Selective Chirality-Driven Photopolymerization of Diacetylene Crystals

Abstract

Crystal engineering of two diacetylene monomers was achieved by branching two chiral groups [R = PhC*MeNH(CO₂)CH₂] exhibiting an enantiopure configuration of S,S-(DA2) and an achiral R,S-meso-isomer (DA4). The X-ray structures of DA2 and DA4 reveal the presence of supramolecular arrangements driven by intermolecular H-bonding. A significant intermolecular closer proximity in DA4 than that in DA2 is depicted, ultimately resulting in a slow thermal (days) and swift (min) photochemical polymerization of DA4 to form PDA5, whereas DA2 is unreactive. DFT computations indicate that in both cases the lowest energy-excited state is the charge-transfer state [CT; PhC*MeNH(CO₂) → π*(-C≡C-C≡C-)]. Therefore, this outcome illustrates a drastic selectivity via a settle change in a carbon configuration. Analysis demonstrates that PDA5 is nonemissive and that its coloration arises from a π → π* excitation of the polymer backbone (DFT computations).

Figure 1

Stacking process and optimal geometric arrangement for the crystalline topochemical polymerization of diacetylenes.

Scheme 1. General Synthetic Pathway to DA Isomers (DA2 and DA4). (a) 2-Propyn-1-ol, DIPEA, DCM, 7 days (74%); (b) CuI, TMEDA, DCM, 4.5 h (81%) (c) 2,4-hexadiyne-1,6-diol, DIPEA, DCM, 4 days (32%) (d) (R)-Methylbenzyl Isocyanate, DIPEA, DCM, 4 days (54%)

Scheme 2. Structure of the PDA5 Derived from the meso R,S-Isomer DA4

Note that the thermal and photochemical transformations are, respectively, slow and fast, indicating that the route through the excited state is significantly more efficient. This is due to the inherently higher efficiency of light to populate a reactive excited state compared to the heat energy’s ability to induce reactions.

Figure 2

Geometric stacking of monomer units in the DA2 crystalline state. This stacking is not suitable for topochemical polymerization (carbon, gray; hydrogen, white; blue, nitrogen, oxygen, red). Displacement ellipsoids are drawn at the 50% probability level.

Figure 3

Geometric stacking of monomer units in the DA4 crystalline state. The stacking is suitable for topochemical polymerization (carbon, gray; hydrogen, white; blue, nitrogen, oxygen, red). Displacement ellipsoids are drawn at the 50% probability level.

Figure 4

Crystal structure of the polymeric PDA5 and the corresponding intramolecular HB network (carbon, gray; hydrogen, white; blue, nitrogen, oxygen, red). Displacement ellipsoids are drawn at the 50% probability level.

Figure 5

(a) View of the packing of PDA5 along the (−CR=CR–C≡C−)_n chain. (b) View of a segment of PDA5 illustrating the planarity of the (−CR=CR–C≡C−)_n backbone and the labeling for C1, C2, and O used to calculate the plane of the lateral chain.

Figure 6

FT-Raman spectra of PDA5 obtained through prolonged heating or through UV irradiation as specified above.

Figure 7

Solid-state absorption spectra of DA2, DA4 (note that DA4 started polymerizing), and PDA5 at room temperature.

Figure 8

Representation of the frontier MOs of one molecule of DA4. There is a strong displacement of the electronic density in the molecule in the transition from HOMO to LUMO, suggesting a CT character to this transition.

Figure 9

Absorption spectrum of DA2 in the solid state (red), and TDDFT-simulated absorption spectrum of DA4: green = bar graph, blue = same but applying an arbitrary thickness of 3000 cm^–1 to each bar.

Figure 10

Representation of the frontier MOs for a model (DA4)₂ unit.

Figure 11

Representation of the frontier MOs of a tetramer of PDA5.