Singlet Fission - The Effects of Solvent Polarity and Conformation

Singlet fission (SF) is a process in which one initial high energy singlet excited state is converted into two triplet excited states of roughly half the energy of the initial state. This process has the potential to increase the efficiency of conventional solar cells by modifying the energy of the incident solar radiation to better match the energy input required to generate electricity. However, the SF technology is still in an early stage of its development and a clear picture of the mechanism has not yet emerged. The aim of this thesis is to shed some light on both the mechanistic aspect and in addition investigate the integration of SF materials with semiconductors in a model system. In the work presented herein we have demonstrated that the relative orientation of the molecules involved in the SF process governs both the rate of formation and decay of the formed triplet states in an intramolecular SF system. Transient absorption studies have revealed that it is possible to selectively excite different conformations and observe orders of magnitude different SF rates for the same molecule by changing the excitation wavelength. Furthermore, conformational changes in the excited state have been utilized to increase the lifetime of the triplet pair which could be of importance in future device implementation. Additionally, we have investigated an intermolecular SF system attached to the surface of mesoporous semiconductors. Here, we found that the surrounding solvent polarity plays a crucial part in deciding what photophysical process is favored on the surface. The study suggests that highly polar solvents are detrimental to SF and triplet injection efficiency for this system due to stabilization of charge separated states

Optically Switchable NIR Photoluminescence of PbS Semiconducting Nanocrystals using Diarylethene Photoswitches

Precisely modulated photoluminescence (PL) with external control is highly demanded in material and biological sciences. However, it is challenging to switch the PL on and off in the NIR region with a high modulation contrast. Here, we demonstrate that reversible on and off switching of the PL in the NIR region can be achieved in a bicomponent system comprised of PbS semiconducting nanocrystals (NCs) and diarylethene (DAE) photoswitches. Photoisomerization of DAE to the ring-closed form upon UV light irradiation causes substantial quenching of the NIR PL of PbS NCs due to efficient triplet energy transfer. The NIR PL fully recovers to an on state upon reversing the photoisomerization of DAE to the ring-open form with green light irradiation. Importantly, fully reversible switching occurs without obvious fatigue, and the high PL on/off ratio (>100) outperforms all previously reported assemblies of NCs and photoswitches

Triplet Formation in a 9,10-Bis(phenylethynyl)anthracene Dimer and Trimer Occurs by Charge Recombination Rather than Singlet Fission

We present an experimental study investigating the solvent-dependent dynamics of a 9,10-bis(phenylethynyl)anthracene monomer, dimer, and trimer. Using transient absorption spectroscopy, we have discovered that triplet excited state formation in the dimer and trimer molecules in polar solvents is a consequence of charge recombination subsequent to symmetry-breaking charge separation rather than singlet fission. Total internal reflection emission measurements of the monomer demonstrate that excimer formation serves as the primary decay pathway at a high concentration. In the case of highly concentrated solutions of the trimer, we observe evidence of triplet formation without the prior formation of a charge-separated state. We postulate that this is attributed to the formation of small aggregates, suggesting that oligomers mimicking the larger chromophore counts in crystals could potentially facilitate singlet fission. Our experimental study sheds light on the intricate dynamics of the 9,10-bis(phenylethynyl)anthracene system, elucidating the role of solvent- and concentration-dependent factors for triplet formation and charge separation

Molecular rotational conformation controls the rate of singlet fission and triplet decay in pentacene dimers

Three pentacene dimers have been synthesized to investigate the effect of molecular rotation and rotational conformations on singlet fission (SF). In all three dimers, the pentacene units are linked by a 1,4-diethynylphenylene spacer that provides almost unimpeded rotational freedom between the pentacene- and phenylene-subunits in the parent dimer. Substituents on the phenylene spacer add varying degrees of steric hindrance that restricts both the rotation and the equilibrium distribution of different conformers; the less restricted conformers exhibit faster SF and more rapid subsequent triplet-pair recombination. Furthermore, the rotational conformers have small shifts in their absorption spectra and this feature has been used to selectively excite different conformers and study the resulting SF. Femtosecond transient absorption studies at 100 K reveal that the same dimer can have orders of magnitude faster SF in a strongly coupled conformer compared to a more weakly coupled one. Measurements in polystyrene further show that the SF rate is nearly independent of viscosity whereas the triplet pair lifetime is considerably longer in a high viscosity medium. The results provide insight into design criteria for maintaining high initial SF rate while suppressing triplet recombination in intramolecular singlet fission