To understand the relationship of structure and chromogenic properties of zwitterionic chromophores, dicyanomethane-based (FDCN and MPCT) and tricyanoquinodimethanebased (PpQDM) chromophores were studied. These chromophores are selected based on the design to have different conjugation lengths and thus have the three primary colors: yellow
(FDCN), magenta (MPCT) and cyan (PpQDM). All of the three chromophores exhibited intense absorption in the UV-Vis region with relatively high molar extinction coefficients (ε)
attributed to strong charge transfer (CT) from the negatively charge dicyanomethanide to the positively charged nitrogen in pyridinium.
In this regard, the chromogenic (i.e., color-changing) properties, including solvatochromism, halochromism, and thermo-halochromism of the three chromophores were
studied in solution, polymer gel and solid polymer. Distinctive chromogenic features of these zwitterionic chromophores were governed by structural factors, including the strength
of the donor and acceptor moieties in addition to the nature of the bridging unit. Thermohalochromic behavior of the chromophores was based on the protonation of the
chromophore by acid and consequently deprotonation by heating. Moreover, the colorchanging process could be reversible and irreversible, depending on the nature of a medium.
Using malononitrile and a polymer gel as a medium, thermo-halochromic process was irreversible, whereas in a suitable polymer host (i.e., PS, PMMA and PS-co-PMMA), these
chromophores showed reversible thermo-halochromic behavior.
Strong charge transfer (CT) of the zwitterionic chromophores in UV-Vis range in addition to their easily CT turned on-off properties eventually led to perform an evaluation
based on their potential application as indirect thermo-indicators. FDCN (as the studied chromophore) demonstrated to be capable of reversibly switching on and off the fluorescence
of the commercial fluorescent dyes under externalstimuli of heat and pH changes.