Volume 21, number 1 CHEMICAL PHYSICS LETTERS 1 August 1973

SHORT-WAVELENGTH FLUORESCENCE OF 1,12-BENZPERYLENE

Aklra NAKAJIMA Dir won of Chernrstry, Research IIzstztzlte of Applzed Electnctt~v. Hokhardo oizzl erszty, Sapporo, Japan

Recclved 8 February 1973 Revised manuscript recclved 27 AprJ 1973

Tno relatively strong bands, subbnld 1 and subband 2, obtervcd in lhe short-wavelength regron of the fluorcs-

cence spectrum of l,l2-benzperylene have been studled by varymg the rxcrtatlon wavelength and temperature and

by addmg jr-he\anc gas In soluhon, subbad 1 was observed at hrgher temperatures and can be attrlbutcd to an

emlssron from a thermally-populated vlbronic lcvcl of the tirht exatcd slnglct state In Ihe gas phase, however, It IS

attributed to a u-u - 1 resonance lluorexcnce SubbJnd 2 IS attributed to fluorescence from the second exrted

sln~lct state

In general, fluorescence emlsslon of aromamc mole- cules m condensed phases arises solely from the lowest excited sn-@et state. Well-l;nown cxcept~ons to this

rJe are provided by azulene [l] and related com-

pounds [2], m which the mternal conversion process from the second excited smglet state (S,) to the first excited smglet state (Sl) 1s relatively slow because of the very small Franck-Condon overlap due to the anomalously large energy gap between them Recently a considerable number of studies [3- 111 has been made on the “anomalous” emlsslon sometimes ob- served In condensed aromatic hydrocarbons at a wave- length shorter than that at which the normal fluores- cence aTpears The short-wavelength emlsslon has been mostly attrmuted to fluorescence fromS, -The small energy difference between S, and S, and the large radiative rate from Sl seem to be essenrlal for the appearance of S, emlsslon [3, IO] However, an

alteinatlve explanation may be equally possible In

condensed rnedla at lugh temperatures, a fluorescence hot band may arlse from emlsston from a vlblatlonally excited level of S, [12] In the gas phase at sufficlent- ly low pressures, a resonance fluorescence-may occur from an mltlally excited vIbrationa level of S, to a lower vlbratlonal level of the ground state (So)

Lawson and Kropp [3] reported the observatron of a temperaLure-dependent anti-Stokes fluorescence

200

U-I 1,12-benzperylene, they mvestlgated the changes m the quan?um yields of fluorescence and triplet forma- tlon and III the fluorescence hfetxne with temperature

in polymethylmethacrylate (PMM) matrices -d inter-

preted the occurrence of the anti-stokes band as due to the thermal population of SZ at higher tempera- tures On the other hand, the present author [lo] studied the fluorescence emission of 1,12-benzperylene m the gas phase and observed two bands in the short- wavelength region of the spectrum It was suggested that one of these bands could be ascrlbed to a reso- nance fluorescence and the other to S, fluorescence

In this work an attempt has been made to make an assignment of these bands With and without an Inert gas such #IS n-hexane, which can only accelerate the vlbratlondl relzxatlon, the gas-phase fluorescence was

measured by varying the excn.ltlon wavelength and

temperatnre The fluorescence spectra were also ob-

served 111 several solvents at various temperatures

The ernlsslon spectra were obtained using a Hltachl MPF-2A spectrofluorometer with sensltlvlty correc-

tions, and the absorption spectra were measured with

a Hitachi EPS3 spectrophotometer The gas-phase spectra were measured using a square quartz cell sealed off from a vacuum line and placed m an electric fur- nace [S] _ The vapour pressure of the sample assessed from the temperature of the furnace was usually low

Volume 21, number 1 CHEMICAL PHYSICS LETTERS I A~lpust 1973

-J (kK)

F1.g 1 Fluorescence spectra of 1,12-benzpcrylene ~n the gas

phase and in solution (a) Gas phase, elcitcd at 27 7 kK or II-

hexanc added (45 torr). (b) III rz-hexanc, (c) II-I dlmethylsul- phowde

(0 01-O 1 ton), so that the exerted molecules can be considered to be colhsron-free The fluorescence hfe-

times were measured usmg frequency-doubled ruby

laser pulses

1 ,l Zbenzperylene was purified by repeated vacuum subhmatrons Spectroscoprc grade jr-hexane and dr- methyls~lphox;lde were used wlthout further punficn-

eon Hekadecnne passed through a column of srhca

gel and glycerol of G R grade were drstllled under re- duced pressure

The fluorescence spectra are shown m fig 1 The gas-phase spectrum deprcted m fig 1 was obtamed by

excrtatton at the lowest possrble wavenumber or m the presence of !I-hexane at high pressure The solu- tion spectra doffer consrderably m different solvents,

assocrdted with the very weak transrtron According to the present author’s studres [13] of solvent effects on the mtensrty drstrlbutron m the vrbratronal struc- ture of the emrssron of aromatic hydrocarbons, the mtensltres of the forbidden vlbrornc bands mcludmg

the O-O band mcrease remarkably with an Increase m

the solvent polarrty, whereas therr band shafts are

comparatrvely small Thus 1s clearly shown U-I fig 1, m a non-polar solvent such as rz-hexane, the O-O band 1s weakly observed at 24 59 kK, whrle U-I a hrghly polar

solvent such as drmethylsulphoxlde rt appears strongly

at 24 47 kK On the basis of these observations, the

O-O band for the gas-phase spectrum can be assigned to the relatively small but drstmct peak at 24 60 kK,

which 1s consrstent wrth the results from the absorp- tron spectra (see table 1)

v (kK1

FIN 2 Gas-phase fluorescence spectra of 1,12-bcnrperylene

m the absence of fc-heuane Eaxtatlon wavenumber (kK)

(a) 27 0, (b) 58 4, (c) 31 3, (d) 34 3, (e) 35 5

As seen from fig 1, a strong band IS observed

around 25 2 kK on the higher wavenumber srde of

the O-O transition both m solutron and u-r the gas

phase This band is hereafter referred to as “subbdnd

1” In ahphatrc hydrocarbons, rt exhrbrts a drstmct structure, probably consrstmg of dt least three peaks

as can be clearly seen m fig 1 The shape of thrs band did not change on dllutlng the soluhon. lndtutlng

that the efkt of self-absorption by the second elec- tronlc band [3] IS negllglble at these concentrat[ons (x1CF5 M) In nddltlon, a broad and fauly strong band appears m the gas phase at about 26 4 kR This band IS referred to as “subband 2”

Fig 2 shows the gas-phase fluorescence spectra (:r: the absence of Inert gas) with varymg excrtatron w&Se-

number With mcreasmg evcrtatron wavenumber, the subbands markedly increase m intensity and the whole spectrum gradually broadens and shifrs to the red, as usually observed m gas-phase spectra 1.5, IO] Corre- spondmgly, the excrtatton spectra monitored at these bands tend to greatly increase rn relatrve rntensrty rn

the higher wavenumber region The wavenumbers of the correspondmg peaks agree well among these eycr-

tatron spectra and the vapour absorptron spectrum

WI - Fig 3 shows the gas-phase fluorescence spectra m

the presence of rz-hexane (45 torr) with varying excr-

tatron wavenumber The spectrum changes LittIe m relative intensity, except m the subband region where it varies shghtly The farr agreement between the

spectrum observed by excrtatron at the low wavenum- ber as shown m fig 2 and those cbserved in the pres-

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Volume 2 L , number 1 CHEMICAL PHYSICS LETTERS 1 August 1973

i 20 22 24 26 i

Y (kK)

Fig. 3 Gas-phase fluorescence spectra of 1,12-bcnzperylene m the presence of rl-helane (45 torr) Excltatlon wavenum- her (kK) (a) 28 4, (b) 34 2, (c) 37 0

ence of n-hexane as shown m fig 3 Indicates that the nbratlonal relaxation by colhs:ons m the excited

state occurs to an appreciable extent under a pressure of n-hexane up to 45 torr However, the observation

that subband 1 remains with relatively high mtenslty when mcreaslrg the pressure of n-hexane from 45 to

165 torr may Imply a saturation effect in colhslonal deactlvatlon.

The dependence of the relative mtenslty of the

subbands on excitation wavenumber and temperature m the gas phase 1s shown m figs 4 and 5, respectively

When n-hexane IS added at a pressure of45 torr, the changes in excitation wavenumber and temperature have a rather small effect In so far as the effects of

excltatlon wavenumber and temperature are con-

cerned, the behavlour of subband 2 appears to be very

similar to that of subband 1 It can be seen from these

figures that the effect of a temperature change by 40”

corresponds to that of a change m excitation energy

by 1 kK In order to obtam a knowledge of the orogm of

subband 1, the fluorescence spectra partly shown m fig. 6 were measured ;1~ a function of temperature m

glycerol and hemdecane solutions. The intensity of

subband 1 increases considerably with the rise of temperature, suggesting that it 1s attributable to an errusslon from a certain thermally accessible level lqher than the lowest vibrational level of S1 Sub- band 1 apparently composed of several vlbroruc bands IS not m a mirror-symmetry relation to the S2 absorp-

tion band Particularly m the gas phase, it 1s located too far from the O-O band of the S,-S2 transItion

202

d

0 26 26 30 32 34 36

V(kK) 9

Fig 4 Dependence of the relative mtenslty of the subbands on e\cl:atlon wavenumber (and n-hexane pressure) (a) Sub- band 1, 0 torr. (b) subband 2, 0 torr, (c) subbrnd 1, 45 torr, (d) subband 2,45 torr

200 24C 280-c

Fig 5 Dependence of the relatwe mtensit] of thr subbands on temperature (and rz-heuanc pressure), excited at 34 5 kK (a) Subband 1,0 torr, 0.1) subband 2.0 torr, (c) subband 1, 45 torr, (d) subband 2,45 torr

0 23 24 25 26 27

Y (kK0

Fg 6 Fluorescence spectra cf 1,12-benzperylene In a glycerol solubon as funchons of temperature CC) (a) -196, (b) -110, (c) -36, (d) 23, (e) 54, (f) 114, (g) 180

Volume 21, number 1 CHEMICAL PHYSICS LETTERS I August 1973

\ 3 & 5 6

1 O3 T-’

Fig 7 Varlatlon of the lntenslty ratlo of subband 1 to the main band wth tcnlperature In glycerol solutions Actwatlon energy (cm-‘) (0) 572, (a) 385, (0) 417

Table 1

Jvavcnumbers of the relevant band5 (O-O transltlon) and ac- tivatlon energies in LK

Lmel Gas Hexndecane Glycerol

SI (a’= ) 24 60 a) 24 64 Sl (flu0 ) 24 66 24 53 Sz (abs ) 26 98 25 97 Sub 1 (flue ) 25o4~o1ob) 2508 Sub 2 (flue) 26 50 + o lob) - Sz-S 1 (abs ) 2 38 I 33 Sub l-S1 (flu0 ) 0 38 0 55 AE 0 44

0 43 0 40

24 59 24 49 25 85 25 06

-

1 26 0 57 0 57 0 39 0 42

a) Estimated from the gas-phnsc absorption spectrum mcas- urcd by A~hara et a1 [ 14 ]

5) The wavcnumber varlcs wth the change m excltatlon wnvcnumber

Figs 7 and 8 show the variation of the mtenslty ratlo of subband 1 to the main band (23 75 k!K)* with temperature for three different runs From these plots, the actlvatlon energy AE can be estunated using the

* The mterwty of this wbromc band was httle affected by the change m the solvent polanty mth temperature, ~mxe- as that of the O-O band changed slightly

20 25 30 lo3 T-'

Fg 8 Vsrlntlon of the mtenslty ratlo of subband I to the mnm band wth tcmpcrathre In hexadecanc solutrons Actwa- tlon energy (cm-‘) (0) 442, (a) 427. (0) 397

followmg equation

‘s 141 = A exp (-CLE/kT)

Table 1 gives the actlvatlon energies thus ab tamed. along with the observed wavenumbers of the relevant bands As can be seen from table 1, the values of AE are in closer order-of-magmtllde agreement with the

energy difference between subband I and the O-O

band of the mam fluorescence than w.th the energy separation between S, and S2 From the temperature dependence of the fluorescence hfetlme and yield in

PMM media, Dawson and Kropp [3 j estrmated the actlvatlon energies to be respectively 120 and

800 cm-l These values are, as they also pomted out,

too small to be compared with the S,-S, energy gap

(1300 cm-l m PMM) On the basis of these facts, tt

wn be concluded that m solution, subband I can be attributed to the fluorescence ernlsslon due to thermal

populatmn m the vlbron,c levels of S, in the low-

pressure gas phase, however, an emsIon occurs from the vlbrationally excited level of S, which the excited

molecule has reached by mterna: ccwerslon4 The III- tenslty varlatlon of subband 1 with increasing excfta- tlon wavenumber and by addmg rr-hexane mdlczz

that it should be ascribed to a resomce fluorescence

from a lugh vlbroruc level of S, to the vlbroruc levels lower by one vlblatlonal quantum of Sot I e , a fluores-

cence of the u-u - 1 type Judging from the wavenumber at which subband 2

was observed, it can be regarded as S2 fluorescence

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Volume 21, number 1 CHEhlICAL PHYSICS LITTERS 1 August 1973

In solu’tlon subband 2 may be reduced m mtenslty and obhterated by the tal of subband 1, since the

O-O band of the So-S, transltlon sl-Efts from 26 98 kK m the gas phase to 25 97 kK m hexadecane Its Intensity varlatlon with mcreasmg excltatlon wave- number and temperature and by addmg an mert gas 1s m accordance with the previous observations in other molecules [5, lo]

In view of its high mtenslty, subband 1 should anse from vlbroruc transItIons allowed to a large ex- tent The large density of states at the hlgher vlbra- tlonal levels in S,-, and S, may also play an unportdnt role. On the other hand, subband 2 IS due to emlgs1op.

with appreciably Hugh radlatlve rate from S2

The fluorescence llfetlmes measured had almost the same values in the mam and subband regons of the fluorescence spectrum The values were 115 nsec (no n-hexane), 90 nsec (45 torr rl-hexane) m the gas phase, and 120 nsec m a degassed hexadecane solution The observed decays were dll exponential However, no further useful mformatlon could be obtamed from the hfetlme data

The author IS mdebted to the referee for the mter- pretntlon of subbnnd 2

Addendum

Quite recently a sundar study [15] has been made of the short-wavelength emlsslon of 3,4-benzpyrene, mcludmg many mterestlng results m connection with thus work

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204