engleski

Ultrafast Transient Absorption of Phenolic and Adamantyl Compounds

Ultrafast dynamics of phenolic and adamantyl compounds was studied by measuring time- resolved absorption changes using transient absorption spectroscopy technique (TA). Phenolic and adamantyl compounds used were adamantyl-phenol, BODIPY- phenol, naphtol and adamantyl-naphtol. Probe pulse spectrum was supercontinuum generated in calcium- fluoride (CaF2) window pumped by 800 nm or 400 nm. In the case of pumping with 400 nm white light spectrum shifted more towards UV but is inherently more unstable because the second harmonic (400 nm) is less stable than fundamental (800nm) by itself. Pump beam was frequency doubled and prism compressed output of the NOPA. NOPA output gave wavelength variability in the range 500-700 nm which was then compressed by chirped mirrors and frequency doubled in BBO crystal which resulted in wavelength variability in the range 260-300 nm. After prism compressing the pump pulse, durations of sub 20 fs were obtained which roughly defined the resolution of the measurement. Measurements were performed at 1kHz repetition rate with pump pulse energies at around 100 nJ. All of the samples undergo photochemical reaction of quinone methide generation after excitation in UV at around 275 nm. Significance of the product molecules is that quinone methides alkylate and cross-link DNA strands, which underpins the antiproliferative activity of some antitumor antibiotics. BODIPY- phenol is interesting because it does not undergo any photochemical reaction when excited into its S1 state (500 nm) but it does undergo a quinone methide generation reaction when excited into its S3 state (276 nm). At last we wanted to investigate the difference in the dynamics of naphtol and adamantyl-naphtol samples. All of the reactions are reversible, i.e. quinone methides transfer back to its initial molecules after some time so the samples are preserved but required to be flown because the lifetime of quinone methides is comparable to laser’s repetition rate so by flowing the sample we ensure that every pump pulse gets absorbed by unexcited (‘fresh’) part of the sample and we are avoiding multiple excitations. Currently, partial quantum chemical TD-DFT simulations of the photochemical reaction of the adamantylphenol are available as calculations are still in progress. Simulations of the reactions of the other 3 samples are still yet to be done. Goal of the TA measurements of the first sample was to investigate which reaction pathway do molecules take as adamantyl- phenols can undergo a reaction in which products (quinone methides) are generated in its excited state then relaxed to its ground state and transferred back to its initial molecules or initial molecules can take a reaction pathway which includes conical intersection by which products are generated immediately in their ground state in which case TA measurements show one transient state less, etc. Spectral dynamics of the first sample was quantitatively investigated and the results show fast dynamics at around 800 fs which will be assigned to one of the reaction pathways after TD-DFT quantum chemical calculations are finished. Other 3 samples were qualitatively investigated, spectral dynamics and time dynamics is shown and compared. Due to ultrashort pump pulses (very large intensity at the focus) coherent artifacts of the TA method, such as cross-phase modulation and two-photon absorption were present and largely interfered with the global analysis which had to be done to have a complete description of the photochemistry of adamantyl-phenol. Extraction and smoothening of coherent artifacts is currently being done which will greatly help with the global analysis and time dynamics analysis such as fitting the exponential decay at a certain wavelength and obtaining decay rates.

Ultrafast transient absorption ; quinone methides ; conical intersection

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