Insights on the Auxochromic Properties of the Guanidinium Group

Abstract

UV/vis spectra of phenylguanidine (PHGU) in the gas phase and in acetonitrile have been simulated by TD-DFT calculations. Several DFT hybrid and long-range corrected functionals were tested with respect to CASPT2 gas phase calculations. Solvent effects were considered using polarizable continuum model (PCM) and compared with the measured data in acetonitrile. Comparison with isoelectronic phenylurea and related phenyltiourea was done as well. The PBE0 and long-range corrected CAM-B3LYP functionals were selected to investigate the effect of protonation on the excitation energies and absorption intensities of PHGU and several guanidine derivatives with different aromatic chromophoric groups (naphthyl, anthracenyl, quinolinyl, anthraquinonyl, and coumarinyl). Also, the effect of complexation and specific interactions through hydrogen bonds with different anions was examined. It was shown that the protonation of the guanidine subunit shifts the low energy absorption bands toward higher energies (hypsochromic shift). The shift is reduced upon complexation with anions. In phenylguanidine salts, λmax values are correlated to the anion basicity and strength of H-bonding. The observed changes diminish upon increase of chromophoric size (naphthyl, anthracenyl). Theoretical predictions of UV/vis spectra correlate well with experimentally measured spectra of selected guanidine derivatives and their salts.

Benzoylguanidines as Anion‐Responsive Systems

“…While deprotonation is not desirable in context of supramolecular chemistry and geometry‐based selectivity,,, it may be useful in indicators when a large UV bathochromic shift is preferred over the actual binding. Deprotonation generally leads to larger shift in absorption maximum with respect to the anion binding ,. Drawback related to the incorporation of the benzoyl (acyl) moiety is the formation of intramolecular hydrogen bonds (IHB) involving protons usually aimed to bind anions which reduces usefulness of the receptor.…”

“…This is not surprising having in mind the expected large difference in p K a between acetate, (Ac), dihydrogenphosphate (DHP) and fluoride (F) anions and target benzoylguanidines of more than 2 p K a units. This result is in line with our previous work where ionic complexes (salt bridges) of arylguanidines were observed only for phenylguanidine/acetate and naphthylguanidine/formate; i. e. with the anions having the basicity within 1–2 p K a units from the target guanidine derivative. Regardless, we tried to fit our titration experiments both to the formation of complex according to the Eq.…”

“…Namely, for the calculations of excitation energies in a donor‐acceptor system, the optimized long‐range corrected functionals outperformed the default ones . In our previous work, we also obtained better agreement of the calculated and experimental UV spectra in a series of arylguanidines using optimized CAM‐B3LYP approach ( ω =0.20 a 0 −1 instead of default value 0.33 a 0 −1 ) . Compound 1 was taken as the model system for optimization of ω parameter and its value is varied from 0.10 to 0.20 in steps of 0.01 a 0 −1 .…”

“…For successful design of new anion sensors, a good computational model is needed which can simulate electronic spectra of both neutral and protonated structures with similar accuracy. Recently we have investigated changes in UV spectra of arylguanidines and their salts employing modern TD‐DFT approaches . Comparison with CASPT2 and experimental data revealed two well‐established DFT functionals (PBE0, and CAM‐B3LYP) as the best choice from the viewpoint of computational cost, efficiency and accuracy.…”

Redox‐Active Guanidines with One or Two Guanidino Groups and Their Integration in Low‐Dimensional Perovskite Structures

“…This large hypsochromic shift could be rationalized by the interactions between the N–H protons and the two carbonyl oxygen atoms. In this context it is worth mentioning that hypsochromic shifts upon protonation of not‐redox‐active mono‐guanidine compounds, including guanidino‐benzene, and the influence of intermolecular hydrogen‐bonding to anions were recently analysed . A similar hypsochromic shift was observed upon protonation of 7 [from λ max = 536 nm for 7 to λ max = 462 nm for ( 7 +2H)(PF 6 ) 2 ].…”