Thermochromism in the triplet excited state of poly(3-octylthiophene)

SyntheticMetals

Thermochromism

101 (1999)

177

in the triplet excited state of poly(3-octylthiophene)

Marwijn P. T. Christiaans, Bea M. W. Langeveld-Voss and RenC A. J. Janssen* Laboratory for Macromolmtlar and Organic Chemistry Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands Tel. +3/-40-2473597, Fax: +31-402451036, E-mail: tgtor~~chem.tue,nl

Abstract Aggregation of poly(3-octylthiophene)(P30T) in 2-methyltetrahydrofuran(2MeTHF) at low temperatureresults in ordered microcrystalliteswhich exhibit a red-shiftedtriplet-triplet absorptionspectrumin comparison with molecularlydissolvedchains. Keywords: Polythiophenes,

Photoinduced

absorption

(PIA) spectroscopy, UV/Vis/nearIR

Thermochromismof the linear absorption of substituted polythiophenesis a well-established phenomenon that originates from a transitionbetweenmolecularlydisorderedpolymerchains and aggregated,moreordered,possiblymicrocrystalline,chains that form when the temperatureis reducedand absorblight at longerwavelengths[l]. Thermochromism hasbeenobservedin thin solid films and in solutionsand is accompanied by similar changesin the photoluminescence [2]. Here,we demonstrate that thermochromism of P30T occursin the triplet excitedstate. Figure 1 showsthe linear absorptionof P30T (85% head-totail) in 2MeTHF upon cooling. Three different temperature regimescanbe distinguished.Initially, the absorptionshiftsfrom 2.857eV at 300K to 2.800eV at 250K togetherwith an increase in absorbance as a result of shrinkageof the solvent. Between 240and 190K, a significantlyred-shiftedabsorptiongrowsin at the costof the bandat 2.8 eV. The new spectrumexhibitsa peak at 2.358eV, with two vibronic shouldersat 2.184and 2.000eV. In this temperaturerangean isosbesticpoint is found at 2.G2eV, indicatingthe interconversionof two species.Apart from a small redshift, the spectrumremainsessentiallyunchangedbelow 190 K. The changesbetween240 and 190K are ascribedto a phase transition betweenmolecularly dissolvedpolymer chainsand aggregated chainsin solution. Similarchangesare observedin the triplet-triplet (T+T*) PIA spectrumwhen exciting at 2.71 eV (Fig. 2). After an initial

absorption,

Thermochromism

----.2@J L

120

K

.. . ..

260

0.0010

s 4 0.0005

-.-.-..

220 K 240 K L&d

240

_.--_.

220

.. .

. . 200

-

180

-

160 -

140

----.,*o

0.0000

1,oo

1so

1.25

Energy

1.75

2.00

(eV)

Fig. 2. T+T* PIA spectraof P30T in 2MeTHF at different temperaturesrecorded with excitation at 2.71 eV. The inset showsselectedPIA spectrawith excitationat 2.00 eV. increaseand a smallred shift, the T+T* spectrumat -1.5 eV decreases dramaticallyin intensitybetween240 and 180K, while a new, weak,absorptionat -1.25 eV comesup. The formationof the new band at low temperatureis more pronouncedwhen excitationisperformedat 2.00 eV (insetFig. 2), corresponding to the first vibronic of the aggregatedphase,but well below the onsetof the absorptionof molecularlydissolvedP30T chains. No PIA signalsareobservedbelow0.9 eV andthe 1.25eV band is assignedto the T+T* spectrumof P30T in the aggregated phase. The interconversionof the T+T* bandsat I .50 eV and 1.25 eV occursin the sametemperaturerangeaschangesin the linear absorption. The reduction of the PIA intensity at low temperaturesmatchesthe previously observed reduction of photoluminescence intensity [2] and indicates that in the aggregated phasenon-radiativedecayis enhancedascomparedto molecularlydissolvedchains. References

2.0

2.5

Energy

3.0

3.5

[l]

(eV)

Fig. 1. Absorption spectraof P30T in 2MeTHF at different temperatures. 0379-6779/99/Z? - see front matter 0 1999 Elsevier PII: SO379-6779(98)00795-4

[2]

Science S.A. All rights reserved.

S. D. D. V. Rughooputh,S.Hotta, A. J. Heeger,F. Wudl, J. Polym. Sci. B. Polym. Phys.25 (1987) 1971. G. Rumbleset al. Synth.Met. 76 (1996)47.