Optical Limiting in Modified meso-alkynyl Porphyrins
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respect to the strong pump. The transmission data collected represent the average of 20 shots at each delay of the probe. Time-resolved fluorescence spectroscopy is performed by using a fluorescence lifetime spectrometer (TimeMaster, Photon Technology International), which has a system response time of - I ns. Fluorescence quantum yields are obtained from a fluorimeter (Luminescence Spectrometer LS50B, Perkin Elmer), following standard procedure. 5 Input fluence dependent single beam transmission measurements are performed by employing -24 ps or - 7ns pulses at 532 nm with long focus geometry. Samples are prepared by dissolving weighed powder into HPLC grade chloroform and subsequently transferred into lmm photometer cells except for time-resolved fluorescence measurement, in which 1cm cuvette with four polished surfaces are used. NMR is performed on each sample to confirm the structure. Absorption spectra are obtained in a number of different concentrations to ensure that the samples tested either fall within or at least do not significantly deviate from the linear regime. RESULTS We summarize our findings on the ground state behavior in Table I. The weight of the center atom(s), Wcenter is also listed for reference. The oscillator strength for both the Q-band and the B-band Q,,,, and •S(-s2) are calculated following established procedure. 6 For comparison purpose, we also include the tetraphenylporphyrin (TPP) and its metallo version Zntetraphenylporphyrin (ZnTPP) whose conjugation lengths are believed to be confined to the ring. It can be seen that the increase in the conjugation from the TPP family to the TMSP family shifts both the Q and the B band to longer wavelength. This trend is represented by Fig. 1, where the Q bands of ZnTMSP, ZnDTMSP, and ZnTPP are compared. Evidently these three chromophores combined cover almost the entire visible wavelengths. It is noticed that the fluorescence quantum yield 77reduces as the conjugation length increases. Apart from a few exceptions, the increase in the weight of the center metal also serves to red shift the absorption maxima. It can be envisioned that the electron rich metal helps to push the electron cloud distribution away from the center and effectively lengthens the conjugation. In a generic three-level model, the difference between the effective absorption crosssection of the excited-state and the ground state, Or - -g, together with the excited-state lifetime T, can be extracted from the transient transmission measurement. 4 We tabulate our findings in Table II. The fluorescence life time T, , obtained by measuring the decay at the fluorescence maxima, is also included. A typical time-resolved fluorescence spectrum is depicted in Fig.2. In all samples tested, we find T, > T', except for TPP (in which the two are identical, indicating the excited-state is mostly of singlet nature). RSA actions in all other samples are more or less influenced by the triplets, although the first few nanoseconds may see more contribution from the excited singlet. The long T, in me
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