J. Phys. Chem. 1995,99, 16991-16998
16991
Size-Dependent Properties of Oligothiophenes by Picosecond Time-Resolved Spectroscopy D. Grebner, M. Helbig, and S. Rentsch* Friedrich-Schiller-UniversityJena, Institute of Optics and Quantum Electronics, Max- Wien-Platz 1, 07743 Jena, Germany Received: May 26, 1995; In Final Form: August 18, 1995@
Thiophene oligomers nT with n = 2-6 repeat units were studied in solution upon excitation with UV pulses S I , induced fluorescence, and (349 nm, 100 pJ, 25 ps). Each nT showed transient absorption spectra S , delayed appearing triplet absorption bands. The transient bands shift bathochrom with increasing repeat units n. Band shapes and extinction coefficients of the transient singlet and triplet bands were determined. The SI absorption bands decay with a single-exponential function. The decay times agree with the transient S , corresponding fluorescence lifetimes of the nT. They increase from 5 1 ps for 2T to 1100 ps for 6T with the SI bands. The spectral shift of transitions oligomer size. The triplet bands appear during the decay of the S , from the ground and excited singlet and triplet states can be excellently described by the extended FEMO model. Even average bond lengths of the conjugated chain of oligothiophenes at ground and excited states could be estimated using the FEMO model.
-
-
1. Introduction Conjugated organic polymers have been investigated intensively in the past few years because of their unusual electrical and optical properties. Of special interest are electrolumine~cence,'-~ photoconductivity, photovoltaic effects, and the electrical conductivity of oxidized or reduced polymer^.^-^ Moreover, the nonlinear optical effects of conjugated polymers have been studied in detail, especially the susceptibility of third order, x ( ~ because ), these materials are promising in scientific and technical fields as ell.'^-'^ It was found that the response of these materials is much faster than that of semiconductors. From this reason it is of interest to study laser pulse induced processes with high time resolution. The first light-induced time-resolved studies on polymers showed that the obtained transient spectra contained the superposed information of subsystems, Le., a distribution of linear chain segments in heterogenic surrounding^.'^-'^ One way to get understanding of light-induced processes in conjugated polymers is the investigation of model substances of well-defined size. Thiophene oligomers of different size can be prepared,I9 and now small oligomers with a well-defined number of repeat units are available. They are soluble and highly stable against intensive irradiation. The stationary absorption behavior of oligothiophenes (nT, n = 2-6) in solution at room temperature are They will be summarized and completed by known studies, especially from steady-state and time-resolved fluorescence measurement^^^.^^ below. The radical ions of oligothiophenes chemical and electrical generated and also light-induced were investigated in the past few years4.24-27in order to contribute to the understanding of electrical conductivity of oligo- and polythiophenes. Radical ions of oligothiophene exhibit well-defined absorption bands in the visible and near infrared spectrum, their spectral position remains unchanged by the preparation. It was found that the electronic properties of thiophene oligomers ( n = 6-8) are comparable with that of polythiophene.28 These properties are determined by the lowest electronic states, which can be investigated by laser spectros@
Abstract published in Advance ACS Absrmcfs, October 15, 1995.
0022-365419512099-16991$09.0010
-
copy. Therefore, we think, a systematic study of light-induced processes on a series of thiophene oligomers by time-resolved measurements can help to understand electronic processes in polythiophene, too. In previous picosecond time-resolved studies on thiophene oligomers we observed transient absorption bands with short and long decay components, but an exact assignment of the unknown spectra was not unambiguous at that time.29,30 In femtosecond studies the increase of induced fluorescence and excited-state absorption during excitation with pulses of 350 fs at 308 nm was o b ~ e r v e d . ~ Only ' . ~ ~ for 6T a slightly delayed induced fluorescence was found. It gave hints to intramolecular relaxation processes between higher electronical excited levels.32 We found such levels within quantum chemical calculations using the CNDOIS method.33 In this paper we report the results of detailed picosecond timeresolved spectroscopic studies on a series of oligothiophenes with n = 2-6 repeat units, using the spectrometer briefly described below. The fast rising transient absorption bands which appeared at all thiophene oligomers nT could be identified by their kinetics as transitions from SIto higher singlet states. Their decay behavior and spectroscopic features, as band positions, half-widths, and absorption coefficients will be reported in this paper. Moreover, we found delayed appearing transient absorption bands for all oligothiophenes which were proved to be triplet absorptions known from nanosecond flash experiment^.^^-^^ In this work we observed the kinetics of the emergence of triplets after picosecond pulse excitation. The kinetic and spectral features alter systematically dependent on the oligomer size and seem to exhibit convergent terminal values which probably can be used to define the size of optical or spectroscopic relevant segments of polythiophene.
2. Experimental Section 2.1. Picosecond Laser Spectrometer. The experiments were carried out using the pump-probe technique. The apparatus for obtaining transient absorption was a double beam spectrometer based on a picosecond laser system described elsewhere.38 A picosecond pulse at 1047 nm was generated by a passively mode-locked Nd:YLF laser and amplified by a 0 1995 American Chemical Society
16992 J. Phys. Chem., Vol. 99, No. 46, 1995 three-stage amplifier with 10 Hz repetition rate. The pulse width was measured by autocorrelation to be 25 ps. The third harmonic at 349 nm with a pulse energy of about 100 pJ was used as pump pulse in the transient absorption measurements. This pump pulse (349 nm) was separated with a dichroic beam splitter from the not-converted fundamental pulse (1047 nm). The fundamental was used to generate a white light continuum as probe pulse, by focusing it into a 10 cm cell containing D20. The white light continuum, located between 400 and 970 nm, was split up into two beams. One of them, the test beam, passed through the sample cell to monitor the absorbance change upon excitation, and the other was used as reference beam. The pump pulse reaches the sample cell after passing an optical delay with a maximum time difference to the white light pulse of 2 ns. Test and reference beams were focused into the entrance slit of a polychromator. The spectra were detected with an OMAsystem (OMA 4, EG&G). To improve the accuracy, about 40 test and reference spectra were accumulated and then used to determine the difference spectrum at each delay setting. From the transmission measurements with and without excitation the difference of the optical density was calculated. The optical density difference AD(d,t) averaged on n pulses was calculated by
Grebner et al.
500 250 300 350 400 450 500 550 600
X
/nm
Intensity / a. u. 1.21 I 1:
-2T
I
F1
1h\
0.8 : AD(A,t) = -log
k= I
0.6 1
F1
n
:
'ET\
0.4:
\
0.2 : where the subscripts are as follows: we, with excitation; ne, no excitation; n, number of selected pulses; test, ref, test and reference beam. The lowest variation of the energy of the fundamental pulse results in an accuracy of AD(d,t) of about 0.01. 2.2 Materials. Unsubstituted nT (2T-6T) and didecylsexithiophene (6T1, P-substituted on rings 2 and 5) were synthesized by Naarmann et al. by Grignard c ~ u p l i n g . 'They ~ were characterized by standard methods.I9 The nT were solved in 1,Cdioxane (Aldrich, spectroscopic grade). Concentrations m o m in the solutions of 2T, 3T, and 6T1 and were c = m o m at 4T, c = 4 x moVL at 5T, and c = c =6 x 8 x moVL at 6T. Because of the poor solubility of 6T the transient absorption measurements were performed on 6T1. Test measurement showed that both the stationary and the transient absorption behavior of 6T and 6T1 are very similar. The cuvette thickness used with the transient absorption measurements was 5 mm.
3. Spectroscopic Features of nT Whereas the thiophene monomer absorbs at about 250 nm, the oligomer bands are located at longer wavelengths and shift bathochrom with increasing repeat units per molecule due to the lengthening of the conjugated system. The structureless absorption bands exhibit increasing absorption coefficients e with increasing oligomer size as shown in Figure la. The fluorescence bands also shift bathochrom with size, but they are well structured with a very similar band shape for all nT, as seen in Figure lb. At room temperature several nonplanar conformers were observed at the electronic ground state with small differences of their energy minima.38.39 Rotational vibrations of neighboring thiophene rings of the conformers cause the structureless absorption bands at room temperature. At low temperature the absorption bands are well structured
nl
,
-,
500 350 400 450 500 550 600 650 X /nm Figure 1. Absorption (a) and fluorescence (b) spectra of oligothiophenes (nT)in dioxane for n = 2-6 repeat units.
and reveal the mirror symmetry of the fluorescence bands which were proved to stem from a planar SI The reason for the different spectra of absorption and fluorescence is mainly the benzoic structure of the electronic ground state with a single bond between the neighboring rings, but the more quinoid structure of the first excited singlet state with double bond character of the ring connecting bonds. These features results also from quantum chemical calculations by QCFF of the electronic structure of nT at ground and excited electronic states, e.g. ref 45, where the rigid planar structure of the first excited singlet state was clearly demonstrated. The influence of solvents on both absorption and fluorescence spectra is insignificant as demonstrated for absorption in Table 1. The experimentally determined fluorescence lifetimes and quantum yields of the nT are summarized in Table 2 and were already given These experimental data were used to determine the radiative ( k ~ and ) nonradiative ( ~ N R ) decay constants of fluorescence states of nT. Moreover, we used a simplified Strickler-Berg formula for an independent determination of k R by the integral absorption of the lowest energy transition. This method is applicable exactly for molecular systems with mirror symmetry of absorption and fluorescence spectra. For nT both absorption and fluorescence belong to the same transition, however, because of the different band shape of absorption and fluorescence a certain deviation of radiative rate constants calculated from integral absorption and obtained from experiment has to be
J. Phys. Chem., Vol. 99, No. 46, 1995 16993
Size-Dependent Properties of Oligothiophenes
TABLE 1: Band Maxima and Absorption Coefficients of Oligothiophenes in Various Solvents nT I(dioxane)/nm E(dioxane)/M-' cm-' I(CHC13)N-' cm-I e(CHCl#M-' cm-l I(benzene)/nmz0 c(benzene)/M-I cm-' 2T
3T 4T 5T 6T 6T 1
306 355 392 414 435 426
14 OOO 25 OOO 32 000 42 000 50 000 46 000
302 355 390 416 432 425 (ref 53)
12 500 25 OOO 45 500 55 200 60 OOO
305 355 391 418
12 100
21 900 32 000 38 100
TABLE 2: Calculated Radiative Rate k ~ (and ~ )Oscillator StrengthfA) and Radiative and Nonradiative Rates k~ and ~ N R Determined from Experimental Fluorescence Quantum Yield QF and Fluorescence Lifetime cs of nT nT PA' kR(*)/10+9s-I @F tdns kR110+9s-1 kNR/10+9s-I 2T 0.8 f 0.1 0.27 f 0.04 0.001 (ref SO) 0.04 (ref 50) 0.25 25 0.10 (ref 50) 0.10 9.9 3T 1.4 f 0.2 0.35 f 0.05 0.056 (ref SO) 0.15 (ref 50) 0.37 6.3 0.53 f 0.05 0.38 f 0.1 1 1.5 4T 1.7 & 0.3 0.36 f 0.05 0.20 f 0.04 0.87 & 0.09 0.33 f 0.10 0.82 5T 2.4 f 0.4 0.44 f 0.07 0.28 f 0.06 1.04 f 0.10 0.32 f 0.10 0.64 6T 2.9 f 0.4 0.49 LI 0.07 0.34 f 0.07 1.1OfO.11 0.34 f 0.10 0.57 6T 1 2.8 & 0.4 0.48 f 0.07 0.37 f 0.07 expected. Nevertheless, they agree within the error limits of both methods. The radiative lifetime of the nT, except 2T, where the experimental accuracy was low, alters only a little (f6%). It seems to be almost independent on the molecular size within the nT series, including the alcyl derivative of 6T. In our opinion, the rigidized structure of the fluorescence state is the reason for this behavior. That means, the strong alteration of fluorescence lifetime and quantum yield with the repeat units n is caused by nonradiative processes, expressed by the nonradiative constant ~ N in R Table 2. With picosecond time-resolved spectroscopy we have an excellent tool to study radiationless processes. We have to search spectra of electronic or vibronic states involved in the radiationless energy degradation upon laser pulse excitation.
A D
0.81
-0.2' 430
" "
4. Results and Discussion
"
"
' "
" "
480
"
"
"
"
530 /nm
X
4.1. Transient Spectra. From determination of the radiative and nonradiative rate constants it is obvious that the former remains nearly constant, whereas the last decays more than 1 order of magnitude. Picosecond time-resolved spectroscopic measurements by pump and probe technique as described in the Experimental Section are suitable to study radiative and nonradiative processes as well. Figures 2-5 show the transient spectra of the optical density difference hD(A,t) in the visible spectral region at selected delay times after excitation for (3T - 6T). In Figure 2a the increase of the absorption band A1 during excitation with the 25 ps pulse at 349 nm is shown which reaches the optical density of 0.6 at the maximum wavelength of 600 nm. The negative hD values below 480 nm indicate the induced fluorescence F which also arises during excitation. Figure 2b shows the decay of A1 and F. The induced fluorescence F is superposed by the delayed appearing band A2. At 960 ps only this band was recorded. In Figure 3 the results of AD measurements on 4T are shown. Here the decay of the fast rising wide band A1 (600-800 nm) is to be seen at four selected delay times. At 973 ps this band is nearly vanished, whereas again a delayed band A2 emerges in the short-wavelength side at 560 and 595 nm. This band is partly superposed with the induced fluorescence F. Figure 4 demonstrate the processes of 5T. The A1 band at 845 nm decays more slowly than for 3T and 4T; therefore, at even 973 ps a defined AD value was recorded (Figure 4a). Figure 4b exhibits the short wavelength part of the ALI spectrum between 500 and 700 nm at 40, 200, 733, and 1933 ps. The A1 decay at the red side is recognizable; moreover,
"
'
"
'
I
" "
580
'
'
"
'
630
AD
0.7f
0.4 0.3
A1 -
-'
427 ps
-960 ps
(b) ' .
-0.21', 430 480 ,
,
,
,
,
"
X
"
"
"
"
530 /nm
"
"
"
"
580
'
"
'
"
'
630
Figure 2. Transient optical density difference spectra of 3T at various delay times. the whole fluorescence band of 5T was recorded. It decays, whereas the band A2, located between F and A1 at 635 nm, increases. Similar behavior was measured at 6T1. Figure 5a shows the decay of the wide A1 band and Figure 5b the arise of A2 during the decay of Al. The fluorescence of 6T1 is visible only at the beginning, later on it is covered by the A2 band. At all studied nT (n = 3-6) the first spectra which arise during absorption of the exciting pulse (fwhm 25 ps) are the
16994 J. Phys. Chem., Vol. 99, No. 46, 1995
Grebner et al.
A D I
1
0.6
600
650
750
700
X
800
/nm
A D
0.21,
A1 .. .
'1%
-0.05F -0.1 "
' ' ' , ' , , '
500
' ' ' , '
,
' ' '
, , , ' ' ' '
600
550
X
, , ' '
,
,
I
.
/nm
Figure 3. Transient optical density difference spectra of 4T at various delay times.
induced fluorescence F and the absorption A l . With a long delay the second transient spectrum A2 appears in the spectral region between F and Al. The increase of the band A2 is accompanied with the decay of Al. A decay of A2 was not observed within the checked delay time up to 2 ns. The transient spectra A1 and A2 of the oligothiophenes are drawn in Figures 6 and 7 in dependence on photon energy of the probe radiation. Both light-induced spectra shift with molecule size to longer wavelengths. The spectral positions and fwhm of A1 are summarized in Table 3 compared with the S I SOband parameter. It is recognizable that the A1 band width are smaller than those of the S I SO band except for 4T. The band parameters of A2 will be discussed later on. We tried to estimate the absorption coefficient E E S A for the excited state absorption on the following way. The excitation was carried out in a 5 mm thick sample of about M concentration. Under these conditions, holds the assumption that each absorbed photon of the exciting pulse at 349 nm forms an excited molecule during excitation, and it is possible to estimate the absorption coefficient or cross section of the excited state. To this aim the pulse energy and beam diameter on the entrance of the sample cell were measured. With (40 f 4) pJ pulse energy and (2.5 f 0.5) mm beam diameter one obtains a photon area density N p = (1.43 f 0.43) x lOI5 cm-*. The values obtained for the cross section UESA, and extinction coefficients are shown in Table 4 and compared to the values from ref 46. For 3T and 4T we could calculate also the oscillator strength of the excited state absorption. The values were comparable to that of the S I SOtransitions. Our values were determined with an deviation of about 30%, the estimations
-
-
-
200 ps
'
700
650
-0.06 -0.08 500
550
600 /nm
650
700
Figure 4. Transient optical density difference spectra of 5T at various
delay times. in ref 46 given without any deviation measure are 2-5 times higher. 4.2. Kinetic Behavior of the Excited-State Absorption. The A1 bands were measured at selected wavelengths near their maxima in dependence on time. All nT except 2T have been measured under identical experimental conditions with 349 nm excitation wavelengths and 25 ps pulse duration. However, 2T does not absorb at 349 nm. Therefore 2T was excited by 350 fs pulses at 308 nm using the femtosecond spectrometer described e l s e ~ h e r e . ~A' . logarithmic ~~ plot of AD versus time results in straight lines for all oligothiophenes for n = 2-6 (Figure 8). The decay times of these single exponential functions of nT are listed in Table 5 along with the fluorescence life times measured by various author^.^^.^^ Both these decay times of each member of the nT series are identically. For 2T the A1 decay time z = 51 ps yields the most reliable value, because the fluorescence data have been obtained with lower time resolution. The comparison of both fluorescence and absorption decay times demonstrates that the A1 band of each nT stems from the corresponding fluorescence state SI.In agreement with quantum chemical c a l ~ u l a t i o nand ~~ low-temperature this state is of 1B symmetry for 2T-6T. The attempt of an assignment of the observed excited state absorption to transitions between calculated states was also done successfully in ref 33. The increase of the S I
Size-Dependent Properties of Oligothiophenes
J. Phys. Chem., Vol. 99, No. 46, 1995 16995
AD
A D I a. u.
-40 ps -173ps --440ps -973 ps
800
850
X
5T
900
950 "
Inm
1.5
1.7
1.9
AD I ~9
ni
I
6T1
-
A1
0
nT
650
700
X
750
800
/nm
Figure 5. Transient optical density difference spectra of 6T1 at various delay times.
A D I a. u. 1.21
1
6T1 5T
4T
3T
ESA
2T
1
OS8l 0.6
1
0.4
I
1.1
1.5
1.9
2.3
E I eV Figure 6. Transient absorption bands A1 of the ( S , for oligothiophenes with n = 2-6.
2.7
-
Em,IeV (Ilnm)
SI
fwhd eV
- so
Em,,IeV (Ilnm)
2T 2.50 (4.95) 0.38 4.05 (306) 3T 2.07 (600) 0.14 3.49 (355) 4T 1.75 (710) 0.57 3.16(392) 5T 1.47 (845) 0.30 2.99 (414) 6T 1.37 (905) %0.31 2.85 (435) 6T1 1.38 (900) %0.32 2.91 (426)
I
U
2.7
TABLE 3: Spectral Positions and Widths (fwhm)of SI SI and RadicaPs5 Absorption Bands of Oligothiophenes
0.05
0.2
2.5
with a 349 nm laser pulse.
S,
1
2.3
2.9
Figure 7. Triplet absorption bands of nT recorded 2 ns after excitation
I
600
2.1
E I eV
0.151 "I
4T
.
, - - - /
750
6T
3.1
SI) transitions
lifetime t A ] with oligomer size, shown in Figure 9, exhibits a nonlinear increase of lifetime with ring number.
4.3. Formation of the Triplet Absorption. The bands A2 arise during the decay of the corresponding A1 bands, and then they remain constant in shape and value during observation time of 2 ns. These long-living transients could be identified as triplet absorption bands from the corresponding lowest triplet state TI of nT by comparison with l i t e r a t ~ r e . ~A~ -comparison ~~ of A2 with triplet bands at microsecond delay is shown in Figure 10 for 3T. Table 6 summarizes the data available for the other nT in comparison with our transient spectra A2. Band position and half width of the reported triplet absorption bands agree very well with our bands A2. By picosecond time-resolved studies we have observed the emergence of the triplet bands. The triplet formation proceeds during the depletion of the
fwhd eV 0.66 0.63 0.61 0.60 0.67 0.64
-
SO,
nT+ Emaxlev f w h d (Ilnm) eV 2.17 (572) 1.88(660) 1.74(712) 1.59 (780)
0.23 0.13 0.15 0.14
primary excited S I level. It was described by a simple model and corresponding rate equations.49 The A2 spectra, Le., the triplet absorption bands of nT, n = 3-6 are shown in Figure 7 at 2 ns. We tried to estimate the absorption coefficient as follows. Our results demonstrate a very effective triplet formation during the depletion of the excited singlet state S I , no hints for other radiationless processes were found. From studies of triplet and fluorescence quantum yield of nT23,34-37,51,54 it can be concluded that nonradiative decay processes are dominated by intersystem crossing. Rossi et al. found a triplet quantum yield of (PT= 0.95 and a fluorescence quantum yield of @F = 0.05 in the case of 3T. Oelkrug et al. found a stable value of 80% for 2T-6T for the sum of fluorescence quantum yield and the yield of produced singlet oxygen after quenching of the triplet state. In conclusion of this it can be assumed that other relaxation processes beside triplet formation and fluorescence are neglectible for all nT. Triplet and fluorescence quantum yields are also measured by Becker et al.54 for 2T7T. They found internal conversion rates which are very small in comparison to the intersystem crossing rates. We used directly measured triplet quantum yields as well as fluorescence quantum yields (@T = 1 - (PF) as a measure for the triplet formation (Table 7). On the other hand, from our picosecond measurements we know the AD values of S , SI absorption after excitation as a measure for the number of primary excited singlet states. At long delay time AD at the maximum of A2 is a measure for the triplets. Moreover, we have determined the singlet excited state absorption EESA. The ratio of both AD values
-
allows us to determine the triplet absorption coefficient UTA by using the known triplet quantum yields @T = [TI]/[SI].
Grebner et al.
16996 J. Phys. Chem., Vol. 99, No. 46, 1995
TABLE 4: Experimental Determined Excited-State Absorption Cross Sections of nT Compared with the Literature& nT 2T 3T 4T 5T 6T 1
Lmaxlnm
UEsAlcm2
495 600 710 845 900
5.32 2.63 2.33 3.13
E~AIM-'cm-l 139 000 f 42 000 68 600 f 20 600 60 800 f 18 200 81 900 f 24 600
x x
x x
Lm,x/nm46
fESA
EESAM-]
46
90 000 250 000 280 000 390 000 500 000 (6T)
498 603 720 836 890 (6T)
1.62 3.25
cm-I
AD 900 nm
- 960 ps, dioxane - . 4 ps, meth. chloride
-4 X
6T1
-5 0 0.1 0.2 0.3 0.40.5 0.6 0.70.8 0.9 1 t 1 ns
\.
0.021
!
,.
I
4415, '475, '525' ,575 ,%5 I
Figure 8. Logarithmic presentation of the excited state absorptions A1 of nT, AD(r), versus delay time t at wavelengths near the transient absorption maxima.
,
, ,
'
,
,
,
,
x /nm
Figure 10. Transient absorption A2 at 960 ps of 3T compared with triplet absorption bands at 2 and 4 p s d e l a ~ . ~ ~ . ~ '
rlns
TABLE 6: Triplet Absorption A2 Band Position and Half-Widths of nT at 2 ns ComDared with Literature Data TTA A2
ZTTAl
~
2T 3T
4T
nT
E,,,,,/eV (d/nm)
fwhd eV
2.70 (460)
0.43
2.21 (560) 2.08 (595)
0.35
1.97 (630) 1.82 (680)
0.21 0.18
E,,,/eV (hm) 3.35 (370) 2.75 (450) 2.64 (470) 2.70 (460) 2.72 (455) 2.19 (565) 2.03 (610) 2.21 (560) 2.05 (605) 1.91 (650) 1.82 (680)
fwhd eV
PS (ref)
0.75 0.50 0.40 0.41 0.40(37) 0.32
27 (51) 30 (51) 50 (35) 57 (36)
0.25
43 (36)
0.22 0.23
8 (52) 24 (35)
35 (35)
Figure 9. Excited state lifetime tAmeasured by transient absorption kinetics in dependence of the repeat units of the thiophene oligomer.
5T 6T (6T1)
TABLE 5: Comparison of Absorption Decay Times Z A with ~ Fluorescence Lifetimes TS
or double bond number. This graphic yields a fast convergence of values with lln to polymer chains of infinite length. More subtile is the question at which finite number of monomers the optical properties does not underlie further alteration. The answer to this question is of practical and theoretical importance, as well. It gives hints for practical preparation of optical devices and allows to prove theoretical models. For our studies of the series of small oligomers, n = 2-6, we utilized various models for the representation of size dependence, e.g., the Lewis-Calvin formula for the absorption wavelengths ilrz z/n , the energy gap dependence AE lln , and the extended FEMO model of Kuhn and Kuhn47,48which yields for the energy gap
nT
ZAIIPs
tdps
2T 3T 4T 5T 6T 1
(51 f 5) (135 13) (531 f 53) (880 f 88) (1018 4r 102)
40/100 (ref 50) 150 (ref 50) (530 f 53) (870 f 87) (1100 i 110)
*
Table 7 contains the triplet absorption coefficients estimated as described. The values obtained for 3T by quite different method^^^-^^ agree within the error limit with our values.
5. Size Dependence of Stationary and Transient Spectra of Oligothiophenes Size-dependent steady-state spectroscopic properties of thiophene oligomers have been reported by many authors. The goal of our studies was the behavior of excited levels in dependence on the oligomer size. In literature mostly can be found a graphic representation of spectral energies in dependence on the inverse of the monomer
-
h2 AE=-(2DB 8mL2
1 + 1) + VO( 1 --2DB)
(3)
where h is Planck's constant, m the electron mass, L the box length, DB the double bond number, VOthe constant value). This formula was designed for a linear polyene chain. We tried to apply it for the oligothiophenes and found that this
J. Phys. Chem., Vol. 99, No. 46, 1995 16997
Size-Dependent Properties of Oligothiophenes
TABLE 7: Triplet Quantum Yields and Extinction Coefficients of nT 2T 3T
0.99 f 0.20 0.93 f 0.19
4T 5T 6T 1
0.80 f 0.16 0.72 f 0.14 0.63 f 0.13
0.93 (54) 0.95 Z!C 0.19 (ref 37) 1.01 f 0.20 (ref 51) 0.90 (54) 0.71 (54) 0.63 (54)
3:
2.5: 2:
RAD
1.5:
A
6
4
8
10
12
14
16
DB versus number of repeat units.
TABLE 8: Fit Parameters and Effective Bond Lengths of the Electron Boxes of Thiophene Oligomers at Ground and Excited States (See Text) AleV vo least-squares sum d/lO-’O m Rad TTA ESA
39 OOO f 19 500 38 400 f 19 500 65 OOO Z!C 32 500
6. Summary
€SA
Figure 11. Energy gaps of stationary and transient absorption of nT
Abs Fluo
34 500 = 7 OOO i (ref 37) 36 799 f 5 OOO (ref 37)
absorption appears smaller, but in that of singlet absorption larger. A qualitative explanation is possible: radical ions “feel” a stronger Coulomb field because one electron is missing, therefore, the box will be smaller. At the triplet configuration the parallel spins need different orbitals (perpendicular to the bond axis), probably this triplet orbital configuration shortens the box, as well. At excited singlets the box appears larger because of the greater distance of electrons to the nucleus back bone.
3.5F
1 2
37 400 f 18 700
2.167 1.750 0.797 0.942 0.747
19.41 19.03 20.10 22.96 17.65
3.55 x 10-4 2.41 x 1.99 x 10-3 6.49 x 10-3 1.29 x
1.39 1.41 1.37 1.28 1.49
formula excellently fits our experimental data. We tried to fit the stationary and the transient spectroscopic transition as well with the different functions mentioned above. Using Kuhn’s formula, we obtained in any case the lowest least-square sum which was clearly less than by fitting with the other formulas. The value L in Kuhn’s equation is the electron box length of the oligomer given by the sum of all bond length of the linear polyene chain extended by the terminal regions, i.e. L = (2DB 1)d with a standard average value do = 1.4 8, of a linear polyene. Equation 3 now gets the form
+
AE=- A
n+l
+ V0(l -);
(4)
using 2 DB = n and
This equation is with except of the constant VOnearly identical with the simple ( l h ) function. Nevertheless the experimental results fit better with this formula. The results are drawn in Figure 11. The curves for absorption and the fluorescence run parallel as expected; however, the transient singlet and triplet absorption and radical ion absorption decay are steeper with n. The optimized parameters A and VOare shown in Table 8. From A we estimated the corresponding bond length. It is surprising that the electron box in the case of triplet or radical
The thiophene oligomers show the following features after excitation by a picosecond laser pulse (349 nm). By absorption of the short UV light pulse (25 ps) the occupation of the S I state occurs. It is demonstrated by the absorption bands A1 in the visible region which correspond to S, S I absorptions. These bands were identified by their time course, they exhibit the same single exponential decay constants as the fluorescence, Le., A1 stems from the fluorescent state. The decay of A1 is accompanied with the increase of the bands A2 located nearby the fluorescence bands for all studied thiophenes. These long-living bands were identified, by comparison with literature, as triplet absorption bands of the lowest triplet state. We have observed the formation process of triplet absorptions at the oligothiophenes for the first time. The formation proceeds during the depopulation of the SI level. The lifetime of the excited S I state and the formation time of the T I state, increase with the thiophene oligomer size from 50 ps at 2T to about 1 ns at 6T. These results are in accord with the observed increase of the fluorescence quantum yield @F with the oligomer size, whereas the triplet quantum yield decreases. The triplet formation is the main, probably the unique radiationless process which depletes the S I state, except the fluorescence emission. We observed size-dependent effects not only at the stationary absorption and fluorescence bands, but also at the transient bands A1 and A2. All bands show systematic long-wavelength shifts which can be described excellently by the extended FEMO model of K~hn.4’,~* The extension of the function AE(DB) allows the estimation of an effective conjugation length of polythiophene derivatives in solution on the basis of the well-known absorption and fluorescence bands of this materials. Using the values of the short wavelength fluorescence maxima of poly(3-hexylthiophene) in solution?5 we found an effective conjugation length of about 10 thiophene rings. That means larger oligomers show no optical relevance.
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