Squaraine Compounds: Tailored Design and Synthesis towards a Variety of Material Science Applications

The interest in squaraine compounds, a relatively old class of dyes, has been recently renewed due to their potential usefulness in a large number of technologically relevant fields such as two-photon absorbing materials, field-effect transistors, solar cells, NIR-emitting fluorescent probes and sensitizers for photodynamic therapy. Aim of the present review is to summarize the most convenient strategies for squaraines and polysquaraines syntheses and to discuss the design strategies enabling for the preparation of squaraines having properties relevant for a given material application. In particular, in the first section of the review we will describe the various synthetic approaches for the preparation of squaraine and polysquaraine derivatives. The second section will focus on squaraines applications. We will describe in detail the use of squaraines in bioimaging and photodynamic therapy. Within the topic of bio-related applications we will also discuss the applications of squaraines as histological probes. Within the NLO-related applications, we will focus on squaraine-based electro-optic and two-photon absorbing materials. Dealing with photoconduction related applications, we will discuss the most recent applications of squaraine compounds in various kinds of devices such as solar cells, hybrid solar cells and NIR-responsive photodetectors. In all of the cases we will mainly focus on structure-property relationships highlighting the role of the active molecule design in the final device performances.