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Fluorescence excitation spectra and decay times of jet-cooled dibenzofuran and the dibenzofuran-water complex
Voltune 112. number 6
CHEMICAL
FLUORESCENCE
EXCITATION
OF JET-COOLED
DIBENZOFURAN
Andrew
R. ALJTY. Anita
SPECTRA
PHYSICS
26 Dccembcr
LETTERS
AND DECAY
TIMES
AND THE DIBENZOFURAN--WATER
C_JONES
1984
COMPLEX
and David PHILLIPS
Tlrc Royal ltrsritlrrion,21 Alhemadc? Street. Lomiot~ IVlX IBS. UK Received
3 August
1984:
in fiial
form
17 October
1984
The fluorescence excitation spectrum of dibenzofuran cooled in a supersonic jet has been obtained, and fluorescence transition and that at +-?I1 cm-’ : the decay d:cJ) times mcasurcd rls 14.8 + 0.2 ns for both the origin band of the AI -At tinle for t11c444 cm-’ band was found to bc 11.S + 0.3 ns. In the presence of water. new bands in the fluorescence exitadecay time of this Con spectra appear at 17s cm-l to the blue of both the orgin and 444 cm -’ bands. The fluorescence assumed dibcnzofuran-water, hydrogen-bonded complex, is 6.5 2 0.5 ns, indicating strong perturbations to the decay charactcrictics of dibcnzofumn by the water.
1. Introduction Jortneret
excitation
2. Experimental
al. [ 11 recently reported the fluorescence spectrum of S-methyl-indole-water Co~ll-
Dibenzofuran (DBFN) (Aldrich Gold Label, used without further purification) was heated to 100°C in a stainless steel sample chamber: the vapour produced was mixed with helium or argon carrier gas and aliowcd to cxpdnd through a continuous flow nozzle to produce a supersonic molecular beam. The background pressure in the expansion dumber ~3s maintained at
plexcs under supersonic jet conditions. They have shown that the spectral shifts of the excitation of the complex with respect to the O-O transition and to a bmd at 7 10 cm-l above the origin are identical. This casts doubt on the identification of the latter as the origin of the 1 La + So transition. There have been several
bonded
reports
=30-t
of fluorescence
complexes
from other hydrogenin jet conditions [Z-51.
As a part of a study
investigating
effects
of hctero-
atom substitution on aromatic systems such as fluorcne, we have sought to create aromatic--water complexes in a jet. We report here the fluorescence of such a complex between dibenzofuran and water. The electronic spectroscopy of dibenzofuran has been investigated in the vapour phase at elevated temperatures. in durenc. cyclododecane and biphenyl host crystals at 4 K [6-81, and in microcrystal form. Two transitions are seen in the 300-180 nm region. The first a short-axis polarised (Al + Al) transition at 9 the second a long-axis polarised (B, f33645 cm-’ At) transition with an onset 1740 cm-* above the A, +A, onset. A prominent band at 44$cmm1 above
Science Publishers Division)
vapour booster
pump
400 and 1600 mbar with a circular nozzle. dimctel 300 jml. DBFN-H,O ducing 27 mbar before it entered
were produced by introvapour into the carrier Sas the sample chamber. This was achieved by passing the carrier gas over distilled water maintained at 23°C. Under the expansion cor.clitions produced using a slit nozzle and argon carrier gas. it was found impossible to observe complex formation. presumably due to inadequate cooling of the iIIterXtiIIg molecules. Complex formation was. however, observed using helium carrier gas and with PD > 20 Torr cm,
the A, + Al origin has been assigned to a vibronic transition induced by a vibration of b, symmetry on the basis of polarisation data 191. 0 009-2614/84/S 03.00 0 Elsevier (North-Holland Physics Publishing
mbar by an Edwards-type
(pumpingspeed 4000 P s-l) backed by and Edwardstype rotary pump. The following range of expansion conditions were used: (i) argon carrier gas at stagnation pressures between 20 and 200 mbar with a 4 111111 X 500 ,ull slit nozzle. or (ii) helium carrier gas between
complexes
of water
where P is the carrier gas pressure and D the nozzle diameter_ 13-V.
529
Volume 112, number 6
CHEMKAL
FIuorcscence excitation spectra were produced using. as the excitation source, a 450 W high-pressure xenon arc Lamp (continuous output) in conjunction with a Rank-Precision Monospek 1000 scanning 1 m monochromator with a grating blazed at 300 nm. reciprocal dispersion 8 W/mm. The output from the ll~o~~oci~rol~~ator ws focused into the molecufar beam a1 ct distance of 7 mm downstream from the nozzle. Torsl. unfiltered fluorescence was collected at right angles to both excitation and expansion by anf/ 1 quartz Icns and focused onto 211 Eh$I XP2020Q photonwltipher lube operated in single photon counting-mode. The pho~omuItipltcr output was rccordcd on a Canbcrra-type ~~~ultic~~anIle1scaler which w~ssynchronised !vith the monoohromator stepphI: motor. This allowed Couuts to be accul~lu~ted over a suffkict~t period to obtain an aeocptsblc signai to noise ratio. Fluorcscc~~cc lifctinxs were mcdsurcd using the tune-torrclatcd sirlgle photon counting technique. The cxc~tation source ~4s the frcqucney-doublcd output of a Spectra Physics s~nchroIiously-pumped, cavitydunlpcd. .ugon ion dye laser system, opcr.ltl?d at d lcpetitiort rate of4 Mi-lz. The frequency doubled laser Irght (IO cm -I fwhm) wzs Cocuscd into the expansion bedill iIIld fhC flUOrCSCCt1CC COIICCtCd aIld detected aS Jbove with the i]i~lusioI~ of an interference filter bcfurc the photomult~plier tube to cut off scattered I.WZI Iight. A detailed descri~~tioii of the iile~s~lreil~en~ ~~1 ,mtlysis ot~lluorcsocwx decay data has been given clwsllcIr 1IO] _ Fluore> . et~cc spcetra were produced using the frcyucri~y-dor~blcd laser output for cscitation; the Icsuftiuy fkto~tscencc was wavcIc11gt21 dispersed then dctcctcd and recorded. as described above.
3. RsuIts
WAVELEN
Gil4
1984
ii
l’ig. 1. I‘luoresccnce ewitlttion spectrum from the origin {O-O) of DBI’N up to 1150 cm-‘. Resoiution is 12 cm-’ fwhm. Prcwurc of argon carrier pas was 700 mbnr and flte 1107xk 0rifk.c n1C.wrcd 4 mm X 500 grn.
pared with those obtained from gas phase absorption spectra [G]: there is good ngreement. A lower-cncl~y absorptiorl system at 31GSO CIW-~ was obscrvcd in the vapour by Pinkhn and Wait [ 11)I this absorption could not be located m a subsequent invcsti~tion by Brce et al. [8]. however_ axxl was not detected in the present cxpcrimcnts. The lowest energy band at 33617 CID-~ is therefore assigned to the origk of the St *So, A, + A, transition [8j_ The high relative intensity of the ori@n band indicates the pure electronic transition to be synlit~etr~’ alloxved.
and discussion
The or~girl legion of the fluorcscencc escltation spcctrunl of jet-cooled dtbcnzofuran is sl~ow~~in fig. I. the rcsoturion of the instrument being limited to I1 cm- * fwhrn. It c’.m be seen ill& this spectrum is comptc~cly free from the considerable sequence con~WIUII prcscnt in previously recorded gas phase absorption spectra (fig:. 2). In table 1 the frequcucies of the observed excited stnte vibrational rnodcs are corn-
530
28 December
PHYSICS Ll3-I-ERS
i 2972
2900
2935 w!vGmlgth
F-i:_ 3.
Room-tempcraturc
DBIW
from
the origin
pas
(marked
2860
A
phse absorption O-O)
fo 1200
+pcctrum of cm-’
adapted
from ref. [Z}. Vertical fines mark the positions of peaks mezured by us. These correspond with the assignments made in ref. 19-1 to within
5 cm-l.
Volume
112. number
CHEMICAL
6
PHYSICS
LEITERS
Vapour (cm-‘)
33647 a) O-O 71-1 b)
33645 209.5 349 444 550 651 691 713
;48 444 549 649 690 709 85Oqh 860 877 901 986 1017 1079 1102 1150 al Accuracy
b, I‘rcquencies
frequency
O-O
I-
3092 Fi. 3. The fluorescence spectrum of DBFN following (a) O-O excitrltion, (b) excitation of 313 cm-l band, (c) ewzit;ltion of 444 cm-l band. Resolution is instrument limited to 20 cm-’ fwhm.
857 876 898 983 1017 1080
(fig. 3b)is the&=0 transition (wliereuis~~ibrational quantum number). This band is slightly red-shifted (=7 cm-l) from the O-O fluorescence transition, as a result of the difference in ground and e.xcited state
1151
f 6 cm-’ _ in this column
accurate
1984
cl
Table 1 Comparison of the vapour phase absorption spectrum frequcntics [6] and the fluorcscencc excitation frequencies mcasurcd here Jet frequency (cm-’ )
28 December
to i2 cm-‘.
in the excitation spectra involving vibrations with a1 symmetry are thus expected. [9] been asThe &kI cm -1 mode has previously signed as being of b, symmetry appearing in the spcctrum as a result of vibronic coupling with the S2 electronic state which is of B, symmetry [S], lying
vibrational
frequencies.
No fluorescence
was observed
Progressions
< 1800 cm-l the anomalous
Table 2 Assig3mcnt
excitation
ofthe
fluorescence spectrum resultingfrom
into the O-O band. Origin at 33647 cm-’
cm-’
above S, [9,12]. This could account for intensity of this band in our excitation
Assignment
214
a1
(218)
427
a,
(425)
557 615
bz b2
(556) (616)
659
al
(659)
742 850
31 31
(746) (851)
spectrum. In order to investigate the extent of vibronic COUphng between the S, and S2 and the possible activity mode in this coupling, dispersed of the 444 cm-l fluorescence spectra following excitation of the O-O, 212 cm-1 and 4.44 cm-l bands were measured. These spectra are shown in fig. 3. An assignment of the fluorescence spectrum arising from the zero-point level of Sl (fig. 3a) is given in
871 1007
h, (871) a, (1010)
1067 1114
al (1064) bz (1114)
1199 1124
b, (1193) a1 (1242)
table 2; a large number
1383
b, (1282)
of b, vibrational
in the spectrum, indicating of vibronic coupling between the states_ The highest-energy, and most fluorescence from the 2 12 cm-1
active
models arc
a considerable degree SI(AI) and SZ(B2) intense feature in vibrational level
a) Ground
1308
“1 (1308)
1358 1414 1460
a1 (1350) 214+1199 214-I-1244
state frequencies
from ref.
a)
[IS]. 531
Volume
112. number
CHEMICAL
6
PHYSICS
from tlic 212 cm-* level at the excitation wavclengrh. Fran&--Condon factors clearly play a dominant role in determining the transition ptobabilities from this vibrational
state
and
the
prccloIninancc
of the
&
=
that there is no significant change in the normal coordmate origin in excitation. Similar behavior 11~sbeen observed in the fluorescence spectra of cscltcd vlbronic levels of other jet-cooled ~nolecuics e.g. dnthwne [ 131 and free base phthaiocyaninc [ 141. in the latter case Fetch ct al. found the position of the fluorcsccnce spectrum to be independent of‘ excitation cncrgy for excitation of lcvcls with up to i 134 cm- i of vibrational energy. Thesan~e vibrational Intcrv& arc present in the 2 12 cm-* spectrum as in the O-O fluorcscencc, with the addition of ;i band at X33 c111-l which IS assigned as a combination: 427 cm1 (a,) f 557 cnl-I (b,). As si&~Il in fig. 3c, the-highest-energy and most intcnsc feature in fluorcscencc 110111 the 444 ctii-I level is not the Au = 0 transition. but rcsonancc fluorcsc‘ence, corresponding to a transition from the pumped level to the zero-pomt jcvcl of the ground state. This supports the d5sgnmenr ot the 444 c111-~ n10dc as one of b., symmetry which 1s actIvc iI vibronicaily coupltng the S, and S2 stdtcs. The othcl prominent h,md in this spcctmn colIcsponds to a gound state b, I‘~~~~ld~iic~il,~l of frcqucn~y 557 cm-l _Tile high rclJtivc intcnrity 0i this transttlon leads us to assign it as tJlc LUJ= 0 Franck-Condon favoured transitIon, 0 bdnd indicates
ag~l11 coIlfirnlIIlg
Itic b2 assigIlIllent
of
rhc 434
LETTERS
28 Dcccmbcr
the origin frequency was a single exponential with lifetime of 14.8 +- 0.3 ns. This is notably
1981
function, shorter
than the corresponding
lifetime of 23 ns for jet-cooled fluorene. measured by Amirav el al. [ 131, probably as 3 result of the greater oscillator strcngtli for tlic St c S, transition in DBFN [ 1 I] or possibly an increased rate of intersystem crossing associated with the presence of the oxygen heteroatom. The lifetime offluorescence produced byexcitinginto the 2 12 c111-~ band was identical, within experimental error. to that of tlic origin, whereas the fluorescence lifctimc of the 444 cnlml band wasslightly shorter. namely 1 I_8 f 0.3 ns. Non-radiative decay of the S, state of DBFN in condensed phases occurs as a result of intersystm classing to T, which lies sonic 8500 cm-’ 1owc1 in energy [ ll.Jlj. The T, state has B-, symmetry; thus. dil ect spin-ol bir coupling between-a sta tc of A1 symmetry and T, issyrmnctry aliowed.whercas spirlorbit coupling between a state ofoveralJ B2 symmetry and T, issymmetly forbidden. The popdat~on ofa b2 vibronicstatc mght thus be expected to lead to a decrease in the rdte of radiationless decay. unless vibj onitally
induced
intcrsystem
crossmg
proceeds at a substantial c~ce~ice
Jifctime
from
this state
rate. The decrease in fluol-
asso&tcd
with popuintion
of the b9
specks cd11 bc attributed to an increase in the radiative decay ralc as a result of the strong induced vlbronic
coupling
between
the S, and S-, states.
cn-’
Illrldc.
LXC~ Cl ai. [is]
~JVC
predicted
coupJ111g Icrltis IO a dccrcase c~urnc~c~ ofvibIo111~aJJy l‘rcc~ue~lcy
bring
gvcrl
In the
dcIIvc
that viblonic excited state
modes.
apJ~rosimatcly
The introductton gas flow
lic-
tile dccrcxc
111
by
&_I = --2oZIM.
Af:’
11~ diffctcIiLc
tiic c~uplcd
coupling
bctwccn
clcctronic
matris
11~ /clo-point
cicnicnt lcvcls
md of
ground mid cxcitctl slate lrcqucnmode” in dibcmofur,m is LOIISISICII~wth ti11s tlicory dnd the use of tile above CApXiOIl allows the nlagIlitude of the vibronic Ltlupling ni.ktri?r eicnicnt to bc cstimatcd ds ==313 The dhprity
LIC!l l-01 ti1c
ill1
-1
“434
in
c111-’
.
lhorcscence dLIy of DBFN, cooled by cxp.~nsron 1111GO0 mhar helium. following cxcrtation
Tllc
at
by
mto
the canicr
the appcarancc
of
two
weak
higlicr
in energy
than
the 444
cm-l
transition
with a vibrational feature in the lA1 * A1 spectrum. However, with water vapour present in the expansion. a shouidel appears - at this resoiuLIOII - indicating the prcscncc of the water-DBFN conq~lex. No further new bandswere apparent at higher energies as a result of the increased congestion of the spectrum at higher vibrational energies and the relatively Jaw intensity of the other vibronic bands. These addItiona transitionsare attributed to a van der Waais nloiecuiar co11lJ~kX formed bctwccn DBFN and water. the water molecule being Jlydrogcn-bonded to tllc lhcrc
states.
of water vapour
xcompanicd
bands in the fluorcscencc excitation spectrum. As shown in fig. 4, the first band was bhteshifted by 178 cm-* from the origin band. At 178 cm-l
u Is. lhc ~Ibronic
wilele
Jddilionai
was
is
OVCrldp
Volume
112. number
CHEMICAL
6
PHYSICS
LETTERS
28 December
1984
implies that the quantum yield of emission is greatly reduced by complexation. While it is possible that hydrogen-bonding of water reduces the oscillator strength for the A, +- A 1 transition, the fact that the decay time shortens drastically must imply that non-radiative decay is enhanced by complexation. 235
212
lee
Relet
165 we
I‘d25
Frequency
212
ree
105
ld2
Acknowledgement
km-‘)
I‘&!. 4. The fluorcseence cxitation spectrum of DBI‘N around the 212 cm-’ line shows the effect of adding water vapour to the c\pansion. Conditions in 3.1 arc 1 atm of hehum miwd with DBFN .I[ 100°C expanding through a 300 pm circular nozzle and excited at 7 mm downstream from the nozzle. The new peak at 178 cm -* Ls produced by adding 27 mb.u of Hz0 lo the helium carriergds. A simile peak appcdrs asan unresolved shoulder 178 cm-’ to the blue of the peak a1 444 cm-’ _ The intcnsily of the vibrational pedk in 3b is less than half that of 3d
oxygen heteroatom of DBFN. The blue-shift in the excitation spectrum produced by complex formation . indicates that hydrogen-bonding of this type leads to stabilisation OF the ground state of the complex rclativc to the excited state: This suggests that the S, +-So transition in DBFN, although largely n--i?* in nature, has sonic charge transfer character, giving rise to a decrease in electron density on the oxygen atom, and
hence a weakening of the hydrogen bond, in the St state. In the case of the water complex with S-mcthylindolc, the opposite is true; increased electron density on the nitrogen hcteroatom could explain the observed red-shift_ There is a clear need for improved calculation of electron density in the excited electronic states of such systems. The fluorescence lifetime of the DBFN-Hz0 complex. following excitation into the origin band, was found to be 6.5 -+0.5 ns. o‘he large uncertainty in this value is due to the relatively poor quality of the data associated with the low fluorescence intensity.) Clearly complex formation has a significant effect on thC dynamics of the excited state decay of DBFN, resulting in a decrease in the fluorescence lifetime by a factor of two. Under conditions such that the intensity of fluorescence from the bdre molecule is reduced by half in the presence of water. the resulting intensity of fluorescence from the complexes is less than ten percent of that from excitation of the bare molecule. This
We are grateful to the Science and Engineering Research Council for generous financial supporr, and to Mr. David Madill and Mr. Bruce hlorris for their excellent technical assistance with the costruction of the jet apparatus. References [ 1J R. Bersohn. !2 j [3J [SJ [5j [6] j7j [8 J 19 J [ 101 Ill] 117-l 1131 [ 141 115 j [16j
[ 171 f 181
U. Even and J. Jortner. J. Chem. Phys. 80 (1984) 1050. H. Abe, N. Mikami and Y. Udapawa, Chem. Phys. Letters 93 (1982) 217. N. Gonohe, H. Abe, N. Mikami and M. Ito. J. Phys. Chem_ 87 (1983) 4406. Y. Nrbu, H. Abe, N. hlikami and hl. Ito, J. Phys. Chem. 87 (1983) 3898. Y. Tomioha, H. Abe, N. hlikami and hl. Ito. J. Phys. Chem. 88 (1984) 2263. A.R. Lxcy. A.E.W. Kni$Jtt and LG. Ross. J. hlol. Spectry. 47 (1973) 307. C. Taliani, A. Brec and R. Zwanch, J. Phys. Chem. 88 (1964) 2357. A. Bree, V.V.B. Vilkos and R. Znarich. J. Mol. Spectry. 48 (1973) 135. A. Bree, A.R. Lacey. 1-G. Ross and R. Z\\arich, Chem. Phys. Letters 76 (1974) 329. D.V. O’Connor and D. Phillips, Time-correlated singlephoton counting (Academic Press, New York, 1984). CA. Pinkham and SC. Wait Jr., J. hlol. Spectry. 27 (1968) 326. C. Ausscms. S. Jaspcrs. G. Leroy and F. v.m Rcmoortere. Bull. Sot. Chim. Bclgcs 78 (1969) 479. W.R. Lambert, P.M. Fclkcr and A.H. Zewdil. J. Chcm. Phys. 75 (1981) 59-58. P.S.H. Fitch, L. Wharton and D.H. Levy, J. Chum. Phys. 70 (1979) 2018. A.R. Lace)‘, E-1‘. McCoy and 1.G. Ross. Chem. Phys. Letters 21 (1973) 233. A. Amirav, U. Even and J. Jortner. J. Chem. Phys. 67 (1982) 1. R.N. Nurmukhamctor and C.V. Gobo, Opt. Spectry. 18 (1965) 126. A_ Brec, V.V.B. Vilkos and R. Zwarich, J. hlol. Spcctry. 48 (1973) 124.
533
CHEMICAL
FLUORESCENCE
EXCITATION
OF JET-COOLED
DIBENZOFURAN
Andrew
R. ALJTY. Anita
SPECTRA
PHYSICS
26 Dccembcr
LETTERS
AND DECAY
TIMES
AND THE DIBENZOFURAN--WATER
C_JONES
1984
COMPLEX
and David PHILLIPS
Tlrc Royal ltrsritlrrion,21 Alhemadc? Street. Lomiot~ IVlX IBS. UK Received
3 August
1984:
in fiial
form
17 October
1984
The fluorescence excitation spectrum of dibenzofuran cooled in a supersonic jet has been obtained, and fluorescence transition and that at +-?I1 cm-’ : the decay d:cJ) times mcasurcd rls 14.8 + 0.2 ns for both the origin band of the AI -At tinle for t11c444 cm-’ band was found to bc 11.S + 0.3 ns. In the presence of water. new bands in the fluorescence exitadecay time of this Con spectra appear at 17s cm-l to the blue of both the orgin and 444 cm -’ bands. The fluorescence assumed dibcnzofuran-water, hydrogen-bonded complex, is 6.5 2 0.5 ns, indicating strong perturbations to the decay charactcrictics of dibcnzofumn by the water.
1. Introduction Jortneret
excitation
2. Experimental
al. [ 11 recently reported the fluorescence spectrum of S-methyl-indole-water Co~ll-
Dibenzofuran (DBFN) (Aldrich Gold Label, used without further purification) was heated to 100°C in a stainless steel sample chamber: the vapour produced was mixed with helium or argon carrier gas and aliowcd to cxpdnd through a continuous flow nozzle to produce a supersonic molecular beam. The background pressure in the expansion dumber ~3s maintained at
plexcs under supersonic jet conditions. They have shown that the spectral shifts of the excitation of the complex with respect to the O-O transition and to a bmd at 7 10 cm-l above the origin are identical. This casts doubt on the identification of the latter as the origin of the 1 La + So transition. There have been several
bonded
reports
=30-t
of fluorescence
complexes
from other hydrogenin jet conditions [Z-51.
As a part of a study
investigating
effects
of hctero-
atom substitution on aromatic systems such as fluorcne, we have sought to create aromatic--water complexes in a jet. We report here the fluorescence of such a complex between dibenzofuran and water. The electronic spectroscopy of dibenzofuran has been investigated in the vapour phase at elevated temperatures. in durenc. cyclododecane and biphenyl host crystals at 4 K [6-81, and in microcrystal form. Two transitions are seen in the 300-180 nm region. The first a short-axis polarised (Al + Al) transition at 9 the second a long-axis polarised (B, f33645 cm-’ At) transition with an onset 1740 cm-* above the A, +A, onset. A prominent band at 44$cmm1 above
Science Publishers Division)
vapour booster
pump
400 and 1600 mbar with a circular nozzle. dimctel 300 jml. DBFN-H,O ducing 27 mbar before it entered
were produced by introvapour into the carrier Sas the sample chamber. This was achieved by passing the carrier gas over distilled water maintained at 23°C. Under the expansion cor.clitions produced using a slit nozzle and argon carrier gas. it was found impossible to observe complex formation. presumably due to inadequate cooling of the iIIterXtiIIg molecules. Complex formation was. however, observed using helium carrier gas and with PD > 20 Torr cm,
the A, + Al origin has been assigned to a vibronic transition induced by a vibration of b, symmetry on the basis of polarisation data 191. 0 009-2614/84/S 03.00 0 Elsevier (North-Holland Physics Publishing
mbar by an Edwards-type
(pumpingspeed 4000 P s-l) backed by and Edwardstype rotary pump. The following range of expansion conditions were used: (i) argon carrier gas at stagnation pressures between 20 and 200 mbar with a 4 111111 X 500 ,ull slit nozzle. or (ii) helium carrier gas between
complexes
of water
where P is the carrier gas pressure and D the nozzle diameter_ 13-V.
529
Volume 112, number 6
CHEMKAL
FIuorcscence excitation spectra were produced using. as the excitation source, a 450 W high-pressure xenon arc Lamp (continuous output) in conjunction with a Rank-Precision Monospek 1000 scanning 1 m monochromator with a grating blazed at 300 nm. reciprocal dispersion 8 W/mm. The output from the ll~o~~oci~rol~~ator ws focused into the molecufar beam a1 ct distance of 7 mm downstream from the nozzle. Torsl. unfiltered fluorescence was collected at right angles to both excitation and expansion by anf/ 1 quartz Icns and focused onto 211 Eh$I XP2020Q photonwltipher lube operated in single photon counting-mode. The pho~omuItipltcr output was rccordcd on a Canbcrra-type ~~~ultic~~anIle1scaler which w~ssynchronised !vith the monoohromator stepphI: motor. This allowed Couuts to be accul~lu~ted over a suffkict~t period to obtain an aeocptsblc signai to noise ratio. Fluorcscc~~cc lifctinxs were mcdsurcd using the tune-torrclatcd sirlgle photon counting technique. The cxc~tation source ~4s the frcqucney-doublcd output of a Spectra Physics s~nchroIiously-pumped, cavitydunlpcd. .ugon ion dye laser system, opcr.ltl?d at d lcpetitiort rate of4 Mi-lz. The frequency doubled laser Irght (IO cm -I fwhm) wzs Cocuscd into the expansion bedill iIIld fhC flUOrCSCCt1CC COIICCtCd aIld detected aS Jbove with the i]i~lusioI~ of an interference filter bcfurc the photomult~plier tube to cut off scattered I.WZI Iight. A detailed descri~~tioii of the iile~s~lreil~en~ ~~1 ,mtlysis ot~lluorcsocwx decay data has been given clwsllcIr 1IO] _ Fluore> . et~cc spcetra were produced using the frcyucri~y-dor~blcd laser output for cscitation; the Icsuftiuy fkto~tscencc was wavcIc11gt21 dispersed then dctcctcd and recorded. as described above.
3. RsuIts
WAVELEN
Gil4
1984
ii
l’ig. 1. I‘luoresccnce ewitlttion spectrum from the origin {O-O) of DBI’N up to 1150 cm-‘. Resoiution is 12 cm-’ fwhm. Prcwurc of argon carrier pas was 700 mbnr and flte 1107xk 0rifk.c n1C.wrcd 4 mm X 500 grn.
pared with those obtained from gas phase absorption spectra [G]: there is good ngreement. A lower-cncl~y absorptiorl system at 31GSO CIW-~ was obscrvcd in the vapour by Pinkhn and Wait [ 11)I this absorption could not be located m a subsequent invcsti~tion by Brce et al. [8]. however_ axxl was not detected in the present cxpcrimcnts. The lowest energy band at 33617 CID-~ is therefore assigned to the origk of the St *So, A, + A, transition [8j_ The high relative intensity of the ori@n band indicates the pure electronic transition to be synlit~etr~’ alloxved.
and discussion
The or~girl legion of the fluorcscencc escltation spcctrunl of jet-cooled dtbcnzofuran is sl~ow~~in fig. I. the rcsoturion of the instrument being limited to I1 cm- * fwhrn. It c’.m be seen ill& this spectrum is comptc~cly free from the considerable sequence con~WIUII prcscnt in previously recorded gas phase absorption spectra (fig:. 2). In table 1 the frequcucies of the observed excited stnte vibrational rnodcs are corn-
530
28 December
PHYSICS Ll3-I-ERS
i 2972
2900
2935 w!vGmlgth
F-i:_ 3.
Room-tempcraturc
DBIW
from
the origin
pas
(marked
2860
A
phse absorption O-O)
fo 1200
+pcctrum of cm-’
adapted
from ref. [Z}. Vertical fines mark the positions of peaks mezured by us. These correspond with the assignments made in ref. 19-1 to within
5 cm-l.
Volume
112. number
CHEMICAL
6
PHYSICS
LEITERS
Vapour (cm-‘)
33647 a) O-O 71-1 b)
33645 209.5 349 444 550 651 691 713
;48 444 549 649 690 709 85Oqh 860 877 901 986 1017 1079 1102 1150 al Accuracy
b, I‘rcquencies
frequency
O-O
I-
3092 Fi. 3. The fluorescence spectrum of DBFN following (a) O-O excitrltion, (b) excitation of 313 cm-l band, (c) ewzit;ltion of 444 cm-l band. Resolution is instrument limited to 20 cm-’ fwhm.
857 876 898 983 1017 1080
(fig. 3b)is the&=0 transition (wliereuis~~ibrational quantum number). This band is slightly red-shifted (=7 cm-l) from the O-O fluorescence transition, as a result of the difference in ground and e.xcited state
1151
f 6 cm-’ _ in this column
accurate
1984
cl
Table 1 Comparison of the vapour phase absorption spectrum frequcntics [6] and the fluorcscencc excitation frequencies mcasurcd here Jet frequency (cm-’ )
28 December
to i2 cm-‘.
in the excitation spectra involving vibrations with a1 symmetry are thus expected. [9] been asThe &kI cm -1 mode has previously signed as being of b, symmetry appearing in the spcctrum as a result of vibronic coupling with the S2 electronic state which is of B, symmetry [S], lying
vibrational
frequencies.
No fluorescence
was observed
Progressions
< 1800 cm-l the anomalous
Table 2 Assig3mcnt
excitation
ofthe
fluorescence spectrum resultingfrom
into the O-O band. Origin at 33647 cm-’
cm-’
above S, [9,12]. This could account for intensity of this band in our excitation
Assignment
214
a1
(218)
427
a,
(425)
557 615
bz b2
(556) (616)
659
al
(659)
742 850
31 31
(746) (851)
spectrum. In order to investigate the extent of vibronic COUphng between the S, and S2 and the possible activity mode in this coupling, dispersed of the 444 cm-l fluorescence spectra following excitation of the O-O, 212 cm-1 and 4.44 cm-l bands were measured. These spectra are shown in fig. 3. An assignment of the fluorescence spectrum arising from the zero-point level of Sl (fig. 3a) is given in
871 1007
h, (871) a, (1010)
1067 1114
al (1064) bz (1114)
1199 1124
b, (1193) a1 (1242)
table 2; a large number
1383
b, (1282)
of b, vibrational
in the spectrum, indicating of vibronic coupling between the states_ The highest-energy, and most fluorescence from the 2 12 cm-1
active
models arc
a considerable degree SI(AI) and SZ(B2) intense feature in vibrational level
a) Ground
1308
“1 (1308)
1358 1414 1460
a1 (1350) 214+1199 214-I-1244
state frequencies
from ref.
a)
[IS]. 531
Volume
112. number
CHEMICAL
6
PHYSICS
from tlic 212 cm-* level at the excitation wavclengrh. Fran&--Condon factors clearly play a dominant role in determining the transition ptobabilities from this vibrational
state
and
the
prccloIninancc
of the
&
=
that there is no significant change in the normal coordmate origin in excitation. Similar behavior 11~sbeen observed in the fluorescence spectra of cscltcd vlbronic levels of other jet-cooled ~nolecuics e.g. dnthwne [ 131 and free base phthaiocyaninc [ 141. in the latter case Fetch ct al. found the position of the fluorcsccnce spectrum to be independent of‘ excitation cncrgy for excitation of lcvcls with up to i 134 cm- i of vibrational energy. Thesan~e vibrational Intcrv& arc present in the 2 12 cm-* spectrum as in the O-O fluorcscencc, with the addition of ;i band at X33 c111-l which IS assigned as a combination: 427 cm1 (a,) f 557 cnl-I (b,). As si&~Il in fig. 3c, the-highest-energy and most intcnsc feature in fluorcscencc 110111 the 444 ctii-I level is not the Au = 0 transition. but rcsonancc fluorcsc‘ence, corresponding to a transition from the pumped level to the zero-pomt jcvcl of the ground state. This supports the d5sgnmenr ot the 444 c111-~ n10dc as one of b., symmetry which 1s actIvc iI vibronicaily coupltng the S, and S2 stdtcs. The othcl prominent h,md in this spcctmn colIcsponds to a gound state b, I‘~~~~ld~iic~il,~l of frcqucn~y 557 cm-l _Tile high rclJtivc intcnrity 0i this transttlon leads us to assign it as tJlc LUJ= 0 Franck-Condon favoured transitIon, 0 bdnd indicates
ag~l11 coIlfirnlIIlg
Itic b2 assigIlIllent
of
rhc 434
LETTERS
28 Dcccmbcr
the origin frequency was a single exponential with lifetime of 14.8 +- 0.3 ns. This is notably
1981
function, shorter
than the corresponding
lifetime of 23 ns for jet-cooled fluorene. measured by Amirav el al. [ 131, probably as 3 result of the greater oscillator strcngtli for tlic St c S, transition in DBFN [ 1 I] or possibly an increased rate of intersystem crossing associated with the presence of the oxygen heteroatom. The lifetime offluorescence produced byexcitinginto the 2 12 c111-~ band was identical, within experimental error. to that of tlic origin, whereas the fluorescence lifctimc of the 444 cnlml band wasslightly shorter. namely 1 I_8 f 0.3 ns. Non-radiative decay of the S, state of DBFN in condensed phases occurs as a result of intersystm classing to T, which lies sonic 8500 cm-’ 1owc1 in energy [ ll.Jlj. The T, state has B-, symmetry; thus. dil ect spin-ol bir coupling between-a sta tc of A1 symmetry and T, issyrmnctry aliowed.whercas spirlorbit coupling between a state ofoveralJ B2 symmetry and T, issymmetly forbidden. The popdat~on ofa b2 vibronicstatc mght thus be expected to lead to a decrease in the rdte of radiationless decay. unless vibj onitally
induced
intcrsystem
crossmg
proceeds at a substantial c~ce~ice
Jifctime
from
this state
rate. The decrease in fluol-
asso&tcd
with popuintion
of the b9
specks cd11 bc attributed to an increase in the radiative decay ralc as a result of the strong induced vlbronic
coupling
between
the S, and S-, states.
cn-’
Illrldc.
LXC~ Cl ai. [is]
~JVC
predicted
coupJ111g Icrltis IO a dccrcase c~urnc~c~ ofvibIo111~aJJy l‘rcc~ue~lcy
bring
gvcrl
In the
dcIIvc
that viblonic excited state
modes.
apJ~rosimatcly
The introductton gas flow
lic-
tile dccrcxc
111
by
&_I = --2oZIM.
Af:’
11~ diffctcIiLc
tiic c~uplcd
coupling
bctwccn
clcctronic
matris
11~ /clo-point
cicnicnt lcvcls
md of
ground mid cxcitctl slate lrcqucnmode” in dibcmofur,m is LOIISISICII~wth ti11s tlicory dnd the use of tile above CApXiOIl allows the nlagIlitude of the vibronic Ltlupling ni.ktri?r eicnicnt to bc cstimatcd ds ==313 The dhprity
LIC!l l-01 ti1c
ill1
-1
“434
in
c111-’
.
lhorcscence dLIy of DBFN, cooled by cxp.~nsron 1111GO0 mhar helium. following cxcrtation
Tllc
at
by
mto
the canicr
the appcarancc
of
two
weak
higlicr
in energy
than
the 444
cm-l
transition
with a vibrational feature in the lA1 * A1 spectrum. However, with water vapour present in the expansion. a shouidel appears - at this resoiuLIOII - indicating the prcscncc of the water-DBFN conq~lex. No further new bandswere apparent at higher energies as a result of the increased congestion of the spectrum at higher vibrational energies and the relatively Jaw intensity of the other vibronic bands. These addItiona transitionsare attributed to a van der Waais nloiecuiar co11lJ~kX formed bctwccn DBFN and water. the water molecule being Jlydrogcn-bonded to tllc lhcrc
states.
of water vapour
xcompanicd
bands in the fluorcscencc excitation spectrum. As shown in fig. 4, the first band was bhteshifted by 178 cm-* from the origin band. At 178 cm-l
u Is. lhc ~Ibronic
wilele
Jddilionai
was
is
OVCrldp
Volume
112. number
CHEMICAL
6
PHYSICS
LETTERS
28 December
1984
implies that the quantum yield of emission is greatly reduced by complexation. While it is possible that hydrogen-bonding of water reduces the oscillator strength for the A, +- A 1 transition, the fact that the decay time shortens drastically must imply that non-radiative decay is enhanced by complexation. 235
212
lee
Relet
165 we
I‘d25
Frequency
212
ree
105
ld2
Acknowledgement
km-‘)
I‘&!. 4. The fluorcseence cxitation spectrum of DBI‘N around the 212 cm-’ line shows the effect of adding water vapour to the c\pansion. Conditions in 3.1 arc 1 atm of hehum miwd with DBFN .I[ 100°C expanding through a 300 pm circular nozzle and excited at 7 mm downstream from the nozzle. The new peak at 178 cm -* Ls produced by adding 27 mb.u of Hz0 lo the helium carriergds. A simile peak appcdrs asan unresolved shoulder 178 cm-’ to the blue of the peak a1 444 cm-’ _ The intcnsily of the vibrational pedk in 3b is less than half that of 3d
oxygen heteroatom of DBFN. The blue-shift in the excitation spectrum produced by complex formation . indicates that hydrogen-bonding of this type leads to stabilisation OF the ground state of the complex rclativc to the excited state: This suggests that the S, +-So transition in DBFN, although largely n--i?* in nature, has sonic charge transfer character, giving rise to a decrease in electron density on the oxygen atom, and
hence a weakening of the hydrogen bond, in the St state. In the case of the water complex with S-mcthylindolc, the opposite is true; increased electron density on the nitrogen hcteroatom could explain the observed red-shift_ There is a clear need for improved calculation of electron density in the excited electronic states of such systems. The fluorescence lifetime of the DBFN-Hz0 complex. following excitation into the origin band, was found to be 6.5 -+0.5 ns. o‘he large uncertainty in this value is due to the relatively poor quality of the data associated with the low fluorescence intensity.) Clearly complex formation has a significant effect on thC dynamics of the excited state decay of DBFN, resulting in a decrease in the fluorescence lifetime by a factor of two. Under conditions such that the intensity of fluorescence from the bdre molecule is reduced by half in the presence of water. the resulting intensity of fluorescence from the complexes is less than ten percent of that from excitation of the bare molecule. This
We are grateful to the Science and Engineering Research Council for generous financial supporr, and to Mr. David Madill and Mr. Bruce hlorris for their excellent technical assistance with the costruction of the jet apparatus. References [ 1J R. Bersohn. !2 j [3J [SJ [5j [6] j7j [8 J 19 J [ 101 Ill] 117-l 1131 [ 141 115 j [16j
[ 171 f 181
U. Even and J. Jortner. J. Chem. Phys. 80 (1984) 1050. H. Abe, N. Mikami and Y. Udapawa, Chem. Phys. Letters 93 (1982) 217. N. Gonohe, H. Abe, N. Mikami and M. Ito. J. Phys. Chem_ 87 (1983) 4406. Y. Nrbu, H. Abe, N. hlikami and hl. Ito, J. Phys. Chem. 87 (1983) 3898. Y. Tomioha, H. Abe, N. hlikami and hl. Ito. J. Phys. Chem. 88 (1984) 2263. A.R. Lxcy. A.E.W. Kni$Jtt and LG. Ross. J. hlol. Spectry. 47 (1973) 307. C. Taliani, A. Brec and R. Zwanch, J. Phys. Chem. 88 (1964) 2357. A. Bree, V.V.B. Vilkos and R. Znarich. J. Mol. Spectry. 48 (1973) 135. A. Bree, A.R. Lacey. 1-G. Ross and R. Z\\arich, Chem. Phys. Letters 76 (1974) 329. D.V. O’Connor and D. Phillips, Time-correlated singlephoton counting (Academic Press, New York, 1984). CA. Pinkham and SC. Wait Jr., J. hlol. Spectry. 27 (1968) 326. C. Ausscms. S. Jaspcrs. G. Leroy and F. v.m Rcmoortere. Bull. Sot. Chim. Bclgcs 78 (1969) 479. W.R. Lambert, P.M. Fclkcr and A.H. Zewdil. J. Chcm. Phys. 75 (1981) 59-58. P.S.H. Fitch, L. Wharton and D.H. Levy, J. Chum. Phys. 70 (1979) 2018. A.R. Lace)‘, E-1‘. McCoy and 1.G. Ross. Chem. Phys. Letters 21 (1973) 233. A. Amirav, U. Even and J. Jortner. J. Chem. Phys. 67 (1982) 1. R.N. Nurmukhamctor and C.V. Gobo, Opt. Spectry. 18 (1965) 126. A_ Brec, V.V.B. Vilkos and R. Zwarich, J. hlol. Spcctry. 48 (1973) 124.
533