基于时间分辨发射光谱技术对光解反应的研究

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HORIBAScientificTime-Resolved Fluorescence Application Note TRFA-7 Investigating photocleavage using time-resolved emission spectra The choice of protecting group is of crucial importance in the success of many steps in organic synthesis and themanipulation of polyfunctional molecules， since they can prevent the formation of undesired side products andreactions. Photoremovable protecting groups exhibit numerous advantages， such as the relatively soft conditionsrequired for their deprotection and orthogonality with respect to acid- or base-sensitive groups. They have foundapplication for the controlled release of a variety of molecules in the areas of materials science， the caging andrelease of biologically significant compounds and in synthetic organic chemistry. Photocleavage reaction The photocleavage reaction typically involves the formationof an ion pair from the excited ester， which can eitherrecombine or split to form the corresponding alcohol， seesimplified scheme for an amino acid- heterocycle systembelow Scheme l. Simplified photocleavage scheme It can then be the case that there will be a mixture offluorescing species present. Potentially each will have acharacteristic decay time and spectrum. An objective is to attain control over the cleavage conditions；good photochemical yield， good cleavage rate and selectivityvia choice of wavelength. Considering the wavelength it isadvantageous for this to be longer than ~350nm as it canavoid， in the case of biological applications， cell damagecaused by UV light. It offers the possibility of the use of two-photon excitation， which is advantageous for biologicalapplications， and enables an improvement in selectivity， asonly a small (femto litre) volume will be excited. Time-resolved fluorescence measurements Sinceit is likely that the chosen system， uponphotoexcitation， will contain several spectrally overlappingspecies， a simple steady state measurement will not besufficient to characterise the system and elucidate thedynamics involved. The use of a time-resolved technique，such as measuring the time-resolved emission spectrum(TRES)， is useful in obtaining this information. The TRES acquisition is dependent on the presence of an emissionmonochromator. A system that can be used is the HORIBAScientific FluoroCube shown below. Fig. 1. FluoroCube-01 with DeltaDiode excitation A typical measurement involves incrementing themonochromatorrinnfixed wavelengthissteps， wvith time-resolved decays acquired for either fixed time intervals or toa predetermined peak count at each wavelength. To obtainintensity information， the option of fixed time interval shouldbe chosen. Acquisition of the instrumental response (IRF orprompt) allows further analysis using reconvolution. It is alsopossible to obtain the decay associated spectra， shownschematically in Fig.2-also see relevant HS Technical Note. Fig. 2. Scheme for obtaining decay associated spectra This enables spectra to be related to the different decaytimes and is helpful in elucidating the species present. Photocleavage of novel amino acid ester derivativesThis example is based on the work of Costa andGoncalves (see reference) using compounds shown inScheme Il. R=CH(CH3)2， CH2Ph， R1=CO2CH2Ph， H Scheme Il. Compounds used and expected reaction Time-resolved decays， measured using a FluoroCube，showed that the subsituent moieties on the amino acidinfluenced the decay kinetics. This is demonstrated inthe data shown in Fig. 2. This shows the effect of asubstituent change on the time-resolved fluorescencebehaviour. Amodel compound (to represent thephotocleaved HET-OH moiety) was also measured. Oneoftheeaadvantages(of)fttime-resolved fluorescencemeasurements is that visually a qualitativeinterpretation of the data can be quickly made. Fromthe top panel it is easily seen that the decay kinetics aredifferent. DAS calculated from a TRES measurement aredisplayed in the other panels. Summing the individualDAS enables the overall (=steady state) spectrum to beobtained. These data allow comparisons， with modelcompounds for example， and assignment of species tobe made. In the bottom panel it is apparent that theDAS for the longer lived decay is similar to the spectrumof the model compound. Also from the lifetime values it is possible to estimatethe rate constants shown in Scheme I. This gives a briefindication of the usefulnessoffttime-resolvedfluorescence techniques in this field of application. Fig. 2. Example data from time-resolved lifetime andDAS analysis of compounds shown in Scheme ll The results shown in this note are based on thefollowing paper， A.M. Piloto， A.M.S. Soares， G. Hungerford， S.P.G. Costaand M.S.T. Goncalves，6，2011. Photolysisatlongwavelengths of amino acid ester derivatives based on 4-methyl-6-methoxy-2-0x0-2H-naphtho[1，2-b]pyrans. Eur.J. Org.Chem.5447-5451. www.horiba.com/scientific France：+33 (0)1 64 54 13 00 Germany： +49 (0)89 462317-0Italy：+39 2 5760 3050 Japan： +81 (0)3 62064717Brazil： +55 (0)115545 1514 Other： +33(0)1 64 54 13 00 HORIBAExplore the futureAutomotive Test Systems Process & Environmental Medical Semiconductor l Scientific