Dissertation. zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) vorgelegt der

Transcription

1 Cyclic nd liner photoioniztions of cridine derivtives nd xnthone investigted y nnosecond lser flsh photolysis Disserttion zur Erlngung des kdemischen Grdes Doctor rerum nturlium (Dr. rer. nt. vorgelegt der Mthemtisch-Nturwissenschftlich-Technischen Fkultät (mthemtisch-nturwissenschftlicher Bereich der Mrtin-Luther-Universität Hlle-Wittenerg von Herrn M.Sc. Bell Hussein Mohmed Hussein ge. m..97 in Egypt Gutchter:. Prof. Dr. Mrtin Goez, Hlle. Prof. Dr. Hns-Gerd Löhmnnsröen, Universität Potsdm Hlle (Sle, 5 Dtum der Verteidigung: 6..5 urn:nn:de:gv:3-43 [

2 Contents Contents. ntroduction... Prt Experimentl methodology nd theoreticl nlysis...5. Experimentl section Stedy-stte mesurements Lser flsh photolysis Mterils Determintion of triplet-triplet extinction coefficients Reltive ctinometry Energy trnsfer Determintion of extinction coefficients nd quntum yields of rdicl ions Anlysis of triplet stte decy kinetics First-order decy Self-quenching Triplet-triplet nnihiltion Determintion of electron sorption Liner nd cyclic photoioniztions Anlysis of light intensity dependences Kinetic formultion of light sorption Liner photoioniztion processes Consecutive two-photon ioniztion Prllel photoioniztion of ited singlet nd triplet sttes Stepwise photoioniztion vi three-photon process Cyclic mechnisms of electron donor photoioniztions Singlet or triplet stte undergoes photoioniztion Both singlet nd triplet sttes undergo photoioniztion Cyclic mechnisms of electron cceptor photoioniztions... 6

3 Contents Prt Donor nd cceptor photoioniztions Results nd discussions...9 Prt.A Electron donor photoioniztions Photoioniztion of cridone derivtives vi their singlet stte3 4.. Spectroscopic chrcteriztion Triplet nd rdicl ction sorption spectr Triplet energy trnsfer Electron trnsfer Cyclic photoioniztion of N-methylcridone in SDS Possile rection mechnisms Anlysis nd simultion of cyclic photoioniztion mechnism Cyclic photoioniztion of cridone in SDS Two-lser pulse experiments Liner photoioniztion of cridone derivtives in lcohol-wter solution SDS s scrificil electron donor Photoioniztion of xnthone vi its triplet stte Stedy-stte sorption spectr Trnsient sorption spectr Triplet decy nlysis Determintion of extinction coefficient of xnthone triplet stte Liner photoioniztion in methnol-wter (: v/v y ner UV Cyclic photoioniztion in queous SDS solution t 38 nm Photoioniztion in queous SDS t 355 nm Two-lser two-color lser flsh photolysis Effect of xnthone concentrtion Effect of SDS concentrtion on the photoioniztion Photoioniztion of cridine through singlet nd triplet chnnels Asorption spectr Fluorescence spectr... 6

4 Contents 6.3. Trnsient sorption spectr Triplet energy trnsfer Photoioniztion of cridine in lkline wter Simultion study ccording to results of Kellmnn nd Tfiel nterprettion of devitions for the triplet stte Photoioniztion of cridine in lkline methnol-wter mixture Cyclic photoioniztion in SDS Photoioniztion quntum yields Photoioniztion of monoprotonted Proflvine Asorption nd fluorescence spectr Trnsient sorption spectr Photoioniztion mechnism t 355 nm Prt.B Electron cceptor photoioniztion Photoioniztion of xnthone/mine systems Direct genertion of xnthone rdicl nion Rte constnts for xnthone triplet quenching y mine Bck electron trnsfer Secondry rection of the xnthone ground stte Cyclic photoioniztion of xnthone rdicl nion in methnol-wter solution Cyclic photoioniztion of xnthone nion rdicl in queous SDS solution Comined triplet stte nd rdicl nion of xnthone photoioniztion pthwys Two-color two-lser flsh photolysis of xnthone /mine system Liner photonioniztion of X - t green light (53 nm Summry / Zusmmenfssung Summry Zusmmenfssung References...9

5 Chpter : ntroduction. ntroduction Photoioniztion plys n importnt role in iologicl processes [-7] e.g., light interction in chloroplsts during photosynthesis [8] nd photoioniztion of dihydronicotinmide denine dinucleotide (NADH [9]. Lser ittion of deoxyrionucleic cid, DNA, my produce reks, either in single strnd or in complementry doule strnds [, ]. Therefore, photoioniztion of vriety of orgnic molecules in polr solvents upon visile nd UV-lser ittion hs een of considerle interest for kinetic nd mechnistic studies [-6]. Photoejection of n electron occurs vi different mechnisms, depending on the ville photon energy, the ited stte properties of the sustrte, nd the nture of the solvent [7-3]. f the photon energy of the ittion wvelength is greter thn the photoioniztion threshold, mono-photonic electron ejection from the relxed or unrelxed lowest ited singlet stte is energeticlly fesile [3-34]. n other words, incresing the ittion photon energy cuses chnge from two-photon into one-photon ioniztion process with n increse in the photoioniztion quntum yield [34]. The two-photon ioniztion is reltively common process with visile or ner-uv light ittion, where the sustrte sors the first photon to give the ited stte which in turn is ionized y the second photon [35-37]. Lser flsh photolysis (LFP with opticl detection is widely used to distinguish etween mono- nd iphotonic ioniztion. The nonliner ehvior of the electron yield upon vrition of the lser intensity indictes iphotonic ioniztion [38, 39]. More precisely, the electron yield increses linerly with the squre of the lser intensity [4, 4]. Rel-time oservtion of chemicl rection, which constitutes primry key for understnding rection mechnisms, still remins chllenge. Lser techniques re not only importnt for their study, ut hve lso developed into pivotl tools in science nd technology, with significnt pplictions in industry, communiction, nd medicine. One importnt exmple is multi-photon spectroscopy [4], where high-intensity lser with short pulses cuses the sorption of two or severl photons. Multilser flsh photolysis cn e used to: generte new rective intermedites, give more informtion out photorection mechnisms,

6 Chpter : ntroduction identify the chemicl species during the rection pthwy, nd open up new rection pthwys. n ddition, the selectivity of lsers llows the multilser method to e pplied to iologicl ctivities such s photodynmic therpy [43]. Two-lser chemistry hs een extensively investigted y vrious groups [44-5]. Goez nd his group hve reported tht photoinduced electron trnsfer etween ketones (such s 4-croxyenzophenone [5] or,5-nthrquinonedisulfonte [53] nd donor molecules produces rdicl nions nd rdicl ctions y the first lser (38 nm. The rdicl nion sors second photon from the second lser ittion (387.5 nm giving the originl ketone s well s the hydrted electron. The use of two-color lser flsh photolysis llows the investigtion of the rection pthwys of trnsient intermedites. The first pulse is djusted to generte the intermedite, which cn then e selectively ited y the second pulse t different wvelengths. The oserved leching of emission or sorption signls resulting from the second lser stems from the ited stte tht tkes prt in the photoioniztion. The lifetime of either oth ited sttes or one of them must e long enough to sor the second photon. The resulting rections cn e studied in qulittive nd/or quntittive mnner. Lser ittion cn e comined with different detection methods [54, 55] to otin more informtion out the photorection system. The rdicl ions resulting from electron trnsfer nd photoioniztion processes cn e studied y trnsient photoconductivity [56], electron prmgnetic resonnce (EPR [57,58], nd chemiclly induced nucler polriztion (CDNP [59,6]. n this work, we studied the photoioniztion mechnisms of some heterocyclic compounds in the presence of scrificil electron donor. n ddition to the verifiction of known liner photoioniztion mechnisms, the min gol of this thesis is to investigte the cyclic photoioniztion mechnisms which were first oserved through our experiments. Our mesurements were sed on pplying the nnosecond lser flsh photolysis technique (LFP with opticl detection. We lso compred the ehviour of the detectle species resulting from cyclic photoioniztions with tht in liner photoioniztions. This thesis is orgnized into two prts. The first prt contins description of the experimentl methodology nd the theoreticl nlysis: Chpter descries the nnosecond lser flsh photolysis pprtus with opticl detection used in our experiments nd discusses

7 Chpter : ntroduction 3 the physicl properties of the exmined sustrtes (e.g., quntum yield nd lifetime of the triplet stte. n Chpter 3, we nlyse the light intensity dependence of the concentrtions of the ited sttes, rdicl ions, nd electrons. n this nlysis, we distinguish etween liner nd cyclic photoioniztion mechnisms. The solution of differentil equtions for the kinetic models usully gives closed form expression tht cn e fitted to the experimentl dt. n some of our mesurements, the solution of these kinetic equtions ws cumersome nd we used mthemticl softwre pckge (Mthemtic 4. to fit the numericl solutions to the experimentl dt. The second prt comprising Chpters 4 to 8 includes the min results of this thesis on complex cyclic nd liner photoioniztion mechnisms of cridine derivtives nd xnthone. Wheres cridine derivtives serve solely s electron donors in our investigtions, we used xnthone oth s donor nd cceptor depending on the experimentl conditions. We will show tht the high lser intensities open new rection pthwys of photoioniztion. For instnce, the electron yield in the photoioniztion for n electron donor (e.g., cridone derivtives in queous sodium dodecylsulfte (SDS micellr solutions eeds the initil concentrtion of the sustrte due to cyclic mechnism (Chpter 4. Furthermore, the rdicl ction produced during the photoioniztion process exhiits sturtion ehviour t high lser intensities while the electron yield increses linerly. These oservtions re inconsistent with the liner two-photon ioniztion process reported previously [36-4, 5], where the concentrtions of oth electron nd rdicl ction must e identicl under the sme experimentl conditions. Chpter 5 descries the ctlytic cyclic photoioniztion mechnism of xnthone in queous SDS micellr solution, while its photoioniztion in lcohol-wter solution proceeds vi liner two-photon ioniztion process. n order to give complete description of the effect of SDS concentrtion on the photoioniztion mechnism, we compred the electron yield of the photorection system in vrious SDS concentrtions s well s in lcohol-wter solution i.e., studied the electron yield s function of the lser intensity in micelle concentrtion ove nd elow the criticl micelle concentrtion (cmc. n ddition to the cyclic photoioniztion of the triplet stte (in the cse of xnthone or the singlet stte only (in cse of cridone derivtives, we will lso show tht the cyclic photoioniztion of cridine nd proflvine proceeds vi oth singlet nd triplet sttes (Chpters 6 nd 7.

8 Chpter : ntroduction 4 n Chpters 5 nd 8, we compre the results of two-lser ittion of the exmined systems with those otined from single lser ittion under similr experimentl conditions. Chpter 8 is devoted to the cyclic photoioniztion of n electron cceptor in the presence of n electron donor. We studied the effect of the mines triethylmine (TEA nd,4-dizicyclo[..]octne (DABCO on the photoioniztion of xnthone. The presence of the mine in this system leds to n electron trnsfer. The photoioniztion of xnthone/mine systems t high lser intensity is interesting, s the electron yield is much higher thn in the sence of mine under the sme experimentl conditions. Finlly, in Chpter 9 summry of the results of this thesis is given.

9 5 Prt Experimentl methodology nd theoreticl nlysis

10 Chpter : Experimentl section 6. Experimentl section.. Stedy-stte mesurements UV-visile sorption spectr were mesured using Shimdzu UV- spectrophotometer. The sorption ws mesured directly efore irrdition nd there ws no sign of ny chemicl interction etween the components in their ground sttes. The sorption spectr of the exmined compounds cn e compred to known spectr s reported in the literture. n ll cses, the Beer-Lmert plots were found to e liner in the mesured rnge of the ground stte concentrtions. The concentrtion of the smples for trnsient sorption mesurements ws djusted to give n sornce less thn. t the ittion wvelengths in the -cm opticl pth length of the cell. Stedy stte fluorescence mesurements were crried out using Perkin-Elmer LS5B spectrometer. The fluorescence quntum yields nd the fluorescence lifetimes of the exmined sustnces re known... Lser flsh photolysis The trnsient sorption nd fluorescence spectr were mesured using lser flsh photolysis pprtus with n imer lser (Lmd Physik LPX-i, 38 nm, lser pulse durtion 6-8 ns, mximum energy pproximtely 6 mj per pulse nd/or Nd:YAG lser ( Continuum Surelite -; 355 or 53 nm with pulse width of 6 ns ; the mximum energy ws pproximtely 5 mj/pulse for 355 nm nd 8 mj/pulse for 53 nm s the ittion sources. The intensity of imer lser ws vried y using metl-grid filters while the intensity of the Nd:YAG lser ws chnged y vrying the voltge on the flsh lmp pumping the lser hed. A prt from intensity dependent mesurements, the filters were used to suppress second-order rections, or to minimize the photoioniztion of the rection system, when desired. The dimensions of the ited volume were x 4 x mm, nd the opticl pth length of the detection system ws 4 mm. For the two lser pulse experiments, home-mde dely genertor ws used. The dely etween the pulses could e chosen in the rnge of few nnoseconds to severl microseconds. The lser pulses pssed through the opticl cell in colliner geometry. The lser flsh photolysis setup is shown schemticlly in Figure.. The dimensions of the lser pulse t the front of the cell were 3 x 4 mm, from which the centrl prt ws selected

11 Chpter : Experimentl section 7 y n perture. The lser pulse impinges on the cell t 9 o with respect to the monitoring em nd ws optimised for homogeneous illumintion of the detection volume. Xe-Arc Lmp Filter Shutter Cell Filter Monochromtor Photomultiplier Mirror Mirror Oscillscope Nd:YAG Lser Photodiode Mirror Photo diode Excimer Lser 38 nm Dely Genertor PC Figure.. Digrm of the experimentl setup for nnosecond lser flsh photolysis The detection system mesures the chnge in the photocurrent due to emission or differentil sorption of the trnsient versus time t selected wvelength. By plotting the intensity of signls versus the monitored wvelength trnsient spectrum cn e otined which chrcterizes this species. A negtive sorption y the trnsient corresponds to leching, where the sorption of the products nd/or trnsients produced in this rection is smller thn tht of the strting mterils. The trnsient spectr otined under our experimentl conditions cn e compred to known spectr. Detection of sornce is done with high pressure xenon lmp, suitle opticl system, filters to suppress the stry light, monochromtor, nd photomultiplier with n incresed sensitivity in the red (Hmmtsu R 98. The xenon rc lmp for monitoring is used with shutter to protect the smple from undue photolysis. The shutter ws controlled y the PC. The side-on photomultiplier tue (PMT is plced t the exit slit of the monochromtor, nd genertes n electricl signl corresponding to the light intensity striking the cthode of the PMT; the photocurrent is mplified in the PMT, controlled y the voltge pplied to the dynodes. A digitl oscilloscope (Tektronix is used to convert the PMT output to digitl signl, nd to trnsfer the dt to computer for nlysis nd storge. The time response of the detection system is 5 ns.

12 Chpter : Experimentl section 8 These experiments were generlly crried out using flow system in order to void depletion of rectnts or ccumultion of products. The flow rtes were chosen such tht ech lser shot ited fresh solution, nd were mintined during the experiments. Depending on the mgnitude of the sorption chnges, the trnsient signls were ccumulted t lest 64 times in order to otin sufficient signl to noise rtio. The solutions were prepred using ultrpure Millipor MilliQ wter (resistnce 8. MΩ cm -. Despite its unique properties nd ovious iologicl relevnce, one of the mjor fctors tht limit the photochemicl studies of orgnic compounds in queous solutions is their poor soluility in this medium. Anionic micelles of sodium dodecylsulfte, SDS, cn overcome this prolem. n ll experiments, the micelle concentrtions were t lest times higher thn the sustrte concentrtions. This ensures tht no micelle contins more thn one molecule [6, 6]. The micelle concentrtion [M s ] is given y Eqution (. [S t] cmc [Ms ] = Eq. (. N g where [S t ], cmc, nd N g re the concentrtion of SDS, the criticl micellr concentrtion (8 x - M [6], nd the ggregtion numer of SDS (6-6 [6], respectively. All experiments were crried out t room temperture in neutrl solution or sic solution, where the ph vlues were djusted y the ddition of NOH. Simultion fits of the kinetic models were performed y softwre pckges (Origin 6. nd Mthemtic Mterils Most of the chemicls were otined commercilly in the highest ville purity nd used without further purifiction. N-Methylcridone, xnthone, triethylmine,,4- dizicyclo[..]octne, nd enzophenone-4-croxylte were purchsed from Aldrich. Acridone ws purchsed from Lncster. Acridine ws otined from Acros. Methylviologen (,`-dimethyl-4,4`-ipyridinium chloride ws Sigm-Aldrich product. Sodium dodecylsulfte ws otined from Sigm. Sodium hydroxide, methnol, nd proflvine hemisulfte dihydrte were otined from Fluk. Xnthone nd cridine were crystllized severl times from queous methnol.

13 Chpter : Experimentl section 9.4. Determintion of triplet-triplet extinction coefficients.4.. Reltive ctinometry The extinction coefficient of triplets ws determined using the comprtive technique [63-66]. This method of determintion is vlid only if smll frction of the molecules is ited, so tht the sornce chnges remin liner with the lser energy. A sustnce whose quntum yield (ϕ T nd extinction coefficient (ε T of the triplet stte re known is used s the stndrd, i.e., s n ctinometer. n these experiments [67], the concentrtions of stndrd nd sustnce studied re chosen to yield the sme opticl density t the ittion wvelength. Then, for ny given lser energy the numer of photons sored y ech of the two solutions will e identicl. Hence, the quntum yield of triplet formtion of the sustnce studied is proportionl to the triplet concentrtion, nd Eq. (. cn e used to estimte the triplet extinction coefficient of the sustnce under study, M, or its quntum yield, depending upon which of them is known. ϕ ϕ M T st T 3 * [ M ] = 3 * [ st ] Eq. (. where M 3 * ϕ T,[ M ], ϕ T st,nd[ 3 st * ] re the quntum yield nd triplet concentrtion of the sustnce under study, quntum yield nd triplet concentrtion of stndrd sustnce, respectively. Replcing concentrtions with sornces ccording to Lmert-Beer s Lw in Eq. (. will give ϕ M T M T st T st T M T st E. ε = ϕ T Eq. (.3 E. ε st ε T M ε T st E T where,,, nd re the triplet extinction coefficients nd the mximum sornces of the triplet stte M E T.4.. Energy trnsfer The ited triplet stte of the exmined sustnce cn e generted y energy trnsfer from the ited triplet stte of sensitizer [63, 68, 69]. To e n pproprite donor for n energy trnsfer experiment, its triplet stte should e t lest 8-5 Kcl/mol higher thn tht of the cceptor [7]. n this cse, the reverse trnsfer cn e neglected. The triplet life of the donor should e long, nd the intersystem quntum yield should e reltively high. ts

14 Chpter : Experimentl section extinction coefficient, ε D, should e lrge t the oservtion wvelength in order to otin ccurte mesurement vlues. k 3 D * isc D Re. (. k 3 * D + A q D + 3 A * Re. (. n this experiment, the solution contins oth D nd A. Only D should e ited to its singlet stte, which converts rpidly to the triplet stte which then undergoes triplet-triplet energy trnsfer. The cceptor molecules should sor s little s possile of the ittion light, in order to improve the ccurcy of the clcultions. The decy of the sensitizer triplet will e ccompnied y the growth of the trnsient sorption due to the cceptor triplet. Assuming the two decy pthwys of (., nd (., for the sensitizer triplet 3 D * efficiency of energy trnsfer cceptor through energy trnsfer is η En (i.e., the proility tht the triplet donor rects with T the η EnT = q k q [A] k [A] + ( τ D Eq. (.4 where [A] is the cceptor concentrtion nd τ D is the triplet lifetime of the donor in the sence of the cceptor. η En pproches unity if k T q [A] >> (τ D - which shows tht [A] should e s high s possile. The concentrtion of the cceptor triplet formed vi energy trnsfer, is given y Eq. (.5. [ 3 A * ] = 3 * [ D ] k os k + os D ( τ Eq. (.5 where k os is the decy rte of the donor triplet in the presence of the cceptor [7]. By expressing concentrtions in terms of sorptions, one cn clculte the triplet extinction coefficient of the exmined sustnce s with Eq. (.6, D E 3 * ε 3 * A D kos + ( τ ε 3 * = Eq. (.6 A E k 3 D * os where 3 *, E 3 *, ε 3 * nd ε * re the sorptions of donor nd cceptor triplet nd the E D A D extinction coefficients, respectively. 3 A

15 Chpter : Experimentl section.5. Determintion of extinction coefficients nd quntum yields of rdicl ions Time-resolved lser flsh photolysis permits the direct oservtion of rdicl ion intermedites. t is esy to determine the extinction coefficients of rdicl ions y n electron trnsfer rection [7-74], if the extinction coefficient of the other rdicl ion formed in tht process is known. n these experiments, the concentrtions of rdicl ctions nd rdicl nions re equl. Therefore, the extinction coefficient of one rdicl ion cn e clculted y ppliction of the Lmert-Beer s Lw s in Eq. (.7, E ε + M A ε + = Eq. (.7 M E A where ε, + ε, E nd re the rdicl ction nd rdicl nion extinction + M A M E A coefficients nd the mximum sornces for the rdicl ction nd rdicl nion, respectively. The sorption of the rdicl ions should e monitored t the wvelength of their mximum sorptions to give more ccurte results. where The quntum yield of the electron trnsfer cn expressed s in the Eq. (.8 [75,76] ϕ = ϕ. Eq. (.8 ET T η ET ϕ ET nd ϕt re the quntum yield of electron trnsfer process nd intersystem crossing, respectively. η ET is the efficiency of electron trnsfer..6. Anlysis of triplet stte decy kinetics.6.. First-order decy n dilute solution nd in the sence of quencher, the triplet stte 3 A * will decy y interction with solvent molecules. This gives unimoleculr decy rte constnt, k st, ccording to the following rection k 3 * A st A Re. (.3 Since the difference in sornce of triplet nd ground stte, ΔE ( 3 A *, is directly proportionl to [ 3 A * ], it follows 3 * 3 * Δ E A = ΔE ( A exp[ k t] Eq. (.9 ( st 3 * where ΔE ( A is the initil sornce difference, i.e., t t =.

16 Chpter : Experimentl section By fitting the experimentl sorption decy curve to Eq. (.9, the rte constnt for decy of the ited triplet stte, k st, is otined..6.. Self-quenching For ittion t low lser intensity in the sence of quencher, the ited triplet stte will decy ccording to first-order, ut the decy rte increses with n increse of the sustrte concentrtion under the sme experimentl conditions, i.e., the triplet stte cn e quenched y the sustrte in pseudo first-order process (Re. (.4 [77]. The experimentl sorption of the triplet cn e fitted s mono-exponentil decy (Eq. (. with n oserved decy rte constnt (k os, k sq 3 A * + A A Re. (.4 3 * 3 * Δ E( A = ΔE ( A exp[ k t] Eq. (. os where k = k + k [A] Eq. (. os st sq The rte constnt of self-quenching, k sq, cn e determined from the slope of plot of k os s function of the sustrte concentrtion. The intercept of this plot yields the intrinsic firstorder decy constnt k st Triplet-triplet nnihiltion At high lser energies, significnt concentrtion triplet is generted. Their trnsient sornce no longer exhiits mono-exponentil decy, nd the decy rtes re incresed y n increse in lser intensity, the reson is tht in ddition to the descried first-order nd pseudo first-order dectivtion processes, triplet-triplet nnihiltion occurs [78-8] ccording to Re. (.5. 3 A * k T 3 * + A T A Re. (.5 For mixed first- nd second-order decy kinetics, Eq. (.3 is pplicle. The decy rte of 3 A * is the sum of the individul rtes, 3 dδe( A dt * = ( k st + k sq 3 * kt [A ] ΔE( A + Δ T ( ΔE(. d ε λ 3 A * Eq. (. where 3 * 3 * k ΔE ( A ΔE( A = k 3 * T T kt T 3 k + ΔE ( A exp( k t ΔE ( A Δε λ. d Δε λ. d * Eq. (.3 k = k + k [A ] nd kt-t is the rte constnt of the triplet-triplet nnihiltion. ( st sq

17 Chpter : Experimentl section 3 f only triplet-triplet nnihiltion rection occurs, the decy should follow simple secondorder kinetics, 3 * 3 * [ A ] [ A ] = Eq. (.4 3 * + k [ A ] t T T 3 * where [ A ] is the triplet concentrtion immeditely fter the end of the lser pulse..7. Determintion of electron sorption The difference etween the sorption signls otined in rgon- nd in N O-sturted solutions under otherwise identicl experimentl conditions is tken s the true electron sorption signl s shown in Figure. (s n exmple. Clcultion of the concentrtion of the hydrted electron ws crried out using literture vlue of 85 M - cm - for the molr extinction coefficient t 7 nm in queous solution [83]. All mesurements sed on the study of the electron formtion were crried out y mesuring its trnsient sornce t 83 or 89 nm, ecuse the emission intensities of Xenon lmp t these wvelengths re much higher thn tht t the sorption mximum of electron (7 nm, i.e., t these wvelengths, the signl/noise rtio is more etter thn tht t 7 nm. The reltive extinction coefficients t 7 nd 89 (83 nm were determined experimentlly. n N O-sturted solution, the electron sorption signls re quenched in few nnoseconds due to Re. (.6[83, 84]. N O + eq + H O N + OH + OH Re. (.6 When necessry, the hydroxyl rdicls produced in Re. (.6 were removed y dding tert-utnol (.5 M, which rects with OH producing non-rective rdicl [85]. Figure.. Kinetic trces recorded t 89 nm of.6 x -4 M xnthone in methnol-wter (: v/v following 38 nm lser pulse (87.7 mj/cm. ( in rgon-sturted solution, ( in N O-sturted solution, nd (c the difference of them. E (89 nm,8,4, ( ( (c Time / ns

18 Chpter 3: Anlysis of light intensity dependences 4 3. Liner nd cyclic photoioniztions Anlysis of light intensity dependences Photoioniztion t high lser intensities cn occur y other mechnisms thn t low intensities, e.g., with cridone derivtives [86] or the tris-, -ipyridyl ruthenium ( ion [87, 88], xnthone [89] or enzophenone-4-croxylte [9] in the presence of the electron donor. This chpter descries different liner nd cyclic photoioniztion mechnisms. We distinguish etween these mechnisms kineticlly y studying the dependences of the concentrtions on the light intensity. 3.. Kinetic formultion of light sorption The ground stte molecule (A sors photon to give its ited stte (A * s expressed in Re. (3.. A * + hv A Re. (3. The rte, r i, of conversion of the ground-stte molecules into their ited stte y light d of wvelength λ, depends on the rte s d t quntum yield ϕ i, of the rection [9] r of light sorption y the ground stte nd on the d[a] d s = = Eq. (3. dt d t V i ϕ i where V is the ited volume. The totl intensity of the iting light is given y τ = ( t dt Eq. (3. where τ is the durtion of the lser pulse nd. (t is the envelope of the lser pulse. All solutions in our experiments re opticlly thin, so ll the sorption steps cn e treted s first order processes. The Lmert-Beer lw cn e linerized to give s =.33 ε [A] d Eq. (3.3 λ where ε λ,, nd d re the extinction coefficient of the sustrte t the ittion wvelength nd the opticl pth length of cell. nserting Eq. (3.3 into (3. yields

19 Chpter 3: Anlysis of light intensity dependences 5 r i.33ϕi ε λ = λ [A] Eq. (3.4 A hc N l A where λ nd A re the wvelength of the lser pulse nd the irrdited re. N A, c l, h re the Avogdro numer, speed of the light nd Plnck s constnt. Therefore, the rte r i of ny photoinduced rection step is proportionl to the concentrtion of the species tht is ited nd the light intensity. The ssocited rte constnt k i is time dependent ecuse it depends on (t, k ( t = ( t Eq. (3.5 i i where i is constnt of proportionlity. By comprison with Eq. (3.4, the reltionship etween kinetic constnt i nd the quntum yield ϕ i of the corresponding process is given y Eq. (3.5 [9]..33ϕi ε λ λ i = Eq. (3.6 A hc N l A The constnt of proportionlity i hs the dimension of reciprocl light intensity, re per energy (cm /mj. t is directly proportionl to the pertining extinction coefficient nd quntum yield. n our clcultions, we will ssume rectngulr lser pulse i.e., t = /τ Eq. (3.7 ( where is the totl lser intensity. With tht solutions of the kinetic equtions cn e otined in closed form nd depend only on. Thus the rte constnt of light-driven step cn e expressed s true constnt with Eq. (3.8. k i = i Eq. (3.8 τ 3.. Liner photoioniztion processes n this section, we will clssify the exmined sustrtes s either n electron donor (D or n electron cceptor (A, depending on the experimentl conditions. D sors photon to generte its ited stte (D* tht cn e ionized y further photon. The ited stte of n electron cceptor (A* in the presence of n electron donor (e.g., n mine is quenched y electron trnsfer to produce the rdicl nion (A -, which cn e ionized y second photon.

20 Chpter 3: Anlysis of light intensity dependences Consecutive two-photon ioniztion n this process [9-94], the molecule is ited to give D * y the sorption of one photon with rte constnt (t. Asorption of the second photon y D * results in formtion of n electron nd the rdicl ction (D + with rte constnt (t. The mechnism of tht ioniztion is depicted in Scheme 3., which consists of the sequence of two first-order rections y which the ited stte uilds up nd decys. D (hv D * (hv D + _ + e q Scheme 3. The solution of the differentil rte equtions of this scheme is given y Eqs. (3.9 nd (3.. * [D ] = { exp( exp( } Eq. (3.9 c [e c q ] [D = c + ] = exp( exp( Eq. (3. The kinetic constnt of formtion of the ited stte cn e directly clculted using Eq. (3.6, ecuse the sorption quntum yield is usully equl to unity nd the extinction coefficient of the sustrte ( ε ground t the ittion wvelength cn e determined experimentlly. The kinetic constnt for photoioniztion of the ited stte could e clculted in the sme wy, if the extinction coefficient of the ited stte, ε ited, t the ittion wvelength nd the quntum yield of photoioniztion were known. Experimentlly, is otined y fitting Eqs. (3.9 nd (3. to the experimentl dt. Eqution (3. shows tht the concentrtions of oth electron nd rdicl ction re identicl. Therefore, the mesurement of either concentrtion suffices. The electron curves re identicl when nd re interchnged, s shown in Figure 3.. Also, the form of the curves for D * remins the sme, so they only wy to determine which constnt is the lrger of the two is y mesuring the solute concentrtions of D *.

21 Chpter 3: Anlysis of light intensity dependences 7 Figure 3.. Concentrtions c of the ited stte (D *, nd the electron (e - q reltive to the sustrte concentrtion c s functions of the lser intensity for consecutive two-photon ionitztion. The curves were simulted with Eqs. (3. 9 nd (3.. The prmeters of simultion were =.5 cm /mj, =.5 cm /mj, solid lines; =.5 cm /mj =.5 cm /mj, dshed lines. The electron curves re identicl in these two cses. At low lser intensity, the electron curve shows wht is clled n induction period in kinetic plot (Figure 3., ecuse electron formtion requires the formtion of n intermedit first. Therefore, in order to distinguish etween mono- nd iphotonic processes, the lser intensity dependence of the electron yields should e mesured t low lser intensities. However, more relile informtion is otined from the concentrtion of the intermedite t high intensities. For the specil cse =, the intensity dependences of the concentrtions re given y Eqs. (3. nd (3. for Scheme 3.. * [D ] = exp( Eq. (3. c [e c q ] [D = c + ] = exp( ( + Eq. (3. The ctul light intensity experienced is ffected y D or D*, other soring species (i.e., y n inner filter effect or re-sorption of emitted rdition, especilly when sustnce exhiits high fluorescence quntum yield. When is treted s n djustle prmeter in fit to the experimentl dt, the est-fit vlue of my e difference from the

22 Chpter 3: Anlysis of light intensity dependences 8 clculted vlue ecuse of these effects. However, Eq. (3.6 cn e used s chemicl ctinometer for ll light-driven steps [9]. Assuming tht the quntum yield of ittion of the ground stte is unity, the quntum yield of ioniztion ( ϕ clculted from Eq. (3.3. ε ion in Scheme 3. cn e ground ϕ ion = Eq. (3.3 ε ited Also, Eq. (3.6 cn e used to otin the rtio of the quntum yield of photoioniztion for the sme system t different ittion wvelengths s represented in Eq. (3.4 ϕ ϕ ion, λ ion, λ ( / λ (ground λ (ground λ (ited = Eq. (3.4 ( / λ λ ε. ε ε. ε λ (ited The descried two-photon ioniztion process my e ccompnied y dectivtion processes of the ited singlet stte such s fluorescence emission nd triplet stte formtion ( 3 D * with rte constnts k f nd k isc, respectively. The pproprite modifiction of Scheme 3. is shown in Scheme 3.. D (hv D* (hv D k f k isc + + e _ q 3 D* Scheme 3. The lser intensity dependence sed on Scheme 3. for the singlet nd electron concentrtions re given y Eqs. (3.5 nd (3.6, respectively. * D ] = (exp( exp( Eq.(3. 5 c ( [ ] = [ e q c exp( ( exp( + ( Eq. (3.6 where, = k fτ k isc τ ± ( + + k f τ + k isc τ 4( + k isc τ Eq. (3.7

23 Chpter 3: Anlysis of light intensity dependences 9 The functionl forms of Eqs. (3.5-(3.6 nd (3.9-(3. re identicl (,, only the leding constnt fctors re slightly different. The concentrtion of the ited triplet stte is 3 * [ D ] c kisc τ = exp( ( exp( + ( [e = c q ] k isc τ Eq. (3.8 The term k isc [ D * ] cn e omitted from the differentil rte equtions tht represent Scheme 3., when the fluorescence quntum yield is unity. The solutions of the kinetic equtions in tht cse give the sme functionl from for the singlet nd the electron s in Eqs. (3.5 nd (3.6, ut with k fτ τ = + + ± ( + + k f, 4 Eq. (3.9 At low intensity (, <<, the intensity dependences cn e pproximted y Eqs (3. nd (3.. * [ D ] c = Eq. (3. [ e q ] = c Eq. (3. Eq. (3. shows tht the concentrtion of the ited singlet stte increses linerly with incresing the lser intensity, while Eq. (3. shows tht the electron concentrtion increses linerly with the squre of the lser intensity Prllel photoioniztion of ited singlet nd triplet sttes Scheme 3.3 represents tht process k isc 3 D* (hv D (hv D * (hv D k f + + e _ q Scheme 3.3

24 Chpter 3: Anlysis of light intensity dependences The differentil rte equtions for ech species re d[ D] * = ( t[d] + ( k f [ D ] dt Eq. (3. * d[ D ] * = ( t[d] ( k f + kisc + ( t[ D ] dt Eq. (3.3 3 * d[ D ] * 3 * = kisc [ D ] ( t[ D ] dt Eq. (3.4 + d[ e q ] d[d ] * 3 * = = ( t[ D ] + ( t[ D ] dt dt Eq. (3.5 These equtions cn e solved y the method of prtil frctions. The result for the ited singlet stte hs lredy een shown in Eq. (3.5.The solution for the other species re 3 * [ D ] c k isc τ = ( k ( exp( ( ( exp( ( exp( + ( Eq. (3.6 [e [ c ] [D = ] [ c q ] = ] + ( exp( ( ( ex kisc τ exp( ( ( ex ( exp( ex + ( ( Eq. (3.7 where, re given y Eqution (3.8. f ecomes zero, Eq. (3.7 reduces to Eq. (3.6 ecuse ( + isc τ / / reduces to unity under our conditions Stepwise photoioniztion vi three-photon process Goez nd Zurev hve descried the following photoioniztion [95] with n intervening chemicl step tht produces second photoionizle intermedite S. D (hv (hv (hv D * S e _ q D + P e _ q Scheme 3.4 To simplify the kinetic model, the formtion of S is ssumed to e fst on the time scle of the lser pulse, so (t cn e regrded s the rte constnt of the formtion of the rdicl

25 Chpter 3: Anlysis of light intensity dependences species (S. Therefore, D + cn e omitted from the kinetic model. The concentrtion of ech species reltive to the initil sustrte concentrtion, c is given y ( ( exp( ( ( exp( ( ( exp( ] [c [S] + = ex ex ex Eq.(3. 8 ( ( exp( ( ( ( exp( ( ( ( exp( ] [c ] [e q + = ex ex ex Eq.(3.9 ( ( exp( ( ( exp( ( ( exp( ] [c [P] + = ex ex ex Eq. (3.3 For D *, Eq. (3.9 holds. Figure 3.. Concentrtions c of ited stte (D*, intermedite (S, hydrted electron (e q nd photoproduct (P reltive to the sustrte concentrtion c s functions of the lser intensity for Scheme 3.4. The curves were clculted with Eqs. (3.9 nd (3.8-(3.3. The prmeters of the simultion were: =.5 cm /mj, =.5 cm /mj, =.5 cm /mj, (solid lines. =.5 cm /mj, =.5 cm /mj, =.5 cm /mj (dshed lines. The electron curves remin identicl when nd were interchnged. Agin, the electron curves cnnot help to discriminte etween nd prmeters s illustrted in Figure 3.. The electron curves re identicl if nd re interchnged, while this interchnge hs n effect on the curve for the ited stte s shown in Figure 3..

26 Chpter 3: Anlysis of light intensity dependences 3.3. Cyclic mechnisms of electron donor photoioniztions Singlet or triplet stte undergoes photoioniztion We hve recently reported [86] tht the irrdition of n electron donor D in the presence of scrificil electron donor D sc cn produce electrons y the ctlytic cycle mechnism shown in Scheme 3.5. The rdicl ction (D + resulting from the ioniztion process plys key role for the rection mechnism, ecuse it sors third photon nd is then reduced y the scrificil donor (e.g., SDS to regenerte the ground-stte molecule. D D D D sc D sc.+ e _ q (hv (hv (hv *.+ Scheme 3.5 The solutions of the corresponding system of differentil equtions re given y Eqs. (3.3- ( = exp( ( ( exp( ( ( ] [D * k c Eq. ( = + exp( ( exp( ( ] D [ c Eq. ( = c q ( exp( ( ( exp( ( ( ] [e Eq. (3. 33 where ( 4( (, ± + + = Eq. (3.34 f ecomes zero, Scheme 3.5 is trnsformed into consecutive two-photon ioniztion (Scheme 3..

27 Chpter 3: Anlysis of light intensity dependences 3 Figure 3.3. Concentrtions c of ited stte (D*, electron (e q nd photoproduct (D + reltive to the sustrte concentrtion c s functions of the lser intensity for the cyclic mechnism of Scheme 3.5. The curves were simulted with Eqs. (3.3 (3.33 The prmeters of the simultions were: =.5 cm /mj, =.5 cm /mj, =.5cm /mj (solid lines. =.5 cm /mj, =.5 cm /mj, =.5 cm /mj (dshed lines. Both the electron nd the rdicl ction curves remin unchnged when nd re interchnged. At low lser intensity, the concentrtions of the electron nd the rdicl ction re equl, ut t high intensities, the electron concentrtion is lwys higher. The concentrtions oth of the ited stte nd of the rdicl ction rech sttionry vlue etween zero nd c t high intensities while the electron curve increses without ounds nd eventully surpsses the initil concentrtion of the sustrte. As opposed to liner mechnism, the ehviour of the electron curve is thus quite different from tht of the rdicl ction. As Figure 3.3 shows the concentrtion of the ited stte increses linerly t first efore reching pek, followed y grdul decrese towrds the stedy stte. The liner increse indictes tht the formtion of the ited stte is monophotonic process s expected, nd the ltter oservtion provides relile evidence tht regenertion of the ited stte occurs through cyclic rection.

28 Chpter 3: Anlysis of light intensity dependences 4 Figure 3.4. Concentrtion c of the species in Schemes 3.5 nd 3. reltive to the sustrte concentrtion c s functions of the lser intensity. The curves were clculted with Eqs. (3.9-(3 nd (3.3-(3.33. Solid lines: cyclic; =.5 cm /mj, =.5 cm /mj, =.5 cm /mj; dshed lines: consecutive two-photon ioniztion, =.5 cm /mj, =.5 cm /mj. Figure 3.4 displys the ehviour of ll species for the cyclic rection on the one hnd, nd the consecutive two-photon photoioniztion process on the other. At low light intensity, differentition etween the two mechnisms is impossile. However, tht differentition is esy t high light intensity, where the concentrtion of the ited stte pproches stedy stte vlue in the cyclic rection, ut pproches zero in the liner rection, nd where the electron concentrtion increses linerly in the former ut reches c in the ltter rection. The dependence of the electron curve on is shown in more detil in Figure 3.5. Figure 3.5. The electron concentrtion reltive to the sustrte concentrtion c s functions of the lser intensity. The curves were simulted with Eq (3.33. The kinetic prmeters of the simultion were: =.5 cm /mj, =.5 cm /mj (ll curves; =.5 cm /mj (solid line; =.5cm /mj (short dshed line; =.5 cm /mj (long dshed line.

29 Chpter 3: Anlysis of light intensity dependences Both singlet nd triplet sttes undergo photoioniztion The cyclic mechnism of donor in the presence of scrificil donor for ioniztion of oth ited singlet nd triplet sttes is displyed in Scheme 3.6. The rte constnts for oth ioniztion steps re (t nd (t. The rdicl ction sors photon to regenerte the sustrte y rection with scrificil donor with rte constnt 3 (t. 3 D * k isc (hv D * ( hv D + _ + e q k f ( hv 3 ( hv D Scheme 3.6 The kinetic re descried y Eqs (3.35 (3.39: d[d] dt * + = ( t[d] + k [ D ] + ( t[d ] Eq. (3.35 f 3 * d[ D ] * = ( t[d] ( ( t + k f + k isc [ D ] dt Eq. ( * d[ D ] * 3 * = kisc[ D ] ( t[ D ] dt Eq. (3.37 d[ e q ] * 3 * = ( t[ D ] + ( t[ D ] dt Eq. (3.38 d[d dt + ] * 3 * + = ( t[ D ] + ( t[ D ] ( t[d ] Eq. ( Closed-form solutions of these equtions re cumersome ecuse they contin the roots of cuic eqution, so it is etter to solve them numericlly. An exmple is shown in Figure 3.6.

30 Chpter 3: Anlysis of light intensity dependences 6 Figure 3.6. Numericl simultion of the lser intensity dependence of the concentrtions c reltive to the initil concentrtion of sustrte c on the sis of Scheme 3.6 with rte constnts =.5 cm /mj, =.5 cm /mj, =.5 cm /mj, 3 =.5 cm /mj, nd k isc = k f =5x 7 s -, t =6 ns. Qulittively, D *, D + nd e - q re seen to ehve very similr to D *, D + nd e - q in Scheme 3.5. n ddition, the curves for D * nd 3 D * in Figure 3.6 hve similr shpe Cyclic mechnisms of electron cceptor photoioniztions Photoinduced electron trnsfer from electron donors, such s mines to romtic ketones in polr solvents produces the rdicl nion of the ketone nd the rdicl ction of the electron donor [96-]. Recent studies hve reveled tht the photorection of certin romtic ketone/mine systems yield electrons, in concentrtion tht surpsses the initil concentrtion of the ketone t high lser intensity. Goez nd Zurev [9] were le to show tht the electron concentrtion s function of the lser intensity is strongly dependent on the quenching process nd the rte constnt of ited stte formtion. f the rte constnt of ited stte formtion is much lrger thn the rte constnt of the quenching process, the electron ehviour exhiits sturtion t high lser intensity wheres the electron curve increses linerly when the quenching process is fster thn ited stte formtion. Scheme 3.7 summrizes the mechnism. The ited stte A * is quenched y electron trnsfer from the electron donor (D in its ground stte with rte constnt k q to produce its rdicl nion (A - nd the rdicl ction (D +. The rdicl nion sors second photon to give n electron nd regenertes the sustrte with the rte constnt 4 (t. Rditionless decy of the ited stte (k d is too slow to compete with the quenching, so it cn e neglected. Becuse of the very low concentrtion of the sustrte A, the smll pth length of

31 Chpter 3: Anlysis of light intensity dependences 7 the rection cell, nd the high concentrtion of quencher the light dependent steps re first or pseudo-first-order rections. By using the mss lnce, the rte equtions re found to e * d[a ] * = ( t[c ] ( ( t + kq [A ] ( t[a ] Eq. (3.4 dt d[a dt ] * = kq[a ] 4 ( t[a ] Eq. (3.4 D D.+ k q A * _ A (hv k d A 4 e _ q (hv Scheme 3.7 The concentrtion of the electron cn e clculted y the integrtion of the rdicl nion concentrtion over the effective lser pulse width (τ fter multipliction with the rte constnt of electron formtion ( 4 (t. τ q ] = 4 ( [A ] [ e t Eq. (3.4 The efficiency of electron trnsfer increses with the increse of the quencher concentrtion, pproching unity. At high quencher concentrtion, intermedicy of the ited stte cn e neglected. Thus, the rte of rdicl nion formtion is given y Eq. ( d[a dt ] = ( t[c ] ( + ( t[a ] Eq. (3. 43 Solving Eq. (3.43 gives 4 [A ] [c ] = ( exp[ ( + 4 ] Eq. ( nd y integrtion, one otins the concentrtion of the electron

32 Chpter 3: Anlysis of light intensity dependences 8 [e q ] [c ] 4 = ( + 4 ( exp[ ( + ] + ( Eq. (3.45 Figure 3.7 represents the ehviour of oth electron nd nion rdicl curves for fst quenching. n tht cse, the electron concentrtion increses linerly with incresing lser intensity, while the rdicl nion exhiits sturtion ehviour t high lser intensity. This ehviour will e encountered in such cyclic pthwy if oth quencher concentrtion nd lser pulse intensity re sufficient. Figure 3.7. Simultion of the lser intensity dependence of the concentrtions c reltive to the initil concentrtion of the sustrte c for the rdicl nion ( Eq. (3.44 nd the electron (Eq. (3.45, with kinetic constnts =.5 cm /mj nd 4 =.5 cm /mj. The solid nd the dshed lines represent the hydrted electron nd the rdicl nion. Appliction of the stedy-stte pproximtion for the rdicl nion in Eq. (3.43 gives the limiting concentrtion of rdicl nion. [A ] [c ] lim = ( + 4 Eq. (3.46 The liner rise of the electron concentrtion under these conditions is given y [e q ] = [c ] ( + 4 Eq. (3.47

33 9 Prt Donor nd cceptor photoioniztions Results nd discussions

34 Chpter 4: Photoioniztion of cridone derivtives vi their singlet stte 3 Prt.A Electron donor photoioniztions 4. Photoioniztion of cridone derivtives vi their singlet stte Despite the lrge numer of studies on the electronic relxtion process of cridone derivtives in condensed phse [-4], lser flsh photolysis study of the photoioniztion of N-methylcridone (MA nd cridone (ACO hs not een reported so fr. Much reserch hs een directed to define the mechnism of the photoioniztion of orgnic molecules in solution [5-8]. n recent yers, it hs een shown tht the ittion of romtic compounds in fluid medi t high lser intensity my induce muti-photonic ioniztion, which cn mke new rection pthwys ville [86, 87]. n this chpter, we descrie for the first time the cyclic nd liner photoioniztions of MA nd ACO. O O N H (ACO N CH 3 (MA 4.. Spectroscopic chrcteriztion The UV-visile sorption spectrum of n queous SDS solution of MA, for exmple, shows sorption peks t 59 nd 4 nm with high molr extinction coefficients (ε 58 nm = 5.5 x 4 M - cm -, ε 4 nm = 8.89 x 3 M - cm - s shown in Figure 4. [9, ]. This suggests tht the nds hve π-π * chrcters. The concentrtion of N-methylcridone in queous SDS solution ws determined from its opticl sorption []. n ll cses, the sorption ws mesured efore irrdition, nd there ws no sign of ny chemicl interction etween the components in their ground sttes.

35 Chpter 4: Photoioniztion of cridone derivtives vi their singlet stte 3 Figure 4.. Stedy-stte sorption spectr of N- methylcridone in.5 M queous SDS (Solid line nd in ethnol-wter mixtures (:4 v/v (dshed line, t room temperture ε / (M - cm - 6x 4 5x 4 4x 4 3x 4 x 4 x λ / nm The fluorescence quntum yield, ϕ f, depends on the nture of the solvent [-3]. The fluorescence spectrum of MA in queous SDS solution exhiits n emission mximum t 43 nm, wheres for ACO, the emission mximum lies t 45 nm [, ]. We mesured the fluorescence spectr of oth MA nd ACO t vrious lser intensities nd different lser ittion wvelengths. The fluorescence spectr for ech species hve the sme generl form ut possess different intensities s shown in Figure 4., indicting tht the MA nd ACO re the only emitting species. Furthermore, the fluorescence is not quenched y SDS under our mesurement conditions. 4 ( ( Rel.ntensity Rel.ntensity λ / nm λ / nm Figure 4.. Fluorescence spectr of cridone derivtives otined y 355 nm lser ittion t different lser intensities in N O-sturted solution. ( 5.5 x -5 M MA in.5 M SDS (squres, 6.6 mj/cm ; circles, 538 mj/cm nd tringles, 33 mj/cm. (.4 x -5 M ACO in queous solution (circles, 33 mj/cm ; squres, 495 mj/cm.

36 Chpter 4: Photoioniztion of cridone derivtives vi their singlet stte Triplet nd rdicl ction sorption spectr The T-T sorption mximum of N-Methylcridone exhiits strong lue shift with incresing solvent polrity [3,4]. Following lser flsh photolysis of MA in queous SDS solution t low lser intensity, we oserved tht the trnsient spectrum hs mximum sorptions t 58 nd 3 nm nd leching t round 4 nm due to the depletion of the ground stte of MA s shown in Figure 4.3. The trnsient sorptions t 58 nd 3 nm were quenched in oxygen-sturted solution, nd were not ffected in N O-sturted solution. Therefore, the sorption nds t these wvelengths re ttriuted to the T-T sorption signls. Figure 4.3. Trnsient sorption spectrum of 9.89 x -5 M MA in N O-sturted.5 M queous SDS solution otined y 355 nm lser ittion t low intensity (c., 3. mj /cm t room temperture. ΔE, (,5,,5, -,5 -, -, λ (nm The trnsient sorption spectrum otined (Figure 4.4, circles y lser photolysis t high intensity of n queous SDS solution of MA with N O sturtion following 355 nm light exhiits n dditionl sorption nd t round nm. The lifetime of tht trnsient is not ffected y the presence of oxygen nd is ttriutle to N-methylcridone rdicl ction, MA +. The low sorption t 58 nm illustrtes tht the min trnsient is MA + with miniml shre of 3 MA*. MA + lso sors t 3 nm. The spectr of MA + nd 3 MA* cn e seprted y mesurements t two different lser intensities (Figure 4.4. At low intensity, oth the triplet stte nd MA + re present, while t high lser intensity the trnsient sorption results minly from MA + with very smll contriution of the triplet stte. The sorption of the triplet stte in the rnge 76-8 nm is negligile. Therefore, the sorptions t these wvelengths chrcterize MA +. Hence, within experimentl error, one cn deduce the pure MA + sorption spectrum t low lser intensity y multiplying the trnsient spectrum otined t high lser intensity y certin fctor. Then, the triplet spectrum cn e otined y sutrcting the pure MA + spectrum from the