POP-Type MMX-Chain Compounds with Binary Countercations and Vapochromism - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 203

oxidation number is used in this chapter unless otherwise noted. Strictly, 3+ and 2+
should be represented as (3- )+, respectively. AV state is classified into

class III by Robin-Day classification, while CDW, CP and ACP are classified into
class II [ 17 , 18 ]. Because each electronic state exhibits various physical properties,
MMX chains have the potential to be multifunctional switching materials.
MMX chains synthesized to date have been categorized into two ligand systems:
dithioacetate (dta) [M 2 (RCS 2 ) 4 I] (M

d )+ and (2+ d

¼ ¼

alkyl chain group), which
are discussed in Chap. 9 , and pyrophosphite (pop) Y 4 [Pt 2 (pop) 4 X]

Ni, and Pt; R

n H 2 Oor

Y 0 2 [Pt 2 (pop) 4 X] n H 2 O(Y ¼ alkali metal, etc.; Y 0 ¼

alkyl ammonium,

alkyldiammonium; X ¼ Cl, Br, and I; pop 2 ¼ P 2 H 2 O 5 2 ).
In the dta system, as mentioned in Chap. 9 , a high conductivity and a
metal-semiconductor transition have been reported so far. Recently, Zamora
et al. reported the formation of nanowires on the substrate, which is promising for
the future molecule-based devices [ 19 - 22 ]. However, establishing the method to
obtain the desired electronic states and accompanying physical properties of MMX
chains is remaining issue that needs to be solved. The negative chain in the pop
system requires the countercations, which have advantages to synthesize the vari-
ous derivatives and to control the electronic states of MMX chains. Furthermore,
the removal of lattice water can also change the electronic states of MMX chains.
Although many pop-type MMX chains have been synthesized so far [ 23 - 30 ], in this
chapter, we focus on the chemistry of pop-type MMX chains containing two kinds
of countercation (binary countercations), which was discovered recently. We also
focus on the response of pop-type MMX chains to chemical external stimuli that is
water-vapor-induced changes of the physical properties of pop-type MMX chains
as represented by vapochromism. The other important response to physical external
stimuli, that is photoinduced phase transition, is explained in Chap. 11 .

10.2 Structure and Electronic States of MMX Chains
with Binary Countercations

Since all MMX chains thus far synthesized are in one of the four electronic states
shown in Fig. 10.1 without exception, searching the new electronic states in MMX
chains are very important for bringing about new physical and chemical properties
of low-dimensional materials. It should be noted that all of the known pop-type
MMX chains contain only one kind of countercation (unitary countercation). In
order to extend the system, introducing two kinds of countercation (binary

countercations) was performed.

10.2.1 Forming Condition of MMX Chains with Binary
Countercations

The general formula of MMX chains with binary countercations are

A 2 B[Pt 2 (pop) 4 I] n H 2 O(A ¼ K and Rb; B ¼ alkyldiammonium (Scheme 10.1 );