Absorption and phosphorescence spectra of 4-nitropyridine N-oxides and 4- and 3-nitroquinoline N-oxides

Transient absorption spectroscopic studies on 4-nitroquinoline N-oxide: From femtoseconds to microseconds timescale

The singlet excited state of 4-nitroquinoline N-oxide (¹NQNO*) has been characterized by different spectroscopic techniques, combining transient absorption with steady state and time-resolved emission spectroscopy. The energy of ¹NQNO* has been established as 255 kJ/mol from the fluorescence spectrum, whereas its lifetime has been found to be 10 ps in the femto-laser flash photolysis (LFP) experiments, where a characteristic S₁S_n absorption band with maximum centered at 425 nm is observed. In a first stage, the triplet excited state of NQNO (³NQNO*) has also been characterized by emission spectroscopy in solid matrix, at low temperature. Thus, from the steady-state phosphorescence spectrum the triplet energy has been estimated as 183 kJ/mol, whereas the setup with time resolution has allowed us to determine the phosphorescence lifetime as 3 ms. Formation of ³NQNO* by intersystem crossing in solution at room temperature, has been monitored by femto-LFP, which shows the appearance of a band with maximum at 560 nm (T₁-T_n). It increases with the decreasing intensity of its precursor 425 nm (S₁S_n) band, giving rise to an isosbestic point at 500 nm. The characterization of ³NQNO* has been completed by nano-LFP, using xanthone as photosensitizer and oxygen as well as β-carotene as quenchers. In addition, quenching of ³NQNO* by electron donors (DABCO) is also observed in aprotic solvents, leading to the radical anion of NQNO (^-NQNO). If there is a proton source in the medium (Et₃N as electron donor or MeCN:H₂O/4:1 as solvent system) protonation of the radical anion results in formation of the neutral radical of NQNO (NQNOH). In general, all processes are slower in protic solvents because of the solvation sphere. Overall, this information provides a deeper insight into the formation and behavior of excited states and radical ionic species derived from the title molecule NQNO.

Femtosecond Studies of Charge-Transfer Mediated Proton Transfer in 2-Butylamino-6-methyl-4-nitropyridine N-Oxide

We have unraveled the effects of an amino substituent in the ortho position on the excited-state dynamics of 4-nitropyridine N-oxide by studying the picosecond fluorescence kinetics and femtosecond transient absorption of a newly synthesized compound, 2-butylamino-6-methyl-4-nitropyridine N-oxide, and by quantum chemical calculations. Similar to the parent compound, the S(1) state of the target molecule has significant charge-transfer character and shows a large (approximately 8000 cm(-1)) static Stokes shift in acetonitrile. Analysis of the experimental and the theoretical results leads, however, to a new scenario in which this intramolecular charge transfer triggers in polar, aprotic solvents an ultrafast (around 100 fs) intramolecular proton transfer between the amino and the N-O group. The electronically excited N-OH tautomer is subsequently subject to solvent relaxation and decays with a lifetime of approximately 150 ps to the ground state.

Theoretical investigation of the linear and second-order nonlinear susceptibilities of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal

The linear and nonlinear optical properties of the 3-methyl-4-nitropyridine-1-oxyde (POM) crystal have been evaluated using semiempirical quantum chemistry techniques. The scheme includes (i) the evaluation of the polarizability and first hyperpolarizability of increasingly large one-dimensional, two-dimensional, and three-dimensional clusters of POM, (ii) the use of the time-dependent Hartree-Fock approach to determine the static and dynamic responses in combination with semiempirical Austin model 1 Hamiltonian, (iii) the assessment, for the POM monomer and dimer, of the electron correlation effects using second-order Moller-Plesset perturbation theory with several basis sets containing polarization and diffuse functions, (iv) the assessment of the validity of the multiplicative scheme and its application to get effective polarizability and first hyperpolarizability of the POM unit cell in the crystal, (v) the use of a sum-over-states approach to attribute the first hyperpolarizability to a dominant charge-transfer excited state, and (vi) comparison with experimental data as well as with calculated values obtained using the oriented gas approximation.

Crystal structure and spectral considerations of 5-methyl-2-nitraminopyridine N-oxide

The crystal structure of 5-methyl-2-nitraminopyridine N-oxide (5M) has been determined and its electronic absorption spectra in the solution have been investigated. In the solid, two monomeric proton-transferred (PT) units of 5M are perpendicularly located in the crystal lattice. They are connected by two intermolecular hydrogen bonds of different strength (O-H···N 2.647(3) and 2.695(3) Å). In the gas phase the same relative arrangement of both 5M (PT) monomers has been found. As results from semiempirical PM3-CI calculations (12–14), the prototropic amino (H) [Formula: see text] imino (PT) equilibrium may occur in the solution. Assignment of experimental absorption bands to the calculated transition energies of both forms of 5M reveals that hydrogen bonded 5M (PT) dimers are dominant in cyclohexane while imino (PT) monomers seem to prevail in acetonitrile solution. The different character of electronic transitions and different relative arrangement of both monomers in amino (H) than in imino (PT) dimers are responsible for very low oscillator strengths and dipole moments of the former.Key words: crystal structure, methylated nitraminopyridine N-oxides, semiempirical PM3-CI computations, intermolecular hydrogen bonding.