The 13C CP/MAS NMR spectrum of the parent material showed an intense peak centered at 147 ppm corresponding to the ethenylene (–CHdouble bond; length as m-dashCH–) bridging groups into the
MELANOTAN-I framework (Fig. 4) [32] and [33]. After Soxhlet extraction with acetone, the e-PMO material also exhibited additional signals at 76, 71 and 30 ppm which are characteristics of the residual surfactant (P123) left in the material. Two more signals at 17 and 58 ppm can be assigned to ethoxy groups (Si–O–C2H5) which were not completely hydrolyzed under the synthesis conditions. After the Diels–Alder reactions with the pyrrole derivatives, all samples gave an intense signal at 147 ppm demonstrating that the ethenylene units embedded into the pore walls remained mostly unalterable through the Diels–Alder reaction. Additionally, the presence of new bands in each different material corroborated the formation of surface adducts. Fig. 4 depicts the 13CP/MAS NMR spectra and the assignments of each signal for each surface Diels–Alder adduct. Thus, all modified ethenylene-bridged PMOs showed new signals in the upfield region associated to the saturated carbons present in the surface adduct. For instance, P-e-PMO showed two new upfield signals at 46 and 17 ppm associated to the surface Csp3 carbons. Meanwhile, these carbons were observed at 52 and 13 ppm in the material after Diels–Alder with 1-phenylpyrrole. In this latter, the carbon signals of the aromatic ring fell within the main peak which makes it not possible to identify. Furthermore, this material showed less intense signals due to the steric hindrance of phenyl groups in the Diels–Alder cycloaddition. For MP-e-PMO and DMP-e-PMO three signals corresponding to the saturated carbons appeared in the range 65–10 ppm. These were more intense for MP-e-PMO which confirmed the extent of the Diels–Alder reaction in a major proportion by using N-methylpyrrole as diene. In the case of TMP-e-PMO, new peaks were appreciated in the upfield region. Thus, signals at 26, 37 and 19 ppm were associated to the CH3– groups bonded to the quaternary carbon, to the N-atom or Si-atom, respectively. For this case, the signal of the quaternary carbons around 70 ppm matched up with the surfactant signals. Moreover, additional resonances at 137 and 130 ppm appeared in all samples after the Diels–Alder reactions. These were similar to the signals observed in a pure vinylsilica [31], thus corroborating the thermal
transformation of some –CHdouble bond; length as m-dashCH– double bonds to –CHdouble bond; length as m-dashCH2 vinyl groups. As can be observed in the assignments of each signal in Fig. 4, the external Cdouble bond; length as m-dashC group from the Diels–Alder adduct would be situated at 135 ppm for P-e-PMO, MP-e-PMO and PhP-e-PMO and 137–138 ppm for DMP-e-PMO and TMP-e-PMO. For these latter, an increase in relative intensity of this peak at 138 ppm confirmed the effectiveness of the Diels–Alder reaction. Signals associated to unreacted diene adsorbed on the material were only present for P-e-PMO (i.e. 108 and 120 ppm).
