Crystal Structure, Spectroscopic, and Theoretical Investigations of Excited-State Proton Transfer in the Doubly Hydrogen-Bonded Dimer of 2-Butylamino-6-methyl-4-nitropyridine N-Oxide

Abstract

The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.

Theoretical Analysis of the Electronic Ground and Excited Franck-Condon State of 2-Alkylamino-(3, 5, or 6)-Methyl-4-Nitropyridine N-oxides

“…This process was estimated as a very fast one in polar solvents and described as being triggered by creation of the excited intramolecular CT state. From the other hand, in nonpolar medium such process was shown to be much slower [25,26]. An interesting conclusion was drawn out, namely, it was suggested that the observed excitation wavelength dependence of the quantum yield is due to existence of the two different intramolecular proton transfer processes [26].…”

“…Some of EDA systems showing the dual fluorescence can be used in the fabrication of molecular switches [21]. The N-oxides molecules, under considerations, show the dual fluorescence, too [20,25,26].…”

“…1). The fluorescence spectra are composed of normal and anomalous (with large Stokes shift) bands [20,25,26]. Both, absorption and fluorescence, spectra are strongly solvent dependent [20, 25, and 27].…”

“…The crystal structure of considered N-oxides was described only for 2B6M [25] and for 2-ethylamino-4-nitropyridine N-oxide (2E) molecules [28]. In both cases the crystal packing takes place with participation of intermolecular hydrogen bonds but in the case of 2B6M, the molecule crystallises in the especially beautiful form of a double hydrogen-bonded dimer, which consists of two internally hydrogen-bonded monomers [21].…”

“…Namely, electronic spectra of 2M3M and 2M5M were calculated by intermediate neglect of differential overlap (INDO) method [27] whereas dipole moments of the same molecules were calculated by means of vectorial summation of the group moments and by ab initio Hartree-Fock (HF) 3-21G method [29]. The density functional theory (DFT) B3LYP/6-31G(d,p) method was used for calculation of IR spectrum of 2-ethylimino-4-nitropyridine N-oxide [28] and electronic spectrum of 2B6M [20,25]. The RHF/6-311G++(d,p) method was used in the studies on acid-base interactions for 2M6M [30].…”