We have designed, synthesized and characterized new carbazole-based dyes for combined two-photon fluorescence and second-harmonic generation microscopy, one of which performs very well and selectively in mitochondria in both channels.

A combined UV-visible, cyclic voltammetric theoretical and hyper-Rayleigh scattering study has been carried out on a new series of ferrocenyl chromophores (2a–2d and 7–11) to develop a structure–property relationship, by varying the acceptor strength and the conjugation path length between the donor and the acceptor and by disubstitution of the ferrocene moiety. This allowed systematic establishment of the contribution of the conjugation bridge, acceptor strength and substitution on both rings of ferrocene towards non-linear optical response as well as its redox switchability with one of the chromophores (8) having an on/off ratio of 25, the highest for ferrocenyl chromophores, and also thermal stability up to 300 °C. To deduce the participation of the available polarizable electrons towards the overall non-linear optical properties (with electron contribution from both the rings as observed from average bond length calculation from crystal data of 2c and 7 and the absorption studies), respective intrinsic hyperpolarizability (β₀ ^int) of the dipolar chromophores was calculated. Interestingly, the shorter chromophore 2c showed exceptionally high non-linear optical response and intrinsic hyperpolarizability compared to its longer counterparts.