Benzylamine

The organic precursor salt was
synthesized by slow evaporation of an equimolar mixture of HI (Sigma-Aldrich;
57 wt % in H₂O) and benzylamine (Sigma-Aldrich; 99%). After
slowly adding HI to benzylamine, swirling to mix both components and
heating on a hot plate at 60 °C, a white salt was formed. Once
the solvent was fully evaporated, the crystallized salt was washed
three times with diethyl ether (Avantor) and dried in a vacuum oven
at 120 °C overnight. PbI₂ (Sigma-Aldrich; 99%) was
used as the inorganic precursor. The organic/inorganic precursor solution
was prepared by dissolving the organic and inorganic salts in an equimolar
ratio in dimethylformamide (DMF, Sigma-Aldrich), yielding a concentration
of 0.1 M.

Single crystals of
(C₆H₅CH₂NH₃)₂PbI₄ and (C₆H₅CH₂NH₃)₄Pb₅I₁₄·2H₂O were grown at room temperature using the same layered solution
technique as used in our previous work.^{19 (link)} PbI₂ (74 mg, 0.16 mmol; Sigma-Aldrich; 99%) was dissolved
in 3.0 mL of concentrated (57 wt %) aqueous hydriodic acid (Sigma-Aldrich;
99,95%). Absolute methanol (3.0 mL; Lab-Scan; anhydrous, 99.8%) was
carefully placed on top of the PbI₂/HI mixture, without
mixing the solutions. A sharp interface was formed between the two
layers due to the large difference in densities. Benzylamine (Sigma-Aldrich;
99%) was added in great excess by gently adding 15 droplets, using
a glass pipet, on top of the methanol layer. The reaction mixtures
were kept in a fume hood under ambient conditions. After 2 d, a small
number of crystals started to form. The crystals were collected after
two weeks by washing three times with diethyl ether (Avantor). A mixture
of three types of crystals was obtained: bright orange platelets ((C₆H₅CH₂NH₃)₂PbI₄), colorless needles
(an unidentified phase), and nearly transparent, light yellow bar-shaped crystals ((C₆H₅CH₂NH₃)₄Pb₅I₁₄·2H₂O). Figure S1 shows a photograph of all
three types of crystals.