Colorless to All-Black Full-NIR High-Contrast Switching in Solid Electrochromic Films Prepared with Organic Mixed Valence Systems Based on Dibenzofulvene Derivatives

Functional electrochromic materials which allow energy modulation both in the visible and in the near infrared (NIR) spectral ranges are attracting increasing interest both for the fundamental scientific aspects related to their spectroelectrochemistry and for their technological applications. Vis-NIR dimmable windows based on these materials are very promising for tunable shading, thus allowing lighting and heat energy use saving. Organic mixed valence compounds (MVs) are an interesting class of small molecules with NIR electrochromism arising from optically induced intervalence charge transfer transitions (IVCT). Here we report the synthesis and Vis-NIR electrochromic properties of new organic mixed valence systems, with two and three amino redox centers bridged by a dibenzofulvene (DBF) unit. We studied the neutral and charged state characteristics of these MVs in solution by spectroelectrochemical experiments, theoretical TD-DFT investigations and, in the solid state, through electrochromic devices (ECDs). We show that a fine-tuning of the electrooptical properties of these MVs can be obtained by different functionalization on the exocyclic fulvene bond of the DBF moiety, including the introduction of a third redox centre, leading to compounds where all the three redox centers participate in the electron transfer processes as a function of the applied voltage. As a proof-of-concept, the above MVs were used to form solid thermoplastic laminable films in order to fabricate transmissive-to-black switching electrochromic devices, with intermediate colour switching characteristics, enabling us to cover all the color palette. Beyond this important exploitability in the Vis region, useful in many applications, the most important characteristic of these devices is their absorption in almost the whole NIR range (800-2200 nm) through the excitation of highly charged radical species which show intense IVCTs. Importantly, all the devices show high optical contrast, response times of a few seconds and excellent switching stability over 10000 cycles.