Mobile charge-transfer triplet excitons in biphenyl-tetracyanobenzene single crystals

Volume

21, number

1

CHEMICAL

PHYSICS LETTERS

MOBILE CHARGE-TRANSFER IN BIPHENYL-TETRACYANOBENZENE

Recerved

1 August 1973

TRIPLET EXCITONS SINGLE CRYSTALS

27 Aprd 1973

The lcwest evcltcd trlplct state of a mlxed biphenyl-tetrncyanobenze?e smg!e crystal has been studred. The form&on of mob& charge-transfer evatons with a h@ly temperature dependent dlffuslon rate (actlkahon energy 700 cm-l) IE estabhshed A hoppmg frequency of 5 X 10’ ~urnps per second at 250°K was estimated A very stro% optlal spm polanzatlon allows ESR-spcctroscoplc studies of the evatons at hfetlmes smaller than 0 2 msec

1 Introduction In a number of cases electron-donors and acceptors form 1 1 mxed crystals, where the two types of moleCU!Z; d&r &ernatlvely stacked III hnear arrays with the molecular planes being paralIe1 Thus molecular arrangement allows the formation of charge-transfer (CT) complexes d the distance between the molecular planes 1s small enough (3 5 A) Crystals of such complexes have been studied to some extent [ 1,2] by

optical methods In a recent paper ESR-spectroscopic studies of a 1 1 mixed single crystal contammg naphthalene and 1,2,4,5-tetracyanobenzene (TCNB) have been reported It was shown that the lowest excited triplet state of thus crystal is a long-lived (hfetime r about 1 set) charge-transfer (CT) state, localized at all temperatures (77 to 120°K) at which an ESR sgnal could be observed In that work we took advantage of the fact that ESR spectroscopy 1s a very sensittve tool to establish the charge-transfer character of triplet states [3] The prrmary goal of the present BSR work is to report the formation of mobile charge-transfer tnplet excrtons u-t a 1 1 mixed smgle crystal of brphenyi (donor) and TCNB (acceptor) In then proneermg work on tnplet excrtons m anthracene and naphthalene crystals, Haarer and Wolf [4, S] showed that ESR spectroscopy yrelds valuable mformatlon on the

nature and the dynamics (excrton hopping frequency) of triplet excitons. From the pomt of view of tnplet excttons, the charge-transfer single crystals have some common features with the single crystals of I ,4dibromonaphthalene [G] , where the molecules are ako stacked m hnear arrays with the molecul3i planes aemg parallel In crystals wrth such a molecular arrangement, one-dimensional (ltnear) excltons are formed wluch migrate preferentially parallel to the stack axis. A prmcipal difference IS, however. *at the complexes in our crystals are translationally equivalent, whereas the dibromo-naphthalene crystal has two groups of differently crrented molecules per umt cell Thus the exclton motion in the CT crystal affects the hyperfine structure of each ESR hne and it IS not possible to di.stmgulsh between the exciton motion parallel and perpendicuIar to the stack axis. The dtbromo-naphthalene crystal yreids two pans of ESR hnes corresponding to the two translationaIIy mequ:valent molecules of the urut cell and the ESR spectra yield only mformatron on the exciton dtffusion perpendtcular to the stack axes

2 Crystals TCNB was prepared

m tis

laboratory

by Dr W.

43

Volume

21, number

1

View alorq

CHEMICAL

I August 1973

PHYSICS LETTERS

ternatlvely III lmear arrays with the molecular planes being oriented perpendicular to the stack axis [7,8] Such a molecular arrangement m the blphenyl-TCNB crystal IS also supported by our fluorescence polanzation expenments which showed that the ermsslon from the lowest excited singlet state IS polarized parallel to the b axis Cperpendlcular to the molecular planes) From the above results we tentatively assume the molecular arrangement of fig 1 We cannot decide if the crystal has more than one complex per unit cell

b-axls

3 Measurements a=73?028. Molecular

b=68+012 planes

3 1 Optrcal stzrdles

1 b-ax&

Fig 1 Structure and molecular arrangement

assumed for the 1 1 moved smglc crystal of blphenql nnd TCNB The damennonsa=73~02A,b=68_cOlAandc=15~2Ahnve ken determmed by X-ray analysis The plants of the molecules arc pcrpendlculz1 to the b nw The rclahve onentatlon of thr molecules m the comple\ has been arbltrarlly assumed

Kuhnle and was purified by recrystalhzatlon from ethanol and by subsequent subhmatlon The undeuterated blphenyl (Aldrich) used m the present work was purified by zone refining whereas the perdeuterated blphenyl (Merck, Sharp & Dohme) was used wlthout further punficatlon Smgle crystals conslstmg of an equunolar mixture of blphenyl (donor) and TCNB (acceptor) were grown from spectroscopltally pure acetone by slow solvent evaporation They had the typical dlmenslons 2 X 1 X 0 2 mm3 The results obtamed with crystals contammg zone refined and non-purified blphenyl were essentially Identical By X-ray expenr?ents, based on the rotating-crystal method, two perpendicularly orlented crystal axes G and b ~th

the umt

cell dunenslons

The long-hved emlsslon spectrum (taken with a phosphoroscope of the crystal obtamed by uradlatmg mto the lowest excited CT smglet state (absorption maxunum at 390 nm) exhlblts a broad structureless band with a maxunum at 530 run Tne O-O bands of the donor and the acceptor phosphorescence are located at 420 [9] and 440 nm, respectively The phosphorescence spectrum of a solid solution of blphenyl and TCNB obtained by irradiating mto the lowest excited CT band consists also of a broad band which nearly comcldes with the crystal phosphorescence These results suggest that the lowest excited triplet state of the blphenyl-TCNB crystal 1s a chargetransfer state Prellmmary 1:fetune measurements at 77°K of this metastable state have been performed with a phosphoroscope and with a 30 Dsec flash apparatus It was found that the phosphorescence spectrum consists of a long-lived (“1 set) and a short-lived (=0.2 msec) component

Only

the latter

could

be observed

H-I the

ESR spectrum

Q = 7 3 f 0 2 ?i

and b = 6 8 + 0 1 A could be Identified The thtird axis could not be located by the X-ray method However, the ESR experiments described below malcate that the c ams IS perpendicular to the b axis and forms an angle of j3 E=120° witi &thea axis From the X-ray pattern obtamed by rotatmg the crystal about an axis normal to the ab p1ar.e the dlmenslon m the c dlrection was estunated as c = 15 +-2 A The unit-cell dunenslon of b = 6 8 A 1s characterlstlc for CT crystals m which the donors and acceptors are stacked d44

and results

c=15+2&

3 2 Zero-field-sphttmg

(ZFS) parameter

ESR spectra were taken with a standard X-band spectrometer (Vanan V-4502) Irradiation was achieved with a 100 W l-&-pressure Hg lamp (Osram HBO 100 W/2) In order to prevent lrradlatlon mto locally excited donor or acceptor states a cut-off filter (Scho.t WC 375) with a transmlsuon of less than 1% at 350 nm was used In an attempt to determme the orientation of the prmclpal axes of the zero-field-

Volume

21, number

CHEMlCAL

I

PHYSICS LETTERS

1 August 1973

3200

28M

F1g 2 Angular dependence of the resonxxe fields of the trlplet state obtamcd by rotatIn:, the magnetic field Ho ,n the bc pknne (a), m the o’b plane (b) and in iha c’c plane (c) cyIS the angle (in degrees) between Ho and tl-e 3~s u’, b and c The sold tunes have been calculated using cq (29) of ref [ 10) wth the ZFS parameters and g-fdLtors gwen m ldble 1 The m~uwave frcquercy was 9184 9 MHz (a), 9230 4 MHz (b) and 9230 2 MHz (c) The crosses (emlsslon Imcs) Jnd the dots (absorption Imes) are c\pcr~mental values The arrows mdlcatc nhcre absorption lmes Invert Into enusslon lmes and WCCversa

Table 1 Comparison of zero-field-splitting parameters and Dm = (D’+~I?~)“~ D

Species

(m cm-‘)

J!I

D, E

Dm

axes system These experrments showed that wlthm ztz accuracy of about +lO’ tie axes u’, b, and c (cf _ fig 1) dre parallel to the prmclpal axes of the ZFS tensor The magnetic resonance fields obtamed by rotating

blphcn>

l/TCNB

naphthalene/TCNB

DLA/benzonltnlc blphenyl TCNB

d

50

[ II]

IS grx = 2 0085,g,,, b) The Isolated complex c) DEA = dlcthylandme

a) It

*00358

[3]

[ 201

0527

c)

+o

0168

o

0603 b)

200224

005

0 047

000s

0049

0 1092

0 0036

0 1094

-

0 1317

= 2 0073 and g,, has a value

= 2 0066

Dm = 0 064

+ 0 01

spllttlng (ZFS) tensor with respect to the crystal axes, we recorded ESR spectra for z large number of onentatlons of the statx mapetlc fleId Ho m the crystal

the

crystal

about

these

axes

are

e~~+n

in fig.

3-

ZFS parameters D and E have been delermmed from the resonance fields for the canonxal onenta-

The

tlons usmg the standard equations (cf. ref. [IO]. eq (29)) The results are given m table 1 and are compared there with the ZFS parameters of the naphthalene-TCNB charge transfer compIex and the diethylarulme-benzomtrlle exclplex [I I ] . For comparison we also took the tnpIet ESR spectrum (at 77°K) of a mlxed solrd solution ofbiphenyl and TCNS m a hquld crystal solvent [3] When the n-radiation was achieved with the WG 375 cur-off filter only a very broad ti = 2 spectrum, chancteristlc of CT tnp!et states, could be observed From this 4.5

Volume

21. number

1

CHEMICAL

I August

PHYSICS LETTERS

1i.

1973

Motronal Narrowrng Modulation

Amplitude 006 Gauss

Resonance

i i

10

05

I-lg 3 Am = 1 ESR spectra of the blphcnyl-TCNB smglc crystal for drffercnt or~cntnt~ons ofHo m thca’c plane at 77°K Q = 90” corresponds to an orlentatlon of II6 parallel to the 0’ aws (_v aws) The IIIU may be dlstorted due to a relatwely hygh value of the modulation amphtude (“0 6 G) h’ote that for all orlcntatlons thcrc IF one absorption (-j and one

emlshlon (-) hnc. mdlcntlve of optical spin polarlzatlon spectrum

the average ZFS parameter

Dm = (cf , table 1) The Dm-values of the lsoleted complex and the smgle caystal are m good agreement It IS found (cf , table 1) that the average ZFS parameter of the crystal trlplet state IS considerably smaller than the Dm-values of the donor and the acceptor, respectively We conclude therefore that the lowest excited trlplet state of the blphenyl-TCNB crystal is a CT state This conclusion 1s supported by the sLrmlarlty &the D and E values obtnmed for the typical compieles summarized in table 1

(D2-+3E2)1~2 could be determmed

3 3 Optlcai spm polanzatlorl Typical AIPZ = 1 spectra of the crystal are presented m fig 3 for different orlPntatlons of the magnetic field m the G’C plane For each orlentatlon two imes of about equal Intensity are observed One of these hnes IS absorptive and the other one emlsslve The Intensity drops to zero at IX= 80°, where the absorption hne Inverts mto an emlsslon lme and vice vera These mverslon pomts are mdlcated In fig 2 by arrows The simultaneous appearance ofan absorption and an cmisslon Ime mdlcates optical spm polarlzatlon [4, 12, 131 due to a !arge devlatlon of the population 46

Field HO

\

312C Gauss

j L ,

plane

Ho I” a’c

‘\.

‘a, s

‘.

1

\

‘\ =\

I\

I

‘K_‘_ “;-‘-._.

I 150

100

200 -

250 Temperature

300 PKI

l-q 4 Temperature dependence of the hne uldth &!I’ of the ESR hne obtamed for an orlentatlon of the magetx field m t!le ic plane at a reconance field of 3 120 G The Ime at temperaturcs above 250°K may be broadened due to a prohlbrt~vely lage modulatlun amphtude (-0 06 G) The lines are lorentzlnn at temperatures above 100°K

of the three trlplet Zeeman levels from thermal equlhbrlum Haarer and Wolf [4] derived the followmg necessary condltlon for the appearance of emlsslve hnes l/r Z {e-ip (AE/kT)

-

1 )/3T1

In this equation T, IS the longltudmal relaxation time, 7 IS the tnplet lifetune and AE IS the energy difference between two Zeernan levels (AI? z 0 3 cm-l) At T= lOOoK,fGr Instance, emlsslve lmes can still be expected d the trlplet hfetune IS about 700 times larger than the relaxation time T, The Metune of the trlplet state observable m the ESR spectrum IS shorter than 1 msec (the decay time of the electromc shutter) at T = 77°K Accordmg to opt& measurements the 1lfetLme of this short hved species IS about 0 2 msec. The spm-lattice relaxation tune T, of the trlplet state has been measured for several orlentatlons of the magnetic field with respect to the crystal axes usmg the saturation method [4] RelativeI} large relaxation tunes of the crder of lo-4 set have been obtained at 77OK for all onenta-

Volume 21, number 1

L August 1973

CHEMICAL PHYSICS LE-I-TERS

t~ons and therefore the above condltlon 1s easily fulfilled fol tIus temperature [14] Due to the very short hfetlme of the trlplet states at the lowest temperature avallable with our equlpment, the decay and the growmg-m characterlstlcs of the ESR signals could not be studled It IS thus not possible as yet to get mformatlon on the relative population and depopulation rates of the Zeeman sublevels accordmg to the method of Sill and Schwoerer 1131 3 4 Exczton rnobrllty An outstandmg feature of thetnplet-CT states m the blphenyl-TCNB smgle crystal IS the strong narrowing of the ESR lmes with mcreasmg temperature Lme widths of the order of tenths of gauss are obtamed at room temperature This IS demonstrated m fig 4, w,here the lme width (defined as the field &stance of the extrema of the first-derlvatlve ESR lmes) 1s plotted as a function of the temperature In the temperature range between 90 and 300°K the lme width decreases from M = 2 2 G to M = 0 1 G The lumtmg value (A/?,) of the hne width (A.M) at the lowest avallable temperature of 77% depends strongly on the crystal preparation Values of AH,,, where measured for a number of crystal preparations and It was fotind that Mm varied between 2 and 16 G If the lme width M of a crystal showmg the h&est attamable AH, value of 16 G IS plotted as a function of the temperature, the upper lmut of 16 G (at 77°K) IS approached asymptotically at decreasmg temperature This asymptotic behaxour shows that the CT tnplet excltons characterized by the upper Ilmlt of Mm are nearly munoblllzed at 77°K The relatively small value of Mm = 16 G suggests that the line wdth of these lmmoblllzed excltons 1s determined pnrnanly by the mhomogeneous broadenmg of the ESR lmes caused by the hyperfme mreractlon of the trlplet state electrons ~th the nuclei of the donor and the acceptor This conclusion 1s supported by the observation that the hne widths of the ESR spectra of completely unmoblhzed trlplet states m mixed ndphthalene-ds-TCNB [3] or durene-d14-TCNB crystals are also about 15 to 20 G. From the appearance of only one pair of lines at neghglbIe exclton motion It follows that, whatever the number of complexes per umt cell, all complexes

mus! be translatlonally equivalent (for an example see ref [3]) In thus case the locally exerted (unmoblle) triplet states and the mobde tnplet excrtons have (1) a common prmclpal axes system for the ZFS tensor and (2) identical ZFS parameters [I41 The temperature dependent narrowing of the lmes at T > 77°K 1s mdIcatlve for the onset of exclton dlffusion above the$e temperatures The exclton motion causes a random modulation of the hyperfine mteractlon leadmg to a motional (or exchange) narrowmg of the mhomogeneously broadened ESR imes The observed line width depends on the hmrtmg lme width A!!?,.,, at neghglble exclton motion and on the hoppmg frequency of the excltons According to Haarer [ !5] the following relation holds MNN’12 = m

m’

where N 1s the number of hyperfine states seen by a dlffusmg exaton durmg Its hfetme 7 Accordmng to our optlcal measurements the hfetune of the tnplet state 1s of the order of 0.1 msec between 150 and 250°K Using the lme wraths given UI fig 4 one estimates a hopprng frequency of v- = 5 X lOi Hz at d 250°KandvJ=5X106Hzat150 K The close packmg of the donors and acceptors ?n a direction parallel to the b axis allows a strong mteraction between complexes stacked above each other It 1s thus obvious that the excltons move preferentially us a dlrectlon parallel to the stack axls The coefficient V of tlus onedmenslonal exclton dlffuslon IS deterrmned by the eqrlatlon 20 =vjb2 (b = 6 8 A, cf , fg

1) and D has the values D = I 2 X 10m7 cm2/sec at 250°K and 1 2 X IOm8 cm2/sec at 150°K From the temperature dependence of the dlffusion coefficient cne ob tams an apparent actrvdtlon energy of 700 cm-l for the charge-transfer tnpIet exclton migration This value IS by a factor of 4 Iarger than the actlvailon energy for the CT smgiet exclton transfer in durene-TCNB single crystals [2]

4 Fmal remarks A salient property of the CT tnplet excltons bttidled m tlus work 1s the high temperature sensltlvlty of the exclton

dlffuslon

rate characterized

by a rather high

47

Volume 21, number 1

CHEMICAL PHYSICS LETTERS

value of the activation energy (700 cm-l) In contrast to U-us behavlour, the exclton dlffuslon m anthracene smgle crystals IS nearly temperature mdependent [4] The sensltlve temperature dependence of the exclton moblhty may have essentlaliy two orlgms” First of all, impurltles (which may be present especrally m TCNB) may act as traps or may produce so-called X-traps [17] by lowermg the energy of complexes adjacent to the unpurlty mo!ecules Secondly “self-trappmg” cacsed by a conslde,able difference m the equlllbrlum conflgulatlon [I 81 or m the charge dlbtrrbutlon [I] between the excited CT state and the ground state has to be consldered ExperImental evidence for “self-trdppmg” due to a rearrangement of the charge dlstrlbutlon upon excltatlon has been provided by Hochstrdsser et al [ 1 J _These authors found that the singlet CT states m anthracene-tnmtrobenzene smgle crystals are practically lmmoblllzed at 300°K Changes m the equlllbnum configuratlon upon excltatlon have been consldered as responsible for the fact that the excuner-excuner energy tiansfer m pyrene smgle crystals 1s only effective at high temperatures [IS, 191 We assume that it IP also the “self-trappmg” which hts the triplet exclton dlf~lslon m our crystals for the followmg reasons (1) the fact that the ZFS parameters of the Immobilized dnd the mobtie CT trlplet states comclde rules out impurltles as traus, (2) it 1s hardly conceivable that the X-traps can b~ve a depth of 700 cm-l

* We rule out the posslblhty that the high temperature needed to allow trlplet excltop dlffuslon IS due to the faLt that the donorc and accep!ors arc not arranged m a completely alternating fashlo;l Such crystal faults would hJce to be pre:ent III a very hl$ concentration and they would most probably prevent completely the cxlton dllfu%on

parallel to tha stnch 3x1s

48

1 August

1973

Achowledgement The authors wish to thank Dr W Kuhnle of this laboratory for the preparation of TCNB and Dr E Forster for his generous help with the hfetlme measurements We thank Professor Dr A Weller for most helpful and encouraging dlscusslons Fmally we are greatly mdebted to Dr. D Haarer for many hours of dlscusslons

about

exclton

problems

References 111 R hl Hochstrasser,

S K Loucr and C Reid, J Chem Phqs 41 (1964) 1073 (21 T Kobayashl and S Nagakura, hlol Phys 24 (1972) 695 and 3 Sch.xarz, Chem Phys 131 P hrebs, E Sack.mann Letters 8 (1971) 4 17 141 D IiJarer and H C Wolf, Phys Stat Sol 23 (1969) K117 [51 D Hdarer and H C Wolf hlol Cryst Llquld Cryst 10 (1970) 359 [61 R bchmldberger and H C Wolf, Chem Phys Letters 16 11972) 402 [71 S h;umakura, F Iwasakl and Y Salto, Bull Chcm Sot Jap.m 40 (1967) 1826 [Sl J C A Boeyens and r H Herbstem, J Phys Chem 69 (1955) 2153 191 P J Wsner, J Am Chem Sot 89 (1967) 2820 1101 Ph Kottls, Ann Phys (Parts) 4 (1969) 459 1111 H Ileens, J de Jong and A Weller, Colloque Amperes XV (North-Holland, Amsterdam, 1969) p 289 1131 W S Veeman and J H van dcr Waals, hlol Phys 18 (1970) 63 1131 H Sl\i and M Schwoerer, 2 Naturforsch 2.5a (1970) 1383 [ 141 H hIohi%ald, unpublished results [ 1.51D llaarer, Thesis, Unlverslty of Stuttgart (1969) [16] H Sternhcht and H hI hlcConnel1, J Chem Phys 35 (1961) 1793 1171 H Port and Ii C Wolf, m The trlplet state, cd A B Zahlan (Cambrldgc Umv Press, London, 1967) p 393 [ 181 N Y C Chu. K Kawnoka and D R Kearns, J Chem Phys 55 (1971) 3059 [19] W Klopffer znd H Bauser, Chem Phqs Letters 6 (1970) 279 [20] S Siegel and H S Judelhls, J Phys Chem 70 (1966) 2201