The spectroscopy and the photophysics of 1,2-benzazulene, a derivative of azulene.
Date
2022-02-14
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ORCID
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Thesis
Degree Level
Masters
Abstract
The photophysics and the spectroscopy of 1,2-benzazulene were studied both in the liquid and solid phase using azulene for comparison, with the interest of investigating its quenching mechanism from the S2 state and the possibility of it undergoing singlet fission via S2 + S0 → 2T1. To understand the quenching mechanism, it is important to, first of all, investigate the similarities and differences in the spectroscopic properties of 1,2-benzazulene and azulene. Steady-state absorption measurement reveals that the S1-S0 absorption bands for azulene and 1,2-benzazulene look similar in shape and intensity but very weak compared to their respective S2-S0 absorption spectrum. However, the first absorption bands for 1,2-benzazulene are red-shifted compared to azulene. Steady-state fluorescence measurement of these two compounds shows that azulene and 1,2-benzazulene emit anomalous fluorescence emission from the S2 state. However, unlike azulene, the S0-S2 absorption and fluorescence spectra of 1,2-benzazulene reveal an obvious mirror image relationship implying the same transitions are most favourable for absorption and emission at the S2 state. In addition, there is neither evidence of S2–S1 nor S1–S0 fluorescence emissions observed for both azulene and 1,2-benzazulene, suggesting that similar to azulene, 1,2-benzazulene also undergo rapid internal conversion from S1 to S0 bypassing T1. Also, 1,2-benzazulene and azulene have been subjected to aggregation studies for comparison. This study is to explore the differences and similarities in their spectroscopy and photophysics at higher concentrations. The results obtained from these experiments reveal that at higher concentrations, there is no clear evidence of fluorescence self-quenching from the S2 state of 1,2-benzazulene compared to azulene, where fluorescence self-quenching from the S2 is evident at a similar concentration range. The S2 fluorescence self-quenching in azulene occurs via a diffusion-controlled process with a static quenching component observed from the Stern-Volmer plots.
The time-dependent diffusion coefficient (Kq) was obtained to be 1.5 x 1010 M-1s-1, similar to that obtained by Reis e Sousa, et al.1. Picosecond transient absorption measurement conducted on the solution phase samples of 1,2-benzazulene gives S2 lifetime similar to that obtained by non-linear fluorescence upconversion technique. The calculated non-radiative decay rate constant from the S2 (knrS2) of 1,2-benzazulene at its measured E(S2 – S1) reveals an energy gap correlation following the same trend as azulene and its fluorinated derivatives. This occurrence suggests that, like azulene and its fluoroderivatives, S2-S1 internal conversion is the main decay path for 1,2-benzazulene. Also, the electronic spectra of 1,2-benzazulene show that singlet fission process via S2 + S0 → 2T1 would be near thermoneutral. However, the electronic spectra obtained for the 1,2-benzazulene solid thin film reveal the formation of aggregates with no measurable S2 fluorescence, suggesting ultra-fast processes happening likely, singlet fission. The mechanism for this ultrafast process in 1,2-benzazulene thin film is yet to be explored.
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Keywords
Spectroscopy, photophysic, Franck-Condon factor, quenching, singlet fission, triplet-triplet annihilation, emission, absorption, internal conversion, intersystem crossing, aggregation, pi-stacking, thermoneutral
Citation
Degree
Master of Science (M.Sc.)
Department
Chemistry
Program
Chemistry