2P073
Excitation properties of molecules in organic light emitting device and biosensor
B. Saha1, M. Ehara1, H. Nakatsuji1,2
1Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyou-ku, Kyoto 615-8510, Japan
2Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4 Takano Nishihirakicho,Sakyo-ku, Kyoto 606-8103, Japan

Two types of interesting molecules are studied in this research. First, poly-para-phenylene
vinylene (PPV)-based chromophores can serve as a model for photo luminescent conducting polymers,
widely used in organic light emitting device1. Unsubstituted PPV (1 and 2) has attracted considerable
interest, both in terms of its basic properties and its applications. Second, fluorophores N, N-dimethyl9-Anthracenemethanamine (3) and DANS acid (4) which are promising fluorescent labels forbiosensor applications and frequently used by the scientific community2. SAC-CI method has been
employed successfully to investigate the electronic excitation properties of these molecules. Groundstate was optimized using DFT(B3LYP)/6-31G* while the excited states were optimized using
CIS/D95(d). Absorption and emission spectra have been calculated using SAC-CI/D95(d). Gaussian
03 suite of program was utilized for the computation. In the calculation of excitation properties, the 1s
core of C and O atoms 2s2p core of S atom and corresponding virtual MOs were excluded from theactive space. Absorption and emission
spectra agree well with the available
1
experimental data. A comparison of theexcitation properties and optimized
geometry with the other computations
4
also has been made.
23
Singlet excited states of 2 aredisplayed in Table 1. The lowest singlet transition corresponds to HOMO-LUMO transition withexcitation energy 3.44 eV which shows excellent agreement with the experimental one4.
Corresponding HOMO-LUMO transition energy of 1 is 4.26 eV. This shows that the lowest energy
absorption peak gets red shifted with increasing chain length, in agreement with various theoreticaland experimental studies3 and the line becomes stronger. The central vinylene C=C bond relates tobonding and antibonding overlaps of π-orbitals in the HOMO and LUMO, respectively, whereas theopposite phase relationships are seen along the vinylene C-C bond (Fig. 1).
NMe2

H2C

NMe2

SO3H

Table 1. Absorption spectra of PPV3 (2).
SAC-CIState∆E (eV)fMain CI description (|C| > 0.3)
1
A'
3.44
1.6709 0.95 (H→L)
1
A'
4.32
0.0571 -0.58(H-4→L)+0.57(H→L+2)-0.36 (H→L+4)
1
A'
5.05
0.0000 -0.73(H-1→L) – 0.63(H→L+1)

Exptl.
∆E (eV)
3.444

Other
∆E (eV)
3.455

Stilbene (1) has planar ground state which is corroborated by other experimental6 andtheoretical7 investigations. At the B3LYP/6-31G* level, with average discrepancies of 0.03 Å on bondlengths from the available experimental10 data were observed. The CIS/D95(d) optimization showsthat the lowest two singlet excited states of 1 are planar. In the lowest excited state of 1, the central
vinylene C=C bond increases by 0.015Å while the vinylene C-C bond decreases by 0.057Å. The
ground and excited state geometries agree
well with the other available computational
values8 (Fig. 2). The ground state and twolowest singlet excited states of 2 are also
planar. But in this case the vinylene C=CFig. 1 HOMO (left) and LUMO (right) MOs of PPV3 (2).
bond increases by 0.026Å while the C-C

1.391
1.392
1.409
1.407
1.466
1.453

1.398
1.395

1.395
1.394

1.408
1.402
1.393
1.394

1.381
1.380
1.394
1.404

1.398
1.395

1.433
1.430

1.408
1.393

1.431
1.426

1.409
1.403 1.413
1.400

1.370
1.387

Fig 2. Optimized geometries for the ground (left) and lowest singlet excited state (right) of PPV2 (1). Uppervalues represent present calculation, lower ones from other computation (Reference 8).

decreases by 0.022Å. In the excited state the C=C bond increases due to reduction in bond order while
the C-C bond shrinks. We obtained the vinylene C=C bond length as 1.398Å, although quite low value
1.318Å for this bond was reported experimentally10.
The absorption spectra of 3 have displayed in Table 2. The lowest singlet transition energy 3.16
ev is deviated by 0.21 eV from the experimental value of 3.37 eV9. This is a π-π* transition. We
obtained a very strong absorption at 5.28 eV which is a π-π*. The emission spectra of 3 have displayedin Table 3. We
Table 2. Absorption spectra of N, N-dimethyl-9-Anthracenemethanamine (3).
obtained the lowest
SAC-CIExptl.
Statesinglettransition∆E (eV)fMain CI description (|C| > 0.3)
∆E (eV)energyas2.80 eV
1A
3.16
0.1323 0.95(H→L)
3.379
1
which deviates byA
3.41
0.0006 -0.67(H-1→L)-0.65(H→L+1)
1
0.17 eV only fromA
5.09
0.0600 -0.90(H-2→L)
1A
5.28
1.8636 0.62(H→L+1)-0.60(H-1→L)+0.30(H→L+1)
theexperimental
1A
5.69
0.0053 -0.85(H→L+3)+0.36(H-4→L)
value of 2.97 eV8.
The lowest singlet
transition corresponds to HOMO-LUMO transition. A very strong emission is observed at 5.23 eV
which is also a π-π* transition. Only the MOs from the anthracene moiety are involved in thesetransitions. A weak
Table 3. Emission spectra of N, N-dimethyl-9-Anthracenemethanamine (3).
transition at 4.69 eVSAC-CIExptl.
Stateobtainedwhich∆E (eV)f
Main CI description (|C| > 0.3)
∆E (eV)1
corresponds to n-π*A2.800.1307 -0.94 (H←L)
2.979
1transition.
In thisA
3.37
0.0007 0.67 (H-1←L) + 0.66 (H←L+1)
1case a transition ofA
4.69
0.0023 0.91(H-2←L)
1
thelonepairA
5.23
1.0473 0.66(H←L+2)-0.48(H←L+1)+0.47(H-1←L)electron of nitrogen
atom to the LUMO of the anthracene moiety is occurred.
In conclusion, we can say that the SAC-CI theory is useful to reproduce the experimental spectraquite well for these molecules.
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