- Email: [email protected]
Luminescent quantum yields and vibrational spectroscopy
Synthetic Metals 102 (1999) 1529-1530
Luminescent Quantum Yields and Vibrational Spectroscopy J. Dhote’; K.P. Kretschhz;A.P. Dave?; W. Blat? and H.J. Byrne’* ‘School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland ‘Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland
Abstract The luminescent quantum yield of organic materials is strongly determined by their local environment. This is illustrated for the case of Anthracene and Terphenyl in a range of solvents. Although weak solvato-chromatic shifts of the absorption and luminescence spectra are observable, they do not account for the dramatic variation in luminescent quantum yields. The results suggest that the quantum yields are dictated by the nonradiative processes,particularly the correlation of the vibrational structure of the chromophore with that of its local environment. This is illustrated using a Raman spectroscopy,and the implications on the prospect of controlling the quantum yields of organic materials in the solid state are discussed. Keywords: Optical absorption and emission spectroscopy, conjugated molecules, Light sources.
Infrared
and Raman spectroscopy,
Organic
semiconductors
based on
1. Introduction
2. Experimental
Organic dyes are the most spectrally versatile materials for laser applications, extending Tom the ultraviolet to the near infrared [l]. Their applicability to solid state devices is, however, limited by their poor film quality. The observation of strong electroluminescence [2] and stimulated emission [3] from organic polymers has thus revitalised interest in organics for light emitting deviceswith the prospect of a range of chemically tailored materials, which can be electrically pumped, to yield light emission over a range a broad as that of dyes. Photoluminescent yields as high as 80% can now be obtained in conjugated polymers. However, much of this efficiency is lost in the solid state [4]. Aggregation can o&en account for the differences but the associated changes in the electronic states of the polymer are not always observed, advocating a more complete investigation of factors which control the luminescent yield of molecular materials. Previous studies of dyes in solution have illustrated that solvatochromic effects resulting from dipole interactions, rotary effects and conformational changes can all effect the quantum yield [5]. In this study two simple dye species,anthracene and terphenyl, are chosen to illustrate that a major influence on the radiative quantum yield is the inhibition of nonradiative processes in local environments whose vibrational structure does not correlate with that of the dye moiety. Relative yields are measured in a range of organic solvents and it is shown that the variations with solvent are not solvatochromic in origin, but rather are determined by the overlap between the vibrational spectra of the chromophore and the solvent, as measured by Raman Spectroscopy. Implications in terms of the design of light emitting organic materials are discussed.
Solvents employed for the study were cyclohexane, hexane, toluene, benzene, methanol, ethanol and dioxane. The range of solutions of both dyes were made up to constant concentration, 6.25mgIl in the case of anthracene and 1.6mg/l in the case of terphenyl. In both casesaccuraciesin concentrations are -5%. Uv/visible spectra were measured using a Shimadzu UV2 1OlPC absorption spectrometer. Luminescence was measured using a Perkin Ehner LSSOBfluorimeter. In all casesexcitation wavelengths were at the first absorption maximum and all other parameterswere kept equal. Raman spectra were recorded using an Instruments S.A. Labram IB spectroscopicmicroscopeoperating at 632.8mn.
*Currespnnding Au&r, Ph. +353 14024929 Fax. +353 1 4024988 e-mail: [email protected]
3. Results and Discussion In both the anthracene and terphenyl solutions, a weak solvatochromism in the absorption and emission spectra is observed. The positioning of the spectral maxima, plotted against the Onsager polarisation fimction [6] of the solvent, is well behaved, and for anthracene is well documented [7]. While the oscillator strength is, within the accuracy of the measurement, constant over the range of solvents, there are dramatic variations in the luminescent output of both chromophores. These variations do not correlate with solvatochromaticparameters. Nonradiative decay through internal conversion is one of the principle competing processesto radiative decay in molecules and in order to undergo this process, the local environment of the molecule much be capable of absorbing the vibrational quanta. Figure 1 compares,for example, the Raman spectrum of terphenyl with those of benzene and dioxane. For comparison, there is a strong overlap between the double bond (-16OOcm-‘) vibrations of the terphenyl and those of the benzene, whereas there is no such feature in the dioxane spectrum.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)005 14- 1
1530
H.J. Byrne
et al. I Synthetic
Metals
Figure 1: Comparison of the Raman Spectra of dioxane (top), terphenyl (middle) and benzene (bottom).
80000 /; 3 .a 60000 ?
-
.$43000
-
102 (1999)
1529-1530
essentially a co-polymer, has a high efficiency in solution supports the proposal that it is essential to inhibit vibrational coupling along the backbone. A move towards more complex backbone structuresmay further enhance this efficiency [8], the remaining challenge being to control the interchain vibrational coupling in solids.
/I ------__--
2
A
J
A
1 2oooo O-
,
200
400
600
800
1000
1200
1400
1600
1800
2000
Wa"en"mber/om-~
Figure 2: Plot of the integrated luminescence versus the summed overlap of Raman Spectra for Terphenyl in a range of solvents.
*5
Sumof vibrationaloverlaps(arb.units) Figure 3: Plot of the integrated luminescence versus the summed overlap of Raman Spectrafor Anthracene in a range of solvents.
4.
5e+4
1e*5
2e+5
2~5
Sum of vibrational
3e*5
overlaps
3s+5
4.?+5
h. 4e+5
Conclusions
This study highlights the importance of vibrational coupling to the local environment in determining the luminescent efficiency of a material. It furthermore illustrates how Raman spectroscopycan be employed to make ab initio predictions of the relative yields of a material. It is intended that this will be carried out using the same dye moieties in nonconjugated polymer matrices, and the conclusionswill be employed in the design of novel luminescent materials.
(a&units)
It is proposed that the overlap of the spectra is a gauge of the effX%ncy of the internal conversion and therefore the luminescent efficiency. Spectra for the solvents were taken keeping conditions constant such that a semiquantitative comparison could be made. For all solvents the overlap across the entire spectrum (200 - 3500cm.‘) was calculated. In figure 2 is plotted the integrated luminescenceoutput versusthe overlap of the vibrational spectra for terphenyl. There is a clear correlation between the two parameters, illustrating that the highest luminescent output occurs in solvents which have the weakest vibrational coupling to the dye moiety. A similar correlation is observed for anthracene in the range of solvents, as shown in figure 3. The correlation emphasisesthat eff%%nt light emission in molecules is achievable only by inhibiting vibrational coupling to the local environment. In the fabrication of films in solid matrices, materials which have little or no vibrational correlation with the luminescent chromophore should be employed. The observationsalso give pointers for the design of conjugated polymers for luminescence applications. In such materials coherent vibrational can occur along the backbone of the polymer. That poly para phenylene vinylene,
5. References [l] Kodak Optical Products,Catalogue, Rochester,New York [2] J.H. Burroughes, D.D.C. Bradley, A.R. Browne, R.N. Marks, K.Mackey, R.H. Friend, P.L. Burn, A.B. Holmes, Nature, 347,539 (1990) [3] G. Kranzelbinder, H.J. Byrne, S. Hallstein, S. Roth, G. Leising and U. Scherf, Phys. Rev. m, 1632 (1997) [4] C. Zenz, W. Graupner, S. Tasch, G. Leising, K. Muellen, U. Scherf, Appl. Phys.Lett., 71,2566 (1997) [5] G. Jones II, W.R. Jackson, C-Y. Choi, W.R. Bergmark, J. Phys. Chem., 89,294 (1985) [6] “Solvents and Solvent effects in Organic Chemistry”, C. Reichardt, VCH Verlagsgesellschafi,Weinheim (1988) [7] “Chemistry and Light”, P. Suppan, Royal Society of Chemistry (1994). [8] S. Maier, A. Drury, A.P. Davey, H. J. Byrne and W. Blau, these proceedings.
Luminescent Quantum Yields and Vibrational Spectroscopy J. Dhote’; K.P. Kretschhz;A.P. Dave?; W. Blat? and H.J. Byrne’* ‘School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland ‘Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland
Abstract The luminescent quantum yield of organic materials is strongly determined by their local environment. This is illustrated for the case of Anthracene and Terphenyl in a range of solvents. Although weak solvato-chromatic shifts of the absorption and luminescence spectra are observable, they do not account for the dramatic variation in luminescent quantum yields. The results suggest that the quantum yields are dictated by the nonradiative processes,particularly the correlation of the vibrational structure of the chromophore with that of its local environment. This is illustrated using a Raman spectroscopy,and the implications on the prospect of controlling the quantum yields of organic materials in the solid state are discussed. Keywords: Optical absorption and emission spectroscopy, conjugated molecules, Light sources.
Infrared
and Raman spectroscopy,
Organic
semiconductors
based on
1. Introduction
2. Experimental
Organic dyes are the most spectrally versatile materials for laser applications, extending Tom the ultraviolet to the near infrared [l]. Their applicability to solid state devices is, however, limited by their poor film quality. The observation of strong electroluminescence [2] and stimulated emission [3] from organic polymers has thus revitalised interest in organics for light emitting deviceswith the prospect of a range of chemically tailored materials, which can be electrically pumped, to yield light emission over a range a broad as that of dyes. Photoluminescent yields as high as 80% can now be obtained in conjugated polymers. However, much of this efficiency is lost in the solid state [4]. Aggregation can o&en account for the differences but the associated changes in the electronic states of the polymer are not always observed, advocating a more complete investigation of factors which control the luminescent yield of molecular materials. Previous studies of dyes in solution have illustrated that solvatochromic effects resulting from dipole interactions, rotary effects and conformational changes can all effect the quantum yield [5]. In this study two simple dye species,anthracene and terphenyl, are chosen to illustrate that a major influence on the radiative quantum yield is the inhibition of nonradiative processes in local environments whose vibrational structure does not correlate with that of the dye moiety. Relative yields are measured in a range of organic solvents and it is shown that the variations with solvent are not solvatochromic in origin, but rather are determined by the overlap between the vibrational spectra of the chromophore and the solvent, as measured by Raman Spectroscopy. Implications in terms of the design of light emitting organic materials are discussed.
Solvents employed for the study were cyclohexane, hexane, toluene, benzene, methanol, ethanol and dioxane. The range of solutions of both dyes were made up to constant concentration, 6.25mgIl in the case of anthracene and 1.6mg/l in the case of terphenyl. In both casesaccuraciesin concentrations are -5%. Uv/visible spectra were measured using a Shimadzu UV2 1OlPC absorption spectrometer. Luminescence was measured using a Perkin Ehner LSSOBfluorimeter. In all casesexcitation wavelengths were at the first absorption maximum and all other parameterswere kept equal. Raman spectra were recorded using an Instruments S.A. Labram IB spectroscopicmicroscopeoperating at 632.8mn.
*Currespnnding Au&r, Ph. +353 14024929 Fax. +353 1 4024988 e-mail: [email protected]
3. Results and Discussion In both the anthracene and terphenyl solutions, a weak solvatochromism in the absorption and emission spectra is observed. The positioning of the spectral maxima, plotted against the Onsager polarisation fimction [6] of the solvent, is well behaved, and for anthracene is well documented [7]. While the oscillator strength is, within the accuracy of the measurement, constant over the range of solvents, there are dramatic variations in the luminescent output of both chromophores. These variations do not correlate with solvatochromaticparameters. Nonradiative decay through internal conversion is one of the principle competing processesto radiative decay in molecules and in order to undergo this process, the local environment of the molecule much be capable of absorbing the vibrational quanta. Figure 1 compares,for example, the Raman spectrum of terphenyl with those of benzene and dioxane. For comparison, there is a strong overlap between the double bond (-16OOcm-‘) vibrations of the terphenyl and those of the benzene, whereas there is no such feature in the dioxane spectrum.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)005 14- 1
1530
H.J. Byrne
et al. I Synthetic
Metals
Figure 1: Comparison of the Raman Spectra of dioxane (top), terphenyl (middle) and benzene (bottom).
80000 /; 3 .a 60000 ?
-
.$43000
-
102 (1999)
1529-1530
essentially a co-polymer, has a high efficiency in solution supports the proposal that it is essential to inhibit vibrational coupling along the backbone. A move towards more complex backbone structuresmay further enhance this efficiency [8], the remaining challenge being to control the interchain vibrational coupling in solids.
/I ------__--
2
A
J
A
1 2oooo O-
,
200
400
600
800
1000
1200
1400
1600
1800
2000
Wa"en"mber/om-~
Figure 2: Plot of the integrated luminescence versus the summed overlap of Raman Spectra for Terphenyl in a range of solvents.
*5
Sumof vibrationaloverlaps(arb.units) Figure 3: Plot of the integrated luminescence versus the summed overlap of Raman Spectrafor Anthracene in a range of solvents.
4.
5e+4
1e*5
2e+5
2~5
Sum of vibrational
3e*5
overlaps
3s+5
4.?+5
h. 4e+5
Conclusions
This study highlights the importance of vibrational coupling to the local environment in determining the luminescent efficiency of a material. It furthermore illustrates how Raman spectroscopycan be employed to make ab initio predictions of the relative yields of a material. It is intended that this will be carried out using the same dye moieties in nonconjugated polymer matrices, and the conclusionswill be employed in the design of novel luminescent materials.
(a&units)
It is proposed that the overlap of the spectra is a gauge of the effX%ncy of the internal conversion and therefore the luminescent efficiency. Spectra for the solvents were taken keeping conditions constant such that a semiquantitative comparison could be made. For all solvents the overlap across the entire spectrum (200 - 3500cm.‘) was calculated. In figure 2 is plotted the integrated luminescenceoutput versusthe overlap of the vibrational spectra for terphenyl. There is a clear correlation between the two parameters, illustrating that the highest luminescent output occurs in solvents which have the weakest vibrational coupling to the dye moiety. A similar correlation is observed for anthracene in the range of solvents, as shown in figure 3. The correlation emphasisesthat eff%%nt light emission in molecules is achievable only by inhibiting vibrational coupling to the local environment. In the fabrication of films in solid matrices, materials which have little or no vibrational correlation with the luminescent chromophore should be employed. The observationsalso give pointers for the design of conjugated polymers for luminescence applications. In such materials coherent vibrational can occur along the backbone of the polymer. That poly para phenylene vinylene,
5. References [l] Kodak Optical Products,Catalogue, Rochester,New York [2] J.H. Burroughes, D.D.C. Bradley, A.R. Browne, R.N. Marks, K.Mackey, R.H. Friend, P.L. Burn, A.B. Holmes, Nature, 347,539 (1990) [3] G. Kranzelbinder, H.J. Byrne, S. Hallstein, S. Roth, G. Leising and U. Scherf, Phys. Rev. m, 1632 (1997) [4] C. Zenz, W. Graupner, S. Tasch, G. Leising, K. Muellen, U. Scherf, Appl. Phys.Lett., 71,2566 (1997) [5] G. Jones II, W.R. Jackson, C-Y. Choi, W.R. Bergmark, J. Phys. Chem., 89,294 (1985) [6] “Solvents and Solvent effects in Organic Chemistry”, C. Reichardt, VCH Verlagsgesellschafi,Weinheim (1988) [7] “Chemistry and Light”, P. Suppan, Royal Society of Chemistry (1994). [8] S. Maier, A. Drury, A.P. Davey, H. J. Byrne and W. Blau, these proceedings.