We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.
Fluorescent organic molecules have received great attention due to their largest applications, for example, in DNA and RNA spectroscopies studies, development of new photoluminescence bioprobes, and applications in fluorescence spectroscopy. In specific, purine base analog molecules present high fluorescence quantum yields and significant Stokes shift. Furthermore, the addition of push−pull structures at the purine core could increase the photoluminescence properties, making candidates for photoluminescence bioprobes. To consider this, a complete spectroscopic study was performed on nine push−pull purines, distinguished by different push−pull structures. In specific, for this research, the two-photon absorption (2PA) study showed that the compounds present induced two-photon fluorescence at the therapeutic window, desired for fluorescence microscopy. The brightness property was evaluated, indicating that all chromospheres are fluorescent by a 2PA process. Additionally, ultrafast transient absorption was performed to elucidate contribution of the excited states on the 2PA spectra, and quantum chemistry calculations were performed to corroborate the experimental results.
pela orientação durante o mestrado, pelo apoio, compreensão e principalmente pela amizade. Ao Prof. Dr. Cleber Renato Mendonça pela orientação durante os três anos de iniciação científica e igualmente pelo apoio, compreensão e principalmente pela amizade. Ao Prof. Dr. Sergio Carlos Zílio pela amizade e pelo exemplo de seriedade e profissionalismo. Ao Prof. Dr. Antonio Carlos (Jabah) pelo incentivo no começo conturbado de minha graduação. Aos meus pais pelo amor, sacrifícios e por sempre terem acreditado. Às minhas avós Ana e Rosária, tios(as) e primos(as) pelo carinho com que sempre me recebiam no regresso à minha casa. Ao amigo Ismael pelo companheirismo e amizade sincera, e à sua família. Ao amigo Daniel Silva, companheiro de grupo e república, pela amizade e momentos de descontração. Ao amigo Daniel (Krusty), companheiro da habilitação "Óptica e Fotônica". Creio que aprendi, em boa parte devido ao seu jeito "desencanado", a encarar os problemas com mais tranquilidade. Ao amigo Guillherme (Blane), companheiro de graduação e alojamento.
Abstract:In this work, the synthesis and the nonlinear absorption and population dynamics investigation of a series of zinc phthalocyanines (ZnPcs) dissolved in chloroform are reported. In order to determine the relevant spectroscopic parameters, such as absorption cross-sections of singlet and triplet excited states, fluorescence relaxation times, intersystem crossing, radiative decay and internal conversion, different optical and spectroscopic techniques were used. By single pulse and pulse train Z-scan techniques, respectively, singlet and triplet excited states' absorption cross-section were determined at 532 nm. Furthermore, the intersystem crossing time was obtained by using both techniques combined with the fluorescence lifetime determined by time-resolved fluorescence. The radiative and internal conversion rates were determined from the fluorescence quantum yield of the samples. Such spectroscopy parameters are fundamental for selecting photosensitizers used in photodynamic therapy, as well as for many other applications.
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