Fluorescence-upconversion experiments

The instrument response fiinction (IRF) for the fluorescence upconversion experiment, then, caimot be shorter than the intensity cross-correlation fiinction, which can be obtained usmg an mstniment like that shown in figure B2.1.4... 

First results from our fluorescence upconversion experiments are shown in Fig. 2, which displays the solvation-functions of C343 in bulk water and adsorbed on ZrC>2 nanoparticles. The response in bulk water confirms the previously reported results of bimodal dynamics [8] and a corresponding behaviour can be found for the dye bound to Z1O2, indicating that similar processes are involved. The results from biexponential fits to the solvation function S(t) of C343 in pure water and at the ZrCh-water interface are listed in Table 1. In both cases we find a fast decay time of about 100 fs and a slower decay of about 750 fs. We can see that the individual decay times stay similar and only the relative contributions change, resulting in an overall somewhat faster solvation for adsorbed dyes. 

In this section, I will discuss some of the more recent developments in continuum solvation dynamics in polar solvents. Some of these deal with incorporation of realistic models for chromophores [8,43 16] used in fluorescence-upconversion experiments, others with improvements in modeling of the solution dielectric properties [47,48], including incorporation solvent dielectric response over a wide frequency range [43,44, 46,48] into theories of SD. 

To reveal possible ultrafast processes occurring on a time scale less than 30 ps, femtosecond fluorescence upconversion experiments were performed [30] in toluene under magic angle polarization. To extract complete information of the decay times and their amplitudes in function of detection wavelength, the measurements were performed in three time windows of 5 ps, 50 ps, and 420 ps. 

The results of the measurements for m-C lPj are very similar to those of the para compound and the data sets can be interpreted identically. The obtained time constant of this vibrational/solvent relaxation process is 10 ps as found previously in fluorescence upconversion experiments [30], For the compound m-CIPi, the maximum of the positive transient absorption band attributed to the S,-S absorption is shifted about 5 nm to the blue and also the zero crossing point is shifted from 610 nm in the case of p-CIPi to 602 nm for m-ClPj. 

An important extension to the simplest upconversion experiment at a single detection frequency M2 is the practice of measuring time-resolvedfluorescence spectra, that is, the shape of the fluorescence spectrum... 

The anisotropy fiinction r t) = (/ (t) -1+ 21 t)) is detemiined by two polarized fluorescence transients / (t) and/j (t) observed parallel and perpendicular, respectively, to the plane of polarization of the excitation pulse. In tlie upconversion experiment, the two measurements are most conveniently made by rotating the plane of polarization of the excitation pulse with respect to the fixed orientation of the input plane... 

Sample preparation was given elsewhere [2]. Femtosecond fluorescence upconversion and picosecond time-correlated single-photon-counting set-ups were employed for the measurement of the fluorescence transients. The system response (FWHM) of the femtosecond fluorescence up-conversion and time-correlated single-photon-counting setups are 280 fs and 16 ps, respectively [3] The measured transients were fitted to multiexponential functions convoluted with the system response function. After deconvolution the time resolution was 100 fs. In the upconversion experiments, excitation was at 350 nm, the transients were measured from 420 nm upto 680 nm. Experiments were performed under magic angle conditions (to remove the fluorescence intensity effects of rotational motions of the probed molecules), as well as under polarization conditions in order to obtain the time evolution of the fluorescence anisotropy. 

The apparatus can be divided into three main sections, laser-pulse production, amplification, and fluorescence upconversion. The ultrafast lasers employed for upconversion are those that have been described above. The experiment is a crosscorrelation between the fluorescence pulse and the laser pulse. At time t = 0, the sample is excited by the ultrafast laser pulse. The resulting incoherent fluorescence (frequency = coa) is collected and mixed in a nonlinear crystal (such as KDP) with the gating pulse (frequency = cui) derived from the excitation source and arriving at time t = z. This frequency mixing of the fluorescence and probe beams generates light at the sum frequency, cosum... 

Transient absorption and fluorescence experiments on 1 and 4 and the model carotenoid pigments 3 and 6 yielded convincing results about the energy transfer pathway (Fig 1) (A.F. Moore et al, unpublished). In 1, the lifetime of Sj was measured by fluorescence upconversion to be 45 fs, whereas the Sj lifetime of model carotenoid 3 is 160fs.TheS, lifetime was 8 ps in both 1 and 3. Moreover, the S, rise time for tetrapyrrole 2 was found to be 62 fs by fluorescence upconversion. Therefore, only the carotenoid 83 state was quenched by the attached tetrapyrrole. The observation that the time constant associated with the decay of the energy donor(carotenoid 83) matches... 

Dyad 4 illustrates that both pathways can be active. In initial experiments on dyad 4 the attached tetrapyrrole was found to quench the carotenoid S3 state of6 from 95 to 28 fs and its S, level from 12 to 9 ps. The rise of the 8, level of tetrapyrrole 5 as measured by fluorescence upconversion required a major exponential component (74%) of 41 fs- and a minor component (26%) of4 ps-. While the match between the 9 ps decay of the carotenoid 8, and the 4 ps rise component of the tetrapyrrole 8, is only qualitative, these prelimenary experiments do provide evidence that both states can be energy donors. 

The methods discussed so far, fluorescence upconversion, the various pump-probe spectroscopies, and the polarized variations for the measurement of anisotropy, are essentially conventional spectroscopies adapted to the femtosecond regime. At the simplest level of interpretation, the information content of these conventional time-resolved methods pertains to populations in resonantly prepared or probed states. As applied to chemical kinetics, for most slow reactions (on the ten picosecond and longer time scales), populations adequately specify the position of the reaction coordinate intermediates and products show up as time-delayed spectral entities, and assignment of the transient spectra to chemical structures follows, in most cases, the same principles used in spectroscopic experiments performed with continuous wave or nanosecond pulsed lasers. 

Zinc tetraphenylporphyrin (ZnTPP) has been used recently as a model compound for Sj spectroscopic and dynamics studies, and has been the subject of intensive study. Some interesting questions regarding the actual relaxation mechanism stiU remain, however. Gurzadyan et al. recorded the Sj fluorescence decay and the Sj fluorescence rise profiles of ZnTPP in ethanol solution by fluorescence upconversion at an excitation wavelength of 394 run [119], In their experiment, the Sj state decayed exponentially with a 2.35 ps lifetime, identical to the risetime of its Sj fluorescence. A 60-90 fs rise component in the S 2 fluorescence profile was ascribed to IVR in the S2 state. Gurzadyan and coworkers also showed that Xjj for ZnTPP... 

Ultrashort fluorescence lifetimes are best determined by photon upconversion. An excellent description of the technique for non-specialists wishing to set up an experiment is in preparation as part of a current IUPAC project on ultrafast intense laser chemistry .181 The time resolution is basically limited by the temporal width of the laser pulses that are being used. An intense gate pulse at frequency vq is mixed with the fluorescence at a frequency vp in a nonlinear optical crystal (Special Topic 3.1), to create a short pulse at the sum frequency i j. The gate pulse thus represents a time window for the fluorescence... 

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