Morphology Dependence of the Optical Properties of Dalm Related Materials
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ABSTRACT Diazoluminomelanin (DALM) is an electroluminescent polymer which has shown significant optical activity in response to perturbing fields. The current model for this process features optical excitation of a polymer backbone containing conducting conjugation, with subsequent energy transfer to a luminescent group. In this paper we have performed electronic structure calculations using the AM1 Hamiltonian with configuration interaction to estimate the electronic properties of two potential models for the DALM backbone. Contrary to the conventional picture of conjugation, the phenyl groups in the DALM backbone show significant twist angles (42'-55') depending on substitutional group, resulting in localized electronic excitations. INTRODUCTION Diazoluminomelanin (DALM) is a luminescent conjugated polymer discovered and further developed by Kiel et al'. The molecule is thermochemiluminescent and can be formed chemically and biologically, the latter in both bacteria and mammalian cells 2 -. The optical activity of the DALM system is affected by electromagnetic radiation and temperature' . The sensitivity of the DALM to microwave radiation 3 and its ability to be formed within cells, suggest that when the mechanisms of its luminescence are better understood, it may potentially be useful as a cellular dosimeter in measuring microwave absorption and as a molecular temperature probe 6 . Work by Wright7 suggests that the backbone of DALM is a poly(m-phenylene) structure, more specifically, a poly-tyrosine as shown in figure la . Luminol (5-amino2,3-dihydro-l,4-phthalazinedione) is also present in the polymer but its positions of attachment along the chain are unknown. Physical characterization of DALM has suggested that the luminescence is due to an excitation of the backbone with subsequent energy transfer to the luminol. To investigate the luminescence of the DALM system, we have selected two models for the DALM backbone; the phenol oligomers and tyrosine oligomers shown in figure 1. We have examined and compared the electronic transitions and HOMO-LUMO energies of both of these representative backbones. To determine whether it may be necessary to study these properties in a time-averaged manner, we have also calculated the energy barrier for rotation between neighboring rings in a biphenol system and examined the dependence of the electronic properties on the torsion angle. 909 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
COOH
COOH
cjH--NH 2
t•H--NH
H2
2
0
Etc.
CH-NH2
(a)
COOH
E
TH 2
0
Etc
0"Z' OH
OH
OH
CH
Etc.
OH
(b)
CH-NH2 COOH
Figure 1. A schematic illustration of the (a) poly-tyrosine and (b) poly-phenol system representing a simplified DALM backbone. COMPUTATIONAL METHODS We have used the AMI Hamiltonian 8 as implemented in MOPAC93 9, to calculate both the optimized geometries and the electronic transitions. The AM1 Hamiltonian has been shown to provide accurate descriptions of the optical properties in aromatic molecules . All bond lengths and angles were optim
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