Chain Length Dependent, Reversible Switching of Spiropyran-PMMA Blend Dielectrics for Light-Gated Organic Transistors

The increasing demand for mobile and flexible electronics and the rising interest in the “Internet of Things” promotes research on lightweight organic electronic devices and concepts where the integration of silicon chips is too expensive. One promising approach to introduce additional functionalities for sensing and memory applications into conventional organic electronic devices is the introduction of molecular switches. These small organic molecules undergo reversible isomerization between (at least) two isomers when irradiated with light of different wavelength and this induces drastic changes in physicochemical properties such as frontier orbital energy levels, dipole moment and/or molecular geometry. These reversible changes in molecular properties can be exploited to deliberately modify charge transport in organic field-effect transistors (OFETs) and therefore enable optical control over device characteristics.<br/><br/>Spiropyran is a photoswitch that can be used to fabricate light-switchable organic electronic devices. The system is particularly interesting, because the UV light-induced isomerization of the neutral spiropyran (SP) to the zwitterionic merocyanine (MC) can be exploited to reversibly tune OFET characteristics [1-3]. One possible approach to incorporate spiropyran-based photoswitches into organic transistors is to blend them with dielectric polymers to obtain light-responsive gate dielectrics. However, there is no consistent explanation in the literature for how isomerization of the photoswitch alters charge transport and performance of OFETs, and several mechanisms have been proposed. We address this issue by blending spiropyran molecular switches with alkyl side chains of different lengths (i.e. C₁, C₄, C₁₀ and C₁₆) with poly(methyl methacrylate) (PMMA) and study the reversible switching of these functional blends using UV/Vis absorption spectroscopy, impedance spectroscopy and further electrical characterization in metal-insulator-metal capacitors and OFETs. The UV/Vis absorption data show that all four SP derivatives isomerize to their respective MC form upon UV irradiation. Thermal annealing induces the reverse reaction from MC to SP. In OFETs, the reversible switching between the SP and MC states translates into pronounced, reversible shifts of the threshold voltage.<br/><br/>Surprisingly, we find that the switching of both capacitor and transistor characteristics strongly depends on the length of the alkyl side chain of the photoswitch. This suggests that the change of the dipole moment (low versus high) and the charge (neutral versus zwitterionic) might not be the only cause for the altered device characteristics, but that the side chains must also be considered. This chain length dependent switching is further investigated by bias stress measurements using OFETs and a similar dependence on the chain length is found.<br/><br/>Such a chain length dependent reversal of the switching in light-gated devices has not been reported before. Hence, our results highlight the importance of the chemical design of photochromic molecules for well-controlled switchable devices.<br/><br/>1. Ishiguro, Y. et al., ACS Applied Materials & Interfaces, 2014. <b>6</b>(13): p. 10415-10420.<br/>2. Shen, Q. et al., Advanced Materials, 2010. <b>22</b>(30): p. 3282-3287.<br/>3. Shen, Q. et al., The Journal of Physical Chemistry C, 2009. <b>113</b>(24): p. 10807-10812.