Challenges in Controlled Thermal Deposition of Organic Diradicals

Abstract

We have demonstrated that it is possible to evaporate diradicals in a controlled environment obtaining thin films in which the diradical character is preserved. However, evaporation represents a challenge. The presence of two radical sites makes the molecules more reactive, even in case of very stable single radicals. We have explored the parameters that play a role in this phenomenon. Bulk formation thermodynamics and delocalisation of the unpaired electrons play the major role. The higher the formation energies of the crystal, the more difficult is the evaporation of intact radicals. The larger the delocalization, the more stable is the film exposed

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

“…We have previously focused our efforts on the evaporation of radicals and diradicals. 32,[35][36][37][38][69][70][71][72] Here, we extend our protocol to evaporate triradical 3. We deposit thin films of triradical 3 on SiO2/Si(111) wafers by organic molecular beam deposition (OMBD), 73 which has been proven to be a suitable method for growing radical and diradical thin films.…”

“…It would be unprecedented to successfully fabricate the first thin-film of a high-spin triradical. [35][36][37][38] Figure 1. Thermally robust triplet ground state diradicals 1 and 2, and quartet ground state triradical 3.…”

From radical to triradical thin film processes: the Blatter radical derivatives

“…[15][16][17] We use a specific protocol to assess the intactness. 25 We consider (1) the stoichiometric information from the XPS spectra, (2) the comparison between films and powder XPS spectra (the powder does not undergo evaporation) looking at the shape and the components of the single spectroscopic lines using a best-fit procedure 26 that we have also previously correlated with electron spin resonant (ESR) spectroscopic investigations; 10,15,27,28 (3) the comparison between the experimental and the theoretical curves obtained by DFT calculations. 25 When the films are stable enough under air, they are also investigated by using ESR spectroscopy.…”

“…The onset of the thermogravimetric analysis (TGA) decreases from the Blatter-pyr to the NN-Blatter and the diNN-Blatter from 523 K to 433/439 K, shrinking the size of the temperature window available for the evaporation that must be higher than the energy of formation of the bulk. 25 This allows sublimating the molecules from the powder and targeting the substrate with a molecular beam using a Knudsen cell. Film formation onto a substrate is a phenomenon that is against the thermodynamic equilibrium between adsorption and desorption.…”

“…3). 16,25 Furthermore, the temperature range necessary to evaporate the intact diradical and triradical leads to enhanced island formation and it hinders the growth of films nominally thicker than 2 nm (the nominal thickness is the thickness of the equivalent film assuming an ideal layer growth mode). Growing thicker films needs higher evaporation temperatures that damage the radical moiety (Fig.…”