33% to ~45. Thin film (TF) preparation techniques, steady-state and time-resolved spectroscopic methods, and numerous advanced calculations were used to study the three systems presented here, all of which exhibit polymorphism.
TFs of 1,3-diphenylisobenzofuran (1), were prepared and two polymorphs, α-1 and β-1, were discovered and characterized. α-1films exhibit ΦT near 200% and low ΦF, whereas the dominant photophysical processes in the β-1 polymorph are prompt and excimer emissions, with ΦT around 10%. Absorption fitting revealed that the S1 state of β-1 is lower than α-1, and therefore SF and the correlated triplet 1(TT) is energetically inaccessible to β-1. The SF mechanism in TFs of each polymorph is outlined in great detail.
Polymorphism in tetracene (Tc), a near 200% ΦT SF material, has been previously documented, although morphology considerations have been neglected. While crystallite size has been shown to affect dynamics, the two Tc polymorphs, I and II, have not been analyzed in a thorough comparison of dynamics and photophysics. Tc II films show SF rates that are independent of crystallite size and SF occurs more rapidly than in Tc I. The slower Tc I SF rates are highly dependent on grain size. Coupling calculations suggested that Tc I should be faster, but these calculations are limited,and more sophisticated, multimolecule calculations are needed to support experimental results.
Two extremely stable indigo derivatives, Cibalackrot (2) and a tert-butylated derivative(3) were structurally and photophysically characterized in solution and in TFs. Two crystalline polymorphs (2α, 2β) and an amorphous phase (2a), as well as a crystalline (3α) and amorphous (3a) phase of 3 were deposited by thermal evaporation. ΦT values of less than 25% were observed for all morphologies, except in 2β(ΦT= 50%). Excimer formation dominates relaxation pathways in TFs of 2 and 3.
Ryerson, Joseph L., "Structural and Photophysical Considerations of Singlet Fission Organic Thin Films for Solar Photochemsitry" (2016). Chemistry & Biochemistry Graduate Theses & Dissertations. 194.