Fused azulenes as possible organic multiferroics

We present compelling theoretical results showing that fused azulene molecules are strong candidates for exhibiting room temperature multiferroic behavior, i.e., having both ferroelectric and ferromagnetic properties. If this is experimentally proved, these systems will be organic multiferroic materials with important potential applications.

Figure 1

Structure of (a) fused azulene, (b) DMRG ladderlike scheme for building fused azulene, (c) polyazulene, and (d) oligoacene.

Figure 2

Spin gaps (in eV) vs $\frac{1}{n}$ defined as ${\Delta }_{M_S}=E\left(M_S\right)-E\left(0\right)$, $M_S=1$, 2, or 3 for the PPP model (a) and for the Heisenberg approximation (b). The shaded regions represent the ground state of a particular spin for the chain lengths above which they occur. The spins of states in the shaded region progressively increase in units of 1 from right to left, for fused azulenes. In the case of oligoacenes, ${\Delta }_1$ is always positive, implying a singlet ground state.

Figure 3

(a) Net charge ($p$-$c$) in each ring of fused azulenes with $n=10$, 9, and 8 azulene units (top to bottom)—see text. The even (odd) ring index corresponds to a five- (seven-) membered ring. (b) shows dipole moment in the ground state for a different number of azulene units.

Figure 4

Charge gap (in eV) for fused azulenes vs $\frac{1}{n}$.