N-Salicilideneanilines are among the most studied thermo- and photochromic systems in the solid state. Although thermochromism is a general property of crystalline N-salicilideneanilines, photochromism is known in a limited number of cases. As a method for the construction of thermo- and photo-responsive molecular architectures, the co-crystallisation of 1,2,4,5-tetrafluoro-3,6-diiodobenzene (I2F4) with three selected imines of o-vanillin, named 1, 2 and 3, obtained through a condensation reaction with 3-aminopyridine, 4-bromoaniline and 4-iodoaniline, respectively, is reported herein. All crystals and co-crystals have been characterised by means of solid-state complementary techniques (X-ray diffraction, solid-state NMR spectroscopy, absorption and emission spectroscopy). The role of halogen bonding and crystal packing in the optical and chromic properties of all solid materials is discussed. All solids exhibit thermochromic behaviour, and three of them (2, 2 ⋅I2F4 and 3 ⋅I2F4) are also photochromic. Imine derivative 3 crystallises in two different polymorphic forms (3 A and 3 B) and a solvate (3 ). The bromo and iodo derivatives, 2 and 3 B, are isomorphous and form isomorphous co-crystals with I2F4, but behave differently when exposed to UV light because only crystalline 2 is photochromic. Interestingly, the replacement of bromine with iodine seems to turn off the photochromism because crystalline 3 A and 3 , and even the 2 3 solid solution, do not manifest photochromic behaviour.

The polycyclic aromatic hydrocarbon (PAH) triphenylene (TP) has been reacted with halopentafluorobenzene (XF5, X = Br, I) and 1,4-dihalo-tetrafluorobenzene (X2F4, X = Br, I) to yield the corresponding cocrystals TP•BrF5, TP•Br2F4, TP•IF5, and TP•I2F4 form I. These materials have been synthesized by dissolving TP into an excess of liquid or molten co-former, and single crystals have been grown via seeding chloroform solutions. They have been fully characterized by a combination of techniques including X-ray diffraction, Raman, and luminescence spectroscopy in the solid state. TP•I2F4 form I was found to undergo a single-crystal to single-crystal (SCSC) polymorphic phase transition induced by temperature (when cooled down to 100K) leading to the new form TP•I2F4 form II, which is transformed back into the first structure when brought again at RT. This behavior was confirmed also by Raman spectroscopy. Upon cocrystallization and as a result of the external heavy atom effect, all crystalline materials exhibited bright room temperature phosphorescence clearly visible by naked eye. The latter was almost exclusive for cocrystal TP•I2F4, whereas for TP•Br2F4 both fluorescence and phosphorescence were detected. In TP•Br2F4, the phosphorescence lifetime was of the order of 200 ms, and with the visual outcome of an orange phosphorescence lasting for a couple of seconds upon ceasing the excitation, that makes this compound classifiable as an Ultralong Organic Phosphorescent (UOP) material. The results evidenced the role of the nature of the heavy atom in governing the phosphorescence output from organic cocrystals.

A set of molecular salts with general formula [1H]nA·xH2O (1 = 4-amino-cinnamic acid, A(n-) = NO3(-), BF4(-), PF6(-), SO4(2-), x = 0, 1) was prepared and structurally characterized. [1H]Cl and [1H]2SO4·H2O(II) were found to undergo an SCSC stepwise [2+2] photodimerization, which was followed by X-ray diffraction; a kinetic analysis was performed on single crystals of both salts. In the case of [1H]Cl the photoreaction was also studied on polycrystalline materials.

The possibility of decreasing the solubility of the antidepressant drug venlafaxine hydrochloride by formation of molecular salts with organic acids accepted by the Pharmacopeia has been successfully investigated. Reacting venlafaxine with coumaric, ferulic, oxalic, salicylic, fumaric, and citric acids results in the protonation of the amino group and formation of the corresponding 1:1 molecular salts. All compounds have been characterized by a combination of solid state techniques, i.e., single crystal and powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy and solid-state NMR (1H MAS, 13C and 15N CPMAS, 1H DQ MAS, two-dimensional (2D) 13C–1H HETCOR, and 2D 14N–1H J-HMQC) spectroscopy. Intrinsic dissolution tests were performed on the pure salts, and suitable candidates were selected for the preparation of solid formulations with excipients; dissolution profiles for the solid formulations were measured in water and sodium chloride solution, and compared with that of the commercial form of venlafaxine, showing that the coumarate salt might represent an improvement for extended release administrations.

The [4 + 4] photoreactivity of the anthracene derivative 9-(methylaminomethyl)anthracene (MAMA) has been investigated in solution, gel medium and in the solid state. While quantitative formation of the cyloaddition photoproduct was achieved upon irradiation at λ = 365 nm of ethanol solutions of MAMA, only partial and slow conversion was detected in gels of low molecular weight gelators, and solid-state reactivity was not observed due to the unfavourable relative orientation of the anthracene moieties in the crystal. In hexafluorophosphate, tetrafluoroborate and nitrate silver(i) complexes, however, 9-(methylaminomethyl)anthracene exhibits a more favourable mutual orientation for the aromatic fragments, and [4 + 4] photoreactivity resulted. All compounds were structurally characterized via single crystal and/or X-ray powder diffraction and by Raman spectroscopy; this last technique proved effective in detection of the photoproduct in all solid state complexes.