Many-electron natureMany-electron natureof polydiacetylene colored statesof polydiacetylene colored states

A.A.Hasanova, S.S.MoliverState University Ulyanovsk, 432 970 Russia

Ordered diacetylenes R–C≡C–C≡C–R topo-chemically turn into conducting conjugated polymers PDA.

All PDA solids have similar optical properties, and can be further converted into similar colored phases ("blue", "red", and "purple").

These was explained chemically as two possibilities of backbone structure:[=RC–C≡C–CR=] "acetylenic", and[–RC=C=C=CR–] "butatrienic".

We propose a quantum-chemical model of conducting PDA, which derives geometry of backbone from one of several possible electron configurations.

High point symmetry of unit cell (1-4) gives degenerate molecular orbitals, hence open-shell electron configurations will present conducting states.

NB: substiteted PDA is (RC4R)x

Large unit cell 2(HC4H)4Crystalline class D2d

Monomers are shown with tilt angle =0 (monomer crystal).

Polymer crystal =75-90°

Large unit cell 2(HC4H)4 has k-set {+X}, that gives a reliable quantum chemistry of PDA. Calculations of 2(HC4H)8 with {+4P+2K+X} support it.

All-trans isomer of PDA is always considered to be a product of topochemical reaction in ordered monomer system. Since usual R are large groups (substituting H atoms – circles), this is a valid stereochemical assumption.

However, certain packing of some R-substituted monomers can be suggested, which will produce other isomers.

Quantum chemistry method

MO LCAO open-shell restricted Hartree–Fock (ROHF).

Semi-empirical (INDO) parametrization.

Variation of 2 conjugated bonds within monomer, valence angle of H, and tilt angle of monomers .

Results of total energy minimization at given :

close minima of cis and trans isomers and of 2 many-electron terms of the trans isomer.

1B2 cis

1B2 trans

1B1 trans–0.75

–1.15

–1.45

eV

/mon

om

er

ground term

acetylenic bond, Å

butatrienic bond, Å

intermonomer bond , Å

trans 1B1 1.44 1.47 1.52 90°

trans 1B2 1.46 1.49 1.61 83°

cis 1B2 1.42 1.57 1.48 79°

Dependence of the total energy

– on electron configuration (term of the open-shell multiplet), and

– on isomer pattern,

shows, that rather weak factors in real PDA films may switch electron configuration, that results in certain colored state.

Stabilization of PDA conjugated backbone in one of several possible many-electron configurations is a primary cause of electronic and structural properties of a given colored phase.