Chemical Physics Letters 402 (2005) 197–201 www.elsevier.com/locate/cplett
Characterisation of the triplet state of a fluorene–terthiophene alternating copolymer H.D. Burrows a
a,*
, L.G. Arnaut a, J. Pina a, J. Seixas de Melo a, N. Chattopadhyay b, L. Alca´cer c, A. Charas c, J. Morgado c
Departamento de Quı´mica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal b Department of Chemistry, Jadavpur University, Calcutta 700 032, India c Instituto de Telecomunicac¸o˜es, Instituto Superior Te´cnico, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Received 7 October 2004; in final form 1 December 2004 Available online 24 December 2004
Abstract The triplet state behaviour of the alternating copolymer poly[2,7-(9,9-bis(2 0 -ethylhexyl)fluorene)-alt-2,5-terthiophene] (PF3T) was studied in benzene solution. This shows a transient absorption at 730 nm, which decays with a lifetime of 10 ls. Using the singlet depletion method, a quantum yield UT = 0.335 was determined. This efficient intersystem crossing results from extensive spin–orbit coupling by the sulfur atoms. In the presence of air, the triplet state sensitises singlet oxygen formation in nearly quantitative yield. A lowest triplet state energy of 1.84 eV was determined from time-resolved photoacoustic calorimetry. This is intermediate between polyfluorenes and terthiophene, in agreement with theoretical predictions. 2004 Elsevier B.V. All rights reserved.
1. Introduction Conjugated organic polymers are an important class of materials, with applications ranging from light emitting diodes to sensors, photovoltaic systems and thin film transistors [1,2]. Polyfluorenes are important examples, since they can form liquid crystals, possess high fluorescence quantum yields and emit in the blue [3–8]. Interest is being shown in tuning this emission and modifying charge injection barriers and transport by copolymerisation with other conjugated molecules [4,9–19]. Polythiophenes have interesting optical and electronic properties [20–23], in particular the possibility of changing emission wavelength either through increasing chain lengths [20,21] or via steric interactions [22] and alternating 9,9-dialkylfluorene/thiophene copolymers have recently attracted interest [9–17,19]. Their synthesis
*
Corresponding author. Fax: +351 239 827 703. E-mail address:
[email protected] (H.D. Burrows).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.12.031
and use in devices, such as LEDs [12,16] and solar cells [19] have been reported. For a series of well-defined alternating 9,9-dialkylfluorene copolymers [11,15,16], the photoluminescence quantum yields were lower in thiophene derivatives than in the corresponding phenyl-9,9-dialkylfluorene copolymers. This was attributed to enhanced intersystem crossing to the lowest triplet states in the former case due to the presence of sulfur, and is in agreement with the observation of triplet state formation being much more important with polythiophenes than with polyfluorenes [20,24]. However, little information is available on the triplet states in these compounds, and the only study we are aware of involves their observation by optically detected magnetic resonance (ODMR) [25]. This is a major omission, since information on triplet states is relevant to areas such as energy transfer to dopants in electrophosphorescent devices and sensitisation of singlet oxygen formation. We report a detailed study of triplet state formation and decay in the alternating copolymer poly[2,7-(9,9-
H.D. Burrows et al. / Chemical Physics Letters 402 (2005) 197–201
S S
S
n
Structure 1.
bis(2 0 -ethylhexyl)fluorene)-alt-2,5-terthiophene] (PF3T, Structure 1), whose synthesis, characterisation, and singlet state properties have previously been reported [11,15,16].
2. Experimental The copolymer PF3T (molecular weight Mn 2800; Mw/Mn = 1.5) was synthesised by Suzuki coupling from the fluorene–boron ester and dibromoterthiophene monomer in the presence of Pd(PPh3)4 catalyst. Details are given elsewhere [11,15]. Solvents and other reagents were of the purest grades available, and solvents were purified by standard procedures. Triplet–singlet difference absorption spectra and yields were obtained using an applied photophysics laser flash photolysis equipment pumped by the third harmonic (355 nm) of a Nd:YAG laser (spectra physics). Decay of the triplet state followed first-order kinetics. Transient spectra (300–850 nm) were obtained by monitoring the optical density change at 5–10 nm intervals, averaging at least 10 decays at each wavelength. The triplet molar absorption coefficients obtained in benzene were determined by the singlet depletion technique, according to the relationship [26]: eT ¼
eS DODT ; DODS
eBenzophenone DODPF3T TT max UBenzophenone : Benzophenone T ePF3T DOD TT max
3. Results and discussion Following laser flash photolysis at 355 nm of a degassed solution of PF3T in benzene, depletion of ground state absorption (460 nm), and formation of a new band at 730 nm was observed (Fig. 1). The band at 730 nm, assigned to the PF3T triplet state, was also obtained (unpublished results (2004)) by the pulse radiolysis-energy transfer technique [30,31]. This absorption decayed with an identical lifetime (10 ls) to recovery of ground state absorption at 460 nm. The triplet maximum is close to that of poly[2,7-(9,9-bis(2 0 -ethylhexyl)fluorene)] (PF2/6, 750 nm [31]), but is at longer wavelengths than terthiophene triplet (460 nm [20]). However, its lifetime
ð1Þ
where both DODS and DODT are obtained from the triplet–singlet difference transient absorption spectra. The UT values were obtained in benzene by comparing the DOD at 525 nm of solutions of benzophenone (the standard) and of the compound (optically matched at the laser wavelength) using the equation [27]: UPF3T ¼ T
microseconds. Details of experimental method and data analysis are given elsewhere [21,24,28]. Room-temperature singlet oxygen phosphorescence was detected at 1270 nm by a Hamamatsu R5509-42 photomultiplier, cooled to 193 K in a liquid nitrogen chamber (Products for Research model PC176TSCE005), following laser excitation of aerated solutions at 355 nm (OD355 nm = 0.30), using an adapted Applied Photophysics flash kinetic spectrometer. The modification of the spectrometer involved the interposition of a Melles Griot dielectric mirror (ref. 08MLQ005/345) that reflects more than 99.5% of the incident light in the 610– 860 nm range, and a Scotch RG665 filter. A 600-line diffraction grating was used instead of the standard spectrometer one to extend spectral response to the infrared. The filters employed are essential to eliminate from the infrared signal all the first harmonic contributions from the sensitiser emission in the 500–800 nm region. The dielectric mirror also helps eliminate these bands. Further details and validation of the system have been given elsewhere [29].
ð2Þ
Phosphorescence measurements were made in glasses at 77 K using a Spex 1934D phosphorimeter accessory with a Fluorolog 3-22 instrument. Time-resolved photoacoustic calorimetry (PAC) measurements were performed in a home built apparatus with front-face irradiation [28]. Heat deposition was within a time window of a few nanoseconds to a few
0.15 0.10 0.05 ∆ O.D.
198
2 µs 5 µs 10 µs 30 µs
0.00 -0.05 -0.10 -0.15 300
400
500
600
700
800
λ (nm) Fig. 1. Triplet–singlet difference absorption spectra observed at various times following laser excitation at 355 nm of a solution of PF3T (A355 nm = 0.2) in benzene.
H.D. Burrows et al. / Chemical Physics Letters 402 (2005) 197–201
is much closer to that of typical polythiophenes (tens of microseconds [20,24,31]) or terthiophene (88 ls [20]) than polyfluorenes (>100 ls [31]). The triplet state of PF3T was also populated by triplet energy transfer from benzophenone (triplet energy 3.0 eV [32]) in degassed benzene and an identical spectrum was observed. The second-order rate constant for energy transfer (measured from the decay of the benzophenone triplet state absorption at 530 nm as a function of polymer concentration, Fig. 2) was 5.7 · 109 M1 s1. It is worth noting that the lifetime of the PF3T triplet state in this case (ca. 40 ls) is significantly greater than that obtained on direct excitation of the polymer. This may be due to self-quenching at the higher concentrations used in the latter case. The molar absorption coefficient of PF3T triplet state was obtained by the singlet depletion method. This requires knowledge of the molar absorption coefficient of the lowest absorption band of PF3T. In previous works, it has been argued that due to their high molecular weights, molar absorption (extinction) coefficients of singlet states (eS) of conjugated polymers cannot be determined simply based on the measured absorbance and molecular weights [24,33]. Instead, the degree of conjugation involved within the polymer has been estimated by comparison with appropriate oligomers, and the extinction coefficient of the equivalent oligomer has been taken as that of the polymer [33]. This has been done with a series of polythiophenes [33]. However, the copolymer PF3T is low molecular weight, corresponding to about 4–5 monomers; therefore, as a reasonable approximation we have used its MW to determine eS, which was 44 000 M1 cm1. From this, a value e730 nm = 49 875 M1 cm1 was determined for its triplet state molar absorption coefficient. Using this and benzophenone triplet state as actinometer (for benzophenone
1.30
τ0 / τ
1.20
1.10
1.00 0
6
12
18
24
[PF3T] /10 µM -6
Fig. 2. Stern–Volmer plot for decay of benzophenone triplet state absorption at 530 nm as function of PF3T concentration following flash photolysis of degassed benzene solutions.
199
e530 nm = 7200 M1 cm1, UT = 1 [32]), the quantum yield of formation of PF3T triplet state formation UT = 0.335 was determined. Attempts to determine the triplet energy (ET1) of PF3T from its phosphorescence in methylcyclohexane (MCH), MCH/ethyl iodide or ethyl iodide matrices at 77 K were unsuccessful. No phosphorescence was observed in any of these systems, although delayed fluorescence was seen, with a spectrum similar to the normal fluorescence [11,15], and with a lifetime 10 ls. Instead, time-resolved photoacoustic calorimetry (PAC) following excitation of a solution of PF3T in benzene with a nitrogen laser (k = 337 nm) was used to measure this energy. 2-Hydroxybenzophenone was used as reference. As previously discussed [21,24,28], PAC gives the fraction of heat released in nonradiative processes (U1 in Eq. (1)). The equation used in the present case to determine ET1 was [21] U1 Elaser ¼ ðElaser ES1 Þ þ UT ðES1 ET1 Þ þ UIC ES1 ;
ð3Þ
where U1 is the fraction of laser energy that comes out as prompt heat; Elaser is the excitation energy at the laser wavelength (337 nm) = 84.8 kcal mol1; ES1 is the energy of the lowest singlet state (550 nm [15]) = 52.02 kcal mol1; UT is the triplet quantum yield of PF3T in benzene. With the value obtained (from PAC) for U1 = 0.526, together with the fluorescence quantum yield, Uf = 0.5 [11] and UT = 0.335 (this work), the triplet state energy can be calculated through Eq. (3). From these results, we determine ET = 42.36 kcal mol1 = 14,818 cm1 = 1.84 eV. A very similar value has been obtained by pulse radiolysis-energy transfer (unpublished observations (2004)). This is markedly lower than PF2/6 triplet energy (2.30 eV [31]), but higher than that of terthiophene (1.55 eV [21]), strongly suggesting that in the triplet state, as in the lowest singlet excited state, conjugation exists between fluorene and terthiophene segments on the copolymer, consequently modulating its triplet energy. In addition, there may be some contribution from inductive effects of the terthiophene rings to this energy. As a result of the balance Uf + UT + UIC = 1, it can be seen that UIC (quantum yield of internal conversion) is 0.165, showing that this process is less important than intersystem crossing. The greater efficiency for the spin–forbidden intersystem crossing is probably due to a heavy atom effect of the three sulfur atoms leading to enhanced spin–orbit coupling. Singlet oxygen formation is frequently observed following photolysis of aerated solutions of conjugated polymers [24]. This can have important implications in device functioning, and lead to polymer degradation [34]. However, measurement of singlet oxygen yields can also be valuable in providing an independent check of the yield of triplet state formation [24]. Following photolysis of aerated solutions of PF3T in benzene with
H.D. Burrows et al. / Chemical Physics Letters 402 (2005) 197–201
Emission intensity (a.u.)
Emission intensity
200
calculations on the gradual evolution of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies on formation of fluorene based copolymers [18].
0.008 0.006 0.004 0.002
Acknowledgements 0.000 0
5
10
15
20
Laser energy
1200
1250
1300
1350
We thank POCTI, FCT, FEDER, GRICES and DST (Acordo de Cooperac¸a˜o Cientı´fica e Tecnolo´gica Portugal-India) for financial support. 1400
Wavelength (nm) Fig. 3. Phosphorescence of singlet oxygen observed following laser excitation at 355 nm of an aerated solution of PF3T in benzene. Inset: plots of initial phosphorescence of singlet oxygen at 1270 nm as functions of laser intensity following solutions in aerated benzene of 1 H phenalen-1-one (open circles) and PF3T (solid circles).
pulses from a frequency tripled Nd:YAG laser (355 nm), the characteristic singlet oxygen phosphorescence was observed at 1270 nm (Fig. 3). This decayed with a lifetime 25 ls, in reasonable agreement with literature data for this solvent (29.3 ± 1.8 ls [24]). The yield of singlet oxygen formation (UD) was determined from the initial phosphorescence intensity at 1270 nm as a function of laser intensity, comparing the slope with that for 1 H phenalen-1-one in benzene as standard (Fig. 3, inset) [35]. A value UD = 0.32 was determined. Although the relationship between UD and UT depends upon a number of parameters related to the efficiency of energy transfer between excited triplet states of molecules and molecular oxygen, the excellent agreement between these two values is encouraging, and does provide strong support to the idea that intersystem crossing to form the T1 state is one of the major decay pathways of singlet excited PF3T.
4. Conclusion Relatively efficient triplet state formation is observed following photolysis of the alternating copolymer poly[2,7-(9,9-bis(2 0 -ethylhexyl)fluorene)-alt-2,5-terthiophene]. This provides a ready explanation for the decreased fluorescence quantum yield in this system compared with the corresponding phenyl/fluorene copolymer [11,15]. As with the corresponding oligothiophenes and polythiophenes [20,21,24] this is probably associated with increased spin–orbit coupling due to the presence of the sulfur atom in the thiophene rings. This triplet state sensitises singlet oxygen formation with high efficiency. The triplet energy of the copolymer is intermediate between that of a polyfluorene and that of terthiophene, consistent with quantum-chemical
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