Non-exponential picosecond fluorescence decay in isolated pentafluorobenzene and hexafluorobenzene

CllEhllCAL

NON-EXPONENTLAL

AND HEXAFLUOROBENZENE

Desmond V O’CONNOR, Mmoru SUMITANI, John hl. MORRIS I~rsrm~c for ,Wdecrrlar Scrcrrcc. Alyodarp. Ohorohl 4-M. Japan

1 Introduction In recent

years the photophys~cs

and photocherms-

bcnzenes hdve been the sublect of

much research Interest. Smgle vibromc of monofluorobenzene benzenes

[I

[?I hnvc been cawed

out. Exponential

redistribution

in p-difluorobenzene

suggest that the decay kinetics,

[3,4]

YOSHIHARA

there are slgmficant

discrepancies hfetime

de-

would

at least III this mole-

m the published

data. Values rangmg

from 0.7 to I 6 ns have been reported for pentafluorobenzene excited at 270 M-I [8, I I, 121, wlude the of hehafluorobenzene

ported as 2 2 ns followmg

fronl all

tonal

and Keltaro

(smgle-exponential)

hfetnne

level studies

] and two of the dltluoro-

vlbronlc levels m these nlolecdes was reported, although recem observarrons of p~tlal -bra-

cay

1982

PKOSECOND FLUORESCENCE DECAY

IN ISOLATED PENTAFLUOROBENZENE

try of fluormated

10 Dccembcr

PHYSICS LETTERS

has been variously

265 run excitatron

re-

[S] and

as I 5 ns Followmg 270 run excltatlon [ 111. There IS therefore a need for more accurate decay tune meaof these molecules under colhston-free condltions. In this paper we report the results of such mensure-

eur~ments

cule, are more complex. As the number of fluorine substltuents

ments followmg

the absorption

tempt IS made to drstmgulsh between vlbrational cncrgy redlstrlbutlon and other mechamsms that could m-

Increases spectra become increasingly congested

!~euafluorobenzenc, the well-resolved vlbratlond structure of the benzene absorption untd. in penta- and

IS almost completely

vlbronic

lost. Excltatlon

to known

levels III these molecules IS therefore

sable but they have received some attention members or the series. Quantum

ternal conversion place. Radiative

as lsomerisatlon

Pcntafluorobenzene

[5,7]

although

III-

to the ground state may also take

and non-radlatlve

and hexafluorobenzenc

were

purlLed by trap-to-trap distillation on the vacuum line. Spectrograde n-pentane was fractionally dlstlllcd

with retention of the middle fraction. Decay ttmcs were measured following excltatlon with the Fourth harmomc of a mode-locked Nd .YAG laser. Streak unages of the fluorescence

decay rates for these

molecules have been calculated [S-IO] on the basis of fluorescence lifetime measurements. However, 350

2 Ekperirnental

yields of fluoresence

less than unity, even for excltatlon close to the The competmg process has been tenIdentified

duce non-erponenurdlty.

not posas the last

O-O transitlon. tatlvely

at 266 nm. Non-exponen-

single

and of triplet formatron have been measured [S-7] and 11has been found that the sum of+, and 4_ is much

excitation

rial decay was observed In both molecules and an at-

were obtained

with a silicon vldlcon (SIT) camera, dlgtised with a C-1000 Hamamatsu processor and processed in a NOVA 0 009-26

3 minicomputer.

14/S?/OOOO-0000/S

The streak camera was cali02.75 0 1982 North-Holland

CHEMICAL PHYSICS LWTTERS

Volonlc 93, ntnubcr 4

IO Dcccrr~bcr1982

brated by using a smgle laser puke and an etaion to produce a tram of pulses of accurately known temporal

separationfrom whxh an averagecahbratloncurve wascalculated Whenan e~per~i~ent~curve was obtained it was transferredimmcd~atelyfrom the streak camera to the minicomputer and corrected ~suxg one of these prevrously-stored caltbratron curves. Because the fluorescence from both compounds was weak single-shot decay curves were accumulated a number of times before andyws Low tenlper~(ures were achievedby ~~rcuia~~ng cold mtrogen gas round the sample cuvelre ln a quartz dewar. Absorptron spectra were recorded on a Gary 17 spectrophotometer and emlsslon spectra on a Shlmadzu RFSK! spectroffuorophotonreter.

3.

Resultsand discussion

The absorption spectra of penta- and hexafluoro. benzene measured at high resolutkon are dlustrated m fig. I. There is some evidence for vibrations structure in the penta~uorobenzene spectrum, the shoulders having 3 separation of X490 cm- I. This frequency could correspond to the totalIy symmetrx vs. nbra. tion (u = 474 cm- I) which ts active tn the emtsston specrrum of the pentzAluorobenzene molecular cation

IL

[ 13 J. In the hevafhtorobenzene spectrum, on the other hand. no wbrattond features are evident 11has been esttmatcd [ 14,l S] that the O-O transitrons he at 778 and 190 nm m penrafluorobenzene and he~3~uarobenzcne. respectrue&. ~onsequentiy the excess vrbrattonal energy m S, following evcltailon at %6 nm IS ~1600 CI~-~ rn C6HF5 and 3100 cm-t m CgF6. Moreover rt IS clear from tbc spectra shown m lig 1 that cxcttatron at 11~s wavelength populates both S1 and Sz m CeFG.The fluorescence spectra of both compounds COIISIS~of a broad str~~turelcss band extcndmg from 300 to 500 nm. These spectra were tndepctldent of sample ptcssure in the range I --Xl Torr With decreasmg c~~tat~on w3velengtlk the specfrurtl of IO Torr of CgHF5 rcrnamcd unaltered but tbar of C6F6 slufrcd shghtly to the red, as &mratcd rn fig. 3. Ths shft is c\pccted In a spectrunr donunsted by scquence tr~nsittons stnce excited state vtbrational frequencies are usually slightly smaller than ground st;rte ~requcneies.

In table i arc prcsentcd the results

0

220

230

zt0

2x

zeo

1

ofdecaytune

s

710

280

331

CHEMICAL PtlYSiCS LLTLRS

Volimc 9 3, ntrmbrr .I IIICJSU~CIIIL)IIISof four fluorinated

bcmencs under ISO-

latcd n~olcculc cond1ttons at 2Z°C Double cuponcn-

able dtfficultms particularly

m detecting ennssion at low pressures,

from hcxafluorobenzene.

Consequently

mrasurcd with a p~~oto~~~ul~~pirerand oscdloscope, end 1,I,4-Cr,t13FJ elrhtbttcd purely single exponential decay. The contrlbutton of the short-hvcd decay to tllc total mtenslty was grcatcr m C,F, than m

ll~e lifeme valuesglvcn for ths compound must be regarded as approximate. With the addri~on of 400 Torr of or-pentanc there ts a marked decrease in the mtcnsity of the short-lived component although the double exponential character of the decay pcrslsts. Smce 400 Torr corresponds, at a rough estimate, to a

CJlF$.

collision frequency or5 X IO9 s-l

~3s observed 111bo!h

C611F5 snd C&F& wl~cre.~stsolated p-C,C14Ft, the decay of WII~CII ws &I Jccsy

the effect of colhstonol relowrron on the doublcexpor\enttal decays we rcpc31cd the nwsurcments using dtfferent sample pressures end also using 400 Torr of a-pentanc as a buffer &IS. The results arc prescntcd in table 7, It LW be seen that at pressures bctwecn 200 mTorr and I Torr tbcre 1s In order

to mvestigate

little varirtion, wrthm experimentA

error, rn either

dcczy conskmt. In ~dditlon the co~ltr~bu~~on of exb component to the total intensity 1s also unchrrnyng

in tins pressure regton, so that we arc confident that collwon-rrec conditions perlnin in both CotllpoUtldS at thcsc prcssurcs As the prcssurc mcrcascs the gcncrdl trend, somewhat obscured by e.spcrmxntsl

fluctuattons, IS that

the vdluc of thcshorter lrfetut~e rer~~~tnsconst~nt, that of the longer 11fct1nic mcrrdses whde the mtcnstty contrrbutwn of the short-lived con~pot~e~~t decreases. It sl~ould hcrc be noted that we cxpcrlcnccd constdcr. Tabtc :! i’rc>Wred~p~[Id~~C~ ut the d‘!rJ) _-_

--

Compound -

C~lll 5

_.

-

--

I’rk!awrc (lurr)

011 iI 33 05

I IU ‘II I b) c6l

6

0 ‘2 06

._

-.___

01:f6ftt

r, WI)

_-

I11 r 40 173 * 36 176 i I7 111~ IO 116: 7 164 f 7 I?2 + I8 87i 47r

7 iI

I64 t II

10

l!O* II

I I>)

9J+

51 r

-_ _ _-__-_--3) Rough cstimatr ofmtcmtly

a, Al

rl (PS)

-_-_.---

t

10

5 dfld c6i 6 -_ __A-_

B IS

450 i 71) 7332 375 92Jt 173 195+ 61 710r 60 t115* IO5 970 ?. 64 1736t 564 z 7611

‘71

--u5 05 0.5 06 06 04 02 08

92

05

I-18

06

8l‘t f 59 705 5 42 857 f. 93

05 05 02

amlnhutlon of shor~~bved component h, I Torr wth 400 Tort 01 epcntdne added.

It is not surprismg that total vtbraltonal relaxation IS not aclueved wthm

the ~LIICscale of !_ilefluorescence. Titcr~forc we conthat we have no& measured a lifetime cltaracteristtc of the zero-pomt vtbrattonal level since the value of lhe longer decay tnne IS pressure dependent. Among the mechamsms that might lead to complex decay kmetics m the isolated molecule are the followrng {a) T~vo~onlponent decay has been observed m m-

clude

fermedrate C;ISCmolecules such as pyrazme [ 16). [ 171 and substttuted glyoxltls [IS]. How-

pynmtdinc

ever. these molecules are cllaracterlscd sn~all S,

-T, energy

by a relatlvcly

gap,whereasthis separationIII

CbHFg and CgFb 1s of the same size as the sepamtion In benzene Itself and tn the other fluorinated benzenes.

which are known to exhtbtt the exponentr~ decay expccted from moiecules in the staltstrcal Imut. (b) Vibrational redtstrtbution has been invest&ated in ground-state C,HF, [ 191 snd C,F, [ZO] but not II\ the emted states of these molecules Tlus phenomenon IS currently of intense research interest [71,77] and is expected to give rise to non~~ponentiai elctted-state r&\&ton. It has been observed m #ZCHJ F2 [3,4 j nt excess energtes for wluclt the density of vtbrational states IS as low as 4O/cm-t [“I_ The density ofstales nl 1600 cm-l ewcss energy m C6HFj, calculnted using HadrhoTs formula f23] and the ground-state vibratronal fr~quencles [24], ISalso =40fcm-‘ . A slrnllar calculatron wtb the ground-state frequencies of C,F, [2.5,26] ytelds a densrty of states of a-5 X IO4 al an cwcss cncrgy of 3000 cm-t. It Is lhcrcfore posslblc that,al the e\cltatlon wavelength used in the present study, double cxponcntlal decay could result from ~bratlona1 energy rcdIstr~bulion_ (c) Selecttve excitation of a single vlbromc ICYCIis not possible tn these molecules and the decay of many vlbratlonal lcvcls smmultaneously m&t lead to nonevponcntlal decay curves 1771. (d) Excitation of CGFc at 266 nm populates both the St and theSz evcitedstate. In C6HF, on the other

Volu~nc 93. number 4

CIiEhlICAL

IO Dcrclllircr

l’HYSlCS LLTTLRS

19x1

Table 3 TcmpcrJturr dcpcndLncc or IIIC dcuy ol C6111 5 dnd C61 6 (prowrc m both UIII~ICS = I Torr) -. ----_____ ________ Compounll

TPC)

Tl tp9

c(,llf

12 0 -23 -35

112i IlOi 163 + IlOt

5

22

G.16

-3

______

TZ IP~P

at .a’

4951 61 5282 ?I 738 t 303 670 t 100

06 02 01 01

762’1.88

OS

6402 88 - --~----

05 --

IO 14 17 ‘0

16-12 21

79t

____

10

--

d, Ruupli CS~IIILIIL’ ol ~nlcn~~ly lnntnbution wnlponcnl Energy/cm-’

to mvestigation

I‘lg 3 Thcrmol equtltbrmm ths~r~bul~onover the vtbr.~t~on~l cncrgy ln lhc ground ~ldlr: 01 pcntJtluorobcnzcnc .II (J) 2°C Jnd (b) -3S’C.

kngth cut-off

In ncithcr

It Ilas been pointed

trum

the thermal

equlltbrlum

energy in the electronic

dlstrlbutron ground

that

not bc IS

where

N(E)

bratlonal

exp(-E/k-T)

dE is the number

energy

between

the density of vlbrattonal butlon

functions

at 27’C

for C,HF,

,

of molecules

with

a VI-

of molecules above

22

may have

the zero-pomt

state and hence followmg excitation reach a level capable of mtermedrate strong coupling

with levels m S,. Such a level IS predicted properties

spec-

rcso-

from S1 can bc discounted.

to possess

[29].

(e) In C,F,

The

dcpendencc

of :IIC dec.~ys, lllustratcd m table 3, would seem to support this conclusion. WAm

errors no change was observed in tile apart from a decreased

from

peraturc

the short-lived

was lowered

from

for C6F6

-3°C.

As indicated

energy

distributions

displaced,

27 to -35°C.

tlonal

Fewer

repon between

at 22 and -3S’C

and 266

con-

as the tcmdata

niay

but the same conclusion

In fig. 3 the thermal 11111 CWll~llOn

should not populate

mtcnslty

component,

be drawn for the lempcraturc

drstn-

level In the ground

S, as well as S, radiative

temperature

stales. The calculated

cm-’

fluorcsc~n~c

for short-wavelcnglh

were obtained

fraction

as high as 4000

decay curve

than 390 nm. hlorcovcr,

nor the CgFg

E and E + dE and p(E) IS

m fig 3 It can be seen tllat

a non-neghgble

longer

the C6HF5

at wdvelenglhs

an Identical

Is there any evidence

tribution

at two temperatures,

ore dlustrated

and -35OC,

an energy

dl:‘

the emission

to that at wavelengths

the cxpcrimcnral decay of CgHF5,

means of the formulas dE a p(E)

However

at any rate fluorcsccnce

neglected when the energy of electromc c.\cltation considered. This distnbution can be calculated by

M(E)

W,NC.

by the USC of

nance fluorescence. The mdlcatlons are that m C6HF5

of vIbratIonal

state should

of the

dlffcrcnt

of the Cg HFg cniission

filters.

wavelength.

rcsolutlon

WC lsolatcd

than 290 nm ylcldcd

hand such a compllcatlon would appear to be absent smce lhe Onset of S2 absorpllon lies at a much shorter [X3]

wavclengtli

Accordmgly

rcglons

longer

out however

through

flaorescencc.

01 Wr~4wd

27 and

cqudlbrlum drc s~gmficrntly

of C,HFs

at -35°C

to .my signnlficant cvtcnt

levels m St capable

of couphng

vtbrJ-

WIIII Isoencr-

getlc levels m S2. The most plsuslble

evplandtron

would

appear

for the non-cupo-

nentral

behavlour

to be emrsslon

several

levels m S, CXII having a dlffcrent

from

decay tune.

very fast internal conversion from S, to S, could produce d dlstnbution of vibrational levels different from that produced by partial wbr;ltxon;rl re-

II has been shown that such B superpos~tron of decays can be analysed I” termsof a double nther thnn n WI&

organisatlon

ehponenttal

of mltlally

anism

IS thought

decay

In ahphatlc

Emrsston

cnerges

excited

to be responsible ammes

from S, might

than Sl emkon

Sl levels. Such a mechfor non-exponential

[30]. be expected and therefore

ftlr.7tion

of the mdmdual

decay times are

wdely spaced 130). Levels at higher energies WIN probably have shorter decay tunes and be less heavdy pop-

to he at higher

ulated

at higher

be amenable

sultmg

m a lcngthemng

pressures

or lower

temperatures,

of the longer

decay

rc-

tune un-

Volunic 93. nunlbcr 4

CXIEXIICAL PHYSICS LETTERS

der thcsr condltlons. vlbrattonal

energy

under dlscusslon

ofvarwon pressure III S,

means

There

Tlus ml&t

mcreascd.

be the reason for the lack

IS lowered

the inwstlgauon

between tllesc mechanisms

In order to dls-

results and those of prcvlous two-component

Furthermore

varia-

IS throu&

wavelength.

arc large dlscrepancles between

single e\ponentlal

levels

cannot be ruled out. The obvious

uon of the c\citatlon

We detect

or the

In addltion.couphngbctwecn

of e
There

that

hfcrune

measurements. decay

was reported

whereas

prevrously.

our values for the longer

cay tunes are In general

the present

picosecond

decay

much shorter

of the two dethan the smgle

exponential v,tlues previously reported. Wr cannot ascrlbe the dtscrepancles to dlffennces in the sample prsssurcs Smce our two-component decay- -SW. Aserved over a wde pressure range. We have checked our evpermlental as regards

condltlons

unpunty

very carefully,

and cuvette

especially

fluorescence

and con-

tammation from scattered hght and are satisfied that the observed two-component decay is not an artifact. We must therefore conclude that rcsolutlon of the decays Into two picosecond components was beyond the

capability of the decay time instruments wth wluch the previous measurements were made. This conclusion IS also supported

by the lack of consistency

the previously-reported

data.

In addtlon,

among

the value of

< I 6 m one of these reports [S] would seem to mdlcate that the actual Irl~et~e was shor~r than the time

resolution of the measurmg equipment.

4. Conclusion

It would tlon spectra other

appear that their lughly congested distmgulsh

fluonnated

C6HFS

benLencs

and CbF6

m which

from

absorpthe

monoexponentlal

decay 11x been observed. However, on the basis of the present experm~cnts other effeclscannot definitely be ruled out. Coolmg

of the molecules

in a supersonicjet

or varlatlon of the cxclting wavelength are means by which these effects could be dlstmguished We hope to report puzosecond

354

the results

References

on the time scale

m i-l as the tcmpcrature

and S, in C,F,

mgwh

IS also t!le posslbllity

is redistributed

of measurements

laser In the near future.

with a tuned

1 I ] A S Abraoson. t-I C. Spcas .md S A RIW, J Chcm. Phys 56(1972)X91. 121 C$uttlnann and S A. R~cc. 1. Chcm. Phys 6 t (t974) 131 R.A. CovclcshIc, D A. Dolvz.n and C.S. Pormenkr,

J.

Chcm Phys 72 (1980) 5774 [ 4) S H C,~blu, W.D Lawrence and A E.W. KmShr, J. Phys Chum 86 (1982) 1244.

151 S L. Lcm, C I’. Scmclul dnd I. Ungcr. Can J. Chcm 49 (1971) 1567. 161 Kh Al-Am and D. Phdhps. 1. Phyr Chcm 74 (1970) 4046. 171 D Phdhps, J. Phys Chrm 46 (1967)4679. [Sl C L Loper and C E C. Lcc.Chcm. Phys Lctrcrs 13 (197,) I40 191 G hl. Brcucr and E KC. Lee. Chcm Phya Lctlcrs 1-I (197’)404.

1’01 R C Bra\\” and D. Phdhps. J. I’holoLhur,. 3 (1974) 1’1 R G Oro~n and D. Phdhps. J. Chcm. Sot. 1’xxf;ly

337.

Tram II 70 (1974) 630. R.G lJrown, D. Phdhps and G. Das CupId, J. Phys.Chcm. 78 (197-1) 2407 131 C Co~urr-Xl~gos, D. Couarr and S. Leach, hlol. Phys 37 (197-l) 793. I (1972) 97 ‘-11 D. Phdhpb J. Phorochcm 151 H Sponcr. J. Chcm. Phys 22 (1954) 134.

II]

161 A r rad. F. Labmnni. A TrJmcr 2nd C. Trlc. J Cltcm. Phys. 60 ( 1973) 44 19. 171 K;. Uchlda. I Yam~zdhr Jnd ti tJaba,Chcm. Phys. Lcttcrs38 (1976) 133 181 R van dcr Wcrl rnd J. Kommandcur, Chcm Phys. I6 (1976) I15 191 R.C Sharp, C. Yablonowcl~ dnd N Blocmbcrgcn. J. Chcm Phys. 76 ( 1981) 2 147 S. SPLISUFand E. Crunwld. Chcm Phys. Lcttcrs 73 (1980) 438 J D. UcDonAld Ann Rev Phys.Chcm 30 (19’19) 19.

C S. P.trmcntcr, J Phys Chem. 86 (1982) 1735. PC. Huhoff, Mot Phys 7 (1963) 101. S G. I‘ranhlss and DJ. Hnrrlson. Spccrroclnm. Acln 3lA (1975) 1839. R A R. Pearce, D. Steele and K. Radchffc. J. Mot. SOW. I5 (1973) 409. V-l. Eaton and D Steele. J hlol. Spcclry. 48 (1973) 446. R-P H. Rcllschruck. m: Radutlonlcrs lrrlnslllons, cd. S II. Lm (Acddcmlc Press, New York, 1980) p. I94

T. Dcmum. C J. Wcrkhovcn, J. Lengclaar, R.P.H. Rertschmck and J.D.W. mn Voorst,Chcm. Phys. Lellcrs 27 (1974) 206. 1291 A. NIIZIII. 1. Jorlncr .md PM. Rentzcp~s. Proc. Roy Sot. A327 (1972) 367. 1301 C-l. Cureron. K. Han. D V. O’Connor and D. Phdhps, Chcm. Phys. 63 (1981) 31.