Photophysical Processes in Dynamic Dimers and Carbon Nitride Quantum Dots: From Singlet Fission to Two Photon Absorption

Licentiate thesis, 2026

Photochemistry is the study of how light can be used to drive chemical reactions and photophysical processes, and can play a central role in the development of next-generation light-harvesting and light-emitting technologies. This thesis investigates two complementary processes — singlet fission (SF) and two-photon absorption (2PA) — to elucidate how supramolecular organization, chemical environment, and intermolecular interactions shape the excited-state dynamics relevant to solar-energy utilization. 

Dynamic, non-covalent dimers of pentacene derivatives were designed to explore how weak intermolecular interactions can promote efficient SF in solution at micromolar concentrations. Two thiophene-functionalized 6,13-bis((triisopropylsilyl) ethynyl) pentacene (TIPS-Pc) derivatives, bearing either an aldehyde (PTA) or a carboxylic acid (PTCA) group, were demonstrated to form dimers with reversible bonds capable of undergoing SF. Photophysical characterization reveals rapid triplet-pair formation in both PTA and PTCA, with rates of 1.3 x 1011 and 5.6 x 1010 s-1, respectively. Unlike covalently linked dimers or crystalline systems, these dynamic dimers enable novel tunability of the SF rates through control of concentration, pH and cation coordination. This demonstrates that non-covalent dimerization provides a versatile platform for modulating triplet formation and separation, a crucial step for effective SF.

The other studied process, 2PA, was confirmed in water soluble EDTA-capped graphitic carbon-nitride quantum dots prepared via a microwave-assisted protocol when excited in the near-infrared region (680-880 nm). They produce bright blue emission centered around 400-500 nm. Maintaining the excitation wavelength dependence in emission as observed for one-photon absorption. Solvatochromic studies show modest solvent sensitivity and reveal that 2PA accesses excited states partly different from those reached by one-photon absorption. The 2PA cross-section was estimated to 160 GM at 690 nm excitation.

Together, these studies demonstrate how molecular assemblies and nanoscale structures dictate excited-state formation, separation, and relaxation across chromophore type, excitation regime, and photophysical process. By uncovering the mechanistic factors that control SF and 2PA, this work contributes to the design of adaptive photophysical materials for improved solar-energy conversion and advanced optical applications.