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Fluorescence spectra of perylene-benzo[g,h,i]perylene mixed crystal system
JOURN.1L
OF MOLECULAR
Fluorescence
TAKASHI
SPECTROSCOPY
32, 1-12 (1969)
Spectra of Perylene-Benzo[g,h,i]perylene Mixed Crystal System KAJIWARA,
ICHIMIN SHIROTANI,
The Institute for Solid State Physics,
University
AND HIROO
INOKUCHI
of Tokyo, Roppongi,
Tokyo, Japan
The fluorescence of perylene(guest)-benzo[g,h,i]perylene(host) mixed crystal system was measured at room temperature and at liquid nit,rogen temperature. A sublimed thin film of t,he mixed crystal which contained more than 5 X 1OF mole/mole perylene showed a new fluorescence band at liquid nitrogen temperature. This band, not observed at room temperature, was assigned to transitions between the vibronic states of perylene aggregate in the mixed crystal system. 1. INTRODUCTION
As described previously (1)) commercial benzo[g , h, ilperylene usually contains a small amount of perylene as impurity. Recently, we developed a procedure of purification of benzo[g,h ,i]perylene (1) and found that perylene and benzo[g,h,i]perylene are mixed with each other very easily and that the apparently homogeneous mixed crystal is prepared up to about 30 57 mole/mole perylene concentration (see Appendix ). In the present article, we present the observation of fluorescence spectra of the perylene-benzo[q, h , i]perylene mixed crystal system having various concentration ratios, and further we make some considerations concerning these phenomena. Especially, we report that the fluorescence spectra of a perylene-rich sample observed at liquid nitrogen temperature is due to the excited perylene dimer. 2. EXPERIMENTAL
METHODS
2.1. Sample. Synthesized benzo[g , h, ilperylene (&HI?) was purified carefully by the method reported previously (1). Perylene was synthesized and purified by Iwashima (2). Benzo[g ,A , ilperylene-perylene mixed crystals were prepared by rapid cooling of the melt benzo[g,h ,i]perylene solving a known amount of perylene in glass vessel filled with nitrogen. The mixed crystals obtained k$ the above method were sublimed on a Pyrex glass plate under vacuum of 10 mm Hg. The sublimed thin films were used as a sample specimen through this work. To eliminate reabsorption effect, the sublimed film was made so thin’ that its 1The thickness was about 10-l p. Copyright
@
lQ69 by Scademic
Press, Inc.
KAJIWARA,
SHIROTANI,
AND
INOKUCHI D. Concen
/
I 23
22
21
20
1 19
18
I k~
Wave Number
FIG. 1. The fluorescence spectra of the mixed crystals at 77°K: (-), the concentration (mole/mole) of perylene in the mixed crystal is lo+; (*. * .) 10e3; (-.-), 1w2; (- - - -), 10-l; and (-..-), 0.5. The nomenclatures of the peaks are shown in Table 1.
optical density In IO/Z is less than 0.5 in the spectral response region of fluorescence.’ As pyrelene was sublimed a little easier than benso[g, h, ilperylene, the concentration ratio of the sublimed thin film was different usually from that of the melt mixed crystal itself.3 In the case of rather high perylene concentration samples, higher than lo-’ mole/mole, the concentration ratio was estimated from the absorption spectra of the benzene solution of the whole sublimed specimen. However, for low concentration samples, lower than lop2 mole/mole, the concentration ratio was estimated from that of the melt mixed crystal. d.2. Fluorescence Measurement. A high-pressure mercury lamp accompanying :I 2 A rough calculation using the measured optical density of the film submitted to the fluorescence measurement shows that the variation of the reabsorption effect, is only less than 20y0 in the whole fluorescence spectral region. 3 The vapor pressures of these two aromatic hydrocarbons were observed by Knudsen met,hod as: log P = 11.82 - 6.58 X 103/T (for benzo[g,h,i]perylene); log P = 14.35 6.77 X 103/T (for perylene), where P is described as mm Hg and T absolute temperature (3).
SPECTRA
3
OF PERYLENE-BENZOPERYLENE
Shimadzu DU-2V filter was used as a light source. The fluorescence spectra of the thin-film specimens were measured with a reflection method to eliminate reabsorption effect. In the fluorescence measurement at low temperature, the glassplate specimen fitted onto a quartz cryostat was cooled at liquid nitrogen temperature. 3. RESULTS
AND
DISCUSSION
Figures 1 and 2 show the fluorescence spectra of the mixed crystal systemperylene and benzo[g , h, ilperylene; the concentration of perylene in benzo[g,h,i]perylene matrix changes from about 10e4 to 0.7 mole/mole. As Figs. show, rather the different spectra were observed in the samples having different concentration ratios. Each spectral response observed at liquid nitrogen temperature, has distinct vibrational structures. This is not like the pyrene-perylene mixed crystal system, the fluorescence spectra of which do not show such a clear vibrational spectra (,4). 3.1. The Assignnlent of the Fluorescence Spectra. The spectral response of the fluorescence of the perylene-benzo[g , h, i]perylene mixed crystal at liquid nitrogen temperature appears from about 33.5 kK to about 16.0 kK. The origins of the
p. Concen
23
22
21 Wave
20
19
18
17 kK
Number
FIG. 2. The fluorescence spectra of the mixed cryst’als at room temperature: concentration (mole/mole) of perylene in the mixed crystal is 1OV; (....), 10-Z; and (- - - -), 10-l. The nomenclatures of the peaks are in Table 1.
(---_), the 1OW; (-.-),
KAJIWARA,
4
SHIROTANI, TABLE
AND
INOKUCHI
I
THE CLASSIFICATIONOFTHE FLUORESCENCE BANDS OF~ERYLENE-BEN~O[~,~,~]PERYLF,NE MIXED CRYST.ILAT LIQUID NITROGENTEMPERATURE Band
Origin
A B C I> E F
Benzo[g,h,i]perylene Perylene molecule &O transition” Perylene molecule O-l transition Perylene aggregate Ckl transition’) Perylene aggregate 0-O transition” Perylene aggregate O-2 transition”
e See Fig. 1. b Benxo[g,h,i]perylene may also contribute to this band (see text 1 ’ The vibrational assignment is explained in the text.
Wave
Number
FIG. 3. The absorption spectra of perylene in benzo[g , h,i]perylene: (-), the observed spectra; (-.--), the estimated spectra of perylene in benzo[g,h,i]perylene and (e...), t,he fluorescence spect,ra of benzo[g,h,i]perylene. in this region are classified in Table 1. The intensity of the band about 22.55 kIZ decreased as the concentration of perylene contained in the mixed crystal increased, so it was assigned to benzo[
SPECTRA
OF PEKYLENE-BENZOPEBYLENE
-
23
22 Wave
FIG. 4. Comparison of benzo[g,h,i]perylene
21
PureBerm-(g
20
19
5
h II-Peryiene
18
ItK
Number
of the fluorescence spectra containing 1OW mole/mole
of pure benzo[g,h,i]perylene of perylene at 77°K.
with t,hat.
perylene had to be higher than f, X lo-” mole, but not at room temperature. Because these new bands did not appear at low concentration (lower than lo-” mole/mole), these are assumed to produce from some aggregates of perylene molecules. 3.2. Excited Lh’mer Fluorescence c?f’ Perylene. The new bands appeared at liquid nitrogen temperature resemble the fluorescence peaks of a-type4 perylene found at lower than %‘I< and those of @type4 perylene at lower than SO”I<, reported by Tanaka (5). However, this band must not be “monomer fluorescence” assumed by Tanaka in his kinetic analysis, because it does not appear when the perylene concentration is lo\v. Therefore, the origin of the emitting state might be excited dimer one (but not excimer5). It is not clear horn- many molecules are involved in the fluorescence state of perylene aggregate, but the excited dimer model is assumed to show the main feature of the perylene aggregate fluorescence. The fluorescence level of the perylene crystal at room temperature has been investigated by many researchers and assigned to a forbidden charge-transfer !/e&e electronic level of a pair of perylene molecules, which has inversion symmetry (5,C). The excited dimer state, proposed in this article, must also have the forbidden qe~~le symmetry property. However, the fluorescence bands, noted in this discussion, of a-type and &type perylene and of the present mixed crystal, have very strong intensity and are estimated to be almost the same as that of perylene monomer in the mixed crystal system, the emitting level of which is allowed. We now describe the possible explanation of the strong intensity of the fluores4 See Appendix. 5 By the t,erm “excimer,” dimer pair by some special
we mean some rearrangement and stabilization mechanism such as charge resonance.
of an excited
KAJIWARA,
c,
SHIROTANI,
AXD IKOKUCHI
cence bands and rather their anomalous shapes. We consider the intramolecul:u~ vibrational states of perylene dimer (7, 8). The ith electronic state of molecule A is denoted as cpy’ and the jth vibrational state in the ,ith electronic state of molecule A as x:i’j’. Of course, ,j must be thought to be a set of many quantum numbers of various intramolecular vibration modes, but, we choose orw important, mode. Then, the vibronic dimer \vave function m:ty be writkn :IS Fkl. (1 i by using the Simpson and Peterson’s wale coupling :lpproxim:ltion (8 ).
N = $5
(if
,! 3 (if
i += i’ i = ,i’
or
.i f .i’),
and
j = ,j’).
(1)
In this expression, the effect of the antisymmctrization of the function I\-ith respect to the electron exchange is neglected. If we denote the lowest excited singlet, electronic level of a molecule by i = 1, the emitting excited dimer level will be written as follows, &“;d
=
_ !_
i0)
( &‘xL”‘&‘xa”~“’ -cc.sx.4
iO.0,
(1) (FHXR
Cl,Ob
I’)
).
z/a
The molecular planes of two molecules of :Ldimer pair arc parallel long axes of the two molecules are parallel, thewfore I\-e can write,
(p~)p~’
jsjjt/p~)p~)) =
((o~J&J ,yjj 1 pi’JFgii =
and
yJ?,
dso
the
( :z,I
(here 9X! = eXr). Thus, the transition dipole moments between +P’~““‘~“’and electronically ‘“‘““‘~i’)are calculated as follows: swX:, %+ (i) ,j # 0 and ,j’ # 0, (+JJ.O;[l.O)I gjr j +~.O;j.,‘J/ = 0.
ground
(-1-l
(ii) ,j = 0, ,j’ = 0, (~~,WO)
, & / &Lo;n.oJ) = 0.
(.i 1
state does not exist), (the Q,“‘mo”‘~(‘) (iii ) ,j # 0, ,j’ = 0,”
The integral (x (I’“’ 1x ‘““) ) is generally not zero o\\-ing to the coupling of the intramolecular vibration with the molecular electronic state. Then WE may be :Iblc t,o say th:lt the transition from &‘“‘o”” state to the @~.o~0;‘~0’ st:ltc is :dlowcd.
SPECTR.4
OF PEILYLENE-BEN%OPEItYLENE
7
On the basis of the above consideration, we assign the about lS.7 kl< band of the mixed crystal system, containing 10-l mole/mole concentration of perylenc as an impurity, as the excited dimer @~‘“;o~o)-+ +(o,“;‘30)fluorescence, and the 17.4 kl?; band as the &.o;o,o) ~ $,(0,‘3;2.0) transition one. The about 19.S kI< band was proposed to be assigned to surface state by Tanaka, but this might be the forbidden &,O;O,O) --+ +~‘“;o*o) emission. The selection rule of this spectral band can be easily broken by the decrease of symmetry of the dimer pair from the change of relative position of the two molecules or by crystal field perturbation. Another explanation of this band may be given by the appearance of a secondary progression involving some lower frequency intramolecular vibrations. Although we used the weak coupling model for the above discussion, the results may be applied in the case of intermediate coupling, because, even in the medium coupling case, the state corresponding to +?~“;“,o) is little affected by the resonance exchange of the molecular electronic excitation. It must be noted that the above results do not hold in the limit of strong coupling approximation. In this case, the state corresponding to Ey. (1) must be written as follows:
where, X o’“;ol)denotes some intramolecular vibrational pair. Similarly, the electronic ground state is given by
state of the whole dimer
Thus it is easily seen that the transition between*:“;“’ andi& (“‘o;O) is electronically forbidden irrespective of CYand 0. Therefore, we are able to say that the emitting state of the perylene fluorescence under consideration is interpreted as weak or intermediate coupling excited dimer state. We described that the excited perylenc dimer fluorescence in the mixed crystal system is alike to the fluorescence of (Y- and P-type perylenc crystal at low temperature in the previous section. We will describe how the excited dimer emission can occur in P-type perylene. The previous discussion holds only if the two molecules of dimer pair can exchange each other by one or more of the dimer pair symmetry operation and the related transition dipole moments of the molecules arc parallel. Therefore, if the two neighboring perylene molecules have p-form, some possible dimer pairs can be formed, standing along the u-axis, b-axis, and the (a/2, b/2, 0) direction and so on. Then, an excited dimer type emission can be expected from an emitting level in which the excitation in the crystal is localized in one of the above mentioned dimer pair. Of course, as this localized state does not satisfy the symmetry property of the crystal, the crystal must have some distortion about the dimer pair. The distortion may be small displacement of the molecules from their equilibrium position. If two molecules along the b-axis has a dimer pair, it can take about the same
8
KAJIWARA,
SHIROTANI,
rlNI)
INOKUCHI
geometrical configuration as a dimer pair in a-form perylene by such ;L small displacement. Thus, the fluorescence of p-form perylene at low tempcr:\ture observed by Tanaka may be an emission of dimer pair like that in the a-form pcrylene. Thus, the similarity of the fluorescence shapes of the a-form :md P-form perylene can be explained. We compare the energy level of the emitting state of dimcr pair with that of a state in which only one molecule is excited. The former is stabilized mainly by the ~won:mce excitation exchange and the latter b>, the solvent effect by the surrounding molecules. If the stabilization by the resonance excitation exchange is larger than that by the solvent effect, excited dimer emksion is expected rather than :i molecule one. The fluorescence of perylene in t’he mixed crystal system is th:tb case. Generally, except when the excitation localized in only one molecule :done is largely stabilized by the solvent effect’, the fluorescence level of aromatic molecul:w crystal may be unlocalized excited state. If the lowest lkvydov component. is forbidden for the sake of parallelism of the trttnsition dipoles :md the interaction between tmnslntionally nonequivalent molecules is we~tk or internwdi:Lte, the fluorescence from such a component is cxpccted to h:~vc the b:nne fwturc as the excited dimer emission described in t.his section. .j.,.j. ‘I’fW~KlYZtlLre ~eper&nce oj &Xciff?d I>illlW ~hLolYWf!~lfZ flj ~‘~‘l’,IJ/fVlf~. Ill Section 3.2., we concluded that the fluorescences of a-form and /%-form perylene :mtl :dso perylene in the perylene-rich mixed cr>&ll system are all due to the wistjence of dimer pairs. T:mak:t reported the tempernturc dependency of the fluorescence of a-form pcrylene by wsuming an masim~~ in :I curve of t,hc energy of emitting level uersus intermolecul:w distance of the dimer pair (*7). Though he assumed t,hat the fluorescence at ion- temperat,urc~ is due to the sin& molecllle cwitat,iotl, his kinetic an:tlysis also holds cvcn n-Hun this fluorcsc~nce is t,hct rscitcxtl cm-l 12oot
.
am-
2
.L. 600. c r_ 5 400. % a, CL 200-
/11/1-
10-3
10-2 Concentration
10“
1
of Perylene f moleymore,
FIG. 5. The cmcentraiion dependence of the the perylene aggregitles ill bel,xo[g,h,i]perylerle.
flU(JR?SCCIlW
maximum,
I) in Fig. 1, of
SPECTRA
1 24
23
OF PERYLENE-BENZOPERYLENIX
22 21 Wave Number
20
79
FIG. 6. The fluorescence spectra of pure henzo[g,h,ilperyleue
I 24
23
22 Wave
FIG. 7. The fluorescence perylene
spectra at low t)emperature.
dimer emission
21
20
78 kK
at various temperatures.
19
18
kK
Number
of benzo[g,h,i]perylene
as we proposed,
9
only if we replace
containing
about 1W4 mole/mole
his single molecule
N
b!r a
dimer pair MM and excited single molecule A/* by an excited dimer pair A/M*.
If we assume the excited dimer emission as treated in Section 3.2 and apply his kinetic analysis, P-form perylene crystal and also perylene in the perylene-rich
KA.JIWARA,
10
SHIROTAIL’I,
? i
ilND
INOKUCHI
Solid (Evaporated
Thin Film)
I
)
,
1 ; i
23
22 Wave
21
20
19
18
kK
Number
FIG. 8. The concentratjion dependence of the fluorescence spectra of pure benxo[g,h,i]perylene benzene solllt~ion. The flrlorescence spectra of pt~re benzc)[g,h,i]perylene film is also show-n.
lnixcd crystal must shoed the same temperature dependence as a-form perylene crystal in contradiction with the observed results, therefore his kinetic :malysis seems to be uns:ttisfsctory. The fluorescence behavior of perylene crystal :t~ a function of temperature may be essentiully influenced by the aggregation state, but further investigation needs to solve this problem cluantittltively. 3.4. The Concentration Dependence oj’ l
SPECTRA
11
OE’ PEKYLENE-BEN%OPEl~YLENE TABLE
II Unit cell dimension
Compound
Perylene Renzo[g,h,i]perylcne
&)
b CA,
c (ii,
(“1
11.35 11.72
10.87 11.88
10.31 9.89
100.5 98.5
The shift is nearly proportional to the logarithm of perylene concentration as shown in Fig. 5. Though this large shift must be due to aggregation of perylene dimer pairs, we cannot conclude whether this shift is mainly due to the difference of peryleneperylene interaction from perylene-benzo[g , h, ilperylene interaction or due to such an effect that geometrical configuration of perylene dimer pair changes when the aggregation size of perylene pairs increases. If we attribute the shift only to the difference of interaction energy, the logarithmic dependence of the shift on the concentration is deduced. We assume that the difference of interaction energy can be written as A/RR, here, R is intermolecular distance and constant A is independent on R, and that perylcne in the mixed crystal system forms an emitting site whose size is proportional to the power of the perylenc concentration. Then, the shift AV is roughly calculated:
and
then Av z
;
Aa,,
log C + log g
-
; log Ro
1
This explanation, however, is thought to be unsatisfactory, because the value of Av is usually thought to be about 200-300 cm-‘, which is too small to explain the observed shift, 1100-1300 cm-‘. 3.5. The Fluorescence Spectra of Renzo[q, h,i]perylene: Icigurc ci shows the fluorescence spectra of pure benzo[g , h, ijperylene thin film. The fluorescence in tensity at 150°K is about twice that at room temperature. The intensity of fluorescence band, whose maximum is at 440 rnp, rapidly increases as the temperature decreases from about 106”Ii to about 9O”Ii, and the vibrational subbands become sharper. The appearance and increases of the about 440 rnp band cannot be attributed to the decrease of reabsorption, for we used very thin film and the optical density in the region longer than 420 rnp of this film at liquid nitrogen temperature was the same as that at room temperature. The thin film of benzo-
RAJIWARA,
12
SHIROTANI,
ANI)
INOBUCIII
[y , h , ,i]perylene which contains about 10-4-10-” mole/mole perylene shows this 440 rnp band already at 14O”K, as Fig. 7 shows, and this band at liquid nitrogen temperature is sharper than that of pure benzo[y , /I, i]perylene at the same temperature, as Fig. 4 shows (9). The fluorescence of benzo[g, h ,i]perylene at room temperature wan like the broad band which is found in the fluorescence of concentrated benzene solution of benzo[q ,h ,i]perylene at room temperat,ure, as illustrated in Fig. S. Though this fluorescence is thought to be excimer type, its maximum exists at rather shorter \vavelength and does Ilot satisfy the so-called 6000 cm-’ rule (10). Further investigations are now intended to clarify all these phenomena. APPENDIX
The crystal structures (C$,) of perylene and benzo[cl, h, i]perylene are shown in Table II. The crystal structure of perylene-benzo[q, h? ijpervlene (concentration ratio is about 15 :%) mixed crystal is monoclinic (Cih), with two molecules in a unit, cell of dimension u = 11.63 A, h = 1135 8, L’= 92% & and /3 = 98.6”. The crystal structuml data of perylene, ment,ioned in (Table II ) is that of LYform with four molecules in the unit cell. fl-Form perylene also belongs t)o the ( I:$, apace group and give the following crystal dam (ri) : (I = 11.27 f 0.03 A, /I = MS f 0.02 11, c = 9.65 f 0.03 & p = 92.1 f 0.:3” with two molecules in t,he unit cell.
The :tuthors
wish tr) t,hank Mr. Hni.oshi
Iwashima
for providing
i he pure hydrocarhorr,
t~~rleo[g, h, i]perylena.
RECEIVERS : January
10, 1969 1
REFERENCES I. T. K~.JI~.IR.I, I. SHIROT.\NI, II. INOK~(:HI, .1x1) S. I\~.IsHIxI.\, .I. Jiul. Spect,y. 29. 454 (1969). B. S. Iw.\sIIIM;\, private rommunication. 3. N. W:ZK~~YXVIA AND H. INOKUCHI, Bull. Chem. SW. .Jnpczn, 40, 2267 (1967). 4, 11. M. HOCIISTRASSI~CN, J. Chem. Phys. 36, 1099 (19fi2); K. Iirrn.\o~i.~ AND I>. IL. KURYS, J. (‘hum. Phys. 46, 147 (1966); Y. ISHI ;\NII A. ~~.\%-I, b. I’hys. S’oc-. Japarr. 22, 926 (1967). 5. 6. T.~N~K.I, B’rcll. Chrm. ft’oc. Japan 36, 1237 (1963). 0’. R. M. HOCHHTRASSER, J. Chem. Phys. 40, 2559 (1964); I(. M. HO~HST~USSER .\XD A. M~LLIARIS, J. Cheltl. Phys. 42, 2243 (1965); T. .~ZLTMI.\NU R. P. M&LYNN, J. (‘hem. l’hys. 42, 1675 (1965); J. FEHGUSON, .I. Chem. Phys. 44, 2677 (1966). 7. A. WITKO~SKII AND W. MOFFITT, J. Chern. Phys. 33, 872 (1960); E. f:. Mcl?.aa, ;Ius1diun J. Chen~ 14, 329, 344, 354 (1961); 16, 295 (19H3). 8. J. SIMPSON AND II. L. PKTERSON, J. Chem. J’hys. 26, 588 (1957). Y. H. C. WOLF, Z. Physik, 143, 266 (1955). f 0. J. R. BIRKS ‘\NI) 1,. (+. CHRIS’I’OI’HOI~(I~T,I’/w. /x’n?/. Sw. (Lontlvn), Ser. il 27’7. 571 (1964).
OF MOLECULAR
Fluorescence
TAKASHI
SPECTROSCOPY
32, 1-12 (1969)
Spectra of Perylene-Benzo[g,h,i]perylene Mixed Crystal System KAJIWARA,
ICHIMIN SHIROTANI,
The Institute for Solid State Physics,
University
AND HIROO
INOKUCHI
of Tokyo, Roppongi,
Tokyo, Japan
The fluorescence of perylene(guest)-benzo[g,h,i]perylene(host) mixed crystal system was measured at room temperature and at liquid nit,rogen temperature. A sublimed thin film of t,he mixed crystal which contained more than 5 X 1OF mole/mole perylene showed a new fluorescence band at liquid nitrogen temperature. This band, not observed at room temperature, was assigned to transitions between the vibronic states of perylene aggregate in the mixed crystal system. 1. INTRODUCTION
As described previously (1)) commercial benzo[g , h, ilperylene usually contains a small amount of perylene as impurity. Recently, we developed a procedure of purification of benzo[g,h ,i]perylene (1) and found that perylene and benzo[g,h,i]perylene are mixed with each other very easily and that the apparently homogeneous mixed crystal is prepared up to about 30 57 mole/mole perylene concentration (see Appendix ). In the present article, we present the observation of fluorescence spectra of the perylene-benzo[q, h , i]perylene mixed crystal system having various concentration ratios, and further we make some considerations concerning these phenomena. Especially, we report that the fluorescence spectra of a perylene-rich sample observed at liquid nitrogen temperature is due to the excited perylene dimer. 2. EXPERIMENTAL
METHODS
2.1. Sample. Synthesized benzo[g , h, ilperylene (&HI?) was purified carefully by the method reported previously (1). Perylene was synthesized and purified by Iwashima (2). Benzo[g ,A , ilperylene-perylene mixed crystals were prepared by rapid cooling of the melt benzo[g,h ,i]perylene solving a known amount of perylene in glass vessel filled with nitrogen. The mixed crystals obtained k$ the above method were sublimed on a Pyrex glass plate under vacuum of 10 mm Hg. The sublimed thin films were used as a sample specimen through this work. To eliminate reabsorption effect, the sublimed film was made so thin’ that its 1The thickness was about 10-l p. Copyright
@
lQ69 by Scademic
Press, Inc.
KAJIWARA,
SHIROTANI,
AND
INOKUCHI D. Concen
/
I 23
22
21
20
1 19
18
I k~
Wave Number
FIG. 1. The fluorescence spectra of the mixed crystals at 77°K: (-), the concentration (mole/mole) of perylene in the mixed crystal is lo+; (*. * .) 10e3; (-.-), 1w2; (- - - -), 10-l; and (-..-), 0.5. The nomenclatures of the peaks are shown in Table 1.
optical density In IO/Z is less than 0.5 in the spectral response region of fluorescence.’ As pyrelene was sublimed a little easier than benso[g, h, ilperylene, the concentration ratio of the sublimed thin film was different usually from that of the melt mixed crystal itself.3 In the case of rather high perylene concentration samples, higher than lo-’ mole/mole, the concentration ratio was estimated from the absorption spectra of the benzene solution of the whole sublimed specimen. However, for low concentration samples, lower than lop2 mole/mole, the concentration ratio was estimated from that of the melt mixed crystal. d.2. Fluorescence Measurement. A high-pressure mercury lamp accompanying :I 2 A rough calculation using the measured optical density of the film submitted to the fluorescence measurement shows that the variation of the reabsorption effect, is only less than 20y0 in the whole fluorescence spectral region. 3 The vapor pressures of these two aromatic hydrocarbons were observed by Knudsen met,hod as: log P = 11.82 - 6.58 X 103/T (for benzo[g,h,i]perylene); log P = 14.35 6.77 X 103/T (for perylene), where P is described as mm Hg and T absolute temperature (3).
SPECTRA
3
OF PERYLENE-BENZOPERYLENE
Shimadzu DU-2V filter was used as a light source. The fluorescence spectra of the thin-film specimens were measured with a reflection method to eliminate reabsorption effect. In the fluorescence measurement at low temperature, the glassplate specimen fitted onto a quartz cryostat was cooled at liquid nitrogen temperature. 3. RESULTS
AND
DISCUSSION
Figures 1 and 2 show the fluorescence spectra of the mixed crystal systemperylene and benzo[g , h, ilperylene; the concentration of perylene in benzo[g,h,i]perylene matrix changes from about 10e4 to 0.7 mole/mole. As Figs. show, rather the different spectra were observed in the samples having different concentration ratios. Each spectral response observed at liquid nitrogen temperature, has distinct vibrational structures. This is not like the pyrene-perylene mixed crystal system, the fluorescence spectra of which do not show such a clear vibrational spectra (,4). 3.1. The Assignnlent of the Fluorescence Spectra. The spectral response of the fluorescence of the perylene-benzo[g , h, i]perylene mixed crystal at liquid nitrogen temperature appears from about 33.5 kK to about 16.0 kK. The origins of the
p. Concen
23
22
21 Wave
20
19
18
17 kK
Number
FIG. 2. The fluorescence spectra of the mixed cryst’als at room temperature: concentration (mole/mole) of perylene in the mixed crystal is 1OV; (....), 10-Z; and (- - - -), 10-l. The nomenclatures of the peaks are in Table 1.
(---_), the 1OW; (-.-),
KAJIWARA,
4
SHIROTANI, TABLE
AND
INOKUCHI
I
THE CLASSIFICATIONOFTHE FLUORESCENCE BANDS OF~ERYLENE-BEN~O[~,~,~]PERYLF,NE MIXED CRYST.ILAT LIQUID NITROGENTEMPERATURE Band
Origin
A B C I> E F
Benzo[g,h,i]perylene Perylene molecule &O transition” Perylene molecule O-l transition Perylene aggregate Ckl transition’) Perylene aggregate 0-O transition” Perylene aggregate O-2 transition”
e See Fig. 1. b Benxo[g,h,i]perylene may also contribute to this band (see text 1 ’ The vibrational assignment is explained in the text.
Wave
Number
FIG. 3. The absorption spectra of perylene in benzo[g , h,i]perylene: (-), the observed spectra; (-.--), the estimated spectra of perylene in benzo[g,h,i]perylene and (e...), t,he fluorescence spect,ra of benzo[g,h,i]perylene. in this region are classified in Table 1. The intensity of the band about 22.55 kIZ decreased as the concentration of perylene contained in the mixed crystal increased, so it was assigned to benzo[
SPECTRA
OF PEKYLENE-BENZOPEBYLENE
-
23
22 Wave
FIG. 4. Comparison of benzo[g,h,i]perylene
21
PureBerm-(g
20
19
5
h II-Peryiene
18
ItK
Number
of the fluorescence spectra containing 1OW mole/mole
of pure benzo[g,h,i]perylene of perylene at 77°K.
with t,hat.
perylene had to be higher than f, X lo-” mole, but not at room temperature. Because these new bands did not appear at low concentration (lower than lo-” mole/mole), these are assumed to produce from some aggregates of perylene molecules. 3.2. Excited Lh’mer Fluorescence c?f’ Perylene. The new bands appeared at liquid nitrogen temperature resemble the fluorescence peaks of a-type4 perylene found at lower than %‘I< and those of @type4 perylene at lower than SO”I<, reported by Tanaka (5). However, this band must not be “monomer fluorescence” assumed by Tanaka in his kinetic analysis, because it does not appear when the perylene concentration is lo\v. Therefore, the origin of the emitting state might be excited dimer one (but not excimer5). It is not clear horn- many molecules are involved in the fluorescence state of perylene aggregate, but the excited dimer model is assumed to show the main feature of the perylene aggregate fluorescence. The fluorescence level of the perylene crystal at room temperature has been investigated by many researchers and assigned to a forbidden charge-transfer !/e&e electronic level of a pair of perylene molecules, which has inversion symmetry (5,C). The excited dimer state, proposed in this article, must also have the forbidden qe~~le symmetry property. However, the fluorescence bands, noted in this discussion, of a-type and &type perylene and of the present mixed crystal, have very strong intensity and are estimated to be almost the same as that of perylene monomer in the mixed crystal system, the emitting level of which is allowed. We now describe the possible explanation of the strong intensity of the fluores4 See Appendix. 5 By the t,erm “excimer,” dimer pair by some special
we mean some rearrangement and stabilization mechanism such as charge resonance.
of an excited
KAJIWARA,
c,
SHIROTANI,
AXD IKOKUCHI
cence bands and rather their anomalous shapes. We consider the intramolecul:u~ vibrational states of perylene dimer (7, 8). The ith electronic state of molecule A is denoted as cpy’ and the jth vibrational state in the ,ith electronic state of molecule A as x:i’j’. Of course, ,j must be thought to be a set of many quantum numbers of various intramolecular vibration modes, but, we choose orw important, mode. Then, the vibronic dimer \vave function m:ty be writkn :IS Fkl. (1 i by using the Simpson and Peterson’s wale coupling :lpproxim:ltion (8 ).
N = $5
(if
,! 3 (if
i += i’ i = ,i’
or
.i f .i’),
and
j = ,j’).
(1)
In this expression, the effect of the antisymmctrization of the function I\-ith respect to the electron exchange is neglected. If we denote the lowest excited singlet, electronic level of a molecule by i = 1, the emitting excited dimer level will be written as follows, &“;d
=
_ !_
i0)
( &‘xL”‘&‘xa”~“’ -cc.sx.4
iO.0,
(1) (FHXR
Cl,Ob
I’)
).
z/a
The molecular planes of two molecules of :Ldimer pair arc parallel long axes of the two molecules are parallel, thewfore I\-e can write,
(p~)p~’
jsjjt/p~)p~)) =
((o~J&J ,yjj 1 pi’JFgii =
and
yJ?,
dso
the
( :z,I
(here 9X! = eXr). Thus, the transition dipole moments between +P’~““‘~“’and electronically ‘“‘““‘~i’)are calculated as follows: swX:, %+ (i) ,j # 0 and ,j’ # 0, (+JJ.O;[l.O)I gjr j +~.O;j.,‘J/ = 0.
ground
(-1-l
(ii) ,j = 0, ,j’ = 0, (~~,WO)
, & / &Lo;n.oJ) = 0.
(.i 1
state does not exist), (the Q,“‘mo”‘~(‘) (iii ) ,j # 0, ,j’ = 0,”
The integral (x (I’“’ 1x ‘““) ) is generally not zero o\\-ing to the coupling of the intramolecular vibration with the molecular electronic state. Then WE may be :Iblc t,o say th:lt the transition from &‘“‘o”” state to the @~.o~0;‘~0’ st:ltc is :dlowcd.
SPECTR.4
OF PEILYLENE-BEN%OPEItYLENE
7
On the basis of the above consideration, we assign the about lS.7 kl< band of the mixed crystal system, containing 10-l mole/mole concentration of perylenc as an impurity, as the excited dimer @~‘“;o~o)-+ +(o,“;‘30)fluorescence, and the 17.4 kl?; band as the &.o;o,o) ~ $,(0,‘3;2.0) transition one. The about 19.S kI< band was proposed to be assigned to surface state by Tanaka, but this might be the forbidden &,O;O,O) --+ +~‘“;o*o) emission. The selection rule of this spectral band can be easily broken by the decrease of symmetry of the dimer pair from the change of relative position of the two molecules or by crystal field perturbation. Another explanation of this band may be given by the appearance of a secondary progression involving some lower frequency intramolecular vibrations. Although we used the weak coupling model for the above discussion, the results may be applied in the case of intermediate coupling, because, even in the medium coupling case, the state corresponding to +?~“;“,o) is little affected by the resonance exchange of the molecular electronic excitation. It must be noted that the above results do not hold in the limit of strong coupling approximation. In this case, the state corresponding to Ey. (1) must be written as follows:
where, X o’“;ol)denotes some intramolecular vibrational pair. Similarly, the electronic ground state is given by
state of the whole dimer
Thus it is easily seen that the transition between*:“;“’ andi& (“‘o;O) is electronically forbidden irrespective of CYand 0. Therefore, we are able to say that the emitting state of the perylene fluorescence under consideration is interpreted as weak or intermediate coupling excited dimer state. We described that the excited perylenc dimer fluorescence in the mixed crystal system is alike to the fluorescence of (Y- and P-type perylenc crystal at low temperature in the previous section. We will describe how the excited dimer emission can occur in P-type perylene. The previous discussion holds only if the two molecules of dimer pair can exchange each other by one or more of the dimer pair symmetry operation and the related transition dipole moments of the molecules arc parallel. Therefore, if the two neighboring perylene molecules have p-form, some possible dimer pairs can be formed, standing along the u-axis, b-axis, and the (a/2, b/2, 0) direction and so on. Then, an excited dimer type emission can be expected from an emitting level in which the excitation in the crystal is localized in one of the above mentioned dimer pair. Of course, as this localized state does not satisfy the symmetry property of the crystal, the crystal must have some distortion about the dimer pair. The distortion may be small displacement of the molecules from their equilibrium position. If two molecules along the b-axis has a dimer pair, it can take about the same
8
KAJIWARA,
SHIROTANI,
rlNI)
INOKUCHI
geometrical configuration as a dimer pair in a-form perylene by such ;L small displacement. Thus, the fluorescence of p-form perylene at low tempcr:\ture observed by Tanaka may be an emission of dimer pair like that in the a-form pcrylene. Thus, the similarity of the fluorescence shapes of the a-form :md P-form perylene can be explained. We compare the energy level of the emitting state of dimcr pair with that of a state in which only one molecule is excited. The former is stabilized mainly by the ~won:mce excitation exchange and the latter b>, the solvent effect by the surrounding molecules. If the stabilization by the resonance excitation exchange is larger than that by the solvent effect, excited dimer emksion is expected rather than :i molecule one. The fluorescence of perylene in t’he mixed crystal system is th:tb case. Generally, except when the excitation localized in only one molecule :done is largely stabilized by the solvent effect’, the fluorescence level of aromatic molecul:w crystal may be unlocalized excited state. If the lowest lkvydov component. is forbidden for the sake of parallelism of the trttnsition dipoles :md the interaction between tmnslntionally nonequivalent molecules is we~tk or internwdi:Lte, the fluorescence from such a component is cxpccted to h:~vc the b:nne fwturc as the excited dimer emission described in t.his section. .j.,.j. ‘I’fW~KlYZtlLre ~eper&nce oj &Xciff?d I>illlW ~hLolYWf!~lfZ flj ~‘~‘l’,IJ/fVlf~. Ill Section 3.2., we concluded that the fluorescences of a-form and /%-form perylene :mtl :dso perylene in the perylene-rich mixed cr>&ll system are all due to the wistjence of dimer pairs. T:mak:t reported the tempernturc dependency of the fluorescence of a-form pcrylene by wsuming an masim~~ in :I curve of t,hc energy of emitting level uersus intermolecul:w distance of the dimer pair (*7). Though he assumed t,hat the fluorescence at ion- temperat,urc~ is due to the sin& molecllle cwitat,iotl, his kinetic an:tlysis also holds cvcn n-Hun this fluorcsc~nce is t,hct rscitcxtl cm-l 12oot
.
am-
2
.L. 600. c r_ 5 400. % a, CL 200-
/11/1-
10-3
10-2 Concentration
10“
1
of Perylene f moleymore,
FIG. 5. The cmcentraiion dependence of the the perylene aggregitles ill bel,xo[g,h,i]perylerle.
flU(JR?SCCIlW
maximum,
I) in Fig. 1, of
SPECTRA
1 24
23
OF PERYLENE-BENZOPERYLENIX
22 21 Wave Number
20
79
FIG. 6. The fluorescence spectra of pure henzo[g,h,ilperyleue
I 24
23
22 Wave
FIG. 7. The fluorescence perylene
spectra at low t)emperature.
dimer emission
21
20
78 kK
at various temperatures.
19
18
kK
Number
of benzo[g,h,i]perylene
as we proposed,
9
only if we replace
containing
about 1W4 mole/mole
his single molecule
N
b!r a
dimer pair MM and excited single molecule A/* by an excited dimer pair A/M*.
If we assume the excited dimer emission as treated in Section 3.2 and apply his kinetic analysis, P-form perylene crystal and also perylene in the perylene-rich
KA.JIWARA,
10
SHIROTAIL’I,
? i
ilND
INOKUCHI
Solid (Evaporated
Thin Film)
I
)
,
1 ; i
23
22 Wave
21
20
19
18
kK
Number
FIG. 8. The concentratjion dependence of the fluorescence spectra of pure benxo[g,h,i]perylene benzene solllt~ion. The flrlorescence spectra of pt~re benzc)[g,h,i]perylene film is also show-n.
lnixcd crystal must shoed the same temperature dependence as a-form perylene crystal in contradiction with the observed results, therefore his kinetic :malysis seems to be uns:ttisfsctory. The fluorescence behavior of perylene crystal :t~ a function of temperature may be essentiully influenced by the aggregation state, but further investigation needs to solve this problem cluantittltively. 3.4. The Concentration Dependence oj’ l
SPECTRA
11
OE’ PEKYLENE-BEN%OPEl~YLENE TABLE
II Unit cell dimension
Compound
Perylene Renzo[g,h,i]perylcne
&)
b CA,
c (ii,
(“1
11.35 11.72
10.87 11.88
10.31 9.89
100.5 98.5
The shift is nearly proportional to the logarithm of perylene concentration as shown in Fig. 5. Though this large shift must be due to aggregation of perylene dimer pairs, we cannot conclude whether this shift is mainly due to the difference of peryleneperylene interaction from perylene-benzo[g , h, ilperylene interaction or due to such an effect that geometrical configuration of perylene dimer pair changes when the aggregation size of perylene pairs increases. If we attribute the shift only to the difference of interaction energy, the logarithmic dependence of the shift on the concentration is deduced. We assume that the difference of interaction energy can be written as A/RR, here, R is intermolecular distance and constant A is independent on R, and that perylcne in the mixed crystal system forms an emitting site whose size is proportional to the power of the perylenc concentration. Then, the shift AV is roughly calculated:
and
then Av z
;
Aa,,
log C + log g
-
; log Ro
1
This explanation, however, is thought to be unsatisfactory, because the value of Av is usually thought to be about 200-300 cm-‘, which is too small to explain the observed shift, 1100-1300 cm-‘. 3.5. The Fluorescence Spectra of Renzo[q, h,i]perylene: Icigurc ci shows the fluorescence spectra of pure benzo[g , h, ijperylene thin film. The fluorescence in tensity at 150°K is about twice that at room temperature. The intensity of fluorescence band, whose maximum is at 440 rnp, rapidly increases as the temperature decreases from about 106”Ii to about 9O”Ii, and the vibrational subbands become sharper. The appearance and increases of the about 440 rnp band cannot be attributed to the decrease of reabsorption, for we used very thin film and the optical density in the region longer than 420 rnp of this film at liquid nitrogen temperature was the same as that at room temperature. The thin film of benzo-
RAJIWARA,
12
SHIROTANI,
ANI)
INOBUCIII
[y , h , ,i]perylene which contains about 10-4-10-” mole/mole perylene shows this 440 rnp band already at 14O”K, as Fig. 7 shows, and this band at liquid nitrogen temperature is sharper than that of pure benzo[y , /I, i]perylene at the same temperature, as Fig. 4 shows (9). The fluorescence of benzo[g, h ,i]perylene at room temperature wan like the broad band which is found in the fluorescence of concentrated benzene solution of benzo[q ,h ,i]perylene at room temperat,ure, as illustrated in Fig. S. Though this fluorescence is thought to be excimer type, its maximum exists at rather shorter \vavelength and does Ilot satisfy the so-called 6000 cm-’ rule (10). Further investigations are now intended to clarify all these phenomena. APPENDIX
The crystal structures (C$,) of perylene and benzo[cl, h, i]perylene are shown in Table II. The crystal structure of perylene-benzo[q, h? ijpervlene (concentration ratio is about 15 :%) mixed crystal is monoclinic (Cih), with two molecules in a unit, cell of dimension u = 11.63 A, h = 1135 8, L’= 92% & and /3 = 98.6”. The crystal structuml data of perylene, ment,ioned in (Table II ) is that of LYform with four molecules in the unit cell. fl-Form perylene also belongs t)o the ( I:$, apace group and give the following crystal dam (ri) : (I = 11.27 f 0.03 A, /I = MS f 0.02 11, c = 9.65 f 0.03 & p = 92.1 f 0.:3” with two molecules in t,he unit cell.
The :tuthors
wish tr) t,hank Mr. Hni.oshi
Iwashima
for providing
i he pure hydrocarhorr,
t~~rleo[g, h, i]perylena.
RECEIVERS : January
10, 1969 1
REFERENCES I. T. K~.JI~.IR.I, I. SHIROT.\NI, II. INOK~(:HI, .1x1) S. I\~.IsHIxI.\, .I. Jiul. Spect,y. 29. 454 (1969). B. S. Iw.\sIIIM;\, private rommunication. 3. N. W:ZK~~YXVIA AND H. INOKUCHI, Bull. Chem. SW. .Jnpczn, 40, 2267 (1967). 4, 11. M. HOCIISTRASSI~CN, J. Chem. Phys. 36, 1099 (19fi2); K. Iirrn.\o~i.~ AND I>. IL. KURYS, J. (‘hum. Phys. 46, 147 (1966); Y. ISHI ;\NII A. ~~.\%-I, b. I’hys. S’oc-. Japarr. 22, 926 (1967). 5. 6. T.~N~K.I, B’rcll. Chrm. ft’oc. Japan 36, 1237 (1963). 0’. R. M. HOCHHTRASSER, J. Chem. Phys. 40, 2559 (1964); I(. M. HO~HST~USSER .\XD A. M~LLIARIS, J. Cheltl. Phys. 42, 2243 (1965); T. .~ZLTMI.\NU R. P. M&LYNN, J. (‘hem. l’hys. 42, 1675 (1965); J. FEHGUSON, .I. Chem. Phys. 44, 2677 (1966). 7. A. WITKO~SKII AND W. MOFFITT, J. Chern. Phys. 33, 872 (1960); E. f:. Mcl?.aa, ;Ius1diun J. Chen~ 14, 329, 344, 354 (1961); 16, 295 (19H3). 8. J. SIMPSON AND II. L. PKTERSON, J. Chem. J’hys. 26, 588 (1957). Y. H. C. WOLF, Z. Physik, 143, 266 (1955). f 0. J. R. BIRKS ‘\NI) 1,. (+. CHRIS’I’OI’HOI~(I~T,I’/w. /x’n?/. Sw. (Lontlvn), Ser. il 27’7. 571 (1964).