A Photophysical Interpretation of the Thermochromism of a Polyfluorene Derivative–Europium Complex

The thermochromism of a polyfluorene derivative complexed with a europium ion was interpreted using the photophysical properties of the noncomplexed polymer and a low molecular mass model compound having the same structure of the complexed site in the polymer. To the naked eye, the thermochromism was characterized by a strong red at low temperatures (170–260 K, due to Eu3+ ion) and a blue color at higher ones (280–330 K, due to fluorene). Absorption and emission spectra, time-resolved measurements, and theoretical simulations showed that the polymer chain does not affect the europium photophysical properties, but the opposite occurred: the ion insertion precluded interchain aggregation, and the backbone emission did not vary with temperature variation, in the range of 170–330 K. To the best of our knowledge, this effect is reported for the first time and opened a new avenue for the design of nanothermometers, since the polymer can act as a "built-in standard", thus allowing the construction of much simpler devices.