The impact of solvent polarity on intramolecular proton and electron transfer in 2-alkylamino-4-nitro-5-methyl pyridine N-oxides.

The crystal structure of 2-butylamino-4-nitro-5-methyl pyridine N-oxide (2B5M) and solution studies of both 2B5M and 2-methylamino-4-nitro-5-methyl pyridine (2M5M) N-oxide are presented. Steady-state absorption and emission measurements were employed for both molecules while a picosecond fluorescence up-conversion technique was used to follow the dynamic behavior of the 2M5M system. The experimental methods were complemented by DFT and TD DFT B3LYP/6-31G(d,p) calculations involving ground and excited-state optimization which in the case of the smaller 2M5M molecule were extended to the CAM-B3LYP/6-31G(d,p) method. The solvent effect is incorporated by applying the polarizable continuum (PCM) model. The data reveal that the 2B5M molecule crystallizes in the monoclinic space group P2(1)/c and its crystal lattice is composed of monomers with intramolecular N-H···O [2.572(3) Å] hydrogen bonds, connected into a polymer network by weak intermolecular C-H…O [3.2-3.4 Å]-type interactions. Quantum-chemical calculations show that the aminoalkyl substitutent in aminoalkyl-pyridine N-oxides is a specific determinant of the CT nature of the lowest-lying excited electronic ππ* state, distinguishing them from other nitroaromatic compounds. The results of both picosecond fluorescence up-conversion experiments in different solvents and quantum-chemical calculations suggest that in nonpolar media the ESIPT process in 2M5M is favored, while in polar acetonitrile, the N* → PT* transition demands barrier-crossing and thus unfavorable thermodynamic conditions do not allow the ESIPT to occur. The signals of picosecond fluorescence up-conversion of 2M5M are solvent- and emission-wavelength dependent. The three time components found in a weakly polar isooctane-dioxane mixture have been attributed to solvation dynamics (∼500 fs), and to relaxation of N* and PT* forms while in acetonitrile, a very rapid fluorescence decay with a time constant (2.3-4.0 ps) indicative of the presence of the normal (N*) form was observed. Much shorter fluorescence lifetimes in alcohols (a few picoseconds) and in D(2)O (less than 200 fs) than in aprotic solvents suggest that in protic media, the solvent molecules participate in the ESIPT, bridging between the methylamine group and the N-oxide group of 2M5M.

Photophysical properties of dipeptides containing substituted 3-(quinoxalin-6-yl) alanine. Spectroscopic studies and theoretical calculations.

The photophysical properties of excited states of two hybrid dipeptides [N-(3-(2,3-diphenylquinoxaline-6-ylo)alanylo) glycine], Pe-DPhQ, and [N-(3-(2,3 (pirydine-2-ylo) quinoxaline-6-ylo)alanylo) glycine], Pe-DPiQ, have been investigated by a combined solution-state study (absorption, emission) and quantum-mechanical (ab initio, DFT) calculations. The RHF and DFT B3LYP/6-31G (d,p) computations of the ground-state isomers of Pe-DPiQ dipeptide (open, half-closed, and closed) indicate that the most stable is the "open"-type structure with approximately equal (-44.43 degrees , -43.05 degrees ) dihedral angles describing rotation of the aromatic side rings with respect to the quinoxaline framework. This agrees with the literature findings that synthetic peptides are mostly unfolded. The experiments show that emission of Pe-DPiQ dipeptide is strongly temperature dependent, and at ambient and elevated temperatures the fluorescence is prevailing while the phosphorescence dominated emission spectra are observed at 77 K. On the basis of the decay curves that in the broad temperature range (rt-77 K) are biexponential (2 and 9 ns), it was concluded that at least its two major excited-state conformations may interconvert on the nanosecond time scale. The third component, of a small amplitude (10%) and a long time constant (25 ns), appears only in a new fluorescence band (570 nm) that grows up with the temperature increase. Analysis of the CIS/6-31G(d,p) results of the excited-state isomers of Pe-DPiQ supports the interpretation of experimental emission spectra and enables one to assign two excited-state conformations, demonstrating a tendency to keep one of their two side rings coplanar relative to the central quinoxaline plane, as Pe-DPiQ-I* (41.9 degrees , 6.3 degrees ) and Pe-DPiQ-II* (40.1 degrees , 4.5 degrees ) isomers contributing to the room temperature (403 nm) and 363 K (570 nm) fluorescence bands, respectively. The calculations also explain the electronic character of the corresponding S(1)<-->S(0) transitions and show that the state ordering of Pe-DPiQ resembles that of other diazines where the first singlet is of the npi* character while the S(2) and T(1) are the pipi* states. The reason for a strong phosphorescence is assigned to an effective spin-orbit coupling of appropriate singlet and triplet states that leads to ISC transitions and in result to population of the T(1) state and a phosphorescence from the T(1) state. From the present study, it was concluded that incorporation of quinoxaline moiety into the model peptides does not change the useful spectroscopic properties of the fluorophore and allows one to design its new analogues with improved activity and specificity.