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External spin—orbit coupling. The T1 state of naphthalene in a dibromobenzene host cyrstal
Volume 15, number 4
CHEMICAL PHYSICS LETTERS
EXTERNAL
SPIN-ORBIT
THE T 1 STATE OF NAPHTHALENE
1 September 1972
COUPLING.
IN A DIBROMOBENZENE
HOST CRYSTAL
Steven D. COLSON and Bruce W. GASH Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA
Received 5 June 1972
The origin of the Tl state of naphthalene in a p-dibromobenzene host crystal is shown to be the strongest line in the spectrum at 20720.9 cm -1 . The vibronic activity of the naphthalene phosphorescence indicates that a spin-orbit coupling mechanism different from the one vibronically induced in an isotopic mixed naphthalene crystal is active in a chemically mixed crystal with a halogen-containing host.
1. Introduction
2. Experimental
Giachino and Kearns [I] (GK) doped crystals of halogen-containing aromatic compounds with naphthalene and studied the naphthalene phosphorescence emission at 77°K under low resolution. Because of their limited resolution, they could only compare general spectral features, which restricted their ability to make careful comparisons of vibronic activity or even to assign the origin line unequivocally. Nevertheless, they concluded that there did not appear to be any major changes in relative intensities of the vibronic bands compared with the phosphorescence from naphthalene dissolved in a rigid glass. The origin of the naphthalene phosphorescence in p-dibromobenzene as reported by GK is at 4830 A. This value is shifted by several hundred wavenumbers from the pure naphthalene crystal T 1 origin. This shift is comparable to a vibrational frequency. The phosphorescence reported by GK could, therefore, arise from a vibronic rather than an electronic origin. Reported in this note are the results of a high resolution study of the naphthalene phosphorescence and T 1 +-S O absorption from less than 1% naphthalene in a p-dibromobenzene host crystal at 4.6°K. These results indicate that the origin given by Giachino is an electronic origin. The naphthalene vibronic activity is different from that given by Hanson [2] for naphthalene in a deuteronaphthalene crystal.
Aldrich Lot No. 081991 zone-refined p-dibromobenzene and Aldrich Lot. No. 032307 zone-refined naphthalene were found to be free of impurities in the spectral regions of interest and were used without further purification. Weighed amounts ofp-dibromobenzene and naphthalene were placed in quartz cells, and the cells were evacuated. The crystals were grown in a Bridgeman furnace modified to cool the crystal to liquid nitrogen temperature as it was grown. A Jarrell-Ash scanning spectrophotometer with a 1200 lines/ram grating blazed at 7500 A was used for all the spectroscopic work. A Janis research dewar was used to cool the samples to the liquid helium temperature region.
3. Results and discussion The highest energy line of the naphthalene phos~ phorescence of a p-dibromobenzene host crystal containing less than 1% naphthalene was found to be moderately weak and occurred at 21 126.1 cm -1 (4732.15,8,). GK reported the highest energy line as a strong line at 4830 A. This line corresponds to the strongest line in the 4.6°K phosphorescence at 20720.9 cm -1 (4824.7,8,). By the use of excitation spectroscopy (i.e., the T 1 625
Volume 15, number 4
CHEMICAL PHYSICS LETTERS
state of naphthalene was excited, and the naphthalene phosphorescence was monitored as a function of exciting wavelength), the T 1 absorption of naphthalene in a p-dibromobenzene host crystal was recorded. The absorption spectrum established that the T 1 origin is at 20720.9 cm -1. The ag fundamentals are the strongest lines in the spectrum as they are in the pure naphthalene crystal T 1 absorption [3]. In contrast to the pure crystal absorption, the lines of the T 1 absorption spectrum from the naphthalene in a p-dibromobenzene host are very strongly associated with phonons. The phosphorescence emission line on the high energy side of the origin may arise from naphthalene molecules in some sort of defect site in the crystal. The remaining lines of the phosphorescence spectrum were analyzed. The vibronic activity was found to be different from that of naphthalene in a deuteronaphthalene crystal [2]. While the ag fundamentals remain the strongest lines in the spectrum, all blg fundamentals are missing in the chemically mixed crystal - including the moderately strong Vl0 (big) fundamental (392.6 cm-1). This indicates the spin-orbit coupling mechanism in a chemically mixed crystal with a halogen containing host is different from the one vibronically induced in an isotopic mixed naphthalene crystal. Hochstrasser and Prasad [4] reported one line of the phosphorescence spectrum at 20715 cm -1 without indicating its assignment. The difference between the energy of this reported line and the origin reported here could conceivably be due to sample preparation. However, the same value for the origin is obtained for all samples grown in this laboratory. The phonon side band built on the origin is similar to that reported by Hochstrasser, however, the structure is more clearly developed indicating a localized phonon mode [5] of about 25 cm - i . The phonon structure associated with the weak line at 21 126.1 cm -1 is
626
1 September 1972
different from that associated with the origin and the other fundamentals. The results presented here indicate that there are apparently no symmetry restrictions on external spin-orbit coupling. The singlet character mixed into the guest triplet could come from either the ringlet manifold of the host or the singlet manifold of the guest. However, the results of GK seem to rule out the latter.
4. Summary (i) By the use of excitation spectroscopy, the phosphorescence line at 20720.9 is established as the T 1 origin. (it) The vibronic activity of the phosphorescence of naphthalene in p-dibromobenzene is shown to be different from that of naphthalene in a deuteronaphthalene crystal. The blg fundamentals are not active in the chemically mixed crystal. (iii) A localized phonon mode of about 25 cm -1 is associated with the T 1 --*SO transition of naphthalene in p-dibromobenzene.
Acknowledgement Financial support from the National Science Foundation is gratefully acknowledged.
References [1] G.G. Giachino and D.R. Kearns, J. Chem. Phys. 52 (1970) 2964. [2] D.M. Hanson, J. Chem. Phys. 51 (1969) 5063. [3] G. Castro and G.W. Robinson, J. Chem. Phys. 50 (1968) 1150. [4] R.M. Hochstrasser and P.N. Prasad, J. Chem. Phys. 56 (1972) 2814. [5] P.H. Chereson, P.S. Friedman and R. Kopelman, J. Chem. Phys. 56 (1972) 3716.
CHEMICAL PHYSICS LETTERS
EXTERNAL
SPIN-ORBIT
THE T 1 STATE OF NAPHTHALENE
1 September 1972
COUPLING.
IN A DIBROMOBENZENE
HOST CRYSTAL
Steven D. COLSON and Bruce W. GASH Department of Chemistry, Yale University, New Haven, Connecticut 06520, USA
Received 5 June 1972
The origin of the Tl state of naphthalene in a p-dibromobenzene host crystal is shown to be the strongest line in the spectrum at 20720.9 cm -1 . The vibronic activity of the naphthalene phosphorescence indicates that a spin-orbit coupling mechanism different from the one vibronically induced in an isotopic mixed naphthalene crystal is active in a chemically mixed crystal with a halogen-containing host.
1. Introduction
2. Experimental
Giachino and Kearns [I] (GK) doped crystals of halogen-containing aromatic compounds with naphthalene and studied the naphthalene phosphorescence emission at 77°K under low resolution. Because of their limited resolution, they could only compare general spectral features, which restricted their ability to make careful comparisons of vibronic activity or even to assign the origin line unequivocally. Nevertheless, they concluded that there did not appear to be any major changes in relative intensities of the vibronic bands compared with the phosphorescence from naphthalene dissolved in a rigid glass. The origin of the naphthalene phosphorescence in p-dibromobenzene as reported by GK is at 4830 A. This value is shifted by several hundred wavenumbers from the pure naphthalene crystal T 1 origin. This shift is comparable to a vibrational frequency. The phosphorescence reported by GK could, therefore, arise from a vibronic rather than an electronic origin. Reported in this note are the results of a high resolution study of the naphthalene phosphorescence and T 1 +-S O absorption from less than 1% naphthalene in a p-dibromobenzene host crystal at 4.6°K. These results indicate that the origin given by Giachino is an electronic origin. The naphthalene vibronic activity is different from that given by Hanson [2] for naphthalene in a deuteronaphthalene crystal.
Aldrich Lot No. 081991 zone-refined p-dibromobenzene and Aldrich Lot. No. 032307 zone-refined naphthalene were found to be free of impurities in the spectral regions of interest and were used without further purification. Weighed amounts ofp-dibromobenzene and naphthalene were placed in quartz cells, and the cells were evacuated. The crystals were grown in a Bridgeman furnace modified to cool the crystal to liquid nitrogen temperature as it was grown. A Jarrell-Ash scanning spectrophotometer with a 1200 lines/ram grating blazed at 7500 A was used for all the spectroscopic work. A Janis research dewar was used to cool the samples to the liquid helium temperature region.
3. Results and discussion The highest energy line of the naphthalene phos~ phorescence of a p-dibromobenzene host crystal containing less than 1% naphthalene was found to be moderately weak and occurred at 21 126.1 cm -1 (4732.15,8,). GK reported the highest energy line as a strong line at 4830 A. This line corresponds to the strongest line in the 4.6°K phosphorescence at 20720.9 cm -1 (4824.7,8,). By the use of excitation spectroscopy (i.e., the T 1 625
Volume 15, number 4
CHEMICAL PHYSICS LETTERS
state of naphthalene was excited, and the naphthalene phosphorescence was monitored as a function of exciting wavelength), the T 1 absorption of naphthalene in a p-dibromobenzene host crystal was recorded. The absorption spectrum established that the T 1 origin is at 20720.9 cm -1. The ag fundamentals are the strongest lines in the spectrum as they are in the pure naphthalene crystal T 1 absorption [3]. In contrast to the pure crystal absorption, the lines of the T 1 absorption spectrum from the naphthalene in a p-dibromobenzene host are very strongly associated with phonons. The phosphorescence emission line on the high energy side of the origin may arise from naphthalene molecules in some sort of defect site in the crystal. The remaining lines of the phosphorescence spectrum were analyzed. The vibronic activity was found to be different from that of naphthalene in a deuteronaphthalene crystal [2]. While the ag fundamentals remain the strongest lines in the spectrum, all blg fundamentals are missing in the chemically mixed crystal - including the moderately strong Vl0 (big) fundamental (392.6 cm-1). This indicates the spin-orbit coupling mechanism in a chemically mixed crystal with a halogen containing host is different from the one vibronically induced in an isotopic mixed naphthalene crystal. Hochstrasser and Prasad [4] reported one line of the phosphorescence spectrum at 20715 cm -1 without indicating its assignment. The difference between the energy of this reported line and the origin reported here could conceivably be due to sample preparation. However, the same value for the origin is obtained for all samples grown in this laboratory. The phonon side band built on the origin is similar to that reported by Hochstrasser, however, the structure is more clearly developed indicating a localized phonon mode [5] of about 25 cm - i . The phonon structure associated with the weak line at 21 126.1 cm -1 is
626
1 September 1972
different from that associated with the origin and the other fundamentals. The results presented here indicate that there are apparently no symmetry restrictions on external spin-orbit coupling. The singlet character mixed into the guest triplet could come from either the ringlet manifold of the host or the singlet manifold of the guest. However, the results of GK seem to rule out the latter.
4. Summary (i) By the use of excitation spectroscopy, the phosphorescence line at 20720.9 is established as the T 1 origin. (it) The vibronic activity of the phosphorescence of naphthalene in p-dibromobenzene is shown to be different from that of naphthalene in a deuteronaphthalene crystal. The blg fundamentals are not active in the chemically mixed crystal. (iii) A localized phonon mode of about 25 cm -1 is associated with the T 1 --*SO transition of naphthalene in p-dibromobenzene.
Acknowledgement Financial support from the National Science Foundation is gratefully acknowledged.
References [1] G.G. Giachino and D.R. Kearns, J. Chem. Phys. 52 (1970) 2964. [2] D.M. Hanson, J. Chem. Phys. 51 (1969) 5063. [3] G. Castro and G.W. Robinson, J. Chem. Phys. 50 (1968) 1150. [4] R.M. Hochstrasser and P.N. Prasad, J. Chem. Phys. 56 (1972) 2814. [5] P.H. Chereson, P.S. Friedman and R. Kopelman, J. Chem. Phys. 56 (1972) 3716.