6. Photoionization of acridine through singlet and triplet channels

Transcription

1 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels Photoioniztion of cridine through singlet nd triplet chnnels Photoioinztion of cridine (Ac) in queous micelles hs not yet een studied, lthough, Kellmnn nd Tfiel [4] hve investigted its photoioniztion in lkline wter solution t room temperture. They found tht iphotonic ioniztion proceeds minly through the singlet nd to smll extent vi the triplet stte. We could show for the first time tht the photoioniztion of cridine in SDS micellr solution proceeds vi cyclic mechnism with high photoioniztion quntum yield. N (Ac) 6.. Asorption spectr The UV-visile sorption spectrum of n queous SDS solution of cridine (Figure 6.) shows n sorption pek t nm with molr extinction coefficient of 67 M - cm -, which suggests tht the nd hs π-π * chrcter [4,43]. At low ph, the sorption spectrum shows rod nd centered round 4 nm which is not oserved in lkline solution. This nd is ttriuted to the formtion of cridinium ion [45]. We hve investigted the photorection of cridine in lkline solutions ecuse it is slightly solule in wter (.8 x -4 M) in sence of SDS [46,47]. Figure 6.. Asorption spectr of cridine solutions t room 6 temperture in ().5 M SDS, () wter, (c) methnol-wter (:2 v/v) nd (d) wter t ph 2. ε / ( 3 M - cm - ) c d λ / nm

2 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels Fluorescence spectr The fluorescence spectr of cridine singlet stte ( Ac*) [46] show mximum emission t pproximtely 43 (lkline solution, ph 2), 42 (queous SDS, ph 2), nd 45 nm (neutrl queous solution) (Figure 6.2). The fluorescence spectrum of cridine in queous SDS solution hs two emission mxim, t 48 nd 45 nm, due to the presence of the protonted nd neutrl form of cridine, indicting tht there is n cid-se equilirium. At higher ph vlue, the emission nd t 48 nm ws not oserved, i.e., the nd t 48 nm is due to cridinium ion (AcH + ). Figure 6.2. Stedy stte fluorescence spectr of cridine t room temperture, c d in.5 M SDS, ph 2 (), wter, ph 2,8 (), wter (c), nd.5 M SDS (d). The smples were ited t 355 nm nd normlized with respect to the mximum intensity. Reltive ntensity,6,4,2, λ / nm Medeiros et l. [45] reported tht the fluorescence decy of cridine in wter nd queous SDS solution t ph 2 ws mono-exponentil with lifetimes of out.4 nd 2.3 ns, respectively. The fluorescence quntum yield vlues of cridine t ph > in SDS nd in wter were.6-.9 [45, 47] nd.24 [45], respectively. We determined the fluorescence quntum yield of cridine in methnol-wter (:2 v/v) sed on the reported vlue in methnol [43] nd it ws found to e.. This vlue is pproximtely equl to tht in methnol-wter mixture (:2 v/v) t ph 2. From the previous studies of the photophysicl properties of cridine, the sum of the rte constnts of the rditionless processes of the lowest ited singlet stte in different solvents ws clculted from the fluorescence lifetime nd the rte constnt of the fluorescence emission s displyed in Tle 6.. t is seen tht the rte constnts of the fluorescence emissions of the lowest singlet stte of cridine in polr solvents re lmost equl.

3 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 6 Tle 6.. Rte constnts of dectivtion processes of Ac * in different solvents t ph 2. Solvent τ f x 9 (s) k f x 7 (s - ) (k isc +k ic ) x 7 (s - ) H 2 O [45] M SDS [45] MeOH-H 2 O :2 v/v 4 () 2.5 () 22.5 (c) () ref. [43] () in this work (c) k ic + k isc + k r = 22.5 x 7 s - ; where k r is the rte constnt of the rection of the singlet stte with lcohol 6.3. Trnsient sorption spectr The trnsient sorption spectr of cridine in queous SDS solution under lser ittion wvelength of 355 nm t high lser intensity show sorption nds t 443 nm due to triplet-triplet sorption of cridine [48], nd rod nd centered t 72 nm due to the hydrted electron [49] (Figure 6.3). These oservtions indicte the ioniztion of cridine. Figure 6.3. Trnsient sorption spectr oserved in 355 nm lser flsh photolysis of cridine solution sturted with rgon (squres), N 2 O (circles) The incident lser intensity ws 59 mj/cm 2 pulse. Both curves were recorded t 6 ns; t tht time solvted electrons were scvenged completely y N 2 O in N 2 O- sturted solution. Their difference sorption spectrum is chrcteristic for hydrted electron (tringles). Δ E (, t ),3,25,2,5,,5, λ / nm Correction of the triplet-triplet sorption spectrum for ground-stte depletion yields the solute sorption spectrum of the triplet stte s shown in Figure 6.4. t hs two mximum sorption nds t 443 nd 355 nm. This correction is sed on the extinction coefficient of the triplet stte of cridine, 3 Ac*, t 443 nm tht is.9 x 4 ±. M - cm - [42,43,5].

4 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 62 Figure 6.4. Corrected sorption spectr of triplet triplet sorption of cridine in.5 M queous SDS solution t ph 2 (strs, dshed line) or in methnol-wter :2 v/v (tringles, solid line), following 355 nm lser pulse 6.4. Triplet energy trnsfer The molr extinction coefficients of T-T sorption of cridine in wter solution were reported to e 9 nd 45 M - cm - t λ mx = 443 nm, y light sturtion method [5] nd the triplet energy trnsfer method [52], respectively. Since the discrepncy in vlues cnnot e ignored; we re-determined the molr extinction coefficient of the T-T sorption of cridine. The energy trnsfer from the ited triplet stte of enzophenone-4-croxylte, BC, to cridine in lkline solution (ph 2) ws monitored. The molr extinction coefficient of the triplet stte of cridine t 443 nm in lkline solution ws found to e 85 ± 5%M - cm - sed on the extinction coefficient of the triplet stte of BC t 545 nm (52 M - cm - ) [33]. Figure 6.5 shows the sorption time profile recorded t 443 nd 545 nm (otined y 38 nm lser ittion) for cridine triplet uilding nd enzophenone-4-croxylte triplet decy s result of energy trnsfer process. Figure 6.5. Trnsient sorption decy of the solution contined cridine (4.96 x -5 M) nd enzophenone-4-croxylte (5.39 x -4,6,4 545 nm M) otined y 38 nm lser ittion t low,2 intensity, () growth trnsient sorption t 443 nm for formtion of cridine triplet nd () Ε(λ), -,2 443 nm trnsient sorption decy of the triplet stte of enzophenone-4-croxylte t 545 nm. The solid curves re the fitting of monoexponentil function giving k os = 6.2 x 5 s -. -,4 -, time / µs

5 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels Photoioniztion of cridine in lkline wter We hve oserved tht the electron yield resulting from photoioniztion of cridine in n queous SDS (ph 2) t 355 nm is greter thn tht in methnol-wter, :2 v/v t ph 2 y lmost fctor of 3 under the sme experimentl conditions. This oservtion prompted us to study nd compre the photoioniztion ehviour of cridine in oth cses s well s in lkline wter. By simulting photoioniztion mechnism of cridine in lkline wter s reported y Kellmnn nd Tfiel [4], we found tht n unknown photoproduct of cridine sors t the wvelength of mximum sorption of the triplet stte Simultion study ccording to results of Kellmnn nd Tfiel The lser intensity dependence of the hydrted electron concentrtion (Figure 6.6) shows n induction period t low lser intensity. Figure 6.6 lso includes the reltionship etween lser intensity nd fluorescence intensity of the ited singlet stte of cridine, Ac *. The fluorescence intensity psses through mximum followed y grdul decline. n ddition, formtion of 3 Ac* is monophotonic process. Kellmnn nd Tfiel [4] reported tht the ited singlet nd triplet sttes re the source of the hydrted electron. Scheme 3.3 in Chpter 3 is thus, proposed for tht photoioniztion. The ited singlet stte thermlly relxes nd returns ck to its ground stte with rte constnt k ic (internl conversion). Therefore, Scheme 3.3 must include this step. The cridine rdicl ction (Ac + ) hs n sorption nd t 9 nm [54]. t is importnt to mention tht our nnosecond lser flsh photolysis pprtus does not hve the cpility to detect the trnsient sorption ove 84 nm. Thus, we could not investigte the lser intensity dependence of the yield of the cridine rdicl ction. The oserved fluorescence intensity is the convolution of the Eq. (3.5) with the response function s given in Eq. (6.). Fl α κ ( ( κ κ ) ( κ ( κ κ )exp( χ t) + χ t )( κ + χ t exp( κ + ) ( κ χ t ) exp( κ ) ( κ χ t Eq. (6.) where ) ) ) κ, = κ 2 k fτ kiscτ k + κ τ ± ic ( κ + κ + k f τ + k isc τ kicτ + ) 2 4( κ κ + κ k isc Eq. (6.2) τ )

6 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 64 The oserved triplet concentrtion, (6.3) [ 3 Ac * ] os, fter the end of the pulse will e given y Eq. [ 3 Ac * ] 3 * * = [ Ac ] + [ Ac ]) Eq. (6.3) os ϕ isc where ϕ isc [ Ac * ] is the triplet concentrtion resulting from the cridine singlet stte vi intersystem crossing fter the end of the lser pulse. t is necessry for the kinetic eqution of fluorescence, Eq. (6.), to include scling fctor since, the luminescence collection efficiency is not known. Reducing the unknown prmeters mkes the fit esier, so tht κ, χ, τ, k f, nd k ic + k isc were treted s constnt vlues during the fit nd were mesured independently. Our ttempts to fit the experimentl dt of the oserved species sed on Eqs. (6.3), (6.) nd (3.27) show tht the resulting fit curves of the triplet stte nd the hydrted electron devite from the experimentl dt (Figure 6.6). Figure Effect of lser intensity on the fluorescence intensity of Ac * (strs) nd on the concentrtion of oth the hydrted electron (tringles) nd the cridine triplet stte (squres) reltive to the initil concentrtion of cridine c (2x -5 M) in queous solution t ph 2 following 355 nm lser ittion. The curves were fitted with Eqs. (6.) for Ac *, (6.3) for 3 Ac * nd (3.27) for. The est-fit prmeters were κ= 2.65 x -3 cm 2 mj -, κ 2 =. x -5 cm 2 mj -, cm 2 mj -, k ic τ = 3.25 x -, k isc τ =.369 x -, scle fctor for fluorescence 2.3. Constnt prmeters were κ = 6.7 x -2 cm 2 mj -, χ τ =.2, k f τ =.38 x -. e q

7 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels nterprettion of devitions for the triplet stte Kinetic nlyses in Figure 6.6 show tht the ehviour of the triplet stte is nlogous to tht of the singlet ited stte. This implies tht the triplet stte could pss through mximum concentrtion nd decrese with incresing lser intensity. However, the sorption t 443 nm increses linerly nd reches sturtion stte, indicting tht there is nother cridine derivtive, Ac d, which hs n sornce t 443 nm. Thus, the trnsient sorption t 443 nm cn e given y Eq. (6.4) Δ E os = E 3 + E Eq. (6.4) 443 d ( Ac*,443) (Ac,443) where nd E re the sornces of the ited triplet stte of Ac nd the E ( 3 d Ac*,443) (Ac,443 ) os cridine derivtive t 443 nm, respectively. is the oserved trnsient sorption t 443 ΔE 443 nm. The trnsient sorption of 3 Ac * t 443 nm cn e represented y Eq. (6.5). E 3 * 3 * ([ ( Ac, 443) ( Ac, 443) rel isc rel 3 * * = ε d C Ac ] + ϕ [ Ac ] ) Eq. (6.5) where ε 3 *, d, nd C ( Ac, 443) re the extinction coefficient of the 3 Ac * t 443 nm, the pth length 3 of the cell used, nd the initil concentrtion of cridine ground stte, respectively. [ Ac nd [ Ac * ] rel re the concentrtions of 3 Ac * nd Ac * reltive to the initil concentrtion of cridine, respectively. The concentrtion of the electron must e equl to the concentrtions of the cridine rdicl ction nd its susequent decy products under the sme experimentl conditions s displyed in Scheme 3.3. Therefore, the concentrtion of resulting cridine derivtive during the photorection must e proportionl to the electron concentrtion. The oserved trnsient sorption t 443 nm in N 2 O-sturted solution cn thus e represented y Eq. (6.6) ΔE os * * = β ([ Ac ] + ϕ [ Ac ] ) + γ [ e ] ) Eq. (6.6) ( rel isc rel q rel β = ε nd γ = ε d C 3 * d C ( Ac, 443) (Ac d, 443) where is scling fctor tht gives the rtio of the concentrtion of cridine derivtive to the concentrtion of the hydrted electron. ε is the extinction coefficient of the cridine- (Ac d, 443) derived species t 443 nm. Fitting Eq. (6.6) to the experimentl dt t 443 nm, Eq. (3.27) for the hydrted electron, nd Eq. (6.) for the fluorescence intensities were depicted in Figure 6.7. The rte constnts of the dectivtion process were tken from literture [45]. The est-fit prmeters show tht * ] rel

8 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 66 the triplet nd singlet sttes re ionized with the kinetic constnts of 2.63 x -3 nd 5.9 x -4 cm 2 mj -, respectively. Therefore, the photoioniztion of cridine in lkline wter proceeds vi liner ioniztion process through the singlet nd triplet ited stte chnnel. Figure 6.7. Effect of lser intensity,355 on the normlized fluorescence intensity of Ac * (strs), the concentrtion of the hydrted electron (tringles) reltive to the initil concentrtion of cridine c (2 x -5 M), nd the sorption intensity t 443 nm (squres) in queous solution t ph 2 following 355 nm lser pulse. The triplet stte sorptions were mesured t 443 nm in N 2 O-sturted solution nd multiplied with fctor of 5. The curves were fitted with Eqs. (6.) for the fluorescence of the Ac *, (6.6) for the sornce t 443 nm, nd (3.27) for eq. The est-fit prmeters were κ= 2.63 x -3 cm 2 m J -, κ 2 = 5.9 x -4 cm 2 mj -, k ic τ = 3.25 x -, k isc τ =.369x -, scle fctor for fluorescence 2.3, γ =.32. Constnt prmeters re κ = 6.7 x -2 cm 2 mj -, χ τ =.2, k f τ =.38 x -, d =.4 cm, ε( 3 Ac *,443) = (.9 ±.) x 4 M - cm Photoioniztion of cridine in lkline methnol-wter mixture The photoioniztion of cridine in methnol-wter (:2 v/v) t ph 2 hs een investigted for the first time in this work. The ehviour of the oserved species (Figure 6.8) ws similr to tht in lkline wter t ph 2. The fluorescence quntum yield of the cridine singlet stte vries from solvent to solvent [55], due to the interction etween the Ac * nd the solvent molecules, which results in the electronic chnge of the ited sttes. An dditionl dectivtion pthwy of the ited stte of cridine in hydrogen-donting solvents such s lcohol ws reported previously [56,43]. The ited singlet stte of cridine strcts hydrogen tom from lcohol to produce N-hydrocridine rdicl (cron centered

9 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 67 rdicl) with the rte constnt, k r, of 5 x 7 s - [43]. Thus, this step must e included in the differentil eqution of the singlet stte (Eq. (6.7)). d [ * * Ac ] = κ ( t)[ac] ( k f + kisc + kic + kr + κ 2( t))[ Ac ] Eq. (6.7) d t Figure 6.8. Effect of lser intensity,355 on the fluorescence intensity of Ac * (strs), the sorption intensity t 443 nm mesured in N 2 O-sturted solution (squres), nd the concentrtion of the hydrted electron (tringles) reltive to the initil concentrtion of cridine c (2x -5 M) in methnolwter (:2 v/v) t ph 2 following 355 nm lser pulse. The fluorescence ws normlized nd mesured in rgon-or N 2 O-sturted solution t 43 nm. The sorptions t 443 nm nd the corresponding eqution were multiplied with fctor of 5. The solid lines were fitted with Eqs. (6.) for Ac *, (6.6) for the sornce t 443 nm, nd (3.27) for e ; the est-fit prmeters κ=.6 x -3 cm 2 mj -, κ 2 = 3. x -4 cm 2 mj -, where the dshed lines were fitted with Eqs. (6.) for Ac *, (6.6) for the e sornce t 443 nm, nd (3.27) for (κ, s in Eq.(6.8)) with the est-fit prmeters q κ =.2 x -3 cm 2 mj -, κ 2 = 3.8 x -4 cm 2 mj -, k r τ =3 x -3. The other est-fit prmeters were equl in oth cses nd hd the following vlues: k isc τ =3.9 x -, k ic τ =., scle fctor for fluorescence 2.47, γ =.6. Constnt prmeters in oth cses re κ = 7.5 x -2 cm 2 mj -, k f τ =.5 x -, χ τ =.2, d =.4 cm, ε( 3 Ac *,443) =.9 x 4 ±.M - cm -. q The solution of the differentil equtions corresponding to the photorection of cridine in methnol-wter solution sed on Scheme 3.3 is given y Eqs. (3.5) for the singlet stte, (3.26) for the triplet stte, nd (3.27) for the electron, where κ nd κ re given y Eq. (6.8)

10 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 68 k fτ k iscτ kicτ krτ κ + κ ± κ, = Eq. (6.8) 2 τ τ kicτ krτ 2 kisc τ krτ ( κ + κ + k f + kisc + + ) 4κ ( κ + + ) n our system the mole frction of methnol in wter is.8. Therefore, the quntum yield of the N-hydrocridine rdicl, ϕ r, my e less thn the vlue reported previously. From the est-fit prmeters, k r nd ϕ r were found to e 5 x 5 s - nd 2 x -3, respectively. The kinetic constnt of the photoioniztion of the singlet stte is greter thn tht of the triplet stte, indicting tht the photoioniztion of cridine occurs minly vi the cridine ited singlet stte with smll contriution from the ited triplet stte Cyclic photoioniztion in SDS Owing to the ejection of more thn one electron from the photoioniztion of cridine in queous SDS solution (ph 2) following 355 nm lser ittion, we focused our ttention on the investigtion of photoioniztion mechnism. t should e mentioned tht the cridine rdicl ction in lkline solution deprotontes to give the corresponding cridine neutrl rdicl (Ac ), Re. (6.), which is stle on the timescle of microseconds ( µs) nd hs n sorption nd t 6 nm [4]. + + Ac Ac + H Re. (6.) The deprotontion process occurs over longer time (2 ns) thn the durtion of the lser pulse. However, oth ction rdicl nd neutrl rdicl could hve very similr spectr s suggested for indole nd tryptophn ction nd their neutrl rdicls y Bent nd Hyon [57]. Thus, the deprotontion step my e difficult to oserve. The cridine rdicl (Ac ) cn sor photon to produce n electron nd the cridine ction, Ac +, s displyed in Re. (6.2) Ac + + hv Ac + eq Re. (6.2) t is cler tht the source of the second electron is not the ittion of the cridine species resulting from the deprotontion process for two resons. Firstly, this process tkes plce over much longer time period thn the durtion of the lser pulse. Secondly, the ehviour of the singlet nd the triplet ited sttes is not in line with the liner photoioniztion mechnism, where the concentrtion of oth sttes does not fll ck to zero with n increse in the lser intensity ut reches constnt level indicting the regenertion of oth sttes during photorection within the durtion of the lser pulse (Figure 6.9).

11 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 69 Figure 6.9. Lser intensity dependence of the fluorescence intensity of Ac * (strs), the sorption intensity t 443 nm (squres), nd the concentrtion of the hydrted electron (tringles) reltive to the initil concentrtion of sustrte otined upon 355 nm lser ittion of cridine c (2 x -5 M) in.5 M queous SDS solution t ph 2. The fluorescence ws normlized nd mesured t 42 nm. The electron concentrtions were clculted from sorption signl t 829 nm. The solid lines were clculted sed on Scheme 3.6. The est-fit prmeters were κ = 7.72x -3 cm 2 mj -, κ 2 =. x -3 cm 2 mj -, κ 3 = 3.x -3 cm 2 mj -, k isc τ =.23, k ic τ =.22, scle fctor of fluorescence is 4.626, γ = 6.74x -3. Constnt prmeters were κ = 6.4x -2, k f τ =.58x -, χτ =.2, d =.4 cm, ε( 3 Ac *,443) =.9 x 4 ±.5 M - cm -. Both the sorption intensity nd respective kinetic model were multiplied y fctor of 5. Furthermore, the electron concentrtion t the end of the lser pulse increses continuously with incresing lser intensity nd eeds the initil concentrtion of the sustrte. These oservtions re very strong evidence tht the photoioniztion of cridine in queous solution (ph 2) involves oth singlet nd triplet ited sttes through cyclic photoioniztion mechnism s displyed in Scheme 3.6. Kinetic nlysis of differentil equtions corresponding to Scheme 3.6 will e crried out y numericl integrtion. All dectivtion processes of Ac * were tken from literture [45,47]. The trnsient sorption t 443 nm nd the fluorescence intensity were treted s in the cse of lkline wter system. The fit curves in Figure 6.9 sed on Scheme 3.6 re consistent with experimentl dt.

12 Chpter 6: Photoioniztion of cridine through singlet nd triplet chnnels 7 We ssumed tht the photoioniztion of cridine in lkline wter occurs vi cyclic mechnism s is the cse with SDS solution. Fitting the experimentl dt of the detectle species sed on the modifiction of Scheme 3.6 shows tht the results re lmost similr to tht sed on Scheme 3.3. The differentition etween the two mechnisms is very difficult under the ville experimentl conditions Photoioniztion quntum yields Photoioniztion quntum yields of oth singlet nd triplet sttes of cridine were determined from the est-fit prmeters nd Eq. (3.3) or Eq. (3.6). t ws ssumed tht the extinction coefficients of the singlet-singlet sorption in the three polr solvents used hve pproximtely the sme vlues. The extinction coefficient vlue of Ac* t 355 nm (2 M - cm - ) ws tken from reference [4]. The net photoioniztion quntum yield (ϕ net ) is given y ϕ = ϕ sin + ϕ Eq. (6.9) net ion ion trip ion sin trip where nd re the photoioniztion quntum yields of oth the singlet nd the ϕ ion ϕ ion triplet sttes, respectively. The photoioniztion quntum yields of cridine re listed in Tle 6.2. Tle 6.2. Photoioniztion quntum yields of cridine in different solvents t ph 2 following 355 nm lser ittion. Solvent sin ϕ ion trip ϕ ion net ϕ ε( 3 Ac*, 355) M - cm - ion H 2 O [4] MeOH-H 2 O (:2 v/v) M SDS The dt in Tle 6.2 show tht the photoioniztion of the ited singlet stte is more efficient thn tht of the triplet stte, where the photoioniztion quntum yield of the singlet stte is three times greter thn tht of the triplet stte. Generlly, the photoioniztion quntum yield of cridine in SDS is greter thn tht in wter or methnol-wter solution, owing to the ility of SDS micelles to enhnce the photoioniztion. This hs een discussed in Chpter 5.