The sensitivity of donor – acceptor charge transfer to molecular geometry in DAN – NDI based supramolecular flower-like self-assemblies

A charge-transfer (CT) complex self-assembled from an electron acceptor (NDI-EA: naphthalene diimide with appended diamine) and an electron donor (DAN: phosphonic acid-appended dialkoxynapthalene) in aqueous medium. The aromatic core of the NDI and the structure of DAN1 were designed to optimize the dispersive interactions (π-π and van der Waals interactions) in the DAN1–NDI-EA self-assembly, while the amino groups of NDI also interact with the phosphonic acid of DAN1 via electrostatic forces. This arrangement prevented crystallization and favored the directional growth of 3D flower nanostructures. This molecular geometry that is necessary for charge transfer to occur was further evidenced by using a mismatching DAN2 structure. The flower-shaped assembly was visualized by scanning electron and transmission electron microscopy. The formation of the CT complex was determined by UV-vis and cyclic voltammetry and the photoinduced electron transfer to produce the radical ion pair was examined by femtosecond laser transient absorption spectroscopic measurements.

Femtosecond laser flash photolysis was conducted using a Clark-MXR 2010 laser system and an optical detection system provided by Ultrafast Systems (Helios). The source for the pump and probe pulses were derived from the fundamental output of Clark laser system (775 nm, 1 mJ pulse-1 and fwhm = 150 fs) at a repetition rate of 1 kHz. A second harmonic generator introduced in the path of the laser beam provided 412 nm laser pulses for excitation. 95% of the fundamental output of the laser was used to generate the second harmonic, while 5% of the deflected output was used for white light generation. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provided an experimental time window of 1.6 ns with a maximum step resolution of 7 fs. The pump beam was attenuated at 5 μJ pulse-1 with a spot size of 2 mm diameter at the sample cell where it was merged with the white probe pulse in a close angle (<10°). The probe beam, after passing through the 2 mm sample cell was focused on a 200 μm fibre optic cable, which was connected to a CCD spectrograph (Ocean Optics, S2000-UV-vis for visible region and Horiba, CP-140 for NIR region) for recording the time-resolved spectra (450-800 and 800-1400 nm). Typically, 5000 excitation pulses were averaged to obtain the transient spectrum at a set delay time. The kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data.

Experimental protocol for synthesis of target molecules
Synthesis of diamine naphthalene diimide derivative (NDI-EA). S1 NDI-EA (1) was synthesized in two steps, firstly by reacting mono Boc protected ethylenediamine with naphthalene anhydride followed by deprotection of the Boc group (Scheme S 1).
The completion of the reaction was monitored by TLC.

Synthesis of Compound 2
In an oven-dried 50 ml round bottom flask, compound 5 (100 mg, 0.20 mmol) was dissolved in dry acetonitrile (10 ml) and cooled to 0 °C. To this reaction mixture TMSBr (395 µl, 3.10 mmol) was added for 30 minutes at 0 °C and further stirred for 18 h at 40 °C.

Synthesis of Compound 6
The solid 1, 6 dihydroxynapthalene (500 mg, 3.1 mmol) was dissolved in (15 ml) acetonitrile and K 2 CO 3 (855 mg, 6.2 mmol) was added, and the resulting suspension was stirred at 40 °C for 30 min. Then, 1, 2dibromoethane (5 ml, 62.1 mmol) was added and the mixture was refluxed for 16 h. After cooling down and filtering of the solid, the solvent was evaporated on a rotary evaporator.