4P055
Optical and geometrical properties of oligomers of poly-para-phenylene vinylene(PPV) and poly-para-phenylene (PP): A SAC-CI study
Biswajit Saha, Masahiro Ehara, Hiroshi Nakatsuji
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, JapanThe optical and geometrical properties of the oligomers of poly-para-phenylene vinylene andpoly-para-phenylene which are frequently used as an active layer for polymer-based light-emitting
diode (LED) have been investigated by the SAC-CI SD-R method. The excitation energies for thelowest singlet and triplet excited states of PPVn andn, n = 1 - 4n, n = 1 - 4
Poly-para-phenylene vinylene (PPVn)

PPn consisting of 1 - 4 repeat units are calculated withplanar C2h symmetry constrained for PPVn and with

Poly-para-phenylene (PPn)non-planar D2 and planar D2h symmetry for PPn. The doubly excited state, 21Ag, of PPV1 wasstudied by SAC-CI general-R method considering up to quadruple excitation operators.1 The SACCI geometry optimization was performed for PPV1, PPV2, PP1 and PP2. For all these calculations
double-zeta basis set due to Huzinaga/Dunning plus one polarization function [4s2p1d/2s] wasadopted.
The transition energies were calculated accurately; maximum
length dependence of transition energies

2.6

for both singlet and triplet excitations

2.4

was reproduced correctly as displayed in

2.2

figures 1 and 2, respectively, althoughearlier

SAC-CIDMRG (Ref. 5)
MRSDCI (Ref. 6)
MRSDCI (Ref. 6)

2.0

1.8

123

MRSDCI

4

Number of repeat unit (n)Fig. 2 Triplet excitations in PPVn.

S0-S1 excitation energy (eV)SAC-CI
Expt. (Ref. 9)
TDDFT (Ref. 4)

5.0(for(forS0→T1)

S0→S1)

3.8
0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Inverse of the number of repeat unit (1/n)

Fig. 4 Singlet excitations in PPn.

3.4
3.2
3.0
2.8
2.6

predicted at lower energy (~1.0-1.5 eV

3.6singletstate

triplet

which
state

indicates

with

large

exchange energy. The present SAC-CI

4.0

3.6

123

Number of repeat unit (n)4calculations Fig. 1 Singlet excitation in PPVn.

4.4
4.2

3.8

4.0

localized

4.6

4.0

failed to reproduce it. The triplet state isthe4.8

4.2

andfor PPVn and ~0.9-1.7 eV for PPn) than

5.4
5.2

TDDFT

SAC-CI
Expt. (Ref. 2)
TDDFT (Ref. 3)
TDDFT (Ref. 4)

4.4

SAC-CI
Expt. (Ref. 7)
TDDFT (Ref. 3)
CAS-CI (Ref. 8)

3.8

S1-S0 emission energy (eV)S0-T1 excitation energy (eV)

2.8

4.6

S0-S1 excitation energy (eV)deviation from experimental results lies within 0.2 eV. The chain

3.4
3.2
3.0
2.8
2.6
2.4

1234

Number of repeat unit (n)emission energy also produced correct Fig. 3 Emission spectra of PPVn.
chain length dependence as shown inFig. 3. The failure of TDDFT is also apparent from this figure as thechain length increases. The singlet excitation energies of PPn have been compared withexperimental and TDDFT results in Fig. 4. The transition energy is almost proportional to 1/n. For
planar case, the excitation energies of PPn decreases more rapidly as the chain length increases. Inplanar case, the π-electrons are more delocalized over the oligomers length than the non-planar caseas a consequence the singlet and triplet states are more stabilized.

Zero-point energy correction to the excitation energy was estimated as -0.07 eV for PPV1 and 0.16 eV for PP1; theoretical value approaches to experimental value with this correction. For
calculating zero-point energy correction, ground-state and lowest

R2
R3

R1

R4

excited-state harmonic frequencies were calculated by using
Hartree-Fock and CIS level of theories, respectively. As in most π-S1
T10.060.04

Δr (Å)

0.02
0.00
-0.02

1For PPV1, the doubly excited 2 Ag state transition energy of-0.04-0.06

10-0.081234

78

9R6R2R1

The change in bond length for lowest singlet and triplet states

R3R10

R50.05

PPV1, the increase in the central vinylene C=C bond length is of0.03

the order of 0.05 Å and 0.10 Å for singlet and triplet, respectively

0.01

R11R9R7R4of PPV1 and PP2 have shown in figures 5 and 6, respectively. Forstates and the decrease in C-C bond length is around 0.04 Å.

6Fig. 5 Geometry changes in PPV1.

state above the 11Bu state.11

the C=C bond length is about 0.04 Å for both singlet and triplet

5Bond number

Contrary to the previous theoretical reports we predict this 21Ag

and 0.07 Å, respectively. In case of PP2, the maximum increase ofR90.08

corresponds to a single excitation from HOMO to LUMO.

R8S1
T1

0.04

0.02

Δr (Å)and the decrease in C-C vinyl bond length is of the order of 0.05 ÅR7
R60.10conjugated systems in PPVn and PPn, the S1 state mainly

4.99 eV agrees very well with the experimental value of 4.84 eV.

R8R50.00
-0.01
-0.02
-0.03
-0.04
-0.05

12

3

45

67

891011

Bond number

Fig. 6 Geometry changes in PP2Comparatively larger geometry relaxation is obtained for triplet
state. Geometry relaxation is localized mainly in the central partof the molecule.
11BuThe SAC-CI electron density difference between the groundand singlet/triplet excited state is calculated and the geometricalchanges are discussed in detail using electrostatic force theory.
The electron density difference in PPV1 is shown in Fig. 7. The

13Buelectron density in the σ-bond region increases and that in the πbond region decreases in general. The change in electron density
due to excitation in the triplet state is more pronounced than thesinglet state as it is apparent from Fig. 7. This is the reason

13

1 Bu-1 Bubehind the larger change of bond length in the triplet state than inFig. 7 Electron density difference in PPV1. the singlet state. From this study, we can say that the SAC-CI
Light - decrement, Dark - increment
interval=0.003 (for 1Ag-1Bu, 3Bu),and interval=0.001 (for 1Bu-3Bu)
Reference1

method is useful for investigating the electronic spectra andexcited state geometries of the oligomers of poly-paraphenylene vinylene (PPV) and poly-para-phenylene (PP).

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