Lattice Phonon Mediation of Solid State Photorcactions in Organic Crystals

REVJEl^

Indian J. Phys. 67B (4), 293— 312 (1993)

L a ttic e p h o n o n m e d ia tio n o f solid s ta te p h o to r e a c tio n s in o rg a n ic c ry s ta ls

Subhasis Chakrabarii, Manash Ghosh and T N Misra

Dcpanineni ol Speciroscopy, Indian A<;Mx:ialion lor die Culuvaiion of Science, Jadavjxjr, ('alcuiu-700 032, India

Rt'CciveiilO May 1993, accepted I6June 1993

Abstract : J^hysical or chemical change in u crystal lallicc maridcsls ilsdf as a change in its phonon spccirum leaser Kaman spectroscopic icchniquc is being successfully used lo study crysiallinc slate photorcaclioris in organic malcrials The mam thrust of such siudy has been lo iiivcstigaic the role ol phonons in such reactions.

Idcciroiiic speciroscopy has generally been used to deicrminc ihc extent ol rcaciion progress and also lo dciemmic the nature of the pholoproeesscs involved iniriired iuid Raman spc.ciroseopy have been used lo characicn/c die reactants and the products Raman phonon S[>ectrosu)pic study has been used lo iwesiigatc whether the reaction occurs by homogeneous nieihanism w'here the product lorm solid solution m the reactant lattice or by heterogeneous mechanism where the reatlaiil and the product form separate lattices

Phonon panicipation in such reactions are shown to occur through strong cxuioii phonon coupling or through a mode softening In some reactions, however lattice phonons do not influence reactivity and lopocheinical control is sufficient lo explain crystalline stale reactivity.

Keywords : l^sei Raman spectroscopy, solid slate phoioreaction, exciion phonon coupling, phonon mode solicning

I’ACS Nos. : 33 20 Fb. 78.30 Jw, 82 5 0 -m

Plan of the article 1. Introduction

2. Theoretical back};round

A. Plionon : delocalisation and relaxation . i) Delocalisation

ii) Relaxation

B. Exciion-phonon interaction

C. Concept of reaction cavity

D. Photoinduced lattice instability

3- Apparatus and measurements

294 Subhasis Chakraharti. Manash Ghosh and TN Misra

4. Specific systems

A . Dimerization:

i) p-chloro cinnamic ac 'ui ii) 2,6 Dimethylp-henzoguinone Hi) 7-Methoxy coumarin

B. Polymerization :

i) Dimethyl ester of a, a ' dicyano p-phenylene diacrylic acid ii) Di n-propyl ester of a ,a ' dicyano p-phenylene diacrylic acid III) Diethyl ester of fh-phenylene diacrylic acid

5. Conclusion

1. Introduction

The reacLiviiy o f organic m aterials in the crystalline suuc when pholoirradiated, depends on molcLular packing arrangement in the lattice. T his requires special attention to the role o f phonons in such reactivity. The study o f phonons p rovid e inform ation about the dynam ic aspects o f such reactions. In this essen tially experim ent oriented article phonon participation in solid state photoreaction is discussed. The discussion on the theoretical background is kept limited but sufficient to be useful for understanding o f the process.

The resca'rch on solid state photoreactions has gam ed appreciable attention only after the pion eerin g w ork o f Sch m id t and his co-w orkers 11,2] w h o in v estig a ted the role o f molecular packing in solid stale reactions. A ccording to Schm idt the reactivity and m olecular structures o f the products depend on separation distance and mutual orientation o f potentially reactive functional groups. This led to what is known as "topochcmical principle”. A ccording to this principle the centre to centre distance between the reactive groups is to be less than 4.2 A for the reaction to occur. H ow ever, this upper lim it is not very restrictive b ecau se o f nonavailability o f data in the range 4.2 - 4.7

A

Indeed in m any c a se s such as Lrans c in n a m ic a cid s, b e n z o q u in o n e s , th ym in e, or - benzylidine-y-butyrolaclone, 2 -b c n z y l-5 -b e n z y lid c n e clyclopentanone, 3 -m e th y l^ -n itr o 5” Styrylisoxazole and many other com pounds the potentially reactive groups are separated by a critical distance and the reacting double bonds arc parallel. P hotodim erization o f 2 -b c n z y l 5 -h cn zy lid cn e cy clo p cn ta n o n c and p h oto p o ly m eriza tio n o f d ia c c ty le n c d e se r v e s special menuon. In thc.se ca.ses structural perfection is very m uch sim ilar to that o f m onom er crystal and the reactant and product form solid .solution over the entire conversion range [4J.

In a large num ber o f so lid state photorciictions apparent v io la tio n o f topochcmical principle is also observed. In p -fo r m y l cin n am ic acid, a l()0% yield o f dim er is obtained although the double bonds have center-center distance greater thari 4 .8 A [5J. S o m e reactions have been reported w here reacting double bonds are n onparallel [6 ,7 ,8 ]. In 7 -m c th o x y

Lattice phonon mediation of solid state phoioreactions etc 295 coumarin reactive double bonds arc rotated with respect to on e anotlier by 65^^ [9J. A gain in 2 -b e n z y lid e n c cyclopentanone though the reacting double bonds are at a distance less than 4 .2 A , no photorcaction occu rs on prolonged irradiation flO ]. T h ese results su g g est that lopochem ical principle is insufficient to account for many solid state reactions. T he m otion o f phonons plays an important role in the photorcactions in these crystals. A ttem pts have been made to explain solid state p hotoreaclivay by considering the orbital overlap rather than parallel double bond separation 110,11 ].

2. Theoretical background

A Phonon-delocalisation and relaxation :

Phonon m odes onginate due to rotational and translational oscillations o f the m olecu les m the lattice. In organic crystals lattice vibrational frequencies arc w ell separated from the internal molcculiir m odes due to the fact that in such cry stills the interm olecular forces are 1 0 -1 0 0 Limes weaker than the inu-amolecular forces. The lattice vibrational frequencies in the organic crystals generally lie b elow 2 0 0 cm '. Internal vibrations arc generally at higher frequency, though som e out o f plane d eform ation and torsional m od es arc a lso a ctiv e in the low (requcncy dom ain. A s any ph ysical change alters the phonon spectrum , solid state phase separation or the ch an ge due to solid state photorcaction can co n v en ien tly be studied by plionon sp ectroscop ic m ethod. In order to characterize the nature o f phonon m o tio n s in organic crystals tw o as|X'cis arc studied : (i) d elocalization o f phonons and (n) relaxation of phonons.

fi) ■

Delocalization

111 a real crystal with im purities or in a m ixed crystal the phonon frequency is no longer precisely defined as in a pure crystal.

If the d illeren cc betw een tlic phonon frequency o f host pure crystal and impurity pure crysuil is sm aller than the phonon band w idlh o f the host, am algam ation o f phonon results 112 151. In this lim it the phonon freq u en cies sh o w a m on oton ic sh ift w ith increasing im[)uniy concentralion. H ie reaction m echanism in the solid state is said to be h om ogen eou s when the product form so lid so lu tio n w ith the reactant in the m o lecu la r le v e l. Such a mechanism has been su ggested for solid state photopolym erization o f several d ia cety len es

Wlicn the perturbation caused by the impurity is large, lo ca lised im purity bands a lso appear alon g with pure host bands. The impurity m odes m ay be lo ca lised high frequency m odes, gap m o d es or p seu d o lo ca lised m o d es 11 9 ,2 0 |. In so lid r^ate p h otorcaction this silLiaiion g iv e s seg reg a ted phonon specira and the reaction m ech an ism is said to be heierogeneous. Such m echanism has been established for solid state photopolym erizaiion o f

1,4 bis l/3‘-*pyridyl (2) vinyl] benzene [2 1 J.

296 SMiasis Chakrabarti, Manash Ghosh and T N Misra

In som e ca ses rcacuon m echanism is hom ogcnct)us in the initial stages and b eco m es heterogeneous as product concentration increases [22,23].

Ib c theory is based mainly on the work o f E lliott e l a l [24], T h ey have used Green's function to solve tlic problem.

For a pure host crysUil, Green's function G is found to satisfy the fo llo w in g matrix equation

(

1

where the m ass c o c lfic ic n i and ({f^ is the force con stan t for the pure crystal and the (ireen's function is defined as

2n (2),

(3)

(4)

When* Oil i') IS the H eaviside unit step function. H ence, introducing the perturbation matrix V^(m) due to both m ass and force constant change

V ( ( o ) - + {<!>- ⁰ ") >

we obUim D yson equation

G - G'U ,

which may be expanded as a jxiwcr series

G = + G^'VG^^ + G^^VG^^VG^ -h - (5)

This expansion is tiivially sum med exactly to give

G = G^TG^ (6)

where 7 matrix is defined by

T = V (I-G^^V)^ . (7)

Where Iis the unit matrix.

In die am algam ation lim it, the p o les o f G corresponds to the p o le s o f G^, sh ifted in Irequericy On the other hand there w ill be new p oles o f

G^^

Green's I unction can be calculated using different approxim ation m eth od s based on configurational averaging. Configurationally averaged <G > can be represented as

<G > = G^ + G^^I<G > , (8)

where L is the s e lf energy.

There are m ainly three approxim ation m ethods for obtaining the appropriate Green's fu n ctio n :

Laltice phonon mediation of solid state phoioreactions cic 297

(i) Virtual crystal approxim ation : T his w as first used in electronic problem by N ordhcim [25]. T h is approxim ation is applied w hen the perturbations

i.c

T

matrix approxim ation : T his is applied w hen the perturbation is large and l(K ali/cd states apjx^ar L26-30]. Here, the self energy is localised and the com plex has a [X)lc near the ix)lc o f

T

T

(li) Relaxation .

The study o f anhiu-momcity o f vibration in crystal reveals important temperature e ffe c t and alre^idy a number o f '.soft' m odes ol vibrations vdiosc frequencies depend upon temperature have been lound in con n ection w ith phase transition study. In m ost o f the ord er-order transitions, the structural distortions arc characterised by an unstable or soft m ode w h o se Ireqiiency g o e s to w a id s zero value as the tem perature approaches the phase transition lempcrature. The .soli m odes arc usually damped. Structural phase transitions arc generally accom panied by large am plitude d isp lacem en t sin ce the restoring force w h ich o p p o se s disblaccm cnt g o e s to zero as the m ode soften s. In solid state ph otorcaction s this large amplitude displacem ent can bring the nu)lecules in favourable configuration for the reaction to occur. Apart from different phase transition studies thermal rearrangem ent rciiciion o f m e t h y l a m m o benzene sulphonate [35], non-tliermal B - A conforrnauon change in D N A double helix [36], solid state photorcactions in ortho-m ethoxy trans cinnam ic acid [37] and 7 niethoxy coumarin [38] arc the cases where ukhIcsoftening is observed.

Eysier and Prohofsky [36] have used the nonbonded interaction m odel to deduce the theory o f non-ihcrm al m ode softening. In this mcxlel, it is a.ssumed that there is no change in die strong chem ical bonds.

The nonbonded interaction term s can be w m ic n as the sum o f pair interactions bctwe^cn every pair o f atom s not bonded to each other by stronger bonds.

The total potential energy for the solvent free case, is given by Vn/. = X X X

n m 7. /

where U, IS ihc fam iliar van dcr Wiials 6 - 12 function and V^'2 is the elcclrosm tic icnn.

i

298 Suhhasts Chakraharti, Manash Ghosh and T N Misra

Expanding about equilibrium |X)siiion, w e oblain

' n h

1 V V

//

- — ^ ^

where is a st|uare diagonal matrix ol harmonic force consuints o f dim ension A/^ and is a colum n vector o f d im en sion w h o se elem en ts arc sm all c h a n g es in the n on b on d ed disuinces. The nonbonded forces can affect the lo w est freq u en cies o n ly sin ce they arc too wcitk to affect the high frequencies. Then our task is to diagonalise the follow in g matrix

E’i i r + Ao, (11)

where A’oand /loiirc the matrices o f unperturbed eigen vectors and eigen values respectively and I I ' is the lorce constant perturbation matrix ex p ressed in m a ss-w eig h ted C artesian coordinates lor various v a lu es o f d ielectric param eters a ssig n ed to the van der W aalfi interaction and elcctrosuitic interaction. W ith change in these param eters o n e o f the lo w e s \ frequency nuxles g o es to zero value i.e the m ode softens.

B Excilon-phonon interaction

The concept o f phonon assisted reaction can a lso be extended to p h otoch em ical aggregation reaction. In such reaction a strong cx citon -p h on on coupling m ay result in the deform ation o f the lattice near the excited m olecule. This deform ation can localise excitation. E’unherm orc, a local conform ation which is precursor to the photodim er lattice is created by strong e x c ito n - phonon coupling. This ty|x^ o f reaction is different from defec t initiated photoreaction.

The m anifestation o f strong ex cilo n -p h o n o n coupling in electronic sp ccu 'a ls the lack o f fine structure and large Stokes' shift b etw een the absorption and e m issio n m axim a or appearance o f phonon side bands and line broadening.

E xciton -p h on on co u p lin g m m olecu lar cry sta ls and m ix ed c r y sta ls h a v e been discussed by Hochstrasscr and Prasad [3 9 ,4 0 |. T his interaction arises from the variation o f cxciuition exchange interaction and the change in van dcr W aals interaction with the variation o f spatial and orientational ccx)rdinatcs o f the m olecu les in the nonrigid lattice.

The exciton-phonon interaction for m olecular crystal under adiabatic approxim ation can be written as

= H M , ) + H^{R) + H^,{R) (12)

Here, R represents the spatial and orientational coordinates o f the m o lecu les and (/?o) the cxciton Hamiltonian in the rigid lattice.

Hi^^(R) depends on excitation exchange interaction betw een the pairs o f m o lecu les and describes the inelastic and clastic scattering ol cx c ilo n s o f different w a v e vectors creating a phonon and con servin g total w a v e vector. 1'his results in the cx cito n dam ping creating line broadening o f Lransiiions from ground slate to the cxciton levels.

Lattice phonon mediation of solid state photoreactions etc 299

is a function o f the total change in van dcr Waals interaction betw een all pairs o f m olecules when one m olecule o f the pair is electronically excited. T his results in the recoil o f m olecule in lattice normal coordinate space during electronic transition. The recoil e x c e ss energy is in' the form o f e x c e s s potential energy w h ose dissipation corresponds to lattice relaxation and its m agnitude contributes to the "Franck-C ondon" factors for phonon sidebands.

When

Hp(R) » Hf^ (R)i\\c

i,e,

( R ) » H p ( R), i hc stationary states arc those o f delocalised cxcitons.

A coustic and optical phonon behave in dilTercnt w ays for cxciton-phonon coupling. In general, in a real m olecular crystal, the peaks in phonon side bands do not necessarily have to show any correspondence to the peaks in the Raman jq^ectrum. H ow ever, the electronic excitation coupling is maximum lor zone center optical phonons. It is therefore, not unlikely Uiat .somc tx:aks in phonon side bands have sam e Irequcncics as the Raman phonon bands.

A lso for large den sity o f states and sm all dispersion o f optical phonons, the phonon sid e bands ficquencies may corresponds to the peaks in the Raman specyum .

The lattice participation in the radiativepnx;css in the crysLil demands lhatab.sorption and em ission freq u en cies arc not sam e in contrast to the free m olecular case. T he lattice contribution m ainly originates from the ch an ge o f norm al coord in ates in tw o electro n ic siaies. T h is freq u en cy sh ift b etw een electro n ic absorption and e m issio n is ca lled the Siokcs' shift. Large Stokes' shift, therefore, is an indication o f strong cx c ito n -p h o n o n coupling.

('

Concept of reaction cavity :

TopcK'hemically controlled solid state photoreactions involve m inim um atom ic or m olecular m ovem ents w hich dem ands the presen ce o f properly juxtaposed reaction centres in the crystals. A gain it is b eliev ed that in p h otoch em ical p ro cesses in m olecu lar crysutls the siruciure and orientation o f ex cited states arc c lo s e to that o f ground states. T his con cep t neglects the role o f nearest neighbours and the changes due to m olecular excitation. C ohen has [)ut forward the concept o f 'reaction cavity' w hich g iv es due im portance to the prc.scnce nemest neighbours [41,42]. This concept is useful to explain variety o f solid state reactions and has bc'cn extensively used to understand the geom etries o f cxcim ers o f p olyarom aiics in ihc crystal [4 3 -4 6 ]. The space occupied by the m olecu le in the crystal is the reaction cavity and the neighbours exert som e 'pressure' on the w all o f the cavity leading to som e distortions which involves a k v g c decrem ent o f attractive forces or large increment o f repulsive forces or hoih. vSome restrictions arc applied on this distortion by c lo sc -p a c k c d environm ent. T h ose reactions w hich in v o lv e m inim um change o f the surface o f reaction cavity arc en ergetically lavoured. This con cep t is very u.scful in predicting the product where m ore than one product lop och cm ically perm itted. T he role o f m olecular environ m en t in the crystal towards

300

rcacijvity has been generalised by C’la v c z /o lii |47,48J according Lo w hom the availability o f free space around the rcacuon sue is a prerequisite for crystal reactivity.

D Photoinduced lailK C instability :

Electronic excitation in a molecular crystal or macromolccuUir chain m ay produce a particular type ol lattice insuibility. The electronic exciUition may be kx:alizcd at one lattice su e for a lim e longer thcin die lattice vibralion lim e scale. The localized cxciuuion then mciins the existen ce o f an ex cited m o lecu le w hich due to u s altered properties appears to its neighbtiurs as an impurity. T h e creation o f this im purity m o lecu le introduces a local instability m the lattice configuration w h ich m ay lead to large m olecular disp lacem en ts. T he d isp lacem en ts m ay in som e ca ses be |)rccursi)is ol the lorm alion o f exciin ers and p h otoch em ical reactions. 1'his logic has been put Forward by Craig and C ollm s and found to be valid for one dim ensional lattice [491.

For an infimie onc-dim cnsional chain o f siru ctu ielcss particles, the typical a tom -atom potential can be wriiien as

V{r)

{- B (r

W hen one m o lecu le is ex cited by photon, an im purity is created ami the ex c ite d m olecule inleraci with its neighbour via Ihe poiential T" (r) given by

l'* ( / ■ ) - / / { [ e xp ( - / ; * (/■ r ; ) ) - | ] - | } . (14) llie n local insmbihty in ihc latiice structure is produced. 1 ‘his instability is relieved by

»

large am plitude displacem ents in more than one p ossib le w ay. In the sim p le ^{u k x}IcI one mcxie leads to new' lattice conliguralion and other yieltis a inctastablc nonequilibrium configuration.

T hese tw o distortions can have significantly differcnl potential en ergies and both distortions can decrease the potential energy im m ediately alter ex ciu u io n . P otential energy ch an ge is confined to the im m ediate neighbourhood o f ex cited particle and a ffe c ts the m igration of electronic excitation. Craig and his cow orkers 15()-52| have cited ex a m p les w here cxcim er and cx c ip le x e s arc lon n cd on excitaiion in the solid .stale but not preform ed in the ground state. The lattice instability helps to drive one m o lecu le c lo s e lo its neighbour prom oting excirner and cxcipicx formation. Relaxation ol this cxcim er or ex cip lex m ay result in ground state dimer.

3. Apparatus and measurements

X -ra y diffraction technique has been w id ely used to study so lid state p h o to rea ciio n s [16,5.3 55). But this melhcxi is beset with certain d ra w b a ck s: (1) It m easures space and time averaged structure w hereby iriicroscopic details ol cluster d yn am ics arc lost. (2 ) It takes several hours to .solve three dim ensional structures. (3) X -ray radiation can cau se solid suite reactions.

Latiice phonon mediaiion o f solid slate photoreacAions etc

3()1

E leclronic and internal vibrational spectroscopy can be used to identify the products 156] but the reaction dynam ics can not be detennined from these studies. Electron m icroscopy has been used by T hom as and his co-w ork ers (571 but its application is lim ited due to possible radiation dam age due to electron beam interaction. In c a se w here reaction in v o lv e s a sin gle crystal to a sin g le crystal transformation three-w ave m ixing sp ectroscop y and laser holography arc show n to be prom ising technique [58].

Laser Raman phonon spectroscopy has recently been introduced as a highly suitable methcxl for studying solid stale photorcactions [3 5 ,5 9 X>()|. It provides inform ations of both statics and d yn am ics o f reaction. T he cxpcrimcriuil iirrangemcnt is also h igh ly suitable for such studies. The advantages o f Raman technique arc : (i) An extrem ely sm all am ount o f sam ple can be used, (ii) N o sp ecial sam ple preparation, like m aking p ellet as m infrared or grinding as required in X -ray diffraction, is required. The disadvantages o f this technique are ; (i) It is not suited for investigations o f dilute concentration o f species. H ow ever, in nutny cases this can be made p ossib le by resonance enhancement, (li) It can not be used for any real quantitative study o f reaction rate. If reaction rate is not the topic o f interest, Raman s |x \ iroscopy is in d e a l a very useful technique for the study o f solid stale photoreaction.

4 . S p e c if ic s y s t e m s

A. O LOW mat urn :

i) p-Chloro cAnnamic acid :

p Chloro cinnam ic acid (/?-C C A ) c r y sta lli/c s m the space group with four m o lecu les jvr unit cell [61). T his jS-lorm o f the crysud grown from acetic acid solution has the reactive double bond .sepivration 3.87 A w hich is the distan ce o f h a x is alon g w hich tw o nearest neighbour m olecu les arc related by mirror sym m eiry. Solid state photoirradiaiion g iv e s mirror symmetric 4 ,4 ’-d ich lo ro ^-Um xinic acid as the only prcxluci.

C haracieri/ation o f the reactant and the prcxluci has

been

The progress o f the reaction w as m onitored by electronic absorption sp ectroscop y.

Broad absor|)tion bands are ob served at 194 nm and 2 7 6 nm in the so lid film o f the m onom er. In the dim er spectrum the higher en ergy band sh o w s a sligh t red shift w h ile the other band disappears. A new band at 2 5 0 nm is also observed in the dim er spectrum.

In the cry sta llin e stale p - C C A sh o w s strong flu o r e sc e n c e at 3 0 0 K . T he broad omission band with = 3 9 2 nm shifts to higher energy with reaction progress and in the dimer crystal it is ob served at 3 5 8 nm. A s sh ow n in F igure 1, the broad stru ctu reless

m Suhhasis Chakrahurti, Manash Ghosh and T N Misra

abs()rj)lion and em issio n spccira in the m onom er crystal and large Stokes' shift b etw een the tw o, provide a clear evidence o f strong exciion-phonon coupling in the m onom er lattice.

Wovtlengthlnm) Wavelength (nm)

I 'ijiurc* 1. IMcUnniic absorpiion and cmissjon spccira of (a) monomer and (b) dimer of p -C C A in ihc tr>'MaIlinc stale al 3(XJ K (Used by permission, Ref |68])

I'hc Raman phonon spectra o f this sam ple w ith reaction p rogress is presented in Figure 2. vSix low frequency phonon bands at 11.2, 17.6, 2 6 ,0 , 3 6 .2 , 4 9 .0 and 77 cm ^ are observed in the m onom er sjxxtrum . With reaction progress these bands suffer a m onotonic shift towards low er frequencies. I'hc 11. 2 cm ' band disaptx'ar m R aleigh w ing. On further reaction progress, the bands broaden appreciably. The phonon ol e x ten siv ely photoreacted cryslal is w eak and d iffu se. A s no new phonon band appears w e can co n clu d e that the prcxluctdocs not lorm any separate lattice. T he nature ol the reaction is h o m o g en eo u s in the iniiial stages but as the product concentration increases, the lattice b eco m es p ro g ressiv ely disordered.

I t ) 2,6 d i m e t h y l p — b e n z o q u i r u m e :

The phoiopuxluci m this crystal depends on die nearest neighbour m onom er geom etry w hich co n u o ls the interaction o f tw o double bonds >C =C < and > C = 0 leading to an oxeuin or a cage dimer. The quantum yield o f the cage dim er is only (62J.

Intramolecular vibrations observed in infra-red and Raman spectra w ere exp loited to get information about the photoproduci. The observation o f the tw o infra-red bands at 1676 cm ' and 1634 cm ' both in the photoproduci and in the oxclan dim er and disappearance ol C = () stretching band at 1668 cm ' in the m onom er crystal on photodim erization point to that oxclan is the primary photoproduci o f 2,6 dim ethyl b en zoq u in on c crystal. A b sen ce o f any band al 1715 cm ' or 1700 cm “ ' m the IR spectrum o f partially d im erized crystal also con fin n s the above conclusion.

From the phonon spectra as a fu n ction o f reaction p rogress it is found that the m on om er bands at 5 5, 6 2 , 6 7 , 7 1 , 7 5 and 88 cm ^ sh o w sig n ifica n t sh ift on dim erization.

Laiticc phonon mediation of solid stale photoreactions etc ₃₀₃

T w o new bands appear a i4 7 and 6 0 cm ^ w hich gam in inlcnsily wiUi rcaciion progress and finally broad band having a little structure and covering a w ide spectral range is observed.

This su ggests that in the initial siage reaction m echanism is hom ogeneous due to the fact that at low concentration the product form solid solution in the monomer lauicc bui as the reaction proceeds the prcxluct segregates out and forms its ow n lattice.

Electronic absorption spectra o f 2 ,6 dim ethyl p -b cn /.o q u in o n c at 6 K is sh ow n in Figure 3. A zero phonon transition at 19952 cm ^ is observed w ith prom inent phonon side bands w hich fit into a progression o f 65 cm ^ phonon. This may corresptmd to on e o f the

199S2 cm“

2.6 DMBQ i

at6K

. ± .

21500 20500 ^{1 9 5 ^ 0}

F p eq u en fy { cm "’)

Hpiirc 2. I he Hainan phonon spectra of p - C C / \ I'i^ure 3. I'leclronie iibsorpiion spectra of 2 6 tiyvial with rizaciion progress at "^OO K (a-<l in ilie tliincthyJ p-hen/txjuinonc cry stal at (S K (Used by imrejsing order of dimeri/.alion) (Used b\ pemnssion, Kel, |S9))

p( miission, Ref [fiSj)

observed phonon bands at 62 and 67 cm ‘ in the Kaman specirum. It apjxiars that the cxciion- [)honon coupling is selective and (Kcurs with this sp ecilic phonon m ode. The.se observations indicate that in this reaction, strong cxcilon-phonon coupling with a sp ecific phonon m ode result,s in the fonnalion ofp o la ro n . This provides a local eonrormational change in the lattice Hir.ch is a precursor to the photoproduct lattice.

lit) 7-m cthoxy c o m u irin :

7-nicthoxy cotimarin (7 ~ M C ) is a typical m em ber o f the fam ily where apparent violation ol Schm idt's top och cm ical criteria is observed. A ccording to X -ra y crystallographic study

|6 3 | the reactive d ou b le bonds o f tw o nearest neighbour 7 -M C m o lecu les in the lattice are rotated by 65'^ with respect to each other and centrc-ccnu'c disiiincc o f rc^ictive double bonds is 3.S8 A. The formation o f only solid state photoproduct, the sym metrical head-tail dim er, w as explained by M unhy et ri/'f64] w ho postulated an inherent n cx ib ilily in the m onom er crystal

304

Tor the m o lecu les lo urulcrgo rotaiion. They also ruled out d c fc c is as ihc p o ssib le loci Inr reaction Irom i)erccnlagc yield versus lime ploL

W e have used inlra-red and Raman spectroscopy for characteri/ation o f reactant and pnxhict. The disaj)pearancc o f C =C stretching vibration at 1556 cm * o f pyronc m oiety in the dimer spectrum indicates that the d ouble bonds betw een C (3 ) and C (4 ) are used to form cyclobutanc ring. Appearance o f cyclobuum c ring breathing vibration at 1016 cm ’ and ring deformation and bc‘.nding mcxles at 710 and 590 cm * rcs^x'divcly add further ev id e n c e o f the cyclobutanc ring fonnaiion. T h e O O stretching vibration at 1716 cm ’ is low er than free lactone carbonyl frequency (1725 cm ') w hich is due lo greater sin g le bond character o f the more polar carbonyl group in presence o f the aromatic structure. T his m ode sh ifts to 1747 cm ’ on dim eri/aiion resulting from the loss o f conjugation.

Tlic blue sh ill o f the broad absorption band with = 35 4 nm with dim erization is a general observation and aiiribuicd to the lo ss o f con ju gation . T he strong (luore.sccnce .spectra even at room tem perature with at 391 nm is due to ab sen ce o f intersystem cro.ssing Vtt* since ';r;r* is signilieanily lowered form stale in the crystal slate.

I'hc em ission band shifts to highei energy on dirneri/ation. C onsiderable overlap betw een em ission and ab.sorplion spectra as shown in Figure 4 suggests that excilon-phonon coupling IS rather weak in this crystal.

200 200 360 440

W a v elen g th ( nm)

F ig u re 4. ahsoqmon and emission spectra ot (a) muiKMiici and (b) dimer of 7-M C in solid fdm ai 300 K (Used by pcnnission, Ref |381)

15.0 5 2 .5 90.0

Raman Shi f t (cm“M

Figure* 5, Raman phonon spectra of 7-jVlC crysirfl with read ion progress at 300 K. (U sed by pennission, Ret |3 S |)

7-M C crysuil grown from b e n /c n e solution b elo n g s to the sp ace group V^l with loui m olecules ]iej unit cell w hich demands that six A^, phonons should be R am an active. Figure 5

Lattice phonon mediation o f solid state photoreactions etc

305

shows lhai Lhc monomer crystal is characterised by four sharp and moderately intense low frequency phonons at J9.K, 29.8, 33.8 and 39.8 cm^ in addition to weak broad bands at 52.5 and 74.2 cm ^ As the rciiction proceeds, except the 19.8 cm^ band, all the other phonon bands broaden appreciably and no significant shift in frequency is observed. The 19.8 cm ' band gradually shifts to lower frequency on reaction progress and finally disappears in the Rclcigh wing. The appearance of no new phonon band and the softening of

19.8 cm ’ band suggest that the photorcaciion is assisted by softening of this phonon mode.

In the monomer crystal the reactive pair may undergo libralional motion about the minor iixes L \ and L

which arc nciirly in the plane of coumtirin ring and about the major axis

/ .3

perpendicular to the ring [651. A pan of energy from the radiation field is used to increase lhc thermal energy of the system which increases the libralional motion. The 19.8 c m '' band is likely to arise from libralional motion of monomer molecules around L

axis and this may bring the reactive double bonds of the molecular pair in a favourable configuration for the read ion to occur.

According to the concepi of ’dynamic preformation' 149-52] of pholoproduct, the motion of electronically excited molecules towards maximum overlap geometry would

less energy than that for the ground stale molecules. It is possible that softening of a

imxle may come alxiut through such a pnx:ess.

B Polymerization

i) Dimethyl ester of a,ex' dicyano p-phenylcne diacryhc acid

The sample is a member of a,(x' dicyano /

-phcnylcnc diacrylic acjd {p-CPA) series which structurally is an extension of tt-cyano cinnamic acid molecule. These samples contain two unsiituraled centres and four ccnlre type polymerization occurs on photoirradiation.

The photopolymcrizalion is by cyclohutanc ring formation as evidenced by disappearance of C=C sirciching mode and HC=CH trans vibration in the polymer sjxcirum. The cyclobuiane ring breathing vibration at KXK) cm"^ in the polymer spectrum also sup[K)ris the above conclusion. The C=0 stretching mode in monomer at 1757 cm ^ shifts to

cm ^ in the polymer. This small shift can be attributed to the fact that the resulting polymer is not ol high molecular weight as cis-trans isomerization may occur during fX)lymcn/ation [

].

The elccuonic excitation and emission .spectra of monomer and polymer arc shown in

Figure

. The monomer emission peaks arc at 462 and 497 nm. The excitation peaks arc at

363 and 394 nm. Since the large Stoke's shift is a clear indication of exciton-phonon

coupling causal by strong lattice distortion in the excited suite the phoioreaction is likely to be

phonon mediated. The polymer emission bands arc at 382, 407 and 450 nm. The excitation

spectra shows two peaks at 361 and 379 nm. In the polymer exciton-phonon coupling is

rather weak as suggested by considerable overlap between excitation and emission spectra.

306 Subhaus Chakrabarti, Manash Ghosh and I N Misra

Figure 7 shows Raman phonon spccua with reaction progress. Al room icmpcraliirc there arc four di.stinci bands at 33.2, 57.8, 78.4 and 128.8 cm ' in the monomer. With reaction progrc.ss the intensity of the bands decrease and the bands shift towards higher wavenumbers. After 6 h irradiation only two phonon bands persist al 37.0 and 65.0 cm '

which ire shifted Irom their initial positions by 3.8 and 6.2 cm * respectively. Progressive broadening of phonon spectra suggest increasing lattice disorder and amorphous nature ol the polymer is confirmed by the ab.scncc of any phonon bands after 14 h of irradiation. So the reaction mcchani.sm is homogencxuis in the initial suigcs and the lattice becomes disordered with increasing photoproduci conccnu-alion.

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

F igure 6. Hlecirumc uxciiaiion and emission spccira of (a) mononuT and (b) polymer ol p- Cl'AMc al 77 K (Used by penmssion, Ref 16‘^1)

F’igurc 7. Raman phonon spcciia of />-(^PAMe polycrysials wilh reaction progress a-d are m ifie increasing order of reaeiion (lixposed for a 0 nmi , b 120 min . e 240 min., d 360 min ). (Used by pcnnis’Mon, Ref. |69|).

ii) Di n~propyl ester of a,a ' dicyano fj-phenylene duicrylic add :

II IS the most reactive member o( p-CPA senes. In this crystal, the separation between rcacuve double bonds is 3.931 A [67|. This sample on phoioirradiation has been found to polymerize into a linear, high mokxiilar weight crystalline polymer containing cyclobuiane ring in the mam chain.

ITie infra-red .syxxira show that the aliphatic C=C stretching vibration at 1603 cm^' and

trans HC=CH vibration at 969 cm ^ decrease m intensity with reaction progress and in

extensively reacted crystal these bands disappear. The cyclobutanc ring formation and four-

centre type photopolymcrizaiion is thus confirmed. The C=0 stretching mode at 1720 cm * in

monomer shifts to 1736 cm ^ on polymerization due to loss of conjugation which suggests

high molecular weight yx>lymer formation m contrast to dimethyl ester of p-CPA where this

shift is only 6 cm k

LaiUcc phonon mediation of solid state phoioreaclions etc ₃₀₇

The monomer is strongly fluorcsccnL m the crysuillinc suite. No phosphorescence emission is observed. As shown in Figure 8 the exciUition and emission maxima arc separated and ihcir separation is smaller than that in dimethyl ester of /^-CPA. So here cxciton-phonon coupling is present though it is not as strong as in the dimethyl ester crystal. As the reaction prcKcc-ds, the emission and excitation peaks arc shifted towards high energy side and in the polymer spectra ccMisiderable overlap between emission and excitation spectra suggests a rather weak exciion-phonon coupling in the polymer lattice.

The evolution of Raman phonon spectra with reaction progress is shown in Figure 9.

The monomer spectrum shows seven bands at 23.4, 32.2, 31.6, 86.6. 103.0, 136.8 and 162 cm^ . With rciiction progress the bands show slight shift towards lower frecjuency and

FiRiirc 8. lilcclrom c excilaiion and emission spctiiii ol (a) monomer and (b) polymer of nPr al 77 K

10

90 170

Raman Shift (cm j

Figure 9. Raman phonon .spectra of p-C PA nl*r poly cry sials with reaction progrcs.s a-d arc in ihe increasing order of reaction. (lixjHWcd for a 0 min., b - 10 mm., c ■ 20 min , d . 30 mm )

alter 20 minutes irradiation, the monomer bands at 23.4 and 32.2 cm ' shift to 22.4 and 30.8 cm ' respectively. So the reaction mechanism is homQgencous in thw initial stages and the

reactant and product fonn solid solution. At tliis stage, a new phonon band at 59.8 cm ' starts

rlevcloping and the 86.6 and 103.0 cm ' bands arc broadened. On further reaction progress

Ihe 59.8 cm*"' phonon gains intensity and another band at 111.6 cm '* appears. The monomer

phonon modes at 23.4 and 32.2 cm ' gradually loose their intensity and disappear in the

308 Siihhasis Chakraharu, Manash Ghosh and TN Misra

|X)lymcr spccLriirn. These resLilLs suggest that the reaction mechanism is heterogeneous in the final stages and ihe phase separation between the rcacltint and the product occurs. Sharpening of the phonon bands to some cxteni in the polymer spcclrum suggests good degree of order in the polymer laiiice. We conclude that the phoiorcaciion in this crystal is mediated by phonon through strong exeiton-phonon coupling.

ui) Diethyl ester of f> phcnylene dtacrylic acid:

Ethanol grown monomer crysUil belongs to the space g r o u p w i t h reacting double bond separalion 3.970 A [671. The photoproduct is a high molecular weight polycrysialline jKiwder.

The aliphatic >C=C< stretching mode at 1640 cm ^ and irans HC=CH vibration at KKKlcrn ‘ arc intense in the monomer spectrum and disappears in the extensively reacted crystal. 'The.se suggest the Ibmiation of cyclohulanc ring. Also a cyclobutane ring breathing at 1040 cm ^ and other ring vibrations appear in the infrared spectrum of the photoreacted crystal. There is a 20 cm ‘ shift of C=0 stretching mode on polymerization which points to the high molecular weight of the polymer.

Ihc emission and exciuuion spectra of the monomer and the polymci crystal at 77 K arc shown in Figure 10. It is observed that the emission spectrum of the monomer is well structured with (0,0) hand at 362 nm and strong vibrational peaks. The exeiton-phonon

Fiplire

to

Idgure It. Kaman phonon spectra of />-PDA1m polycrysials with reaction progress, a-d are in ihc increasing order of reaction. (Exposed for a ; 0 min , b 60 rnin , c ‘ 120 min., d ■ 180 min.). (U.scd by permission. Ref |69|).

coupling is wciik in the monomer lattice as c\ idcnced from the appearance of fine structures in

the emission s|x?ctrum and overlapping between emission and excitation spectra. So the lattice

phonon is unlikely to play any significant role in tliis polymerization reaction.

Lattice phonon mediation of solid state photo reactions etc

The Raman phonon sixxira ai clilTcrcnl reaction stages arc shown in Figure 11. In the initial stages disorder in the lattice with formation of lower-order polymers is manifested in broadening of phonon modes.Frequency shifts in the initial stages also points to the homogeneous nature of initial reaction mechanism. In the later stage, however, new sharp bands appear due to phase separation between the rciictant and the product. Though of poor intensity, the sharp phonon bands suggest that the polymer lattice is highly ordered. C>i further reaction progress, the sharp structure of phonon spectrum persists indicating that the ordered structure of the lattice is retained. The phonon frequencies, however, show a monotonic shift to higher values. This indicates that on further reaction, still higher order fx)lymers are formed. This product is formed in a homogeneous mechanism keeping the high degree of order in the lattice.

5. Conclusion

This article focuses on the novelty of phonon probe m deriving the dynamic concept ol reactivity and the mechanism ol solid stale photoreaclions. Phonon participation m such reactions may occur through strong excilon-phonon coupling oi through a mode softening.

I he strength of excilon-pheinon coupling is different for different systems. In some reactions lattice phonon do not influence reactivity signrficanily. The reaction mechanism may be homogeneous or heterogeneous. In the former the reactant and the product form solid

whereas in the latter the reactant and the product form separate lattices. In many ca.scs mixed behaviour is also observed.

Acknowledgment

The authors gratefully acknowledge the financial support by the Council of Scientific and Imluslrial Research, Government of India through Grant No. 3/677/89-HMR II. Thanks arc also due to other collaborators and many individuals mentioned in the text who generously granted [TcnTiission to quote then works.

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About the reviewers

Piol'T N Misra :

Prof. Misra, Professor and Head of the Spcuroscopy Department, Indian Association lor the Cultivation of Science, Calcutta, worked as a Reader in Physics at North Bengal University (1972-77) and as an associate Professor in Physics at Birla Institute of Icchnology and Science, Pilani (1971-72). P’orcign assignments held by him are many. He worked as Visiting Professor (DAAD) at the University of Munchen and the University of Uusseldorf, Germany (1990) ; as Visiting Professor (JSPS) at the Institute for Solid Slate Physics, University of Tokyo, Tokyo, (1981-82) ; as Research Associate at State University v)l New York at Buffalo, USA (1980-81) ; as Post Doctoral Fellow at University of British Colombia, Canada (1967-69); as Research Assistant Professor at Michigan State University (1906-67) and at Louisiana State University (1964-66), USA ; as Senior Tutor at the Ur.ivcrsiiy of Queensland (1964), as Visiting Research Fellow at the University of Sydney (1963-64), Australia. Other foreign visits made by Prof. Misra arc : France 1990, Bio Raman Conference at Reims ; Italy 1990, Insliluic of Molecular Spectroscopy, Bologna ; USA 1986, Inicrnaiional Laser Science Confcrcnce-II, Seattle ; USSR 1983, Nauka '83', Science Symposium ; Hungary 1974. Visiting Scientist under Indo-Hungarian Exchange of Programme, Institute of Physical Science, Budapest.

Dr. Misra's areas of research interests arc :

I. Laser Raman spectroscopy ; Phonon mediation of solid slate reactions, phase transition and Surface Enhanced Raman Spectroscopy.

2

Electronic spectroscopy of organic molecules & molecular crystals

Excilon Splitting,

exciton-cxciton interaction, triplet exciton dynamics.

312 Suhimsis Chakrabarti, Mamsh Ghosh and T N Mism

3. Vibrational spcclroscopy and m olecular geom etry elucidation.

4 . Charge earner generation and transport in organic conductors, sem icon d u ctors and photoconductors.

5. Organic material based d ev ice d e v e lo p m e n t: S olid state batteries, pholovolatic cells, gas sensors and biosensors.

6 Organised m olecular assem b les and ultra-thin L an gm u ir-B lod gctt m olecular film s ; Charge transport and energy transport

S o lar, m ore than o n e hundred and thirty l i ve papers have been p u b lish ed in international journals and has supervised research works o f fifteen Ph.D. students.

Dr. Subhasis Chakrabarl[ ■

P rcscn ily lecturer in P h y sics, A. B. N, Seal C o lle g e , C ooch b eh ar. H is field s ol interest in clu d e Ram an sp e c tr o sc o p ic stu d ies o f p h ase tran sition , cryst al l i nb slate pholoreaclion and surface enhanced Raman scattering. Dr. Chakrabarti has authored tyiirlecn

papers in iniernational journals on the above topics 1

Manash G hosh :

C ollaborates with Prol. Misra m Raman Sfxictioscopy.