Turn-off mode fluorescent norbornadiene-based photoswitches† †Electronic supplementary information (ESI) available: Synthetic procedures, 1H, 13C and COSY-NMR spectra, UV-Vis absorption, kinetics and Arrhenius analysis. Photoisomerization quantum yield, fluorescence lifetime measurements and TDDFT c

To explore the potential of negative photochromic molecules for possible optical memory storage applications, we have here synthesized and studied a series of four norbornadiene–quadricyclane (NBD–QC) photoswitching molecules.

Then trimethylsilyl acetylene (25 mmol) was added dropwise and the reaction was stirred at 60 o C overnight. The solution was cooled and filtered through a short silica gel column with dichloromethane. The solution was washed with deionized water. The organic phase was collected and the solvent was removed in vacuo. The crude was then submitted to automated chromatography using hexane as the eluent at a flowrate of 50 mL/min to collect the product after removing the hexane in vacuo.
Then potassium carbonate (2.2 eq.) was added and the solution was stirred under nitrogen at room temperature until the starting material was consumed, as indicated by TLC (hexane, 100%). The solid residue was filtered off and the solvent was removed in vacuo. Then the crude product was dissolved using dichloromethane, washed with deionized water and then with brine. The organic phase was separated, dried over anhydrous MgSO 4 , filtered and the solvent was removed in vacuo to afford a crude product, which was further purified using automated chromatography with hexane as eluent at a flow rate of 50 mL/min. The product was collected after removing the hexane in vacuo. 1 Deprotected meta-product (7)

Meta-and para-oligophenylethynyl NBD (9 or 10)
In a 100 mL two-necked round-bottom flask, Pd(PPh 3 ) 4 (0.27 g, 5 mol%) and CuI (45 mg, 5 mol %) were added. The flask was evacuated and refilled with nitrogen three times. Toluene (15 mL) was then added and the solution was stirred. 2,3-Dibromonorbornadiene* (0.59 g, 2.4 mmol) in 5 mL toluene was transferred to the reaction flask using a syringe, followed by the addition of 7 or 8 (4.8 mmol, 2 eq.) dissolved in toluene (5 mL). Triethylamine (3 mL), purged with nitrogen beforehand, was added, and the reaction was stirred at 30 o C overnight. The solution was initially yellow which changed to reddish brown overnight. The solvents were removed using a rotary evaporator. The crude was washed with deionized water and extracted with dichloromethane, dried over anhydrous MgSO 4 , filtered and the solvent was removed in vacuo. The reddish brown crude product was submitted to automated chromatography with a gradient of 0-10% dichloromethane in hexane, with a flow rate 50 mL/min to afford the desired products.

TBAF deprotection
In 100 mL flask containing a magnetic stirbar, compound 9 or 10 (0.66 g, 1 mmol) was dissolved in THF (50 mL) and the solution was bubbled with nitrogen for 15 min. The flask was cooled to -30 o C with an acetonitrile/N 2 bath. Then, tributylammonium fluoride (1.0 M solution in THF, 2.3 mL, 2.3 mmol) was added dropwise, while the yellow solution turned reddish. The reaction was stirred overnight at room temperature. The solvent was removed in vacuo, the crude product was washed with deionized water and brine, then extracted with dichloromethane and dried over anhydrous MgSO 4 . After filtration, the solvent was removed in vacuo. The brown crude product was submitted to automated flash chromatography using a gradient of 0-20% dichloromethane in hexane (100 %). After removing the solvents in vacuo, the products 11 and 12 were collected. 11: brownish oil, 0.28 g (80%) 1

Final coupling of 3 or 4 via a Sonogashira reaction
To a two-necked round-bottom flask equipped with a magnetic stirbar Pd(PPh 3 ) 4 (10 mol %), CuI (5 mol %) and 4-iodophenylthioacetate* (1.7 mmol) were added. The flask was evacuated and refilled with nitrogen three times. To the flask was added dry toluene (15 mL), followed by 11 or 12 dissolved in 10 mL toluene dropwise via a syringe. Then, degassed triethylamine (5 mL) was added to the red solution and the reaction mixture was stirred at dichloromethane as the eluent. The solution was concentrated and subjected to automated flash chromatography using a gradient of 0-30% dichloromethane in hexane. The product was collected after removing the solvent in vacuo. Recrystallization from acetonitrile yield analytically pure product.

Thiophene coupling to NBD via Sonogashira reaction
In an oven-dried Schlenk flask (25 mL) provided with magnetic stirring bar, were added Pd(PPh 3 ) 4 (5 mol%), CuI (10 mol%), 2,3-dibromonorbornadiene* (1.2 mmol) and toluene (10 mL). The solution was purged with nitrogen for 15 minutes. Then, diisopropylamine (1 mL) and 2-or 3-ethynylthiophene (2.6 mmol) were added, at which point the color of the solution turned from yellow to dark red. The mixture was reacted until the starting materials were consumed as indicated by TLC. The solution was then diluted with dichloromethane and filtered through a short silica gel plug. The volatiles were then removed using a rotary evaporator. The crude product obtained was submitted to automated flash chromatography using an eluent gradient of 0-5% dichloromethane in hexane. The product was obtained after the solvent was removed in vacuo.    Figure S13: Molar absorptivity vs wavelength of compounds 1, 2, 3 and 4. Three samples dissolved in toluene were taken for each measurement. The average value is displayed.

Arrhenius Plot
Using the Arrhenius equation and plotting lnk vs 1/T gave a linear correlation.

ln = ln -
The slope correspond to the activation energy E a while the intercept corresponds to the preexponential factor, A.        Figure S25: The photoisomerization of 1 with a 365 nm lamp (photon flux Figure S23) to determine the quantum yield.

V. UV-vis absorption and emission measurements
Emission decay