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Photoinduced absorption measurements of triplet states in poly(p-pyridine)
9!lwrlrINETilC IliiilETRLS ELSEVIER
SyntheticMetals 101(1999) 807-808
Photoinduced
absorption
measurements of triplet states in poly@pyridine)
L. J. Hartwellt, M. E. Vaschetto, L. E. Horsburgh and A. P. Monkman. Organic Electroactive Materials Research Group, Department of Physics, University of Durham, South Road, Durham DHI 3LE, UK ‘Also at: Bede Scientific Instruments Ltd., Bowburn South Industrial Estate, Bowburn. Durham, DH6 5AD, UK.
Abstract
Wereport cw photoinducedabsorption(PIA) measurements of poly@pyridine) (PPY). Threecorrelated,inducedfeaturesarefound belowthe n+ X* transition.All have a strongtemperaturedependence andsimilarlifetimesof ca 10 ms. Comparingto ZINDOIS-CI calculationsof the triplet manifold we find good agreementand so concludethat the PIA representstransitionswithin the triplet manifold.However,asyet we have not beenableto definitively measurea v-’ choprate dependence for the PIA signalsindicativeof monomolecular recombination. Keywords:
Photoinducedabsorption,poly(p-pyridine),triplet states,ZINDO.
1 Introduction
Following on from the first reports on the synthesisand characterisation of poly@-pyridine)by Yamamotoet al [I], and subsequent photophysicalstudiesof Blatchford et al [2] we have reported substantialincreasesin photoluminescence quantum yield (PLQY), achievedby modificationsto the synthesisroute for PPY [3,4] and have madefurther progressyielding polymer with a solid state PLQY of 37f3% as comparedto the initial findingsof ca. 5 % in the solid state.To probethe excitedstates of PPY we have started a seriesof photoinducedabsorption (PIA) measurements. We compareour findings to theoretical calculationsof the singletand triplet manifoldsto ascertainif poiaronsibipolarons play a key part in the photoexcitedstateof PPY or whetherexcitonic(triplet) speciesdominate. 2 Experimental
details
Poly@-pyridine-2,5-diyl)waspreparedasreportedpreviously [4]. Thin films of the polymerwere spin-coatedonto Spectrosil typically from a 20 mg.ml-’ solution in formic acid. PIA measurements were made using mechanicallychopped UV excitation( 360-370nm doubletfrom an AI’ ion laser).A lockin amplifiermeasures the magnitudeandphaseof the ac signalwith the frequency of the chopper (dT) and the dc signal (T) simultaneously.Both UV enhancedSi and InAs photodiode detectorsare used.TemperaturedependentPIA measurements weremadein an exchangegas,liquid nitrogencryostat. 3. Computational
methodology
The gasphasegeometriesof modelhexamerpyridine in the singlet and triplet states have been optimized using the semiempirical ParametrizationMethod 3 (PM3) [5]. This method is based on the Neglect Differential Overlap (NDO) aproximation.The nitrogenatomsof the chainweredistributedin randomway at positionstwo or three.The structuresweretotally optimized without geometryrestrictions.The final geometryis planar showingtorsion angle betweenrings smallerthan 2’,
which bestdescribes PPY in the solidstate[6]. Theseoptimized geometricalstructureswerethen usedto calculationof the UVvis spectraof the hexamers.This was achievedusing Zerner’s IntermediateNeglectDifferential Overlap (ZINDOIS-CI) method [7], using up to 260 singlet configurationsand 300 triplet configurations,respectively.ZINDOS-CI hasbeen specifically parametrized to describethe UV-Vis opticaltransitionof organic molecules. The theoretically obtained wavelengths were broadenedwith Gaussians of variablewidth (proportionalto the peakheight) to yield a comparisonwith the experimentaldata. Finally to comparetheoretical spectrato the measuredPIA spectra a simple first order approximationwas applied. A constantenergyshift was subtractedfrom the triplet spectrum. This energy shift was derived from the differencebetweenthe calculatedsinglet spectrumand the experimentalground state absorptionspectrumof PPY [4]. Using the lowest energy absorptionbandthis shift wasestimatedas0.8 eV. 4 Results
PIA spectrafor PPY over the energyrange0.4 eV to 3.5 eV measured at 300 K and 80 K are shownin figure 1. Thereis an obvious large temperaturedependenceto the observedPIA features. Within the error of the measurementthere are similaritiesbetweenthe two spectra.We alsoobservea blueshift in the positionof the PL emissionbandat 80 K. At 300 K-the PIA is weak and so it is diffGxIt to make definitive
comments
about it. At 80 K three broad induced absorptionbandsare observedat ca 2.1 eV, 1.6 eV and 0.9 eV respectively.All three arefoundto havethe samephasewith respectto the PL emission indicatingthat all three featureshave the sameorigin, Chopper dependencies of all featuresare alsobroadly similar.As in PPV [8] the chop rate dependence has a marked turn over at ca 100
Hz, indicatinga lifetime of 10 msfor the excitedstategiving rise to the inducedabsorption,Plasmalines from the laserprevent meaningfulmeasurements being madeon the bleachingof the lowestenergyabsorptionfeature.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00841-8
808
LJ. Hartwell
O.OBl5
et al. i Synthetic
~Ill~ar<
5
Metals
I01 (1999)
807-808
!
Wavelength (nm) 0.2
0.7
1.2
1.7
22
27
i-?&W
Fig.1 PIA spectra of PPY at SOK and 300 K, Results of the ZINDO/S-CI calculations are shown in figure 2. For the case of singlet transitions, one strong 7~+ R* transition is found at 327 nm (3.8 eV) similar to that found by Blatchford et al. [9]. A weaker feature at 255 nm (4.9 eV) along with a number of transitions at 140 - 200 nm (8.9 - 6.2 eV) are also observed. Scaling the lowest transition to 3 eV as observed experimentally [4] good agreement with the experimentally observed higher energy absorption features is found i.e. 4 eV, 4.9 eV [lo] and 6 eV [4]. The triplet spectrum shows three transitions below the singlet 7c+ z* transition. These are centred at 475 nm (2.6 eV), 530 nm (2.3 eV) and 700 nm (1.8 eV). Scaling these three transitions by 0.8 eV yields three triplet transitions to be expected at 1.8 eV, 1.5 eV and 1 eV respectively. 4 Discussion As all three observed PIA features have the same phase relationship and chop rate dependencies, we must conclude that all three features have a common origin. For the case of photogenerated bipolarons one would expect to observe two induced features below the band edge [S]. Typically a mixture of triplet and bipolaron absorptions are observed [l l] which can be distinguished by their respective lifetimes. As we clearly find three correlated features this is not the case in PPY. The agreement between the scaled calculated triplet manifold and PIA spectra is surprisingly good and it is hard not to assign the PIA spectrum as representmg absorptions within the triplet manifold. The lifetimes and strong temperature dependency of the observed PIA features certainly go along with this assignment. If this is the case, the chop rate dependencies should at some characteristic frequency turn over to yield a 0-I relationship (where o is the chopper frequency) indicative of a monomolecular decay. As yet we have not conclusively shown this as the PIA signals are small and noisy above ca 200 Hz. More work is required to definitively prove this point. The same is true for the room temperature PIA spectra. Further, to account for the large apparent Stokes shift seen in PPY absorption and emission, in both solid and solution state [ l41, we have proposed that in the excited state the pyridyl rings distort such that the nitrogen is bent out of the plane of the pyridyl rings [ 121. As occurs in small pyridyl molecules. This distortion shifts the Frank Condon overlaps giving rise to the observed apparent Stokes shifts. As yet we do not know what effect this has on the calculated triplet manifold.
300
Wavelength
Fig. 2 ZMDOIS-CI
500
(nm )
singlet (upper) and triplet (lower) spectra
5 Summary The three observed PIA spectral features of PPY have a common origin and similar lifetimes. The energy pos~t~ons~ and relative intensities of these induced bands corresponds well to calculated triplet absorption spectrum, once the latter is scaled to the observed singlet n+ n* transition. The observed triplets have lifetimes of order 10 ms at 80 K. 5 Acknowledgements We thank the EPSRC for funding. LJH is an 1851 Fellow 6 References [ 1] [2] [3]
T. Yamamoto et al J. Am. Chem. Sot. 116 (1994) 4832. J. W. Blatchford et al. Phys. Rev. Lett. 76 (1996) 1513. ~M. Halim, I. D. W. Samuel, E. Rebourt and A.P.Monkman, Synth. Met. 84 (1997) 95 1. [4] A. P. Monkman, et al, SPIE Proc. 31TS-(l997) 82. [j] J. J. P. Stewart, J. Camp. C&m. 10 (19S9) 209. and J. ?; P. Stewart, J. Camp. Chem. 10 (1989) 221. [6] A. P. Monkman, S. Daily, M. Halim, I. D. W. Samuel~and L. E. Horsburgh, these proceedings [7] J. Ridley, M. C. Zerner, Theor C&m. Acta. 42 (1976) 223. [S] H. S. Wooet al, Phys. Rev. B, 46 (1992) 7379 [9] J. W. Blatchford, et al, J. Chem. Phys. 105 (1996) 9214. [lo] L. Pettersson, G, Grezsinzki, N. Johanssen, 0. Inganas, W. J. Salaneck and A. P. Monkman, to be published [ 1 l] J. M. Leng et al, Phys. Rev. Let, 72 (1994) 156. i [12] W. J. Buma, et al, J&n. Chem. Sot. l-12 (1990) 5447.
SyntheticMetals 101(1999) 807-808
Photoinduced
absorption
measurements of triplet states in poly@pyridine)
L. J. Hartwellt, M. E. Vaschetto, L. E. Horsburgh and A. P. Monkman. Organic Electroactive Materials Research Group, Department of Physics, University of Durham, South Road, Durham DHI 3LE, UK ‘Also at: Bede Scientific Instruments Ltd., Bowburn South Industrial Estate, Bowburn. Durham, DH6 5AD, UK.
Abstract
Wereport cw photoinducedabsorption(PIA) measurements of poly@pyridine) (PPY). Threecorrelated,inducedfeaturesarefound belowthe n+ X* transition.All have a strongtemperaturedependence andsimilarlifetimesof ca 10 ms. Comparingto ZINDOIS-CI calculationsof the triplet manifold we find good agreementand so concludethat the PIA representstransitionswithin the triplet manifold.However,asyet we have not beenableto definitively measurea v-’ choprate dependence for the PIA signalsindicativeof monomolecular recombination. Keywords:
Photoinducedabsorption,poly(p-pyridine),triplet states,ZINDO.
1 Introduction
Following on from the first reports on the synthesisand characterisation of poly@-pyridine)by Yamamotoet al [I], and subsequent photophysicalstudiesof Blatchford et al [2] we have reported substantialincreasesin photoluminescence quantum yield (PLQY), achievedby modificationsto the synthesisroute for PPY [3,4] and have madefurther progressyielding polymer with a solid state PLQY of 37f3% as comparedto the initial findingsof ca. 5 % in the solid state.To probethe excitedstates of PPY we have started a seriesof photoinducedabsorption (PIA) measurements. We compareour findings to theoretical calculationsof the singletand triplet manifoldsto ascertainif poiaronsibipolarons play a key part in the photoexcitedstateof PPY or whetherexcitonic(triplet) speciesdominate. 2 Experimental
details
Poly@-pyridine-2,5-diyl)waspreparedasreportedpreviously [4]. Thin films of the polymerwere spin-coatedonto Spectrosil typically from a 20 mg.ml-’ solution in formic acid. PIA measurements were made using mechanicallychopped UV excitation( 360-370nm doubletfrom an AI’ ion laser).A lockin amplifiermeasures the magnitudeandphaseof the ac signalwith the frequency of the chopper (dT) and the dc signal (T) simultaneously.Both UV enhancedSi and InAs photodiode detectorsare used.TemperaturedependentPIA measurements weremadein an exchangegas,liquid nitrogencryostat. 3. Computational
methodology
The gasphasegeometriesof modelhexamerpyridine in the singlet and triplet states have been optimized using the semiempirical ParametrizationMethod 3 (PM3) [5]. This method is based on the Neglect Differential Overlap (NDO) aproximation.The nitrogenatomsof the chainweredistributedin randomway at positionstwo or three.The structuresweretotally optimized without geometryrestrictions.The final geometryis planar showingtorsion angle betweenrings smallerthan 2’,
which bestdescribes PPY in the solidstate[6]. Theseoptimized geometricalstructureswerethen usedto calculationof the UVvis spectraof the hexamers.This was achievedusing Zerner’s IntermediateNeglectDifferential Overlap (ZINDOIS-CI) method [7], using up to 260 singlet configurationsand 300 triplet configurations,respectively.ZINDOS-CI hasbeen specifically parametrized to describethe UV-Vis opticaltransitionof organic molecules. The theoretically obtained wavelengths were broadenedwith Gaussians of variablewidth (proportionalto the peakheight) to yield a comparisonwith the experimentaldata. Finally to comparetheoretical spectrato the measuredPIA spectra a simple first order approximationwas applied. A constantenergyshift was subtractedfrom the triplet spectrum. This energy shift was derived from the differencebetweenthe calculatedsinglet spectrumand the experimentalground state absorptionspectrumof PPY [4]. Using the lowest energy absorptionbandthis shift wasestimatedas0.8 eV. 4 Results
PIA spectrafor PPY over the energyrange0.4 eV to 3.5 eV measured at 300 K and 80 K are shownin figure 1. Thereis an obvious large temperaturedependenceto the observedPIA features. Within the error of the measurementthere are similaritiesbetweenthe two spectra.We alsoobservea blueshift in the positionof the PL emissionbandat 80 K. At 300 K-the PIA is weak and so it is diffGxIt to make definitive
comments
about it. At 80 K three broad induced absorptionbandsare observedat ca 2.1 eV, 1.6 eV and 0.9 eV respectively.All three arefoundto havethe samephasewith respectto the PL emission indicatingthat all three featureshave the sameorigin, Chopper dependencies of all featuresare alsobroadly similar.As in PPV [8] the chop rate dependence has a marked turn over at ca 100
Hz, indicatinga lifetime of 10 msfor the excitedstategiving rise to the inducedabsorption,Plasmalines from the laserprevent meaningfulmeasurements being madeon the bleachingof the lowestenergyabsorptionfeature.
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00841-8
808
LJ. Hartwell
O.OBl5
et al. i Synthetic
~Ill~ar<
5
Metals
I01 (1999)
807-808
!
Wavelength (nm) 0.2
0.7
1.2
1.7
22
27
i-?&W
Fig.1 PIA spectra of PPY at SOK and 300 K, Results of the ZINDO/S-CI calculations are shown in figure 2. For the case of singlet transitions, one strong 7~+ R* transition is found at 327 nm (3.8 eV) similar to that found by Blatchford et al. [9]. A weaker feature at 255 nm (4.9 eV) along with a number of transitions at 140 - 200 nm (8.9 - 6.2 eV) are also observed. Scaling the lowest transition to 3 eV as observed experimentally [4] good agreement with the experimentally observed higher energy absorption features is found i.e. 4 eV, 4.9 eV [lo] and 6 eV [4]. The triplet spectrum shows three transitions below the singlet 7c+ z* transition. These are centred at 475 nm (2.6 eV), 530 nm (2.3 eV) and 700 nm (1.8 eV). Scaling these three transitions by 0.8 eV yields three triplet transitions to be expected at 1.8 eV, 1.5 eV and 1 eV respectively. 4 Discussion As all three observed PIA features have the same phase relationship and chop rate dependencies, we must conclude that all three features have a common origin. For the case of photogenerated bipolarons one would expect to observe two induced features below the band edge [S]. Typically a mixture of triplet and bipolaron absorptions are observed [l l] which can be distinguished by their respective lifetimes. As we clearly find three correlated features this is not the case in PPY. The agreement between the scaled calculated triplet manifold and PIA spectra is surprisingly good and it is hard not to assign the PIA spectrum as representmg absorptions within the triplet manifold. The lifetimes and strong temperature dependency of the observed PIA features certainly go along with this assignment. If this is the case, the chop rate dependencies should at some characteristic frequency turn over to yield a 0-I relationship (where o is the chopper frequency) indicative of a monomolecular decay. As yet we have not conclusively shown this as the PIA signals are small and noisy above ca 200 Hz. More work is required to definitively prove this point. The same is true for the room temperature PIA spectra. Further, to account for the large apparent Stokes shift seen in PPY absorption and emission, in both solid and solution state [ l41, we have proposed that in the excited state the pyridyl rings distort such that the nitrogen is bent out of the plane of the pyridyl rings [ 121. As occurs in small pyridyl molecules. This distortion shifts the Frank Condon overlaps giving rise to the observed apparent Stokes shifts. As yet we do not know what effect this has on the calculated triplet manifold.
300
Wavelength
Fig. 2 ZMDOIS-CI
500
(nm )
singlet (upper) and triplet (lower) spectra
5 Summary The three observed PIA spectral features of PPY have a common origin and similar lifetimes. The energy pos~t~ons~ and relative intensities of these induced bands corresponds well to calculated triplet absorption spectrum, once the latter is scaled to the observed singlet n+ n* transition. The observed triplets have lifetimes of order 10 ms at 80 K. 5 Acknowledgements We thank the EPSRC for funding. LJH is an 1851 Fellow 6 References [ 1] [2] [3]
T. Yamamoto et al J. Am. Chem. Sot. 116 (1994) 4832. J. W. Blatchford et al. Phys. Rev. Lett. 76 (1996) 1513. ~M. Halim, I. D. W. Samuel, E. Rebourt and A.P.Monkman, Synth. Met. 84 (1997) 95 1. [4] A. P. Monkman, et al, SPIE Proc. 31TS-(l997) 82. [j] J. J. P. Stewart, J. Camp. C&m. 10 (19S9) 209. and J. ?; P. Stewart, J. Camp. Chem. 10 (1989) 221. [6] A. P. Monkman, S. Daily, M. Halim, I. D. W. Samuel~and L. E. Horsburgh, these proceedings [7] J. Ridley, M. C. Zerner, Theor C&m. Acta. 42 (1976) 223. [S] H. S. Wooet al, Phys. Rev. B, 46 (1992) 7379 [9] J. W. Blatchford, et al, J. Chem. Phys. 105 (1996) 9214. [lo] L. Pettersson, G, Grezsinzki, N. Johanssen, 0. Inganas, W. J. Salaneck and A. P. Monkman, to be published [ 1 l] J. M. Leng et al, Phys. Rev. Let, 72 (1994) 156. i [12] W. J. Buma, et al, J&n. Chem. Sot. l-12 (1990) 5447.