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Optical excitation of a three-dimensional triplet energy funnel
Journal of Luminescence 42 (1988) 217 219 North-Holland, Amsterdam
217
OPTICAL EXCITATION OF A THREE-DIMENSIONAL TRIPLET ENERGY FUNNEL J. KOLENDA and C. VON BORCZYSKOWSKI Fachhere,ch Physik, Freie Unwer5ität, Arnimallee 14, D-1000 Berlin 33, Germany
Received 13 June 1988 Accepted 12 July 1988
Tnplet excitation spectroscopy at 4.2 K of X-traps induced in para-dibromobenzene (DBB) by para-xylene guest molecules reveals that several DBB host states split off from the triplet exciton band. These states form an energy funnel with a depth of 122 cm for DBB triplet energy transport. The relative trap energies within the funnel suggest a three-dimensional structure.
1. Introduction Transport of excitation energy has been one of the outstanding topics of the investigation of organic solid state material during the past decade [1,2]. In many cases the method of sensitized luminescence [3] has been used to determine transport properties of the (host) material by doping appropriate (guest) molecules which serve as energy acceptors. However, as has been recently discussed [4] transport rates determined by this method very often reflect the capture rate of the energy acceptor instead of the diffusion rate of the host material itself, a difficulty inherent to all experiments using probe techniques. The possibility has been discussed that these molecules even induce energy funnels consisting of perturbed host molecules energetically lowered below the exciton band of the host material [5]. The spatial width, dimension and energetic shape of the funnels will clearly influence the detailed properties of the energy capture process. Properties of energy funnels are, however, also of interest for themselves because they might serve as concrete model systems for antenna pigments in photosynthetic material which funnel optical energy down to the photoreactive center [6]. The formation of energy funnels in molecular crystals has recently been reported in the case of triplet [7—9]and singlet [10] energy transport but a detailed analysis is still missing. In this communication we will report on the system p-xylene (PX) in the single crystal
p-dibromobenzene (DBB) which induces DBB triplet X-traps 122 cm below the DBB exciton band [11]. We have performed phosphorescence excitation spectroscopy of these DBB X-traps which revealed a pronounced series of absorption lines energetically between the emitting trap and the exciton band at 27910 cm ‘ [12].
2. Experimental Experiments at 4.2 K have been performed on single crystals of DBB doped with 1000 ppm PX. Direct singlet triplet excitation has been provided via a pulsed nitrogen (Molectron UV22) pumped dye laser (Lambda Physics FL2000). Using a butyl-PBD dye the excitation linewidth was about 0.7 cm Total phosphorescence emission has been detected with a photodiode and an appropriate bandpass filter. The output of the photodiode has been fed into a boxcar at 1 ms delay of the 0.1 ms wide gate. To assure that all absorption lines result in DBB X-trap phosphorescence we have cross-checked the excitation spectra by detecting the phosphorescence with 5 cm resolution on the origin of the X-trap emission. ~.
3. Results and discussion The phosphorescence excitation spectrum of the DBB X-trap is shown in fig. 1. The energeti-
0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
218
J. Kolenda, C. von Borczyskowski
cally lowest absorption line corresponds with the origin of the phosphorescence spectrum at 27 788 cm The emission itself can be clearly identified as DBB X-trap emission [13]. The other lines are only observed in absorption. We assign two of the lines as indicated in the figure to local phonon modes belonging to the lowest absorption origin. This assignment is supported by the observation that the same frequency separation is observed in DBB X-trap excitation spectra when doping pchlorotoluene or toluene, although in these cases all other line splittings are cnmpletely different. Comparison with reported singlet triplet absorption spectra of DBB [13] reveals no coincidence with vibrational lines of DBB. None of the absorption lines besides the excitonic origin [12] are observed in pure DBB crystals prepared from the same material. Decreasing the temperature to 1.9 K changes relative line intensities only insignificantly. The strong increase of absorption at high excitation energies is due to the onset of excitonic absorption which also results in an effective population of DBB X-traps. For all these reasons we assign the absorption lines listed in table I to DBB molecules lowered below the exciton band due to the presence of the guest molecule PX, which itself has its triplet absorption origin about 500 cm above the one of DBB [14]. Assuming that PX is incorporated substitutionally into the crystal lattice, one can calculate the .
I
3
JL ~ib
3
35
~ 5
5
~e rgt~ rv Fig. 1. Excitation spectrum at 4.2 K of p-dibromobenzene (DBB) X-trap induced by p-xylene (PX) monitonng total phos phorescence. Local phonon modes of the lowest absorption origin are indicated. Line numbers correspond with table 1.
Three-dimensional triple! energy funnel
Table 1 Absorption hnes below the DBR excitnn hand and calculated intermolecular distances R ________________________________________________ 3 Line ~E ~\E/cm Site 1 R (norm.) (norm.) c 1 1 1 122.0 b 0.36 2 0.35 42.3 b+ c 0.20 3 0.21 25.0 l/2(a + b) 0.13 4 0.14 17.0 I 2(a + b)+ c 0.06 5 0.08 10.0 2c 0.13 6 0.06 7.1 2b 0.045 (0.014) ~ (1.7) a) ~° Could not be observed in all crystals.
distance R from PX to the surrounding DBB host molecules. According to the crystallographic axes a, b and c [15] we have calculated 1/R3 for the various guest host combinations. The shortest distance is along the c-axis for which we have normalized 1/R3 to 1. A comparison of these values tabulated in table 1 shows almost a one-to-one correspondence with the relative energies of distorted DBB molecules. This suggests that the substituted guest molecule induces a dipole dipole type interaction potential although the origin of this potential is not completely clear at the moment, because no permanent dipole moments should be present in this system. However, a 1/R3 potential has recently also been reported for a very similar experiment on triplet energy funnels induced in naphthalene single crystals [16]. In DBB molecules along the b- and c-axes are translationally equivalent whereas those including a translation along the a-axis are unequivalent. Similar to the theory of dimers in isotopically mixed crystals individual pairs of guest host molecules in the DBB/PX crystal can be divided into these two classes for which one would expect a distinguishable behavior with respect to the polarization of the correspondent absorption [17]. We indeed observed polarization absorption lines a1 different 3 as compared to 4 for andthe5 which is in agreement with the assignment of lines to specific host molecules based solely on the 1/R3 dependence. We therefore conclude that the .
.
.
induced energy funnel is three-dimensional. Additionally observed lines on the onset of the cxcitonic absorption presumably belong to next.
.
J. Kolenda, C. von Borczyskowski
nearest-neighbored molecules along the c- and b-axes. Assuming a symmetrical funnel it will be extended along the c- and b-axes by four lattice constants having the form of a Mexican hat in the b, c plane with PX on the top. A similar behavior has been observed when doping DBB with aniline, benzene, toluene and p-chlorotoluene as will be reported in detail elsewhere. At present, we also investigate the importance of these energy funnels for long-range transport processes, for which mdications have been observed for p-dichlorobenzene in DBB [18,10].
Acknowledgements
/
Three-dimensional triplet energy funnel
219
[4] P.E. Parris and V.M. Kenre, Chem. Phys. Lett. 125 (1986) 18g. [5] R.E. Merrifield, J. Chem. Phys. 38 (1963) 920; D.P. Craig and M.R. Philpott, Proc. Roy. Soc. A290 (1966) 583; A293 (1966) 213; H. Benk, H. Haken, and H. Sixl, J. Chem. Phys. 77 (1982) 5730. [6] A.N. Glazer, Ann. Rev. Biochem. 52 (1983) 125. [7] C. von Borczyskowski, J. Grimm, and T. Kirski, J. de Phys. C7 (1985) 73. [8] J. Grimm, T. Kirski, and C. von Borczyskowski, Chem. Phys. Lett. 128 (1986) 569. [9] J. Gnmm, T. Kirski and C. von Borczyskowski, Chem. Phys. Lett. 131 (1986) 522. [10] P. Argyrakis, D. Hooper and R. Kopelman. J. Phys. Chem. 87 (1983) 1647. [11] H. Shinohara and N. Hirota, J. Chem. Phys. 72 (1980) 4445. [12] R.M. Hochstrasser and P.N. Prasad, J. Chem. Phys. 56
This work has financially been supported by the Deutsche Forschungsgemeinschaft (Sfb 337). Single crystals have been prepared by R. Brunn.
(1972) 2814. [13] G. Castro and R.M. Hochstrasser, J. Chem. Phys. 46 (1967) 3617. [14] Ph.J. Vergragt, J.A. Kooter and J.H. van der Waals, Mol. Phys. 33(1977)1523. [15] 5. Bezzi and V. Croatto, Gazz. Chim. Ital. 72 (1942) 318.
References
[16] P. Fischer and H. Port, Proc. Spring Meeting, Germ. Phys. Soc., Bonn, Vol. 7 (1988) p. 42; B. Scherm-KoIb. Diploma Thesis, Bayreuth (1982). [17] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals (Clarendon, Oxford, 1982). [18] T. Kirski, J. Grimm and C. von Borczyskowski, J. Chem. Phys. 87 (1987) 2062. [19] C. von Borczyskowski and T. Kirski, Phys. Rev. Lett 60 (1988) 1578.
[1] V.M. Agranovich and M.D. Galanin, eds., Electronic Excitation and Energy Transfer in Condensed Matter (North-Holland, Amsterdam, 1982). [2] V.M. Agranovich and R.M. Hochstrasser, eds., Spectroscopy and Excitation Dynamics of Condensed Molecular Systems (North-Holland, Amsterdam, 1983). [3] H.C. Wolf. Advan. At. Mol. Phys. 3 (1967) 119.
217
OPTICAL EXCITATION OF A THREE-DIMENSIONAL TRIPLET ENERGY FUNNEL J. KOLENDA and C. VON BORCZYSKOWSKI Fachhere,ch Physik, Freie Unwer5ität, Arnimallee 14, D-1000 Berlin 33, Germany
Received 13 June 1988 Accepted 12 July 1988
Tnplet excitation spectroscopy at 4.2 K of X-traps induced in para-dibromobenzene (DBB) by para-xylene guest molecules reveals that several DBB host states split off from the triplet exciton band. These states form an energy funnel with a depth of 122 cm for DBB triplet energy transport. The relative trap energies within the funnel suggest a three-dimensional structure.
1. Introduction Transport of excitation energy has been one of the outstanding topics of the investigation of organic solid state material during the past decade [1,2]. In many cases the method of sensitized luminescence [3] has been used to determine transport properties of the (host) material by doping appropriate (guest) molecules which serve as energy acceptors. However, as has been recently discussed [4] transport rates determined by this method very often reflect the capture rate of the energy acceptor instead of the diffusion rate of the host material itself, a difficulty inherent to all experiments using probe techniques. The possibility has been discussed that these molecules even induce energy funnels consisting of perturbed host molecules energetically lowered below the exciton band of the host material [5]. The spatial width, dimension and energetic shape of the funnels will clearly influence the detailed properties of the energy capture process. Properties of energy funnels are, however, also of interest for themselves because they might serve as concrete model systems for antenna pigments in photosynthetic material which funnel optical energy down to the photoreactive center [6]. The formation of energy funnels in molecular crystals has recently been reported in the case of triplet [7—9]and singlet [10] energy transport but a detailed analysis is still missing. In this communication we will report on the system p-xylene (PX) in the single crystal
p-dibromobenzene (DBB) which induces DBB triplet X-traps 122 cm below the DBB exciton band [11]. We have performed phosphorescence excitation spectroscopy of these DBB X-traps which revealed a pronounced series of absorption lines energetically between the emitting trap and the exciton band at 27910 cm ‘ [12].
2. Experimental Experiments at 4.2 K have been performed on single crystals of DBB doped with 1000 ppm PX. Direct singlet triplet excitation has been provided via a pulsed nitrogen (Molectron UV22) pumped dye laser (Lambda Physics FL2000). Using a butyl-PBD dye the excitation linewidth was about 0.7 cm Total phosphorescence emission has been detected with a photodiode and an appropriate bandpass filter. The output of the photodiode has been fed into a boxcar at 1 ms delay of the 0.1 ms wide gate. To assure that all absorption lines result in DBB X-trap phosphorescence we have cross-checked the excitation spectra by detecting the phosphorescence with 5 cm resolution on the origin of the X-trap emission. ~.
3. Results and discussion The phosphorescence excitation spectrum of the DBB X-trap is shown in fig. 1. The energeti-
0022-2313/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
218
J. Kolenda, C. von Borczyskowski
cally lowest absorption line corresponds with the origin of the phosphorescence spectrum at 27 788 cm The emission itself can be clearly identified as DBB X-trap emission [13]. The other lines are only observed in absorption. We assign two of the lines as indicated in the figure to local phonon modes belonging to the lowest absorption origin. This assignment is supported by the observation that the same frequency separation is observed in DBB X-trap excitation spectra when doping pchlorotoluene or toluene, although in these cases all other line splittings are cnmpletely different. Comparison with reported singlet triplet absorption spectra of DBB [13] reveals no coincidence with vibrational lines of DBB. None of the absorption lines besides the excitonic origin [12] are observed in pure DBB crystals prepared from the same material. Decreasing the temperature to 1.9 K changes relative line intensities only insignificantly. The strong increase of absorption at high excitation energies is due to the onset of excitonic absorption which also results in an effective population of DBB X-traps. For all these reasons we assign the absorption lines listed in table I to DBB molecules lowered below the exciton band due to the presence of the guest molecule PX, which itself has its triplet absorption origin about 500 cm above the one of DBB [14]. Assuming that PX is incorporated substitutionally into the crystal lattice, one can calculate the .
I
3
JL ~ib
3
35
~ 5
5
~e rgt~ rv Fig. 1. Excitation spectrum at 4.2 K of p-dibromobenzene (DBB) X-trap induced by p-xylene (PX) monitonng total phos phorescence. Local phonon modes of the lowest absorption origin are indicated. Line numbers correspond with table 1.
Three-dimensional triple! energy funnel
Table 1 Absorption hnes below the DBR excitnn hand and calculated intermolecular distances R ________________________________________________ 3 Line ~E ~\E/cm Site 1 R (norm.) (norm.) c 1 1 1 122.0 b 0.36 2 0.35 42.3 b+ c 0.20 3 0.21 25.0 l/2(a + b) 0.13 4 0.14 17.0 I 2(a + b)+ c 0.06 5 0.08 10.0 2c 0.13 6 0.06 7.1 2b 0.045 (0.014) ~ (1.7) a) ~° Could not be observed in all crystals.
distance R from PX to the surrounding DBB host molecules. According to the crystallographic axes a, b and c [15] we have calculated 1/R3 for the various guest host combinations. The shortest distance is along the c-axis for which we have normalized 1/R3 to 1. A comparison of these values tabulated in table 1 shows almost a one-to-one correspondence with the relative energies of distorted DBB molecules. This suggests that the substituted guest molecule induces a dipole dipole type interaction potential although the origin of this potential is not completely clear at the moment, because no permanent dipole moments should be present in this system. However, a 1/R3 potential has recently also been reported for a very similar experiment on triplet energy funnels induced in naphthalene single crystals [16]. In DBB molecules along the b- and c-axes are translationally equivalent whereas those including a translation along the a-axis are unequivalent. Similar to the theory of dimers in isotopically mixed crystals individual pairs of guest host molecules in the DBB/PX crystal can be divided into these two classes for which one would expect a distinguishable behavior with respect to the polarization of the correspondent absorption [17]. We indeed observed polarization absorption lines a1 different 3 as compared to 4 for andthe5 which is in agreement with the assignment of lines to specific host molecules based solely on the 1/R3 dependence. We therefore conclude that the .
.
.
induced energy funnel is three-dimensional. Additionally observed lines on the onset of the cxcitonic absorption presumably belong to next.
.
J. Kolenda, C. von Borczyskowski
nearest-neighbored molecules along the c- and b-axes. Assuming a symmetrical funnel it will be extended along the c- and b-axes by four lattice constants having the form of a Mexican hat in the b, c plane with PX on the top. A similar behavior has been observed when doping DBB with aniline, benzene, toluene and p-chlorotoluene as will be reported in detail elsewhere. At present, we also investigate the importance of these energy funnels for long-range transport processes, for which mdications have been observed for p-dichlorobenzene in DBB [18,10].
Acknowledgements
/
Three-dimensional triplet energy funnel
219
[4] P.E. Parris and V.M. Kenre, Chem. Phys. Lett. 125 (1986) 18g. [5] R.E. Merrifield, J. Chem. Phys. 38 (1963) 920; D.P. Craig and M.R. Philpott, Proc. Roy. Soc. A290 (1966) 583; A293 (1966) 213; H. Benk, H. Haken, and H. Sixl, J. Chem. Phys. 77 (1982) 5730. [6] A.N. Glazer, Ann. Rev. Biochem. 52 (1983) 125. [7] C. von Borczyskowski, J. Grimm, and T. Kirski, J. de Phys. C7 (1985) 73. [8] J. Grimm, T. Kirski, and C. von Borczyskowski, Chem. Phys. Lett. 128 (1986) 569. [9] J. Gnmm, T. Kirski and C. von Borczyskowski, Chem. Phys. Lett. 131 (1986) 522. [10] P. Argyrakis, D. Hooper and R. Kopelman. J. Phys. Chem. 87 (1983) 1647. [11] H. Shinohara and N. Hirota, J. Chem. Phys. 72 (1980) 4445. [12] R.M. Hochstrasser and P.N. Prasad, J. Chem. Phys. 56
This work has financially been supported by the Deutsche Forschungsgemeinschaft (Sfb 337). Single crystals have been prepared by R. Brunn.
(1972) 2814. [13] G. Castro and R.M. Hochstrasser, J. Chem. Phys. 46 (1967) 3617. [14] Ph.J. Vergragt, J.A. Kooter and J.H. van der Waals, Mol. Phys. 33(1977)1523. [15] 5. Bezzi and V. Croatto, Gazz. Chim. Ital. 72 (1942) 318.
References
[16] P. Fischer and H. Port, Proc. Spring Meeting, Germ. Phys. Soc., Bonn, Vol. 7 (1988) p. 42; B. Scherm-KoIb. Diploma Thesis, Bayreuth (1982). [17] M. Pope and C.E. Swenberg, Electronic Processes in Organic Crystals (Clarendon, Oxford, 1982). [18] T. Kirski, J. Grimm and C. von Borczyskowski, J. Chem. Phys. 87 (1987) 2062. [19] C. von Borczyskowski and T. Kirski, Phys. Rev. Lett 60 (1988) 1578.
[1] V.M. Agranovich and M.D. Galanin, eds., Electronic Excitation and Energy Transfer in Condensed Matter (North-Holland, Amsterdam, 1982). [2] V.M. Agranovich and R.M. Hochstrasser, eds., Spectroscopy and Excitation Dynamics of Condensed Molecular Systems (North-Holland, Amsterdam, 1983). [3] H.C. Wolf. Advan. At. Mol. Phys. 3 (1967) 119.