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Aggregate fluorescence in conjugated polymers
30 June 1995
CHEMICAL PHYSICS LETTERS
| Chemical Physics Letters 240 (1995) 373-378
ELSEVIER
Aggregate fluorescence in conjugated polymers U. Lemmer
a,
S. Heun a, R.F. Mahrt a, U. Scherf b, M. Hopmeier E.O. G/Sbel a, K. Miillen b, H. B~issler a
a,
U° Siegner
a,
a Fachbereich Physik, Fachbereich Physikalische Chemie, and Zentrum fftr Materialwissenschaflen, Philipps-Universit~t Marburg, Hans Meerwein Stra,~e, D-35032 Marburg, Germany Max-Planck-lnstttut fiir Polymerforschung, Ackermannweg 10, D-55021 Mamz, Germany b
.
.
Received 7 February 1995; in final form 18 April 1995
Abstract
We study the dynamics of optical excitations in solid films of ladder-type poly(para-phenylene) (LPPP). A low-energy emission band is observed in the photoluminescence spectrum of solid films of this ~-conjugated polymer, which is absent in LPPP solutions. We show that the low-energy luminescence can be attributed to aggregates between subunits of the polymer chains. Energy transfer into aggregate states strongly affects the dynamics of excitations in solid LPPP. We further discuss the influence of non-radiative defects on the dynamics.
1. Introduction
The optical and electronic properties of rr-conjugated polymers have been intensively investigated [1,2] both for their potential applications in optoelectronics [3] and for fundamental reasons [4-7]. So far, mainly the properties of single polymer chains have attracted attention, thereby neglecting the possibility of interaction between different polymer chains. This interaction can result in the formation of aggregates. Here we study a new material from the class of ladder polymers. We focus both on the intrinsic properties of an isolated polymer chain as well as on the possibility of aggregate formation between the subunits of different polymer chains. The conjugated polymer investigated in this Letter is a fully planar ladder-type poly(para-phenylene) (LPPP). It is available as a soluble and structurally regular polymer with high number average molecular weight [8,9]. The chemical structure of the funda-
mental building block of LPPP is shown in the inset of Fig. 1. The material is synthesized via a polymeranalogous cyclization of suitably functionalized single-stranded precursors [8,9]. The optical properties of LPPP in dilute solution have been previously investigated [9]. The absorption spectrum shows a maximum at 22750 cm-1 assigned to the S1-S 0 (0-0) transition, followed by vibrational replica at higher energies [9]. The photoluminescence (PL) spectrum is the mirror image of the absorption spectrum. The emission from LPPP in dilute solution lies in the blue spectral region. The Stokes shift between absorption and emission is small, reflecting the rigid geometry of the conjugated main chain ribbon. The rather high PL quantum efficiency of more than 39% [10] is comparable to the values known for oligophenylenes [11]. Surprisingly, the PL spectrum of solid LPPP exhibits an additional low-energy emission band in the yellow region of the spectrum [10,12]. The blue
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00512-9
374
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
2. Experimental 2.0
2.0 Absorption
_~
~
1.5
1.5
,.o
o_
o.5
0.0
o.s
~ 16000
PLE
<
0.0 20000
24000
28000
Wavenumber (cm"1)
Fig. 1. Photoluminescence excitation spectrum (PLE) detected spectrally integrated below 19000 cm-1 and absorptionspectrum of a solid LPPP film (temperature 300 K). Inset: chemical structure of the fundamental building block of LPPP (X: n-hexyl, R: 1,4-C6 H4-O-n-decyl).
component in the PL spectrum of solid LPPP is similar to the one in the PL spectrum of LPPP in dilute solution. Consequently, the blue emission band is assigned to isolated polymer chains. We refer to the states, which give rise to the blue emission, as bulk states throughout this Letter. A comparable PL characteristic with an additional emission band was observed for another conjugated polyaromatic system, poly(1,4-phenylene-benzobisoxazole). The additional luminescence band was discussed in terms of excimer formation in this material [13]. The term excimers refers to dimers that exist only in the excited state while the ground state of the pair dissociates [14-16]. In this Letter, we present a detailed experimental study of the optical properties of solid LPPP, applying time-integrated and time-resolved PL spectroscopy as well as site-selective fluorescence, photoluminescence excitation (PLE) and absorption spectroscopy. In particular, our experiments clarify the origin of the additional yellow emission band. This additional luminescence is assigned to the formation of aggregates. The aggregates are stable in the ground state, in contrast to the excimers observed in Ref. [13]. The formation of aggregates is due to intermolecular interaction in the conjugated "rr-systems.
Soluble and structurally well-defined LPPP was synthesized by bridging suitably substituted singlestranded poly(para-phenylene) precursors [8,9]. The LPPP samples have a number average molecular weight of = 12000, corresponding to a degree of polymerization of = 30 para-phenylene subunits. Site-selective fluorescence spectroscopy is performed with a tunable dye laser with a spectral width of 0.2 cm -1. The fluorescence is recorded with a 1 m double monochromator, which allows measurements as close as 10 cm-~ to the laser line. In PLE experiments, the luminescence is measured as a function of the excitation energy, employing the dye laser as excitation source. We use a mode-locked frequency-doubled titanium: sapphire laser producing pulses with a temporal width of 100 fs at 25000 cm-1 to excite the sample in time-resolved PL experiments. Here the luminescence is spectrally and temporally resolved with a 0.64 m monochromator and a Streak camera with a time resolution of 25 ps.
3. Results and discussion In the upper part of Fig. 1 the room temperature absorption spectrum of a solid LPPP film is depicted. The spectrum shows the S1-S 0 (0-0) transition at about 22120 cm -1, followed by vibronic progressions blue-shifted by 1500 cm -1. All bands are inhomogeneously broadened. Importantly, the absorption spectrum exhibits a pronounced low-energy tail extending well below 17000 cm -1. The PLE spectrum detected spectrally integrated below 19000 cm-1 is also displayed for comparison in the lower part of Fig. 1. Remarkably, the relative intensity of the first vibronic transition is significantly enhanced in the PLE spectrum with respect to the absorption. In Fig. 2 the PL spectra of two different solid LPPP films are shown for excitation high above the S1-S 0 (0-0) transition. In contrast to the PL spectrum of LPPP in dilute solution [9], for solid LPPP a broad emission band is observed around 17000 c m - 1, i.e. in the yellow region of the visible spectrum. The relative intensity of the yellow emission band varies from sample to sample and depends on the details of
U. Lemmer et aL / Chemical Physics Letters 240 (1995) 373-378
the preparation route [17]. The yellow band can dominate the PL spectrum. We assign the additional yellow emission band to aggregate states formed by subunits of different polymer chains in closely packed solid LPPP polymer films. The yellow emission centered at 17000 cm-~ stems from species which exist in the ground state as demonstrated by the finite absorption found in this spectral region. In contrast to the findings for poly(1,4-phenylene-benzobisoxazole) [13], we can rule out excimers as the source of the additional luminescence band. The weight of the aggregate band relative to the S~-S 0 (0-0) transition is much larger in the emission spectrum than in absorption. This experimental result unambiguously demonstrates highly efficient energy transfer into aggregate states. Further information on the dynamics of optical excitations in solid LPPP is obtained from site-selective spectroscopy at low temperatures. Fig. 3 depicts three site-selective fluorescence spectra for different excitation energies Vex. For excitation at 21900 c m - l , the PL spectrum consists of a superposition of inhomogeneously broadened bands similar to the spectra shown in Fig. 2. Lowering the excitation energy results in line narrowing of the emission spectrum.
i
l
I
f
0.8 0.6
.~
0.4
¢-
c 0.2 0.0 I
12000
,
I
16000
,
~
20000
,
I
24000
Wavenumber (cm1)
Fig. 2. Low-temperaturetime-integratedphotoluminescencespectra of two different LPPP films. The excitation energy is 25000 cm-
1.
375
1.2 1.0 -~ 0.8 0.6 _ _c
0.4
4
0.2 0.0
., ,
I
,
I
,
I
,
I
i
I
,
14000 16000 18000 20000 22000 Wavenumber (cm-1)
Fig. 3. Low-temperature site-selective fluorescence spectra of solid LPPP. The high-energy spike marks the excitation energy: 21900 cm-1 ( ), 19400 cm-1 (---O---), 18700 cm-1 (- z~-).
The yellow luminescence is observed for all excitation energies, even for energies as low as 18700 cm -1, confirming our interpretation that the yellow emission originates from aggregates which exist in the ground state and which can be directly excited optically. The results of the site-selective fluorescence spectroscopy for excitation close to the S~-S 0 (0-0) transition are summarized in Fig. 4, where the position of the S1-S 0 (0-0) emission maximum is plotted versus the excitation energy Vex. The resonance line for which the emission energy is equal to the excitation energy is also shown. As Vex decreases below 21800 cm -~, the maximum of the emission starts shifting linearly with v~x. Consequently, we identify rio c = 21800 cm -a as localization energy. The localization energy is defined by the condition that the rate of energy transfer within the inhomogeneously broadened distribution of bulk states equals the reciprocal lifetime of an excitation at this threshold. Excitations are unable to undergo energy relaxation within the bulk states during their lifetime if Vex < rioc. A Stokes-shift of 180 -t- 20 cm -1 is obtained from the data in Fig. 4 in the quasi-resonant regime below rio c. This small Stokes-shift indicates
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
376
22200
22000 C
21800 E
UJ 0
~
21600
21400 I
21400
I
I
21600
i
I
=
I
=
I
21800 22000 22200
Excitation Wavenumber (cm1) Fig. 4. Energy of the maximum of the emission versus excitation energy for the S 1- S o ( 0 - 0 ) band of solid LPPP. The data ( I ) are taken from the site-selective fluorescence spectra. The resonance line is also shown.
slight structural relaxation of the LPPP skeleton to a new equilibrium configuration after excitation. In light of the importance of energy relaxation in solid LPPP films, time-resolved PL measurements further elucidate the dynamic processes taking place after excitation. The luminescence decays non-exponentially and the decay strongly depends on the energy within the emission band. The decay times shown in Fig. 5 as a function of the detection energy represent the time after which the intensity has dropped to 1 / e of the initial value. The decay time increases from 60 + 10 ps in the spectral range of the bulk states to 600 ps at the low-energy edge of the aggregate band for a typical LPPP film. For LPPP in dilute solution, the decay time increases from -- 400 ps in the blue spectral region to = 1 ns at 17000 cm- 1. The difference between the PL decay times in solid LPPP, which shows a strong aggregate luminescence band, and the decay times in LPPP in dilute solution where aggregate states are absent, demonstrates again the efficiency of the energy transfer from bulk to aggregate states, in agreement with our previous conclusion. However, energy transfer to aggregate states is not the only channel for depopulation of the bulk states. This follows from our measurements of the
quantum efficiency of solid LPPP films. We have found that the quantum efficiency is less than 10%. This value is much lower than the quantum efficiency of LPPP in solution [10]. Thus an additional non-radiative process is involved in the recombination dynamics. Consequently, we conclude that nonradiative traps are present in solid films of LPPP. Due to the presence of non-radiative defects, trapping of bulk excitations into the non-radiative defects is a competitive process to transfer into radiative aggregate states. Comparison of the absorption and the PLE spectrum in Fig. 1 shows that trapping of optical excitations into non-radiative defects is more efficient at higher energies. The higher vibronic progressions are less intense in the PLE spectrum than expected on the basis of their absorption strength. Since the luminescence was spectrally integrated in the PLE experiment, we can rule out that the relative weakness of the PLE signal at higher energies is due to transfer of excitations to low-energy radiative states. In contrast, we can immediately conclude that the relative weakness of the PLE signal at higher energies is the result of a higher probability for nonradiative deactivation.
1400 12001 1000
800 E >,
600
400! 200 0
16000
18000
20000
W a v e n u m b e r (era "1) Fig. 5. 1 / e decay time of the photoluminescence versus detection energy for solid LPPP at 10 K ( 0 ) and for a dilute solution of LPPP in MTHF at 300 K ( • ) . The excitation energy is 25000 cm-
1.
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
Interestingly, the low-energy peak in the PLE spectrum coincides within 200 cm-~ with the localization energy vtoc obtained from site-selective fluorescence spectroscopy. The decrease of the PLE spectrum at higher energies due to an increased probability for trapping into non-radiative defects is thus correlated with the mobility of the optical excitations. Mobile bulk excitations at energies above the localization energy rio c are increasingly captured by non-radiative defects. The nature of the non-radiative defect is subject to speculation. The most likely candidate is oxygene or an adduct of oxygene with LPPP, acting as electron trap and thus giving rise to dissociation of the onchain singlet state of LPPP. In the following, we summarize the dynamics of optical excitations in solid LPPP films as inferred from the experimental data presented in this Letter. Although LPPP has a more or less rigid structure, an LPPP chain is not a single fully conjugated absorber. In contrast, LPPP can be viewed as an array of subunits where the effective conjugation length L etf is a statistical quantity determined by topological disorder. Variation of L e tf gives rise to a distribution of excitation energies. This distribution manifests itself as inhomogeneous broadening of the bulk states in absorption spectra. Because of electronic coupling among the conjugated segments, excitations can migrate within the density of excited states [18,19]. Site-selective fluorescence spectroscopy reveals that a localization energy exists at 21800 cm -1. Above the localization energy, excitations migrate within the density of states. They are increasingly subject to trapping into non-radiative defects in this energy range. Below the localization energy, the mobility of excitations is strongly reduced and nonradiative energy relaxation is less important than at energies above the localization threshold. However, direct trapping of bulk excitations into non-radiative defects without previous migration processes may also contribute to the depopulation of bulk states below the localization energy. More experimental work to clarify the importance of direct trapping is in progress. Transfer of excitations to aggregate states takes place in the whole energy range, i.e. above and below the localization energy. This is demonstrated by site-selective fluorescence spectra, where the ag-
377
gregate emission band is observed irrespective of the excitation energy. The PL decay time is short over a wide energy range as a consequence of the efficient transfer of excitations into aggregate states and trapping into non-radiative defects. In the yellow region of the spectrum, the optical properties of solid LPPP are determined by the aggregate states which give rise to the broad luminescence band centered at 17000 cm -~. Importantly, these states exist in the ground state and can be directly excited, in contrast to the excimers discussed in Ref. [13]. In conclusion, we have presented a detailed study of the nature of the yellow emission band of solid LPPP. We have demonstrated that this emission is due to aggregates and that the formation of excimers can be neglected in LPPP. Time-resolved PL experiments and site-selective fluorescence spectroscopy show the importance of trapping and energy relaxation processes for the dynamics of optical excitations in LPPP.
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 383, the Stiftung Volkswagenwerk, and the BMFT.
References [1] T. Kobayashi, ed., Nonlinear optics of organics and semiconductors, (Springer, Berlin, 1989). [2] S. Etemad and Z.G. Soos, Spectroscopy of advanced materials, eds. R.J. Clark and R.E. Hester (Wiley, New York, 1991). [3] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn and A.B. Holms, Nature 347 (1990) 539. [4] W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. Letters 42 (1979) 1698. [5] A.J. Heeger, S. Kivelson, J.R. Schrieffer and W.P. Su, Rev. Mod. Phys. 60 (1988) 781. [6] U. Rauscher, H. B~sler, D.D.C. Bradley and M. Hennecke, Phys. Rev. B 42 (1990) 9830. [7] S. Mukamel and H.X. Wang, Phys. Rev. Letters 69 (1992) 65. [8] U. Scherf and K. Miillen, Makromol. Chem., Rapid Commun. 12 (1991) 489.
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U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
[9] U. Scherf and K. Miillen, Makromol. 25 (1992) 3546. [10] J. Griiner, H.F. Wittmarm, P.J. Hamer, R.H. Friend, J. Huber, U. Scherf, K. Miitlen, S.C. Moratti and A.B. Holmes, Synth. Metals 67 (1994) 181. [11] I.B. Berlman, Handbook of fluorescence spectra of aromatic molecules, 2rid Ed. (Academic Press, New York, 1971). [12] J. Huber, K. Miillen, J. Salbeck, H. Schenk, U. Scherf, T. Stehlin and R. Stem, Acta Polymer. 45 (1994) 244. [13] S.A. Jenekhe and J.A. Osaheni, Science 265 (1994) 765.
[14] M. Pope and C.E. Swenberg, eds., Electronic processes in organic crystals (Clarendon Press, Oxford, 1982). [15] B. Stevens and E. Hutton, Nature 186 (1960) 1045. [16] J.B. Birks, Rept. Prog. Phys. 38 (1975) 903. [17] G. Leising, private communication. [18] U. Lemmer, R.F. Mahrt, Y. Wada, A. Greiner, H. B~issler and E.O. GSbel, Chem. Phys. Letters 209 (1993) 243. [19] U. Lemmer, R.F. Mahrt, Y. Wada, A. Greiner, H. Biissler and E.O. Gfbel, Appl. Phys. Letters 62 (1993) 2827.
CHEMICAL PHYSICS LETTERS
| Chemical Physics Letters 240 (1995) 373-378
ELSEVIER
Aggregate fluorescence in conjugated polymers U. Lemmer
a,
S. Heun a, R.F. Mahrt a, U. Scherf b, M. Hopmeier E.O. G/Sbel a, K. Miillen b, H. B~issler a
a,
U° Siegner
a,
a Fachbereich Physik, Fachbereich Physikalische Chemie, and Zentrum fftr Materialwissenschaflen, Philipps-Universit~t Marburg, Hans Meerwein Stra,~e, D-35032 Marburg, Germany Max-Planck-lnstttut fiir Polymerforschung, Ackermannweg 10, D-55021 Mamz, Germany b
.
.
Received 7 February 1995; in final form 18 April 1995
Abstract
We study the dynamics of optical excitations in solid films of ladder-type poly(para-phenylene) (LPPP). A low-energy emission band is observed in the photoluminescence spectrum of solid films of this ~-conjugated polymer, which is absent in LPPP solutions. We show that the low-energy luminescence can be attributed to aggregates between subunits of the polymer chains. Energy transfer into aggregate states strongly affects the dynamics of excitations in solid LPPP. We further discuss the influence of non-radiative defects on the dynamics.
1. Introduction
The optical and electronic properties of rr-conjugated polymers have been intensively investigated [1,2] both for their potential applications in optoelectronics [3] and for fundamental reasons [4-7]. So far, mainly the properties of single polymer chains have attracted attention, thereby neglecting the possibility of interaction between different polymer chains. This interaction can result in the formation of aggregates. Here we study a new material from the class of ladder polymers. We focus both on the intrinsic properties of an isolated polymer chain as well as on the possibility of aggregate formation between the subunits of different polymer chains. The conjugated polymer investigated in this Letter is a fully planar ladder-type poly(para-phenylene) (LPPP). It is available as a soluble and structurally regular polymer with high number average molecular weight [8,9]. The chemical structure of the funda-
mental building block of LPPP is shown in the inset of Fig. 1. The material is synthesized via a polymeranalogous cyclization of suitably functionalized single-stranded precursors [8,9]. The optical properties of LPPP in dilute solution have been previously investigated [9]. The absorption spectrum shows a maximum at 22750 cm-1 assigned to the S1-S 0 (0-0) transition, followed by vibrational replica at higher energies [9]. The photoluminescence (PL) spectrum is the mirror image of the absorption spectrum. The emission from LPPP in dilute solution lies in the blue spectral region. The Stokes shift between absorption and emission is small, reflecting the rigid geometry of the conjugated main chain ribbon. The rather high PL quantum efficiency of more than 39% [10] is comparable to the values known for oligophenylenes [11]. Surprisingly, the PL spectrum of solid LPPP exhibits an additional low-energy emission band in the yellow region of the spectrum [10,12]. The blue
0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0009-2614(95)00512-9
374
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
2. Experimental 2.0
2.0 Absorption
_~
~
1.5
1.5
,.o
o_
o.5
0.0
o.s
~ 16000
PLE
<
0.0 20000
24000
28000
Wavenumber (cm"1)
Fig. 1. Photoluminescence excitation spectrum (PLE) detected spectrally integrated below 19000 cm-1 and absorptionspectrum of a solid LPPP film (temperature 300 K). Inset: chemical structure of the fundamental building block of LPPP (X: n-hexyl, R: 1,4-C6 H4-O-n-decyl).
component in the PL spectrum of solid LPPP is similar to the one in the PL spectrum of LPPP in dilute solution. Consequently, the blue emission band is assigned to isolated polymer chains. We refer to the states, which give rise to the blue emission, as bulk states throughout this Letter. A comparable PL characteristic with an additional emission band was observed for another conjugated polyaromatic system, poly(1,4-phenylene-benzobisoxazole). The additional luminescence band was discussed in terms of excimer formation in this material [13]. The term excimers refers to dimers that exist only in the excited state while the ground state of the pair dissociates [14-16]. In this Letter, we present a detailed experimental study of the optical properties of solid LPPP, applying time-integrated and time-resolved PL spectroscopy as well as site-selective fluorescence, photoluminescence excitation (PLE) and absorption spectroscopy. In particular, our experiments clarify the origin of the additional yellow emission band. This additional luminescence is assigned to the formation of aggregates. The aggregates are stable in the ground state, in contrast to the excimers observed in Ref. [13]. The formation of aggregates is due to intermolecular interaction in the conjugated "rr-systems.
Soluble and structurally well-defined LPPP was synthesized by bridging suitably substituted singlestranded poly(para-phenylene) precursors [8,9]. The LPPP samples have a number average molecular weight of = 12000, corresponding to a degree of polymerization of = 30 para-phenylene subunits. Site-selective fluorescence spectroscopy is performed with a tunable dye laser with a spectral width of 0.2 cm -1. The fluorescence is recorded with a 1 m double monochromator, which allows measurements as close as 10 cm-~ to the laser line. In PLE experiments, the luminescence is measured as a function of the excitation energy, employing the dye laser as excitation source. We use a mode-locked frequency-doubled titanium: sapphire laser producing pulses with a temporal width of 100 fs at 25000 cm-1 to excite the sample in time-resolved PL experiments. Here the luminescence is spectrally and temporally resolved with a 0.64 m monochromator and a Streak camera with a time resolution of 25 ps.
3. Results and discussion In the upper part of Fig. 1 the room temperature absorption spectrum of a solid LPPP film is depicted. The spectrum shows the S1-S 0 (0-0) transition at about 22120 cm -1, followed by vibronic progressions blue-shifted by 1500 cm -1. All bands are inhomogeneously broadened. Importantly, the absorption spectrum exhibits a pronounced low-energy tail extending well below 17000 cm -1. The PLE spectrum detected spectrally integrated below 19000 cm-1 is also displayed for comparison in the lower part of Fig. 1. Remarkably, the relative intensity of the first vibronic transition is significantly enhanced in the PLE spectrum with respect to the absorption. In Fig. 2 the PL spectra of two different solid LPPP films are shown for excitation high above the S1-S 0 (0-0) transition. In contrast to the PL spectrum of LPPP in dilute solution [9], for solid LPPP a broad emission band is observed around 17000 c m - 1, i.e. in the yellow region of the visible spectrum. The relative intensity of the yellow emission band varies from sample to sample and depends on the details of
U. Lemmer et aL / Chemical Physics Letters 240 (1995) 373-378
the preparation route [17]. The yellow band can dominate the PL spectrum. We assign the additional yellow emission band to aggregate states formed by subunits of different polymer chains in closely packed solid LPPP polymer films. The yellow emission centered at 17000 cm-~ stems from species which exist in the ground state as demonstrated by the finite absorption found in this spectral region. In contrast to the findings for poly(1,4-phenylene-benzobisoxazole) [13], we can rule out excimers as the source of the additional luminescence band. The weight of the aggregate band relative to the S~-S 0 (0-0) transition is much larger in the emission spectrum than in absorption. This experimental result unambiguously demonstrates highly efficient energy transfer into aggregate states. Further information on the dynamics of optical excitations in solid LPPP is obtained from site-selective spectroscopy at low temperatures. Fig. 3 depicts three site-selective fluorescence spectra for different excitation energies Vex. For excitation at 21900 c m - l , the PL spectrum consists of a superposition of inhomogeneously broadened bands similar to the spectra shown in Fig. 2. Lowering the excitation energy results in line narrowing of the emission spectrum.
i
l
I
f
0.8 0.6
.~
0.4
¢-
c 0.2 0.0 I
12000
,
I
16000
,
~
20000
,
I
24000
Wavenumber (cm1)
Fig. 2. Low-temperaturetime-integratedphotoluminescencespectra of two different LPPP films. The excitation energy is 25000 cm-
1.
375
1.2 1.0 -~ 0.8 0.6 _ _c
0.4
4
0.2 0.0
., ,
I
,
I
,
I
,
I
i
I
,
14000 16000 18000 20000 22000 Wavenumber (cm-1)
Fig. 3. Low-temperature site-selective fluorescence spectra of solid LPPP. The high-energy spike marks the excitation energy: 21900 cm-1 ( ), 19400 cm-1 (---O---), 18700 cm-1 (- z~-).
The yellow luminescence is observed for all excitation energies, even for energies as low as 18700 cm -1, confirming our interpretation that the yellow emission originates from aggregates which exist in the ground state and which can be directly excited optically. The results of the site-selective fluorescence spectroscopy for excitation close to the S~-S 0 (0-0) transition are summarized in Fig. 4, where the position of the S1-S 0 (0-0) emission maximum is plotted versus the excitation energy Vex. The resonance line for which the emission energy is equal to the excitation energy is also shown. As Vex decreases below 21800 cm -~, the maximum of the emission starts shifting linearly with v~x. Consequently, we identify rio c = 21800 cm -a as localization energy. The localization energy is defined by the condition that the rate of energy transfer within the inhomogeneously broadened distribution of bulk states equals the reciprocal lifetime of an excitation at this threshold. Excitations are unable to undergo energy relaxation within the bulk states during their lifetime if Vex < rioc. A Stokes-shift of 180 -t- 20 cm -1 is obtained from the data in Fig. 4 in the quasi-resonant regime below rio c. This small Stokes-shift indicates
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
376
22200
22000 C
21800 E
UJ 0
~
21600
21400 I
21400
I
I
21600
i
I
=
I
=
I
21800 22000 22200
Excitation Wavenumber (cm1) Fig. 4. Energy of the maximum of the emission versus excitation energy for the S 1- S o ( 0 - 0 ) band of solid LPPP. The data ( I ) are taken from the site-selective fluorescence spectra. The resonance line is also shown.
slight structural relaxation of the LPPP skeleton to a new equilibrium configuration after excitation. In light of the importance of energy relaxation in solid LPPP films, time-resolved PL measurements further elucidate the dynamic processes taking place after excitation. The luminescence decays non-exponentially and the decay strongly depends on the energy within the emission band. The decay times shown in Fig. 5 as a function of the detection energy represent the time after which the intensity has dropped to 1 / e of the initial value. The decay time increases from 60 + 10 ps in the spectral range of the bulk states to 600 ps at the low-energy edge of the aggregate band for a typical LPPP film. For LPPP in dilute solution, the decay time increases from -- 400 ps in the blue spectral region to = 1 ns at 17000 cm- 1. The difference between the PL decay times in solid LPPP, which shows a strong aggregate luminescence band, and the decay times in LPPP in dilute solution where aggregate states are absent, demonstrates again the efficiency of the energy transfer from bulk to aggregate states, in agreement with our previous conclusion. However, energy transfer to aggregate states is not the only channel for depopulation of the bulk states. This follows from our measurements of the
quantum efficiency of solid LPPP films. We have found that the quantum efficiency is less than 10%. This value is much lower than the quantum efficiency of LPPP in solution [10]. Thus an additional non-radiative process is involved in the recombination dynamics. Consequently, we conclude that nonradiative traps are present in solid films of LPPP. Due to the presence of non-radiative defects, trapping of bulk excitations into the non-radiative defects is a competitive process to transfer into radiative aggregate states. Comparison of the absorption and the PLE spectrum in Fig. 1 shows that trapping of optical excitations into non-radiative defects is more efficient at higher energies. The higher vibronic progressions are less intense in the PLE spectrum than expected on the basis of their absorption strength. Since the luminescence was spectrally integrated in the PLE experiment, we can rule out that the relative weakness of the PLE signal at higher energies is due to transfer of excitations to low-energy radiative states. In contrast, we can immediately conclude that the relative weakness of the PLE signal at higher energies is the result of a higher probability for nonradiative deactivation.
1400 12001 1000
800 E >,
600
400! 200 0
16000
18000
20000
W a v e n u m b e r (era "1) Fig. 5. 1 / e decay time of the photoluminescence versus detection energy for solid LPPP at 10 K ( 0 ) and for a dilute solution of LPPP in MTHF at 300 K ( • ) . The excitation energy is 25000 cm-
1.
U. Lemmer et al. / Chemical Physics Letters 240 (1995) 373-378
Interestingly, the low-energy peak in the PLE spectrum coincides within 200 cm-~ with the localization energy vtoc obtained from site-selective fluorescence spectroscopy. The decrease of the PLE spectrum at higher energies due to an increased probability for trapping into non-radiative defects is thus correlated with the mobility of the optical excitations. Mobile bulk excitations at energies above the localization energy rio c are increasingly captured by non-radiative defects. The nature of the non-radiative defect is subject to speculation. The most likely candidate is oxygene or an adduct of oxygene with LPPP, acting as electron trap and thus giving rise to dissociation of the onchain singlet state of LPPP. In the following, we summarize the dynamics of optical excitations in solid LPPP films as inferred from the experimental data presented in this Letter. Although LPPP has a more or less rigid structure, an LPPP chain is not a single fully conjugated absorber. In contrast, LPPP can be viewed as an array of subunits where the effective conjugation length L etf is a statistical quantity determined by topological disorder. Variation of L e tf gives rise to a distribution of excitation energies. This distribution manifests itself as inhomogeneous broadening of the bulk states in absorption spectra. Because of electronic coupling among the conjugated segments, excitations can migrate within the density of excited states [18,19]. Site-selective fluorescence spectroscopy reveals that a localization energy exists at 21800 cm -1. Above the localization energy, excitations migrate within the density of states. They are increasingly subject to trapping into non-radiative defects in this energy range. Below the localization energy, the mobility of excitations is strongly reduced and nonradiative energy relaxation is less important than at energies above the localization threshold. However, direct trapping of bulk excitations into non-radiative defects without previous migration processes may also contribute to the depopulation of bulk states below the localization energy. More experimental work to clarify the importance of direct trapping is in progress. Transfer of excitations to aggregate states takes place in the whole energy range, i.e. above and below the localization energy. This is demonstrated by site-selective fluorescence spectra, where the ag-
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gregate emission band is observed irrespective of the excitation energy. The PL decay time is short over a wide energy range as a consequence of the efficient transfer of excitations into aggregate states and trapping into non-radiative defects. In the yellow region of the spectrum, the optical properties of solid LPPP are determined by the aggregate states which give rise to the broad luminescence band centered at 17000 cm -~. Importantly, these states exist in the ground state and can be directly excited, in contrast to the excimers discussed in Ref. [13]. In conclusion, we have presented a detailed study of the nature of the yellow emission band of solid LPPP. We have demonstrated that this emission is due to aggregates and that the formation of excimers can be neglected in LPPP. Time-resolved PL experiments and site-selective fluorescence spectroscopy show the importance of trapping and energy relaxation processes for the dynamics of optical excitations in LPPP.
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 383, the Stiftung Volkswagenwerk, and the BMFT.
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