Optical absorption in the substituted PPVs: theory and experiment

Synthetic

Optical

absorption

Metals

84 (1997)

in the substituted

603604

PPVs:

theory

and experiment

M. Chandrossn, S. MazumdaraIb, M. Lie&, P.A. Lane’, Z.V. Vardeny’, M. Hamaguchid and K. Yoshinod aDepartment d Department

of Physics ‘Department of Electronic

and ‘Optical of Physics, Engineering,

Sciences Center, University of Arizona, Tucson, AZ 85721, l,BA University of Utah, Salt Lake City, UT 84112, USA Faculty of Engineering, Osaka University, Osaka 565, Japan.

Abstract We investigate theoretically and experimentally the effects of (2,5) ch emical substitution on the optical absorption in the phenylene-based conjugated polymers. The effects of the substituents on the optical absorption spectrum are weak. The 3.7 eV band is predicted to be polarized predominantly along the polymer chain axis. This prediction is confirmed experimentally. Keywords: Poly(phenylene vinylene) models and model calculations

and derivatives;

Optical

1. Introduction The absorption spectra of the polyphenylenes are characterized by multiple bands that extend well into the ultraviolet [l-3]. Recent theoretical work [2-51 has stressed the importance of fitting the absorption spectra of the polyphenylenes over the entire experimentally accessible energy range in order to determine important material parameters such as the exciton binding energy, triplet-triplet energy gap, etc. The bulk of the existing experimental data, however, are for the (2,5)substituted derivatives, and thus it is crucial to distinguish and features in the optical spectra that result from chemical substitution from those that are characteristic of the r-conjugation network. The optical absorption spectra of poly(paraphenylene vinylene) (PPV) and its derivatives show four distinct bands at 2.1-2.4, 3.7, 4.7, and 6.0 eV [2,3,6]. Three out of these four bands are visible in the absorption spectrum of the unsubstituted material [2,6], but in the 3.7 eV region only a weak shoulder on the broad low energy band is seen. Whether or not the feature at 3.7 eV corresponds to a distinct band in the unsubstituted material is not clear from the absorption spectrum alone. This band has been either assumed to be a characteristic of the r-electron network [2,3,6], or ascribed to the broken charge conjugation symmetry 0379-6779/97/$17.00

PII 80379-6779(96)040714

Q 1997

Elsevier

Science

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absorption

and emission

spectroscopy;

Semi-empirical

(CCS) in the substituted materials [5]. In this work we demonstrate that substituents have a weak effect on the absorption spectra and that a true band exists at 3.7 eV in unsubstituted PPV. 2. Theoretical Our theoretical Pariser-Parr-Pople

,a

model calculations are performed within a type Hamiltonian, H, written as,

8

+U ‘7;7 ni,tni,l + f C Kj(na - l)(nj - 1). (1) i a,j We take t = to = 2.4 eV for the phenyl rings and tl(t2) = 2.6(2.2) eV for the vinylene linkage. The parameter E is a site energy term that modifies the realtive site energies of the carbon atoms directly bonded to the substituent groups [7]. We take E = -1.0 eV, which simulates a strong substituent effect [6]. The parameters U and V$ describe the on-site and intersite Coulomb interactions, respectively. We take Kj to be long range and of the form (2)

604

h4. Chandross

et al. /Synthetic

where Raj is the distance in A between carbon atoms i and j. The parameter K determines the decay of the long range part of the potential and is necessary to arrive at a quantitative fit of the experimental absorption spectrum [8]. Th e values U = 11.13 eV and K = 1 correspond to the Ohno parameterization. We use a single configuration interaction (CI) approach with the values U = 8.0 eV and K = 2, which give both a quantitative and qualitative fit to all four absorption bands [8]. 3. Results Within the Hiickel model, unsubstituted PPV has four valence bands (v.b.) and four conduction bands (cb.), out of which one v.b. and one c.b. are localized (I and i”), while the remaining bands are delocalized (d and d*). The outermost d and d’ bands play weak roles in optical absorption, and in what follows we focus on the innermost d and d* bands and the 1 and 1* bands. The Hiickel model predicts only three transitions: (i) d -+ d* (x-polarized), (ii) d + I* and I + d* (y-polarized), and (iii) 1 --i I’ (x-polarized). Note that band (ii) occurs exactly at the center of (i) and (iii). The considerable shift of the 4.7 eV band from the center of the strong 2.4 eV band and the 6.0 eV band indicates the strong role of Coulomb interactions. For nonzero Coulomb interactions, the degenerate d + I* and 1 -+ d* states split into d + 1%f 1 * d* states. Transitions to the “plus” combination gives the 4.7 eV band seen experimentally. The “minus” combination, which occurs at or near 3.7 eV is optically forbidden. There are now two models that attempt to explain the observed 3.7 eV band. Within the broken CCS model [5], the 3.7 eV band is a consequence of substitution, which induces mixing between the above ‘(plus” and “minus” combinations. The “minus” combination acquires oscillator strength at the expense of the “plus” state. We have performed explicit calculations within Eq. 1 that show that any such mixing is weak, as long as we insist that the energies of the 2.4, 4.7 and 6.0 eV bands are nearly unchanged by substitution. A far stronger effect of substitution is the acquisition of delocalized character of the erstwhile localized bands. The loss of localized character leads to CI between the d + d* and the “minus” combination of d ---) I* and 1 ---f d*. Within this second model, the 3.7 eV band is a consequence of finite chain lengths in experimental PPV - optical absorption calculations in the unsubstituted material within Eq. 1 find both a 2.4 eV band due to transitions to the lB, exciton and a 3.7 eV “finite size band.” From explicit calculations within Eq. 1, substitution causes the previously forbidden d + 1’ - 1 + d* states to acquire oscillator strength from the “finite size band.” Within our model, the 3.7 eV band is therefore x-polarized, in contradiction to the broken CCS model

Metals

84 (1997)

603-604

which predicts this transition to be y-polarized [5]. We have measured the polarized absorption of oriented poly(2,5-dinonyloxy-1,4-phenylenevinylene,) (NO-PPV), and confirmed this result. Fig. l(a) shows the experimental difference optical density (OD) obtained by subtracting the OD perpendicular to the chain axis from that in the parallel direction. The spectrum clearly shows that bands I and II are strongly xpolarized while band III is y-polarized. Fig. l(b) shows the calculated difference spectrum. The experimental spectrum is in excellent agreement with the theory, thus confirming the analysis above. A similar polarization dependence is given in reference 9.

Figure 1. (a) Experimental and (b) theoretical tion of oriented PPV. See text.

absorp-

4. Acknowledgments We thank V. Massardier for the unsubstituted PPV films. Work at Arizona was supported by the NSF (Grant No. ECS-9408810), the AFOSR (Contract No. F49620-93-l-0199), and the ONR (Grant No. N0001494-l-0322). Work at Utah was supported by the DOE (Grant No. DE-FG-03-93.ER45490) and ONR (Grant No. N00014-94-1-0853). REFERENCES 1. D.A. Halliday et al., Synth. Metals 55-5’7, 954 (1993). 2. J. Cornil et al., Chem. Phys. Lett 223, 82, (1994). et al., Phys. Rev. B50, 14702, 3. M. Chandross (1994). 4. M.J. Rice and Yu. N. Gartstein, Phys. Rev. Lett. 73, 2504 (1994). 5. Yu.N. Gartstein, M.J. Rice and E.M. Conwell, Phys. Rev. B52, 1683 (1995). 6. M. Chandross et al., submitted to Phys. Rev. B, 1995. and H.C. Longuet-Higgins, Proc. 7. J.N, Murrell Phys. Sot. (London) A68, 329 (1955). and S. Mazumdar, submitted to 8. M. Chandross Phys. Rev. B, 1996.