An Integrated Theoretical/Experimental Study of Quinolinic–Isoquinolinic Derivatives Acting as Reversible Electrochromes

A series of compounds, featuring an ethenylic bridge and quinoline and isoquinoline end capping units possessing systematically varied substitution patterns, were prepared as molecular materials for electrochromic applications. The different structures were optimized in order to maximize the electrochromic contrast in the visible region, mostly by achieving a completely UV-absorbing oxidized state. Density functional theory (DFT) calculations are exploited in order to rationalize the correlation between the molecular structure, the functional groups’ electronic properties, and the electrochemical behavior. It is shown that the molecular planarity (i.e. ring/ring π conjugation) plays a major role in defining the mechanism of the electrochemical charge transfer reaction, while the substituent’s nature has an influence on the LUMO energy. Among the compounds here studied, the (E)-10-methyl-9-(2-(2-methylisoquinolinium-1-yl)-vinyl)-1,2,3,4-tetrahydroacri-dinium trifluoromethanesulfonate derivative shows the most interesting properties as an electrochromophore.

Synthesis of (E)-1,2-di(isoquinolin-1-yl)ethene (46) A stirred suspension of 45 (1.700 g, 5.67 mmol) in Ac2O (15 ml) was heated to 70 °C with a water bath. A clear solution was obtained after 10 min and a solid started to precipitate after 20 min. After 5 h, the mixture was poured in 150 ml of water and NaHCO3 was added under stirring till pH 8. The mixture was stirred overnight, 200 ml of water was added, and the light brown solid was filtered washing with 30 ml of distilled water, EtOH (10 ml) and Et2O (30 ml
The mixture was cooled to room temperature and solvent was removed under reduced pressure ( 1 mmHg) obtaining product as a yellow oil (7.66 g, 34.0 mmol, yield 95%).
The mixture was slowly poured in ice (400 g) and basified till pH 9 adding NaOH. Synthesis of (E)-4-(2-(6-fluoroisoquinolin-1-yl)vinyl)quinoline (32) A suspension of 34 (200 mg, 1.24 mmol) and quinoline-4-carboxyaldehyde (163 mg, 1.04 mmol) in a mixture of AcOH (0.5 ml) and Ac2O (0.2 ml) was heated in a CEM discover microwave reactor for 15 min (P max =15 W, 110 °C. The solution was allowed to cool to RT observing the crystallization of a yellow solid which was recovered and dried on filter paper. Product was purified by crystallization from acetone/AcOH 20:1. The crystalline solid was suspended in 25 ml of a saturated NaHCO3 aqueous solution for 30 min and extracted with 2 × 20ml of CH2Cl2. The organic phase was collected, dried over Na2SO4 and evaporated under reduced pressure at Synthesis of (E)-7-chloro-4-(2-(isoquinolin-1-yl)vinyl)quinoline (33) A suspension of 26 (374 mg, 2.61 mmol) and 35 (500 mg, 2.61 mmol) in 2 ml of a 5:2 AcOH/Ac2O mixture was heated in a CEM discover microwave reactor for 18 min (P max =15 W, 110 °C). The solution was allowed to cool to RT observing the crystallization of a yellow solid which was suspended in 30 ml of water and filtered on a Hirsh funnel. Product was purified by crystallization from acetone/ Synthesis of methyl 1,2,3,4-tetrahydroacridine-9-carboxylate (40) To a stirred suspension of 48 (10.00 g, 44.00mmol) and DBU (6.698 g, 44.00 mmol) in toluene (75 ml), MeI (6.245 g, 44.00 mmol) was added; the mixture was stirred at RT for 20 min and then heated to reflux for 3 h. The change in solution colour to yellow and the formation of a white precipitate were observed. The mixture was cooled to RT and filtered washing with 30 ml of toluene. The filtrate was collected, and solvent was evaporated under reduced pressure obtaining product as a pale yellow solid (8.988 g, 37.25 mmol, yield 85%, m.p.: 68-70 °C).
Under N2 atmosphere, in an oven dried 250 ml 3-necked RBF, fitted with reflux condenser, septum and dropping funnel, a suspension of NaH (828 g, 20.72 mmol) in anhydrous DME (10 ml) was prepared. A solution of freshly distilled 26 (742 mg, 5.18 mmol) and 40 (1.500 g, 6.22 mmol) in 20 ml of anhydrous DME was drop wise added to the refluxing suspension of NaH.
The suspension was refluxed for a total of 46 h observing the formation of a bright yellow solution. The mixture was cooled with an ice-water bath, and AcOH (2.50 g) was added followed by 20 ml of water. A yellow solid precipitated and was filtered washing with H 2 O (50 ml).

Electrochemistry of Q-Q derivate
for the Q-Q compound. The DPV analysis shows a consistent peak to peak separation of 100 mV.
The DPV has been obtained with the same apparatus using a scan rate of 20 mV/s.