Experimental and theoretical studies of absorption and photoluminescence excitation spectra of poly(p-phenylene vinylene)

SyntheticMetals 74 (1995) 7-13

Experimental and theoretical studies of absorption and photoluminescence excitation spectra of poly(p-phenylene vinylene) Jenwei Yu a, J.H. Hsu a, K.R. Chuang b, C.L. Chao b, S.A. Chen b, F.J. Kao ‘, W.S. Fann a, S.H. Lin a aInstitute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan, ROC b Department of Chemical Engineering, National Tsing- Hwa University, Hsinchu. Taiwan, ROC ’National Yat-Sen Sun University, Kaohsiung, Taiwan, ROC Received 14 September 1994; revised 23 February 1995; accepted 23 March 1995

Abstract In this paper, we present the experimental absorption spectra and photoluminescence excitation spectra of poly(p-phenylene vinylene) (PPV) . We also present the theoretically calculated absorption and photoluminescence spectra of PPV. The singlet-to-singlet transition energies and moments calculated from the intermediate neglect of differential overlap with the spectroscopic parametrization (INDO/S) semi-empirical method were used to analyze the absorption spectra. The symmetrical vibrational modes calculated from the abinitio method were used to obtain the vibronic features in the spectra of PPV. It is demonstrated that the inhomogeneity observed in the absorption spectra of PPV can be modeled by summing up the absorption of PPV oligomers with different chain lengths. The difference between absorption and photoluminescence excitation spectra is explained. Keywords: PoIy(p-phenylene vinylene); Spectra; Photoluminescence

1. Introduction Conjugated polymers have been studied extensively both experimentally and theoretically, due not only to their potential applications but also to their interesting electronic properties and geometric morphology. One of them, poly(pphenylene vinylene) (PPV), has attracted much attention recently due to its lighting-emitting properties [ I]. PPV with different compositions and structural components has been characterized by different spectroscopic techniques, e.g., UV absorption, photoluminescence (PL), photoluminescence excitation (PLE) , site-selective fluorescence, IR and Raman spectroscopies [ 2-91. Recently, we have performed a theoretical study of singletto-singlet and triplet-to-triplet electronic transitions in PPV using both the intermediate neglect of differential overlap with spectroscopic parametrization (INDO/S) method and molecular exciton theory [ lo]. The results are consistent with the experimental data and help explain the triplet exciton observed when the long time scale spectroscopic technique is used to study PPV. Similar INDO/S calculations have also been done by Cornil et al. [ 11 I. They presented theoretically calculated absorption spectra of PPV and of dimethoxysubstituted PPV oligomers. However, in these spectra 0379-6779/95/$09.50 0 1995Elsevier Science S.A. All rights reserved SSDI0379-6779(95)03335-H

Absorption j-Y,,,

.z 3 %

15000

20000

25000

30000 35000 WaveNumber

40000

45000

Fig. 1. Experimental PPV absorption (dotted) and PLE (solid line) spectra

at 300 K. the vibronic features and inhomogeneity are not considered. In this report, experimental absorption and PLE spectra, which are similar to previous studies [7], are presented (shown in Fig. 1). The inhomogeneity observed in the electronic absorption spectra of PPV, e.g. the wide first absorption

J. Yu et ~1./Synthetic Metals 74 (199.5)7-13

8

band in the range 20 000-35 000 cm- ’and the kink observed atabout OOOcm-‘, 1sanalyzed theoretically. These observations have been considered to come from the inhomogeneity caused by the PPV segments with different chain lengths (here we will use simply ‘chain length’ to represent ‘effective conjugation length’ observed in PPV polymers) and electron-phonon coupling (i.e. vibronic features) [ 21. We demonstrate theoretically that these effects can create absorption band shape similar to the experimental result. The theoretical absorption spectra which are calculated using the INDO/S method are presented. In order to determine the vibronic features, the ground state vibrational frequencies of PPV monomers and dimers and the first excited state vibrational frequencies of PPV monomers are calculated using the Gaussian 92/DFT program at the SCF level [ 121. For simplicity and demonstration purposes, only one of the totally symmetric CC stretching modes is used to obtain the vibronic features. The calculated PL of PPV tetramers, pentamers, and hexamers is also presented. Finally, a simple kinetic model is given to explain the difference observed between absorption and PLE spectra.

2. Experimental PPV was prepared according to a modified Wessling procedure [ 13,141 as shown schematically:

OH- _ ooc

2

A clean fused silica plate was immersed in the above purified polyelectrolyte solution. This plate was then removed from the solution and dried in a dust-free environment. Subsequently, a thin film of the precursor polymer was converted to PPV 3 by heating under dynamic vacuum at 250 “C for 6 h. The film thickness was measured mechanically to be about 3000 A. The absorption spectrum was measured by a Hitachi U3200 spectrophotometer with substrate (fused silica) as the reference. Using a Hitachi F-4010 fluorescence spectrophotometer, the PLE spectrum was measured by scanning the excitation wavelength, while monitoring the emission intensity at the sample’s luminescence region. A long-pass color glass filter (newport GG.400) was put on the luminescence inlet to eliminate scattered excitation light from the grating’s higher-order diffraction. The PLE spectra are practically the same while monitoring at different emitting wavelengths (530,553,570 nm). Fig. 1 shows the optical absorption and PLE spectra. The PLE spectrum was taken at emission wavelength 553 nm with 5 nm bandwidth, while scanning the excitation wavelength with 1.5 nm bandwidth. However, when the excitation wavelength is close to the luminescence wavelength, the spectrophotometer cannot reject scattered excitation light. Hence, the PLE spectrum shows an upward tuning at the low energy side (below 19 000 cm-‘), and the PLE was not measured below 18 000 cm-‘. Since the spectrophotometer measures the absorption by transmission intensity change, we have also compared the reflection spectrum of the sample with a bare fused silica to estimate the influence of the reflection on the absorption. The ratio of ‘change in reflected light’ versus ‘the absorbed light’ is less than 2% above the absorption edge of the sample. Hence, the reflection from the sample has very little influence on the overall absorption spectrum measurement.

3. Theory and computation The molecular absorption coefficient for the electronic transition from a to b states is given by [ 151

in vacuum 2500c

:

+Q--!k

3

First, p-xylylene-bis( tetramethyl sulfonium chloride) was obtained by reaction cu,cY’-dichloro-p-xylylene at a concentration of 0.75 M with excess tetrahydrothiophene (2.25 M) at 50 ‘C in a methanol/water (8/2 vol.) solution for 20 h. Subsequently, the polymerization reaction to form the precursor poly(p-xylylene-cu-tetramethyl-sulfonium chloride) (2) was carried out in aqueous solution by a reaction of the monomer with an equimolar quantity of sodium hydroxide (0.2 M) at 0 “C for 1 h under rigorous anaerobic conditions. The polyelectrolyte was separated from the residual monomer and NaCl by dialysis tubing (from SpectraCo., Pro 3), which has a lower-limit molecular weight cut-off of 3500 Da.

XW%d,w- @I

(1)

where a is a factor which describes the medium effect, (v, v’) are the vibrational quantum numbers, cc,, is the electronic transition moment, Pa, is the Boltzmann distribution function, I(@,, I O,,) I * is the Frank-Condon factor, c is the speed of light and D( %,,,a,- w) is the lineshape function. The emission intensity for the electronic transition from b to a states is given by

(2)

9

J. Yu et al. /Synthetic Metals 74 (1995) 7-13

Drr(ob,,,ao - w> = Q(%,av-W)

bf

r bv’,aO (%a+ y’“v- w>2

bv’.aO+

r bo,av

= ip

(7)

b0,av+(%a-YWu-W)2

Fig. 2. Structure of PPV monomers.

For displaced oscillators, the Frank-Condon factor at temperature 0 K is given by I(o,,10.0)12=n.~eXp(-Sj) j I’

where S, are coupling constants for thejth vibrational mode. The Lorentzian function:

r

D(ww,,av- WI= : p bv',av+

bu’,av

(%u’,av-w)2

used for the lineshape. For simplicity, we use a single symmetrical vibrational mode to calculate the vibronic features. Hence, for absorption Eq. ( 1) reduces to

is

47?UJ %I( a) = 3cfc IcclX12~ cv’, exp( -S) D,(%,,,,,-o) Y’ .

(5)

and, similarly, for emission Eq. (2) becomes I,,(w) =

s Iccba12C$expC -3

Q(u~o.~~-w)

Y

The lineshape functions are now

(6)

where %, is the energy difference between electronic states a and b, and w, is the vibrational frequency. The ground state equilibrium geometries and vibrational frequencies of monomers and dimers are calculated at the SCF level using the 6-31G basis sets in Gaussian 92/DFT. The geometry of the first excited state of monomers is optimized using mono-excited configuration interaction (CIS) and vibrational frequencies are also obtained. We will not scale the vibrational frequencies by the usual factor of about 0.9 since it is not essential at the present level of study where we determine the coupling constant, S, by fitting to the experimental results. Recently, Zgierski and co-workers have made detailed vibronic studies of benzene [ 161, thiophene [ 171 and polycyclic aromatic hydrocarbons [ 181, in which the properties of ground and excited states and the coupling constants were obtained theoretically. Although it would be interesting to see theoretical results from such calculations, considering the nature of system we studied here, i.e. the numerous factors which can cause structural and compositional variations of PPV polymers, we will not pursue such a study and, instead, demonstrate that qualitative spectra can be obtained through gas phase calculations.

4. Results and discussion The structure of PPV monomers is shown in Fig. 2. It belongs to the Cs symmetry point group, which has only A’ and A” irreducible representations. The ab initio optimized ground and first excited state parameters of the monomers are given in Table 1. The optimized ground state parameters of dimers are essentially the same as those of ground state monomers. One can see that, in the excited state of the monomers, the bond lengths of C1C2,C3C4,C&, C,C6 and C&!, are longer than those of the ground state, whereas the C2C3, C,C5 and C& bonds are shortened. In principle, using the information presented in Table 1, it is possible to calculate the coupling constants S,. From the ab initio CIS calculations, the major contribution to the excited state wavefunction is the highest occupied molecular orbital (HOMO) + lowest unoccupied molecular orbital (LUMO) excitation, i.e. r+ r* excitation. A schematic plot of the HOMO and LUMO molecular orbitals of

Table 1 Structure of monomers (units ate in A)

Ground state

1.327

1.477

1.394

1.387

1.386

1.373 1.430

Exp. [ 191

1.318

1.449

1.394

1.381

Excited state

1.385

1.402

1.442

1.361

1.390

1.384

1.397

1.397 1.395

1.377

1.468

.I. Yu et al. /Synthetic Metals 74 (1995) 7-13

10

ground state monomers is shown in Fig. 3. It can be clearly seen that the C1C2 double bond character is destroyed and v bonding character is built up in C&, C,C, and C&s after the HOMO --) LUMO excitation. Since the major structural changes are between C,, Cz, Cs, C4 and Cs, it could be expected that normal modes which have stretching motion between these carbon atoms contribute to the electron-phonon coupling. Several frequencies of totally symmetric normal modes

LUMO

HOMO

Fig. 3. Schematic pictures of LUMO and HOMO of monomers. The circles represent 2p, atomic orbitals of carbon atoms looking from the direction of the z axis. The shaded ones are negative lobes of 2p, Table 2 Normal mode frequencies of the ground and excited state of the monomers, and of the ground state of the dimers (units are in wavenumbers) Monomer (ground)

Monomer (excited)

Dimer (ground)

1862 1809 1778 1681

1787 1691 1638 1630

1873,186O 1819.1808 1778.1754 1702, 1681

Fig. 4. Ground state vibrational normal modes of monomers. The carbon amplitudes are magnified by a factor of 3.5. The frequencies are (a) 1862, (b) 1809, (c) 1778, (d) 1681 cm-‘.

(modes with A’ symmetry), which correspond to the acetylinic C=C stretch and benzene motion, of monomers and dimers are listed in Table 2. The normal modes are shown in Figs. 4(a)-(d) for ground state monomers. For the ground state of monomers, the 168 I cm-’ mode is IR active and for the first excited state of monomers, the 1691 cm-’ mode is IR active. Since the frequencies are about the same, the displaced oscillator approximation can be used for calculating the vibronic features. Also, from the ground and excited state frequencies of monomers listed in Table 2 one can see that

Fig. 5. Ground state vibrational normal modes of dimers. The carbon amplitudes are magnified by a factor of 3.5. The frequencies are (a) 1873, (b) 1860 cm-‘.

J. Yu et al. /Synthetic Metals 74 (1995) 7-13

10000

15000

20000

25000

Energy

oooi

35000

process and the inhomogeneity caused by the different chain lengths of PPV, there are other effects which also induce inhomogeneity. A more detailed figure of Fig. 6(b) is shown in Fig. 6(d). In Fig. 6(d), the solid curve is the total absorption with contributions from tetramers, pentamers and hexamers. The vertical dash-dot lines represent the relative oscillator strengths of these oligomers from Ref. [ IO]. For example, the three vertical lines between 20 000 and 2.5 000 cm-’ indicate the absorption strengths of the first singlet excitations of hexamers, pentamers and tetramers. The small vertical lines which appear at higher energies are due to higher singlet excitations of these oligomers. Since the exact amounts of these oligomers are not known and considering that the range of (effective conjugation) chain lengths is about 6-10 [4,5], equal amounts of them are assumed. We have also considered the contribution from shorteroligomers; however, due to their small oscillator strengths the total absorption does not change much from that shown in Fig. 6(d) . One can note the qualitative agreement between this plot and the experimental plot in Fig. 1. Under the single vibrational mode approximation, if only one type of oligomer, e.g. hexamer, is used for the calculation, the kink cannot be produced in the plot. This suggests that the kink may be due to the inhomogeneity of chain lengths in PPV polymers. However, this kink may also come from the multimode vibronic coupling between the ground and excited states and we are currently investigating this effect.

40000

(cm-‘)

/ L

30000

:\ :

d

25000

11

I

30000

35000

40000

Fig.6.TheoreticalabsorptionspectraofPPV:(a)S=O.S;(b)S=0.8;(~) S = 2.5 ; (d) enlarged plot of(b) The INDO/S transition energies are shifted by 5000 cm-’ to lower energy.

the force constants in the excited state are smaller than in the ground state, indicating that the conjugation in monomers in the excited state is weakened. The monomeric units in the optimized structure of dimers are essentially the same as monomers. For the normal modes listed in Table 2, one can see that the normal modes of dimers form pairs with small frequency separation. One of the pairs is shown in Fig. 5. The small frequency separation indicates weak coupling between the vibrational modes in each monomeric unit. We use the 168 1 cm- ’mode of the excited state monomers for calculating the vibronic feature and band shape in the absorption spectra and in the PL. The calculated absorption spectra, with contributions from trimers, tetramers, pentamers and hexamers, is shown in Figs. 6(a)-(c) according to Eq. (5) with r= 900 cm- ‘. We see that S = 0.8 gives a better fit to the experimental results. The large magnitude of the r damping constant indicates that, in addition to the dephasing

0

5000

10000 Energy

Fig. 7. Theoretical hexamer.

15000

20000

25000

30000

(cm I)

PL of PPV oligomers:

(a) tetramer;

(b) pentamer;

(c)

J. Yu et al. /Synthetic Metals 74 (1995) 7-13

12

the presence of the factor kZ1I ( k2” + k2,), which is usually smaller than unity. However, when k21 % k2,,, i.e. when the vibrational relaxation kzl is much faster than the electronic process k2,,, this factor becomes unity and we obtain Eq. ( 11) . In this case, we can see that the PLE spectra are proportional to the absorption spectra. In other words, the fact that the PLE spectra and absorption spectra of PPV are different indicates that electronic processes in PPV exist which are faster than or comparable with vibrational relaxation. For g -+ 3 excitation, the steady state approximation yields Fig. 8. Schematic diagram of PLE3processes, k,, designates the absorption process and k,, represents the emission. k,, and k,, are the vibrational relaxation rates. k3,z,k, and k,, are the electronic non-radiative decay rates.

The calculated PL spectra of tetramers, pentamers and hexamers of PPV are shown in Figs. 7(a)-(c). These results are obtained using Eq. (6) with r= 400 cm-‘. Since the r+ n* excitation energies decrease as the (conjugation) chain lengths increase, the O-Obands of PPV PL show progressive red shift. The calculated PL spectra of hexamers agree well with the experimental PL spectra of well-ordered PPV at 15 K [9]. From the experimental spectra (Fig. 1) one can notice that the PLE spectrum has a much narrower width than the absorption spectrum. In a PLE spectrum, varying laser frequencies are used to excite the sample, and the intensity at a fixed frequency in the PL of the sample is monitored. This process is shown schematically in Fig. 8, where a stepping-down model is used to indicate the relaxation and this model is valid for harmonic oscillators. If the excitation laser excites PPV to state 2, the rate equations according to Fig. 8 are d[21 dt

=kg2kl

-&“[21

-b,[21

411 -

=k21[211 -hg[ll

-kJll

dt

That is, we use the emission 1 * g as a probing process and the pumping laser for excitation g -+ 2 can vary. Applying the steady state approximation, the concentrations of excited states are [21=

(9) [II=

Thus, for the emission intensity we have Z,=k,,[l]

=k,,$&-+

In

lg 2n

21

kl

(10)

where k,, represents the radiative rate constant. If k2, P k2” Eq. ( 10) reduces to 4=k,g[ll

‘kg2 k ‘Tk [gl In lg

(11)

From Eq. ( lo), we can see that in general the PLE spectra (i.e. Z,) will be different from the absorption spectra due to

[31=

k,,,?k32

“I

(12)

[21=

[II=

j-J&-g

[2] = -&---!z[3] ki,+k,, kzn+kz

and the radiation intensity is &=k,gEU =kl, 21

=k,g&k

In

32

Ig

k

[21

[3]

(13)

kl,+krg kzn+kzr kl, kz k -= g3 k,, + kl, k,, +ka

k,, k3n + kx

kl

For the general case, i.e. g--t rn excitation, the intensity is Z,=k,,t 11 =~g,[glfi

k, yi.1..

m

1.1 1

(14)

Thus, one can see that PLE spectra will be sharper and narrower than absorption spectra since, as the pumping laser is tuned to higher energy, the PLE intensity, Z,, is multiplied by more kj,i_ 1/ (kin + k;,i_ 1) factors which are all smaller than unity. This feature can be clearly seen in Fig. 1. 5. Conclusions The experimental absorption spectra and PL of PPV can be qualitatively reproduced theoretically. The width of the first absorption bandshape can be partially accounted for by including the vibronic features created by the electron-phonon coupling. The kink in the PPV absorption spectra may be caused by the absorption of PPV segments with different chain lengths. By using harmonic approximation to the nuclear vibrations, where a stepping-down relaxation model can be applied, the difference between the absorption and PLE spectra can be explained using a simple kinetic model. Acknowledgement

This work was supported by the National Science Council of the Republic of China, Contract No. NSC 83-0208-M-001039PL.

J. Yu et al. /Synthetic

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