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tl~in r ¢ ¢ ¢ i ~ c d ii~c'i'¢:isiil~: ~itlenlion in ,c,.'u'nt %'u':.ll ~, I ~,¢~.'. I o r

ex; in lpIc. R¢l's. I I--3 1 ;.ind rolcrcnl:¢,, ci ted Ihercin ) I~oc'~iii,,c o f Ihc pol.Clllilil i l l ihc~¢ l i let l ia ltl ~lller th¢ pllllll~l~h)~ic~ ;.ind t~hotoehcnlical reac t i~ i t \ o f ,,tlbsli;.l lC,, i c l : t i i x c io Ih~il observed in is l l in lp ic i l iedia I~i.~ \~u'll an uther Ic'ilnOll>, 141 i. 1"he dil'l'u'ru, nl I)'pen i~! t l l '~; i l l i /¢d niet l i l i inx¢~liT;.lll.'d inu'hltte inolccul : i r ¢i'Vnlal~,. ill ic'cllu'~ iilil.'rOelliulnioli.,,, l.~ill~lllUil-- Bh~dTcu t i lnls, z¢oli1¢~ ;.ind h~t -Tuu '~ l nll~iu'u-iil;.ilClllilpl~.,x¢~. Of lkene. the l l lucr, and il; i~ar l icul~i lC) 'c lodexlr in I ( '1) ) inchi- ,+illn ct)mt~lcxt~s. ; I r e i i l l l l ~ l l ~ S l Ihc' IIIOsl c'~lCllSi\'¢k' slt ldietl

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71uc'o~¢ uni ls wh ich pllssc'ss inlu'rn:il hydrol~ht lbic c: lxi t i¢~ ~iblc Itl inchld¢ ;i I~lrTc x ~lriu'lv o1 ~tlCSl nlolu'cule.n in agti¢ou,, st l lu l ions I $ -7 I. CDs ClllllpOSod ill" .nix. seV¢ll +.llld ¢i7h1 {'_Ju- t'line uni ln arc ler i l ied rl-C'D. ~I - ( 'D and ~/-CD r¢.~pu'ctix¢l). Changes hI the photoph)sic.,, and pholochcnl is l r ) ' o f the ~tlcnl molecules i l lchlded il l ( ' D --a','ilien [ I ' l - I I I Inay be due Io several el i¢crs: specific- interact ions hCl,,veen Ihe ~llcnl zlnd

::: (.'4.++lTt:~,pt'qli.!ill~ ;+ttllhol',,.

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l I l t ('1),, I I 2 l: ",pet'itic ph,, ' , , ,u: lcl l , ,m, herr , Con tile. ~llcsi ~allct lh¢ ( ' i ) , , I 1.'~-l-'ti" , ,ci~., i ! t \ Jt, e l .._'Hc,,I , ' i 'o ! l , t~"occnncn I~li the p,.~Im'il.x el lh',." ,.'ux )lt,lllll¢lll I I~ I" I "Ht'~lCk-'Ib,Hl lllilll ,Ulucnchin ~.

b.,, ~,\,. ~.,rl ,~r ~li'~¢r ,.lu~.'n,.'hur', 1 15!: c~m,,li,,ii:I,, imposed by III¢ u':l ~. il~. i',ll lJlu' +..,.',Illol'In:Ilit)ll"+. i~l" lilt; ~tl,..',,t .!~: ~io..'I+II++2 ~. ;.llld

OlI lh¢ Ir;iil+,,hlil)IIiIi dlld i'olii|ioIIiII lllOlil.)Ii% ol i]l~." !~llolOiIl-

duo.',.I mierui,.'dim¢>, I 16.17 I. Such chan,_'cn in proI,<_'i tic,, arc

more pronot,,iced iu ~,olid ('I)u'omI',Icx¢.,. I I,g]. h'i the ,,r, lid ',lille. althtm~zh ,,o,u¢ pro,.z'rcs,, in ,,)'nthcliC pht+t~+chelt~.islr>' I:,t,, b,.'cn ma,t¢ I I~1, c.,,_,. ( ' D ut+lnplcxcs inducin,.z sel¢cti,, it v i,I pht+tou'hclntc:d rcactionn, onlx +.i f,cxx nltldics I 14.15.19- 21 I lut'. L" hccn p¢rlull++¢d tm the excited stut¢ dvnaltliC'S by iJl,, c',li,_'atiOll of'the photoch¢li+ical tlan>,icrlts and their kinetic

I+¢hu; itmi, nl~dnl,, u,,hL,- the tcu'hlib, lt!¢ of diI'ftt'+¢ retlcctancc h,',cr llu,,h ilholoI> ,,i,, I DRI .FP) de:ehIped I+v \\ilkiu~,t>t; ;t;id ut,,.,~orkc,,, 122 2--If. These ~,ttlili¢,, h:ivc llectl rc,,triu!,..~! it+

tmi,uol¢culm" deacti~. +llion o f tr iplet exc i ted nl;.ltCs, or to t+ht~ - louh~.'ll+ic+ll I lxdro~¢ri ah+ln abnlt 'aclion frol l l the ( ' l ) bY ih¢ ¢'cclted ,Ztlest m~+lcculc w i th in the cav i lv . I-i it i lCrto. Ire D R I . F P ,qudi¢s h,lv¢ h¢cn rePt,rled on Pht+tt+chcmicul bhnolceular procc,,s¢,, bclxx c¢11 din~,ilnilar I¢~.t~¢llls inc luded in +l sol id ( ' D pha,,¢.

The format i tm o f hlclt tnion complexes w i th ( 'Ds has I '~cn used to i mprt+', ¢ the c l l i c iency \ i f Pht+toinduced ¢l¢ctrol l tr~Jlls-

2J2 M. .t ' ir c: . l . I J . . r . , d . l ' l ' h . t . r h c . . ~ / r ~ aml I'hot,,I,i,,h<e~ ,.t: ClwmAtrv I I I 119~;7~ 241-247

OCH3 OCH 3 { ~ OCH3 hv I~OCH3

+ PhCH(CH3)NH2

H20 NO= NO

Sc l l emc I.

fer I PET) prc~'esses [ 25-281 mainly by lowering the back electron translL'r rate due to the constraints imposed by the cavity on the photogenerated intermediates. Previously. we reported 129] the totally selective phomreduction of certain nitrophenyl ethers by l-phen.vlethylamine (PhEA) in solid fl-CD ternar) inclusion complexes ( Scheme I ). Mechanistic studies in aqueous solution suggest [ 3(1] that a molecule of

PhEA can photoredt,ce the nitr- group of a nitrophenyl ether molecule to the nitroso group via a process initiated by PET from PhEA to the triplet excited state of the nitroaromatic compound. Therefore this photoreduction process constitutes an interesting example for the study of bimolecular processes. and in particular PET processes, in CD inclusion complexes in the solid state.

In this paper, we report a DRI.FP stndv of the solid binary inclusion complexes of 4-nitmveratrole ( NV ) :rod p-dinitro- benzene ( DNB I with/3-( 'D and tlf their fl-( ' i) solid ternary inclusion c~m~plexes with Pht-A. In the case of NV. ~ve compare the results obtained here with Ih~se reported previ-

,u,,Iv 1311 with differenl amines in aqueous solution ttsir'lg conventional laser Ilash photol.vsis.

tration to that employed for the ternary solid complex NV/ PhEA//3-CD was obtained by mixing the appropriate amount of NV and/~-CI) ~ith a P h E A / ~ C D binary complex in a,I agalc mortar. The binary PhEA//3-CD complex was precip- itated directly from water at roonl temperature after 2 h of magnetic stirring at 7(1 °C. These mechanical mixtures were also dried in vacuum at 411 °C for 4 h.

Time-resolved DRLFP experiments were curried out by exciting samples with frequency-tripled ( 354.7 nm. 10 nU ) or frequency-quadrupled (266 nm. 22 m JI pulses from an Nd-YAG laser (HY2IX). l.umonies ) which had ) pulse width of 8 ns. A 275 W xenon lamp was used an the monitoring source, and an R928 Hemanmtsu photomultiplier was used as the detector. Transient data were acquired by a Tektronix 2432A digital o,,cilloscope interfaced to a microct,npuler. The details of this system :rod data treatment can be found else~shere 124 I. The parameter used to quamify the tran.sient diffuse relict)ante spectra was AJ/.I . . the fractional change in the diffuse toilet)ante, where J . is the intensity of the diffuse reflected anah'sin,,, e li~ht~ scattered by the sample before liring of the laser pulse.

Unless otherwise indicated, all the measurements were per- formed z,! room temperature with air-equilibrated samples. Samples were shaken between h, ser shots to eqsure that a fresh surface was exposed each time.

(;round state diffuse reflectance absorption spectra were recorded on a Phillips PU88(XI UV- visible spectrophoto- meter equipped ~ ith .m integrating sphere.

2. Experimental secti .n

/J-CD. cellulose ( po~vder. 2111 JUra ). silica ( grade 6:3.21)(I- 425 mesh. 60 A 1.4-nitroanisole ( NA 1. NV. DNB anti PhEA. supplied b,~: Aldrich. ~ere used a.,, recci~ed. NA. NV and DNB have the structures shtlwn in Scheme 2. l)i,,tilled ~ ater was used to prepare the solutions.

Solid CD complexes were prepared by di,,soh'ing a certain amount ~;1 the compound in 0.(il I M ~.-CI) aqueous solulion with magnetic stirring at 71) ':(" for 4 h. After evaporation t}l water in ~acuo with a mta~' evaporator, the solid obtained was dried in vacuum at 40 ~(" lot 4 h. Sample h~ading', arc given in the text as molar ratios.

Mechanical mixtures of similar concentration to I]lat employed for the binar,, ,,olid complex I)NB//3-( 'D were obtained by mixing ihe appropriate amt~tint of DNB with cellulose ~)r silica. A mechanical mixture tfl" similar concert-

OCH 3 OCH3 NO z

NV NA DNB Scheme 2.

3. Results

It is kno~,sn that nitro)ram)tics 132.331 and PhEA 1341 Is~rm inclusion complexes with CI)s it) aqueous solution. Prexiou,, re,,ul:. 129.351 suggest that all the compounds clnph)yed ill the present ~,~ork Iornl inclusion complexes in atluct~us media and in the solid state.

3. I. ltimtrv romph'.w.~

Irradiation at 355 nm of air-equilil~raled solid hinaD' N V / /3-('I) complexes ill the molar ratio 0.04 : I.(X) yields a tran- nietll with an absorption Inaxinlunl at 4~O nln ( see Fig. I (a) I~r the spectrum and Fig. I ( b ) for the decay trace). Thin transient absorption rcseinbles the triplet state of NV in non- hydrt~gcn-bonding soh ents [ 35 ], xvhere Ihe peak of Ihe tran- sienl ahnorplion occurs, for example, at 4l~(1 nm in acetonitrile solulion.

Irradiation at 355 nm of air-equilibrated solid binary N A / ~-( 'D complexes in ltle nlolar ratio 0.07 : I.(X) yields no detectable transient absorption.

Irradiation at 266 nm of air-equilibrated solid binary DNB/ /3-CD con~ ":exes in the molar nitio 0.03 : I.(X) yields a tran- sient with an absorption maximum at less than 390 nm ( see Fig. 2). It was not possible to record the spectra below 390 nm because the analysing light caused photodegradation of

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Fi.~. I. ( a I "] 'r ; l l l , , i¢lll . . ib~.orrl l i~n . ,pc t ' l rUl l l I t)r hillar.~ N V / / 3 - ( . ' 1 ) . , t) l id i l l t ' l t l-

,,it),l c t ) m p l c x t ~ h l a i n c d ' . , , i lh in 1 7 its, a l t e r t h e l a ' , c r p u l . , e a l 3 5 5 m u . l b) Tran,,ie,'~t decay I ra te mt,nilt~red al 4S0 , .n l~Jr binar'. N V / ~ - ( ' D .,olid hl t ' h l~ , ion ~.'olllpl¢'~.

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• ! . | • i ~ i 400 ~ ~ 0 6 0 o

W a v e l e n g t h t nm

Fig. 2. Tran',icut :,b:,orptton ~,pectrum lbr bimtr~ DNB//:I-('D .,olid inclu.,iun complex obtained within 2t)() its after the la.,,cr pulr, e :l1266 ran. A cul-olT liher ~,,a., w, ed to pre,.ent phtmvdegradalitm t~l" the : , , ample alltl ',,t) dat.t cnl- leelitm helow 3911 nm wa,~ nt)l i~t)xxible.

the sample. For this row, on. a cut-otf tiher with zero trans- mittance below 390 nm was interposed between the arc lamp and the sample. The same transient with a lower signal inten- sity was obtained on photoexcitation at 355 nm.

it is knov.n thai lhe triplet ,,rate oIp-nitroacett~phenonecan abstract hydrogen atoms l'r(m~ CDs ~s ithin the inclusion com- plex tm UV irradiation in aqncou.,, .xolution [361. Nitroaro- matic conlpoundx sub,,tituted with electron-~;thdrawing groups, such as DNB and p-nitroacetophenone, where the lowest lying triplet state is of n.,~ ~ character [37--1,1 I. have a much higher tendency to abstract hydrogen atoms than nitroaromatics with electron-donating substitue,,ts, v+here the lowest lying triplet state is of rr.~"" character 137--41 I. More- over. nitroaromatic comramnds without electron-donating groups, having lowest lying triplet states of n.,, -= character. show very .~hort lifetimes, in the range of picoseconds in .~olution 142 I. In this work. ro transient wa.s observed when the binary complex of NA with ~ET-CD was subjected to la.~er

excitation. NA is a nitroaromatic with n~re tendency than DNB to have a l~-;,,cst lying triplet of "m,'n "~: character and. therelbre, with a triplet .,,late expeclcd to have a sli~htlv longer lifetime 1431 than that of DNB but ~,~'ith le.,,s tendency to abstract hydn~gen atoms ! 37-41 I- This. suggest.,, that the tran- sient observed when DNB/CD santplcs are subjected to

DRLFP may be due to the radical formed ,,ia h~drogen abstraction from /3-CD follo,,~,ing excitation of the nitro group.

This assign,,cnt is supported by the tact that nano~-cond laser llash photolysis l excitatio,! of DNB at 206 nm in 2- propanol) leads to a transient with a maximum around 360

nm. This transient has previously been identitied as the con- jugatc acid of the DNB radical anion using the technique of phtm~chemically induced dynamic nuclear polarization t pho;o-CIDNP) 1391. Therefore the obsen,'ed A ...... of the transient from the solid DNB/~L-~-CD complex is consistent ~ith our assignment that this absorption is due to the proton- ated radical anion formed throt,gh hydrogen abstraction by

the triplet state of DNB. In order to conlirm this assignment of the transient

observed with the DNB/~-CD complex. DRLFP studies of mechanical mixtures of DNB with cellulose and silica were perfornled, the former having a filr greater capacity to act as a hydrogen atom donor 1441. With cellulose, the same tran- sient as obtained with the ~I-CD ct)mplexes was obwerved. With silica, no transient absorption was obtained in the 390- 7(X) nm range following DRLFP. irrespective of whether the sample was aerated or deaemted. These results suggest that the lonttation of this transient is only possible if triplet DNB molecules can abstract hydrogen atoms from a suitable donor. and therefore v,'e assign this transient to the protonated radical anion. The fact that the triplet of DNB could not be detected from a deaerated silica sample on the nanosecond timescale is presumably due to the short triplet lifetimes of nitroaro- mattes without electron-donating grot, ps [42.431. It is worth noting that a deaerated sample of a mechanical mixture of silica and NV. which contains electron-donating groups. showed a transient signal attributable to the triplet state of NV at 480 nm on photocxcitation with the laser at 355 nm.

In order to support hJrther the assignments of these tran- sients as the triplet state for NV arid the protonated radical

( l + "~ +~ 7 244 .Tl. ,If," ,'l al. I . h , , , m d +q'l 'h,m, 'h~,m~trs ,rod I'lu,t,,hi,dok,.~ A. ( 'hemi qrv I I I I I ~ 1/) _ 4 1 - _ 4 ,

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T i m e (ps) Fig. 3. "l'r;m,,icnl tlccax tr:~c¢,, in tmi lorcd a1-1~1 s in for: I I ) binarx NV/ /3 -

( ' D ,,,~lid in¢lu,,iot+ o ,mpl¢ ' , . : t 2 ,~ tcrn;:L~ NV/ , ,u rb l¢ , t¢id/ /~ ( ' ! ) ,.t~lid i rc lu-

,,ion c , ,mplcx : (:~) tem:,r ' , N V / I ) h l - A / / J - ( ' l ) ,,olid Illclu,,ion complc+,. All the2 ,..ample,, ha+,¢ Ih¢ .,.a111¢ cit+llC¢lllralitlll +.~1 ~ V .

anion for DNB. quenching experiments v, i lh sorbic acid xverc performed. Sorbic acid in a conjugated dicne which in an excellent quencher of triplet states through energy transfer 145 I- It has been used i 451 an a quencher for ~,ub~,trates with triplet energies of about 60 kcal tool t and nitrobenzenc.s have similar triplet energies, 142.4(31. It ha.', bceu reported that pt)la,,sium s,)rbat¢ acts an a quencher of II1:: triplet slates O f nitrophcnyl ethers 147.48 I. Sorbic acid ha,, also been used [49] an a quencher in the presence of CDs in aqtteous st)lu- riot. Moreover. it in known 1501 that potassiun) sorbatc in included in the CD cavity in aqueous solution and. presum- ably. its neutral analogue will have a higher tendency Iobc included in the lens polar envirtmment of the CD cavity. Thercfl)re sorbic acid was used in attempts to quench triplet excited nitrobenzencn.

Excitation at 355 nm ol'a solid ternary ctlmplcx (11" NV and sorbic acid wi th /J -CD in the molar ratio I).(14 • 11.64" I.tJl) leads to the same transient an obtained with the binary com- plex of NV. with similar decay kinetics, but with a lower initial transient signal ( Fig. 3 ~, consistent with static quench- ing occurring within the duration of the laser pulse for the NV molecules in clone proximity to ,,orbit acid molecules. Thin result is con,,istent with the assignment of the transient observed an either the triplet state of NV or a transient formed f r o m t h e N V triplet state.

Excitation at 266 mn of a solid ternary complex of DNB and sorbic acid with/~-CD in the mohtr ratio().03 • 0.70 • I.(~) does not ~ead to any transient in the 39(P-7(10 nm range: however, sorbic acid absorbs at 266 nm. Excitation at 355 nm of the same complex leads to the same transient as

obtained with the binary complex of DNB with a similar transient intensity and decay. The fact that the transient was not affected by the triplet quencher is consistent with the assignment of the tran:,ient observed an the protonated radical anion o f D N B , although it in interesting to note that sorbic acid does not affect the hydrogen atom ab:,tractioa reaction which suggests that this reaction is re.,2' f-'.:st indeed.

The decay trace of the binary N V//J-( ' i ) ctunplcx transient. expressed an ~. ! / . I o. does not follow a simple kinetic law. it can be litted as two) mtmoexp,,n,'ntials, yielding lifetimes of 51) pus and 1.8 nts respectively for the short- and long-lived components. The kinetic trace in FL,-z'. II b) was obtained by a combination of four individual decays recorded using dif- !':rent timescales. Experiments with variable laser lluence do not show signilicant changes in the shape of the decay trace. suggesting that triplet-triplet annihilation does nnt conlributc to the triplet decay. The same lransient decay was observed when the sample was degassed prior to excitation, prestm+a- bly due It) the protection offered by the ( 'D to any oxygen quenching uf the triplcl.s included in the CD in the .,olid state I 14.15.19-211. It shonld bc noted that the decay of the NV triplet in the /3-CD inclusion complex in the solid stale ix much slower than in .~olution. where its lifctimc in reported It'+ vary between 1).2 and 2.2 p.n depending on the solvent 131.351. Enhanced triplet lifetimes have been ob..,erved for other suhstrates adsorbed on solid supl~tn'tn or included x,, ilhin ,,olid supports: for example, the triplet lifetime of/3-phcnyl- prt~piophenone ix increased hy l ive orders of magnitude in

the channels of sodalile 151.52 I- The decay trace at 3(+1) nm for the binary I)NB//3-CD

complex show,, that the tnmsien', i,, very long lixcd (not . , , h o w fl ).

3.2. T e r m , v < +mu~h'.v('.+

Excitation al 355 nm of a solid Icrn,wy complex of NV and PhEA w i t h / 3 -CD m the molar n l l io I).()4 : 0.35 : 1.0() yields

a transient absorption with a tuaxir.mm at 4811 nn) ( Fig. 4) monitt)ring tnl short tinlescales. At longer timescales, the transient absorption shifts It'+ oiv¢ a rnaxinlunl absorption ill 451) ntu ( Fig. 4 ). Thin second transient was not observed with binary N V //J-C D complexes. The ft)rmcr transient resembles the triplet state of NV observed pre~ iou,,ly in the binary NV/ /J-CI) complex. The absorption spectra of the triplet of NV and the radical anion of NV. which is the lirst intermediate

01"5-

0 2 0 .

0 1 S •

< 3 0 . t O -

0.405, ~ ~ 2

0.aO

Wavelength I nm

Fig. 4. Tranniem ab',orpti<m spectra for remuS NV/PhI -A / /J - ( 'D ~.;dkl inclu- sion ¢nmpl,'x obtained wJlh, il) 1.7 p.~, ( I ) alld 14() ItS (2) al'ler the laser pulse at 355 nm.

M. .1,lit rt a l . / Jmu'md ++f l>hotochemi.stry am/I'lmt,l,i+,h,~,~ A: ('hrmi.~trv I I I ¢/997~ 241 247 245

0 1 2 -

0.10,

0 ~ .

<3 0.06.

0.04

002 | i ! - - - i - i .~o 600 700

Wavelength I n m

Fig. 5. "[ram, icnt ab , ,o rp l ion Sl~.'clrum f - r t c n u u 3 D N B / P h F . A / ~ - C D • o l i d

illCltl~iOIl ¢Ol|llqe.x o h l i l i n c d x ~,ilhitl 8~1 s " • s . t t. a l t e r the la c r pul. c a l 2 6 6 rim. A

c u t - o f f l i l l e r w a s used Io p r e v e u t p h o t o d e g r a d a i i o n o f Ihe ~,ampl¢ a n d •o d a l a

c o l l e c t i o n b e l o w 3tJO t im xx;.lS ilqUl pos~,tble.

in the photoreduction mechanism 1301. have a similar shape and position in non-hydrogen-bonding solv,:nts 1531. In order to clarify whether this transient is the NV triplet or the NV radical anion, we monitored the transient decay at 46t) nm of an NV//3-CD binary complex and an NV/PhEA//3- CD ternary complex with equal NV loading. These transient decays are shown in Fig. 3. A higher transient absorption intensity in the ternary complex suggests that another tran- sient has been formed, which has a higher absorption coef- licient and a similar absorption spectrum to that of the NV triplet. This new transient may be the NV radical anion. The transient observed on longer timescales is more difficult tO assign, and may be either a transient produced durin,,= the photorcduction process or a supeqr'a)sition of sev+ral transients, inchtding Ihe radical anion and the triplet.

A solid mechanical mixture of NV, j3-CD and the PhEA/ fl-CD binary con'|plex, with similar concentrations as those employed for the ternary NV/PhEA/fl-CD complex, shows a transient assigned to the NV triplet state, but no other transient is observed at longer limes.

Photoexcimtion at 266 nm of the ternary sample of DNB and PhEA with ~-CD in the molar ratio 0.03:0.52" 1.00 leads to a transient absorption formed within the laser pulse dur'ttion. The spectrum shows two absorption maxima around 41<) and above 700 nm (Fig. 5). This transient is clearly different from that observed in the binary DNB//8-CD com- plex. This long-lived transient absorption is assigned to the DNB rudical union on the busis of literature data 1541.

4. Discussion

It is known that CDs can act as hydrogen atom donors lor triplet states of carbonyl compounds in solid complexes [ 14,201. The re:;ults presented here show that fl-CD can also act as a hydrogen atom donor for some nitroaromatic triplets (with low-lying triplet states of n, rr* character) 137--41 ] in

I hv ~ sotid state

CH3

Scheme 3.

solid complexes. A similar conclusion ,.,.'as reached in uquc- otis solution 136 I-

PET l'rom PhEA to excited NV and DNB is observed in the terna~' complexes, presumably because fi-CD inclusion comp!exes generally crystallize with a channel-like structure 1551. in such tubular structures, contact between the two reactants is possible, enabling electron transfer to occur on photocxcitation ( see Scheme 3 for ONB ).

No time-resolved information regarding the rate of the PET process could be obtained due to the fact that. at least in the DN B case. this process occurs within the duration of the laser pulse, in the NV case. the overlapping [ 531 of the triplet and radical anion absorption spectra makes it difficult to abstract infornmtion about the kinetic behaviour.

The fact that the PET process does not occur in a mechan- ical mixture of NV+ 13-CD and P h E A / ~ C D with similar concentrations as those employ,.'d for the ternary NV/PhEA/ 13-CD complex suggests that the photochemical process observed in the ternary complex only occurs if preorganiza- tion of the reactants takes place during the formation of the ternary complex. Tbe driving force for this self-organization is probably the fornmtion of hydrogen-bond interactions between the nitro and amino groups. The existence of inter- molecuhtr hydrogen-bond interact kms between nitroaromatic compot, nds and amines has been reported [56.571 in the literature. It has also been rcportcd[ 58 ] that solid molecular compl-xes between nitroaromatic compounds and amines arc fi')rmed which, on irradiation, give mixtures of several photored,aclion prtxtucts.

The fact that the radical anion of DNB is observed imme- diately foilo,,ving the laser pulse for the ternary DNB/PhEA/ /3-CD complex means that the PET process occurs in less than 8 ns. This result further supports the existence of preor- ganization of the reactants in such inclusion complexes. The fact that. in such ternary complexes, the PET process can be competitive with hydrogen atom abstractien from the ~ C D cavity provides further support for the formation of hydrogen- bond interactions between the nitro and amino groups.

• _ 4 1 - _ 4 / 240, .tl. Mir et ul. / . h m r , , I o /Pl , , ro~ I,,mi.~trv , m l Photoh io loey A." ('hemi.~trv I I I ( I ~)97~ ~ ~ "

because the lormation of hydrogen hond~, stabilizes the rr. ~.:: triplet states of nitroaromatic cot t~potmds 1431 and. as pointed out above. 7r.t#' triplet staten have little tendency to abstract hydrogen atoms [ 37-41 I.

Although PET processes ha~e been studied ;n solid CD complexes by time-resolved electnm spin resonance ( ESR ) ! 28 i. as faras we know. this is the lirst time that the technique of DRLFP has been applied to the study of such processes.

It is interesting to note the remarkaole differences in the photobehaviot,," observed for NV in the presence of amines in solid ternary complexes and in aqueous solution. In aque- ous solution [ 31 !. in addition to the formation of photosub- stttution products during NV triplet decay, an ensemble of ~r complexes with broad absorption in the 4(~)-530 nm range and relatively large lifetimes is observed by conventional laser llash photolysis. Moreover. with primary amines ( meth- vlamine or n-hexylamine), the NV radical anion is not observed directly and its existence is infe,ted through indirect methods. (Prelimina~ laser Ilash photolysis experiments 1351 of the photoreaction ,-f NV with PhEA in aqueous solution show that the photobehaviour is analogous to thai previously reported 1311 for methylamine and n-hcxylam- inc.) By contrast, in the investigalion of the solid ternary NV/PhEA//3-CD complex by DRLFP. only the photo- reduction pn)cess is observed, i.e. Ihe h~rmation of the radical anion and photoreduction intermcdiales, without interference from other competitive processes such as photosubstitution and the fl~rmalion of rr complexes, despile the fact that pho- tosubstitulion products by PhEA are obtained in aqueous solution 1291. This highly selective process in the ternary complexes supports the existence of real inclusion complexes in the solid state in which the selectivity is the result of the constraints imposed by the CD cavity on the included reagents.

In summary, the results presented here show that. provided that interaction between the reactants is possible in solid CD inclusion complexes, the study of such complexes by DRLFP is valuable in the investigation of bimolecular photoinduced reactions, especially when they are difficult to study in solu- tion due to the existence of other competitive processes.

Ackno~'ledgemenLs

Financial support from the DGICYT I Ministerio de Edu- caci6n y Ciencia of Spain ). through project PB93-0895. and from the Generalitat de Catalunya. through project 1995SGR00469. is gratefully acknowledged. We also thank DGICYT for a postdoctoral grant to M.M.

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