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Absorption spectrum of rubrene in different solvents
Syretrochlmfcn Acta, 1951, Vol. 4. pp. 2W to 263. ButteraorthYpringer.
Absorption
Londca
spectrum of rubrene
in
different
solvents
G. M. BADUER and R. S. PEARCE JohneonChemicalL&oratories,Universityof Adelaide
It is n-ell known that the position of the absorption bands of substances in solution varies with the solvent, and attempts have often been made to associate the degree According to KZTNDT [I], the shift is of shift with some property of the solvent. directly related to the refractive index, but there are many anomalies and HOUSTON [2] maintains that the shift is aa often in one direction as another. In some cases it is found [3] that the absorption is shifted progressively to longer wavelengths as the dielectric constant of the solvent is increased. and equations relating the shift and dielectric constant have been used by SHEPPARD [a] and by others. An expression has recently been developed [5] relat.ing the red shift in solution (Av) with the oscillator strength (f), the size of the solute molecule, and the index of refract.ion. Thus : A V= const, (5)
(2sl)
where a is the radius of the solute molecule, and % is the refractive index of the solvent at the wavelength of the absorption band. This implies that there should be a linear relationship between Av and (n2 - l)/( 2n2+l) and it seemed of interest to test this relationship using a suitable polycyclic aromatic hydrocarbon. The bright red hy~ocarbon rubrene (I) is suitable for such a purpose as its longest D~YRAISSE and absorption band is not far removed from the sodium D line. BADOCHE [6] have already examined t,he absorpt,ion spectrum of this hydrocarbon in several solvents, and although several anomalies were found. they concluded that the red shift increases with the index of refraction of the solvent used. Few details. were given, however, and it was necessary to re-determine the spectra in as many solvents as possible.
Results and Discussion There are two main regions of ab.sorption for rubrene (see Figure 1). The first is a region of high int,ensity absorption at about 53000 cm-‘, and corresponds to the Group I absorption bands of BRAUDE [7]. or the P-bands of CLAR [8]. The second region of less intense absorption extends from about 26000 to about 18000 em-‘, and corresponds to the Group II absorption of BRAVDE [7]: or the para-bands of CUR [$I. As a matter of fact, the spectrum resembles that, of the parent hydrocarbon, napht,hacene (II), very closely indeed and this may be taken as evidence that the four phenyl groups are not coplanar with the naphthacene ring system, so that A non-coplanar configuration is also supported conjugation is markedly reduced. by X-ray crystallographic data [9]. 2130
Absorption spectrum of rubrene in different eolventa
We have studied the absorption spectrum of rubrene in 19 different solventa, diethyl ether, hexane, amyl acetate, acetone, methylene chloride, dioxan, anisole, benzene, chloroform, chlorobenzene. bromobenzene, e-tetrachloroa8 follows:
Ph
Ph
I
IX
aniline, 1-bromonaphthalene, carbon ethane , pWyrid.ine, quinoline , iodobenzene, Neglecting the point of inflection at about disulphide! and methylene iodide. 24 000 cm-‘, which can be distinguished only in some of the solvents, the various maxima have been labelled A to’ E for easy reference, and the result8 are given in Table 1. The absorption band8 in ether are at shorter wavelengths than in any of t,he other solvents. This solvent is therefore taken a8 the “ Btrrndard “, and the remaining results are tabulated in order of increasing shift to the red (Le. increasing Av,). The error in determining the position8 of the bands is of the order of 30 to 50 cm-‘. Included in the table are the refractive indices of the 8olv8nt8, and the expression (nP - 1)/(2n2+ 1).
s
.w
u
.w
CB
cc
Wuve numbers I W
Fig. 1. Fig. l--Absorption Fig. 2-Fklationehip
spectrum of rubrene in ether and in e-tetrachloroethane. between the red Rhift in eolution, Av,, to (n*-1)/(2n*+l). The pointa are numbered to coincide with the order of eolventa given in Table 1.
In Figure 2, Avg ie plotted against (n2-1)/(2n*+l), the point8 being numbered to coincide with the order of solvents listed in Table 1. It is 8een that there is an approximate linear relationship. &me of the deviation8 are somewhat greater than the experimental error, 80 that the linear relationship doe8 not hold exactly, but the general tendency a8 predicted by the theory 153 seem8 to hold. The faot that the position of an absorption band depends to some extent on the refractive index of the solvent used, must have an important bearing on the inter281 B
G. M. BADOER
and R. 8. PEARCE
Table I* Wave numbers of abeorption bad.8
-.
A-0.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1s 16 17 18 19
Diethyl ether Hexane &llyl 8C&&0 Acetone Methylene chloride Dioxan Ani8ole Benzene Chloroform Chlorobenzene Bromobenzene a-Tetrachloroethane Pyridine Quinoline Iodobenzene Aniline I-Bromonaphthelene Carbon disulphide Methylene iodide
-___
33560 33510 -33320 33330 32250 33000 33000 32780 32890 -
I
B
-4
.I
E
_I_
:I
23040 23040 22940 22990 22800 22830 22780 22990 22780 22730 22670 22680 22680 22620 22520 22620 22570 22780 22570
21690 21530 21600 21630 21600 21500 21460 21420 21370 21410 21320 21370 21340 21390 21270 21120
I , ! ’
20490 20470 20360 20370 20240 20300 20240 20240 20200 20140 20100 20080 20120 20040 20040 20100 20060 19920 19760
19160 19100 19030 19010 18900 18900 18900 18870 18870 18830 18810 18760 18760 18730 18730 18730 18730 18600 18480
!,
! ,.,
_,
nz
ia” B
_
n*-
1
; 2n”+l
--
-
60 130 150 260 260 260 290 290 330 350 400 400 430 430 430 430 560 680 -
1.3519 1.3764 I.4012 1.3591 1.4237 1.4221 1.5173 1*6017 14457 1.6251 1.5604 1.4921 1.509 1.6283 I.6213 1.5863 l-6582 I.6276 1.742
O*li69 0.1907 0.1956 0.1812 0.2032 0.2030 0.2033
0.2277 0*2105 0.2345 0.2444 0.2256 0.2300 0.2611 0.2603 0.2514 0.2693 0.2623 0.2907
* When no wave number ie given for the A bend. thie implies that the eolvent 7~88 not transparent at this wavelength
pretation of the absorption spectra of organic compounds at low temperatures. R&active indices are strongly temperature-dependent, and for a wide variety of organic solvents a decrease in temperature of lo causes an increase in refractive index of about 4.5 x lo-” [lo]. Very little is known regarding the indices of refraction of most solvents at very low or even at moderately low temperatures, but if the temperature gradient mentioned above is assumed to hold for 200“, (which is unlikely to be true) then a reduction in solvent temperature to - 180°, would result in an increase in refractive index of about 0.090. A marked shift in the positions of the absorption bands to the red would therefore be expected from such a change in refractive index alone. In this connection the recent results of &JR [ll] are of interest. &AR found that the so-called para- and B-bands of the polynuclear aromatic hydrocarbons are shifted about 300 cm-’ to the red when their alcoholic solutions are cooled to about - 180’. In this work a mixture of ethyl and methyl alcohols was used as solvent, and it is obviously impossible to calculate the expected shift accurately, but at least it may be said that the observed shift is in the direction, and is of approximately the magnitude expected, if this shiR is due entirely to the change in refractive index of t.he solvent at the reduced temperature. CLAR did not observe a red shift of the Group III (or a) bands of the aromatic compounds and the present work does not provide any information on this point, for rubrene has no Group III bands. It may be, however, that the difference is connected with the smaller oscillator strength in. this case. We are grateful to the Director of Chemistry, South Australian Government Department of Chemistry (Mr. R. J. COWAN) for permission to use the Beckman spectrophotometer. 282
-Absorption
spectrum
of rubrene
in difierent
solvente
Experimental The solvents used in this investigation were all purified by standard methods, usually by distillation etc? the refractive indices quoted in Table 1 being those observed. The spectra were determined with a Be&nun DC spectrophotometer.
The absorption spectrum of the red plynuclear hydrocarbon rubrene (I) has been determined in nineteen solvents. The positions of the absorption bands are found to be approximately dependent on the refractive index of the solvent, and if one of the solvents is accepted as “ standard “. there is an approximate linear relationship between Au and (nZ -1)/(2n*+l). The bearing of the results on the interpretation of spectra at low temper#ures is discussed.
DBtermination du spectre d’ahsorption de Rubrene (I) en dixtneuf solutions. Les positions dee handee d’absorption dependent approximativement de l’indice de refraction du milieu de solution. 1) est Si une des solutions eat accept& comme ‘%standard ‘* la relation entre Au et (n*-1)/(2n’i pfisque lin&re. Discussion dea r&ultat.s pour l’interpr&ation des spertres B temperatures basses.
Zummmenfassung Das Ahsorptionsspektrum des roten vielkernigen Kohlenwasserstiffs Rubren (I) in neunzehn LWgemitteln wurde beetimmt. Es wurde gefunden, daas die Lage der Absorptionsbande angentiert abh&ngig ist vom Brechungsindex des Lasungsmittels und dass eine angen&herte linenre Beriehung wenn einee der L&nmgsmittel als “ Standard ” angebesteht zwischen AV und (n*-1)/(2n*+l). nommen wird. Die Bedeutung der Ergebnisse fiir die Deutung der Spektren bei t.iefen Temperaturen wird eriirtert.
Referencea [I] KIJXDT.
A.; Ann. Phys. Chem. 1878 4 34-54. [2] HOUSTON. R. A.; .4 Treatise on Light. 7th Ed. Longmans, London: 1938. [3] BROODER, L. G. S. and d~ruom, R. H.; J. Amer. them. Sot. 1941 63 3214-3215. [4] SEEPPARD. S. E. and BRIOHAY. H. R.; J. Amer. rhem. Sot. 1944 66 380-384; SHEPPARD, S. E., SEWSOME, P. T. and BRIGHAM, H. R.; J. Amer. them. Ser. 1942 64 2923-2937; SHEPPARD, S. E.; Rev. Mod. Phys. 1942 14 303-340. [5] Banxss, S. S.; J. Chem. Phye. 1950 18 292-2Q6. [6] DUFIUI~SE. C. and BADOCHE. M.; C.R. Acad. Sci., Paris 1935 200 929-931; see also DUFRAISSE, C. and HOR~LOIS. R.; Bull. Sot. chim. 1936 3 1880-1893. [7] BRAVDE. E. A.: AM. Reports, 1945 42 123. [E] CLAR, E.; Aromatische Kohlenwaeser&offe. Springer. Berlin: 1941. [Q] BERQXANN, E. and HEIILINOER. E. ; J. Chem. Phys. 1936 4 532-534. [lo] WEIWBIRC.ER, -4.; JZd. Physical Methods of Organic Chemistry, Interscienre, Sew York: 1945; see RAUER, S. in Vol. 1, p. 656. [ll J CLAR. E.; Spectrochim. Acta 1950 4 116-121.
283
Absorption
Londca
spectrum of rubrene
in
different
solvents
G. M. BADUER and R. S. PEARCE JohneonChemicalL&oratories,Universityof Adelaide
It is n-ell known that the position of the absorption bands of substances in solution varies with the solvent, and attempts have often been made to associate the degree According to KZTNDT [I], the shift is of shift with some property of the solvent. directly related to the refractive index, but there are many anomalies and HOUSTON [2] maintains that the shift is aa often in one direction as another. In some cases it is found [3] that the absorption is shifted progressively to longer wavelengths as the dielectric constant of the solvent is increased. and equations relating the shift and dielectric constant have been used by SHEPPARD [a] and by others. An expression has recently been developed [5] relat.ing the red shift in solution (Av) with the oscillator strength (f), the size of the solute molecule, and the index of refract.ion. Thus : A V= const, (5)
(2sl)
where a is the radius of the solute molecule, and % is the refractive index of the solvent at the wavelength of the absorption band. This implies that there should be a linear relationship between Av and (n2 - l)/( 2n2+l) and it seemed of interest to test this relationship using a suitable polycyclic aromatic hydrocarbon. The bright red hy~ocarbon rubrene (I) is suitable for such a purpose as its longest D~YRAISSE and absorption band is not far removed from the sodium D line. BADOCHE [6] have already examined t,he absorpt,ion spectrum of this hydrocarbon in several solvents, and although several anomalies were found. they concluded that the red shift increases with the index of refraction of the solvent used. Few details. were given, however, and it was necessary to re-determine the spectra in as many solvents as possible.
Results and Discussion There are two main regions of ab.sorption for rubrene (see Figure 1). The first is a region of high int,ensity absorption at about 53000 cm-‘, and corresponds to the Group I absorption bands of BRAUDE [7]. or the P-bands of CLAR [8]. The second region of less intense absorption extends from about 26000 to about 18000 em-‘, and corresponds to the Group II absorption of BRAVDE [7]: or the para-bands of CUR [$I. As a matter of fact, the spectrum resembles that, of the parent hydrocarbon, napht,hacene (II), very closely indeed and this may be taken as evidence that the four phenyl groups are not coplanar with the naphthacene ring system, so that A non-coplanar configuration is also supported conjugation is markedly reduced. by X-ray crystallographic data [9]. 2130
Absorption spectrum of rubrene in different eolventa
We have studied the absorption spectrum of rubrene in 19 different solventa, diethyl ether, hexane, amyl acetate, acetone, methylene chloride, dioxan, anisole, benzene, chloroform, chlorobenzene. bromobenzene, e-tetrachloroa8 follows:
Ph
Ph
I
IX
aniline, 1-bromonaphthalene, carbon ethane , pWyrid.ine, quinoline , iodobenzene, Neglecting the point of inflection at about disulphide! and methylene iodide. 24 000 cm-‘, which can be distinguished only in some of the solvents, the various maxima have been labelled A to’ E for easy reference, and the result8 are given in Table 1. The absorption band8 in ether are at shorter wavelengths than in any of t,he other solvents. This solvent is therefore taken a8 the “ Btrrndard “, and the remaining results are tabulated in order of increasing shift to the red (Le. increasing Av,). The error in determining the position8 of the bands is of the order of 30 to 50 cm-‘. Included in the table are the refractive indices of the 8olv8nt8, and the expression (nP - 1)/(2n2+ 1).
s
.w
u
.w
CB
cc
Wuve numbers I W
Fig. 1. Fig. l--Absorption Fig. 2-Fklationehip
spectrum of rubrene in ether and in e-tetrachloroethane. between the red Rhift in eolution, Av,, to (n*-1)/(2n*+l). The pointa are numbered to coincide with the order of eolventa given in Table 1.
In Figure 2, Avg ie plotted against (n2-1)/(2n*+l), the point8 being numbered to coincide with the order of solvents listed in Table 1. It is 8een that there is an approximate linear relationship. &me of the deviation8 are somewhat greater than the experimental error, 80 that the linear relationship doe8 not hold exactly, but the general tendency a8 predicted by the theory 153 seem8 to hold. The faot that the position of an absorption band depends to some extent on the refractive index of the solvent used, must have an important bearing on the inter281 B
G. M. BADOER
and R. 8. PEARCE
Table I* Wave numbers of abeorption bad.8
-.
A-0.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 1s 16 17 18 19
Diethyl ether Hexane &llyl 8C&&0 Acetone Methylene chloride Dioxan Ani8ole Benzene Chloroform Chlorobenzene Bromobenzene a-Tetrachloroethane Pyridine Quinoline Iodobenzene Aniline I-Bromonaphthelene Carbon disulphide Methylene iodide
-___
33560 33510 -33320 33330 32250 33000 33000 32780 32890 -
I
B
-4
.I
E
_I_
:I
23040 23040 22940 22990 22800 22830 22780 22990 22780 22730 22670 22680 22680 22620 22520 22620 22570 22780 22570
21690 21530 21600 21630 21600 21500 21460 21420 21370 21410 21320 21370 21340 21390 21270 21120
I , ! ’
20490 20470 20360 20370 20240 20300 20240 20240 20200 20140 20100 20080 20120 20040 20040 20100 20060 19920 19760
19160 19100 19030 19010 18900 18900 18900 18870 18870 18830 18810 18760 18760 18730 18730 18730 18730 18600 18480
!,
! ,.,
_,
nz
ia” B
_
n*-
1
; 2n”+l
--
-
60 130 150 260 260 260 290 290 330 350 400 400 430 430 430 430 560 680 -
1.3519 1.3764 I.4012 1.3591 1.4237 1.4221 1.5173 1*6017 14457 1.6251 1.5604 1.4921 1.509 1.6283 I.6213 1.5863 l-6582 I.6276 1.742
O*li69 0.1907 0.1956 0.1812 0.2032 0.2030 0.2033
0.2277 0*2105 0.2345 0.2444 0.2256 0.2300 0.2611 0.2603 0.2514 0.2693 0.2623 0.2907
* When no wave number ie given for the A bend. thie implies that the eolvent 7~88 not transparent at this wavelength
pretation of the absorption spectra of organic compounds at low temperatures. R&active indices are strongly temperature-dependent, and for a wide variety of organic solvents a decrease in temperature of lo causes an increase in refractive index of about 4.5 x lo-” [lo]. Very little is known regarding the indices of refraction of most solvents at very low or even at moderately low temperatures, but if the temperature gradient mentioned above is assumed to hold for 200“, (which is unlikely to be true) then a reduction in solvent temperature to - 180°, would result in an increase in refractive index of about 0.090. A marked shift in the positions of the absorption bands to the red would therefore be expected from such a change in refractive index alone. In this connection the recent results of &JR [ll] are of interest. &AR found that the so-called para- and B-bands of the polynuclear aromatic hydrocarbons are shifted about 300 cm-’ to the red when their alcoholic solutions are cooled to about - 180’. In this work a mixture of ethyl and methyl alcohols was used as solvent, and it is obviously impossible to calculate the expected shift accurately, but at least it may be said that the observed shift is in the direction, and is of approximately the magnitude expected, if this shiR is due entirely to the change in refractive index of t.he solvent at the reduced temperature. CLAR did not observe a red shift of the Group III (or a) bands of the aromatic compounds and the present work does not provide any information on this point, for rubrene has no Group III bands. It may be, however, that the difference is connected with the smaller oscillator strength in. this case. We are grateful to the Director of Chemistry, South Australian Government Department of Chemistry (Mr. R. J. COWAN) for permission to use the Beckman spectrophotometer. 282
-Absorption
spectrum
of rubrene
in difierent
solvente
Experimental The solvents used in this investigation were all purified by standard methods, usually by distillation etc? the refractive indices quoted in Table 1 being those observed. The spectra were determined with a Be&nun DC spectrophotometer.
The absorption spectrum of the red plynuclear hydrocarbon rubrene (I) has been determined in nineteen solvents. The positions of the absorption bands are found to be approximately dependent on the refractive index of the solvent, and if one of the solvents is accepted as “ standard “. there is an approximate linear relationship between Au and (nZ -1)/(2n*+l). The bearing of the results on the interpretation of spectra at low temper#ures is discussed.
DBtermination du spectre d’ahsorption de Rubrene (I) en dixtneuf solutions. Les positions dee handee d’absorption dependent approximativement de l’indice de refraction du milieu de solution. 1) est Si une des solutions eat accept& comme ‘%standard ‘* la relation entre Au et (n*-1)/(2n’i pfisque lin&re. Discussion dea r&ultat.s pour l’interpr&ation des spertres B temperatures basses.
Zummmenfassung Das Ahsorptionsspektrum des roten vielkernigen Kohlenwasserstiffs Rubren (I) in neunzehn LWgemitteln wurde beetimmt. Es wurde gefunden, daas die Lage der Absorptionsbande angentiert abh&ngig ist vom Brechungsindex des Lasungsmittels und dass eine angen&herte linenre Beriehung wenn einee der L&nmgsmittel als “ Standard ” angebesteht zwischen AV und (n*-1)/(2n*+l). nommen wird. Die Bedeutung der Ergebnisse fiir die Deutung der Spektren bei t.iefen Temperaturen wird eriirtert.
Referencea [I] KIJXDT.
A.; Ann. Phys. Chem. 1878 4 34-54. [2] HOUSTON. R. A.; .4 Treatise on Light. 7th Ed. Longmans, London: 1938. [3] BROODER, L. G. S. and d~ruom, R. H.; J. Amer. them. Sot. 1941 63 3214-3215. [4] SEEPPARD. S. E. and BRIOHAY. H. R.; J. Amer. rhem. Sot. 1944 66 380-384; SHEPPARD, S. E., SEWSOME, P. T. and BRIGHAM, H. R.; J. Amer. them. Ser. 1942 64 2923-2937; SHEPPARD, S. E.; Rev. Mod. Phys. 1942 14 303-340. [5] Banxss, S. S.; J. Chem. Phye. 1950 18 292-2Q6. [6] DUFIUI~SE. C. and BADOCHE. M.; C.R. Acad. Sci., Paris 1935 200 929-931; see also DUFRAISSE, C. and HOR~LOIS. R.; Bull. Sot. chim. 1936 3 1880-1893. [7] BRAVDE. E. A.: AM. Reports, 1945 42 123. [E] CLAR, E.; Aromatische Kohlenwaeser&offe. Springer. Berlin: 1941. [Q] BERQXANN, E. and HEIILINOER. E. ; J. Chem. Phys. 1936 4 532-534. [lo] WEIWBIRC.ER, -4.; JZd. Physical Methods of Organic Chemistry, Interscienre, Sew York: 1945; see RAUER, S. in Vol. 1, p. 656. [ll J CLAR. E.; Spectrochim. Acta 1950 4 116-121.
283