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The polarization of the allowed triplet-triplet absorption in anthracene
CHEMICAL
PHYSICS LETTERS
1 (1968)
597-598. NORTH-HOLLAND
PUBLISHING
CO&rPANY.
THE POLARIZATION OF THE ALLOWED TRIPLET-TRIPLET ABSORPTION IN ANTHRACENE Robin M. HOCHSTRASSER
and Alfred
P. MARCHETTI
AMSTERDAM
*
**
Department of Chemistq and Labouatov for Research on the Stvuctuve of Mattev The University of Pennsylvania, Philadelphia. Pennsylvania 19104, USA
Received 22 January 1968
In this note we report
the polarization
of the
well known triplet-trip@ absorption of anthracene in the 4500 - 4100 A region and the assignment for the total space symmetry of the upper state. The short lifetime of the anthracene lowest triplet state makes this determination slightly more difficult than are polarization measurements on the longer lived (oriented) triplets. The spectra were obtained using single crystals of benzophenone which had been doped with anthracene. This technique was used previously [I] to study the polarization of triplet-triplet bands, but in the earlier work only the rather ambiguous (110) face of benzophenone was employed (and the temperature was just 77OK). The crystals were grown from an ethanol solution and the faces were identified from their habit and by conoscopic techniques. The recent crystal structure determination of benzophenone [2] provides ‘the required molecular orientations, and meas-
4500
4200
4100
4000
J,
Fig. 1. The polarized triplet-triplet absorption of anthracene in benzophenone at 77OK (lower- trace) and 4.2OK (upper traces). The Tl + So absorption of benzophenone starts at 4134 b;. The two emission lines beyond 4500 A are xenon lines from the lamp and the weaker emission lines between 4400 and 4150 A are due to benzophenone phosphorescence.
the a crystal axis and that the short molecular axis (z) should make a large projection (Bzb - 1’7O) onto the b crystal axis. The use of benzophenone as a host appears to make it possible for a readily observable concentration of anthracene triplets to be maintained in rather short (- l-2 mm) path lengths of crystals that are quite dilute (- 2 x 10-4 molar).
The spectra are shown in fig. l._ The polarization ratio (Z,/Zb = 3.0) of the 4425 A band clearIy reveals that the transition is long axis poIarized. The lowest triplet state of anthracene has the spatial symmetry 3B1u [3] (iransforming like z). Thus the assignment for this T-T absorption is 3B3g - 3B lu n _ These results confirm the theoretical prediction of Pariser [5] that the first allG-aed triplet-triplet absorption [S] in anthracene should involve a 3E3g state (3Big in Pariser’s tt We obtained similar polarization results for the naphtalene T-T absorption. The 3B3g c 3BIu assignment also holds for naphhlcne. in agreemert with the work of Craig and Fisher i4].
* This research was supported in part by the Advanced Research Projects Agency Contract SD-69, and in part by a U.S.Department of Health Grant GM-1259203. ** N.I.H. Post-Doctoral Fellow. 1968
4McI x
urements of the polarization of the fluorescence of anthracene in benzophenone confirm that the anthracene molecular plane lies approximately in the ab crystal plane of benzophenone [i]. The host crystal structure suggests that in the mixed crystal the anthracene long molecular axis (v) should make a large projection (eya - 20°) onto
February
4400
597
598
R. M. HOCHSTRASSER and A. P. mRCHETT1
axis system) at 3.28 eV above the lowest triplet (experimental energy of 3B3g is about 2.8 eV). The improved resolution of the singlet-triplet absor tion of benzophenone in going from ‘77OK to 4.2 GC-indicates that this absorption obscures the second band of the triglet-triplet transition at 770K. Since the host Tl - So band absorbs about three times stronger along b than a, the 77’K spectra cannot be easily used to determine the polarization of the 0, 1 triplet-triplet absorption band The presence of the broad Tl -So band of the host most probably provided the anomalous intensity ratio reported previously [l] for the two T + T bands (i.e. 0, 0 and 0, lj of the spectra on the (110) face. The measured intensity ratio in this work (I4425 ji/14150 A z 2.3) in the a polarization is within experimental error of the reported isotropic value af 2.9 [6]. It should be noted that while there was a distinct sharpening of the Tl - 5, absorption of benzophenone on
going from 77OK to 4.2OK, there was no corresponding sharpening of the anthracene triplettriplet absorption; and the bands remained about 500 cm-l wide at half-height.
References [1] R. M. Hochstrasser and S. K. Lower, J. Chem. Phys. 40 (1964) 1041. [Z] E. Fleischer, lJniversity of Chicago, private communication. r31 R. H. Clarke and R. M. Hochstrasser. J. Chem. Phvs. - - 46 (1967) 4532. [4] Craig; and Fisher, Proc. Chem. Sot. (1964) 176. [5] R.Pariser. J. Chem.Phys. 23 (1956) 324. [6] D. P. Craig and 1. G. Ross, J. Chem. Sot. (1954) 1589; M. W. Windsor and J. R. Novak, The Triplet State. ed. A. B. Zahlan (Cambridge University Press, 1967) p. 229.
PHYSICS LETTERS
1 (1968)
597-598. NORTH-HOLLAND
PUBLISHING
CO&rPANY.
THE POLARIZATION OF THE ALLOWED TRIPLET-TRIPLET ABSORPTION IN ANTHRACENE Robin M. HOCHSTRASSER
and Alfred
P. MARCHETTI
AMSTERDAM
*
**
Department of Chemistq and Labouatov for Research on the Stvuctuve of Mattev The University of Pennsylvania, Philadelphia. Pennsylvania 19104, USA
Received 22 January 1968
In this note we report
the polarization
of the
well known triplet-trip@ absorption of anthracene in the 4500 - 4100 A region and the assignment for the total space symmetry of the upper state. The short lifetime of the anthracene lowest triplet state makes this determination slightly more difficult than are polarization measurements on the longer lived (oriented) triplets. The spectra were obtained using single crystals of benzophenone which had been doped with anthracene. This technique was used previously [I] to study the polarization of triplet-triplet bands, but in the earlier work only the rather ambiguous (110) face of benzophenone was employed (and the temperature was just 77OK). The crystals were grown from an ethanol solution and the faces were identified from their habit and by conoscopic techniques. The recent crystal structure determination of benzophenone [2] provides ‘the required molecular orientations, and meas-
4500
4200
4100
4000
J,
Fig. 1. The polarized triplet-triplet absorption of anthracene in benzophenone at 77OK (lower- trace) and 4.2OK (upper traces). The Tl + So absorption of benzophenone starts at 4134 b;. The two emission lines beyond 4500 A are xenon lines from the lamp and the weaker emission lines between 4400 and 4150 A are due to benzophenone phosphorescence.
the a crystal axis and that the short molecular axis (z) should make a large projection (Bzb - 1’7O) onto the b crystal axis. The use of benzophenone as a host appears to make it possible for a readily observable concentration of anthracene triplets to be maintained in rather short (- l-2 mm) path lengths of crystals that are quite dilute (- 2 x 10-4 molar).
The spectra are shown in fig. l._ The polarization ratio (Z,/Zb = 3.0) of the 4425 A band clearIy reveals that the transition is long axis poIarized. The lowest triplet state of anthracene has the spatial symmetry 3B1u [3] (iransforming like z). Thus the assignment for this T-T absorption is 3B3g - 3B lu n _ These results confirm the theoretical prediction of Pariser [5] that the first allG-aed triplet-triplet absorption [S] in anthracene should involve a 3E3g state (3Big in Pariser’s tt We obtained similar polarization results for the naphtalene T-T absorption. The 3B3g c 3BIu assignment also holds for naphhlcne. in agreemert with the work of Craig and Fisher i4].
* This research was supported in part by the Advanced Research Projects Agency Contract SD-69, and in part by a U.S.Department of Health Grant GM-1259203. ** N.I.H. Post-Doctoral Fellow. 1968
4McI x
urements of the polarization of the fluorescence of anthracene in benzophenone confirm that the anthracene molecular plane lies approximately in the ab crystal plane of benzophenone [i]. The host crystal structure suggests that in the mixed crystal the anthracene long molecular axis (v) should make a large projection (eya - 20°) onto
February
4400
597
598
R. M. HOCHSTRASSER and A. P. mRCHETT1
axis system) at 3.28 eV above the lowest triplet (experimental energy of 3B3g is about 2.8 eV). The improved resolution of the singlet-triplet absor tion of benzophenone in going from ‘77OK to 4.2 GC-indicates that this absorption obscures the second band of the triglet-triplet transition at 770K. Since the host Tl - So band absorbs about three times stronger along b than a, the 77’K spectra cannot be easily used to determine the polarization of the 0, 1 triplet-triplet absorption band The presence of the broad Tl -So band of the host most probably provided the anomalous intensity ratio reported previously [l] for the two T + T bands (i.e. 0, 0 and 0, lj of the spectra on the (110) face. The measured intensity ratio in this work (I4425 ji/14150 A z 2.3) in the a polarization is within experimental error of the reported isotropic value af 2.9 [6]. It should be noted that while there was a distinct sharpening of the Tl - 5, absorption of benzophenone on
going from 77OK to 4.2OK, there was no corresponding sharpening of the anthracene triplettriplet absorption; and the bands remained about 500 cm-l wide at half-height.
References [1] R. M. Hochstrasser and S. K. Lower, J. Chem. Phys. 40 (1964) 1041. [Z] E. Fleischer, lJniversity of Chicago, private communication. r31 R. H. Clarke and R. M. Hochstrasser. J. Chem. Phvs. - - 46 (1967) 4532. [4] Craig; and Fisher, Proc. Chem. Sot. (1964) 176. [5] R.Pariser. J. Chem.Phys. 23 (1956) 324. [6] D. P. Craig and 1. G. Ross, J. Chem. Sot. (1954) 1589; M. W. Windsor and J. R. Novak, The Triplet State. ed. A. B. Zahlan (Cambridge University Press, 1967) p. 229.