Femtosecond Laser Transient Spectroscopy of Carotenoids and Carbon Nanotubes

Carotenoids and semiconducting single-wall carbon nanotubes, which are both nanoscale carbon-based quantum mechanical systems, were studied using different optical spectroscopy techniques in this dissertation. In both cases, adelocalized molecularorbital system governs the interaction with the visible light. The ultrafast dynamics of thesedelocalized molecularorbital systems right after the excitation with visible femtosecond laser pulses were investigated. The lineshapes and respective lifetimes of the excited states involved in the decay process of two photoexcited carotenoids, spheroidene and spheroidenone, were examined using transient absorption and transient grating spectroscopy. By employing a model-based lifetime analysis routine, written as part of this dissertation, the lifetimes of the S2 excited states of the molecules were estimated. Considering a 5‑level model, these lifetimes were found to be 40±20 femtoseconds for spheroidenone and 90±20 femtoseconds for spheroidene, which are shorter than the values previously reported in the literature. This indicates the necessity of further measurements using shorter pulses (10 femtoseconds) to understand the dynamics of the S2 excited state of carotenoids more comprehensively. It was also found that the addition of a conjugated carbonyl group to the chain in spheroidenone has several crucial effects on the optical response due to the alteration of the delocalized conjugated molecular orbital system of the molecule. These effects include the shortening of the lifetimes of all the excited states, the change in the absorption of the S1 excited state, and the occurrence of unresolved vibrational peaks in the steady-state absorption spectrum. The decay of the E22excitonic state of (6,5) single-wall semiconducting carbon nanotube dispersed in water with flavin mononucleotide (FMN) was observed using transient grating spectroscopy. The lifetime of this excitonic state was measured to be 450 ± 50 fs. A very strong single-frequency oscillatory component was observed in the decay profile. The period of this oscillatory component varies from 40 femtoseconds to 150 femtoseconds in the laboratory time frame. It was hypothesized that electronic/vibrational coherences of carbon nanotubes or flavin mononucleotide molecules cause this oscillation. The true origin of this oscillatory component is yet to be determined.