Ultrafast excited state dynamics of pyridine N-oxide derivative in solution; femtosecond fluorescence up-conversion and theoretical calculations.

In this study we have investigated 2-ethylamino-4-nitro-6-methyl pyridine N-oxide (2E6M) molecule that belongs to important group of Proton Coupled Electron Transfer (PCET) compounds where both the charge transfer (CT) and proton transfer processes in excited states may proceed. In this case, this is possible due to the donors and acceptors of electrons and protons in this system, as well as due to the presence of intramolecular {N-H… O [2,566(3) Å}, hydrogen bond.Using stationary and time-resolved spectroscopy, as well as quantum chemical calculations on the DFT and TD DFT B3LYP/6-31G (d,p) level of theory, a partial CT nature of the S0 â†’ S1 transition in both tautomeric forms (N and T) has been revealed. Additionally, the excited state intramolecular proton transfer (ESIPT) process shown to be more favorable in apolar and weakly polar solvents than in strongly polar acetonitrile (EN(S1) > ET(S1). The displacement of charge from the amine group and the ring to the nitro group has been observed on the changing shapes of the HOMO and LUMO orbitals involved in this transition what further quantitatively allowed to realize the increase in the dipole moment of both forms in the electronic excited state. The calculations show that in two solvents with radically different polarity (heptane, acetonitrile), dipole moments of both forms are very similar [in acetonitrile uN(S1) and uT(S1) are 11.0 D and 11.5 D, respectively]. Hence, in polar media both forms can be stabilized in a comparable manner. This made it difficult for us to assign a single fluorescent band in acetonitrile to one of the tautomeric forms. However, it seems that due to application of time-resolved spectroscopy, this problem has been clarified. The TCSPC decay curve in acetonitrile with an ultrafast lifetime assigned to the (N) form, along with the femtosecond up-conversion signals that demonstrated only an ultrafast decay without any rise-time of a new excited (T) species, allowed us to conclude that in 2E6M in strongly polar solvent the ESIPT does not occur.The unique fluorescence band origins from the (N) form. In protic solvents, the significant kinetic isotopic effects have provided us with conclusive evidence for the presence of the solvent-assisted ESIPT process. Furthermore, it was noticed that the fluorescence lifetime in D2O (100-120 fs) estimated from the up-conversion signals is about 40 times shorter relative to methanol. This may suggest that the sine qua non for the ESIPT process in 2E6M in protic solvents is the formation of a complex with a solvent molecule in the hydrogen bridge between the proton donor and proton acceptor, respectively.

Luminescent properties of chameleon-like metal-organic framework between zinc(II) dichloride and two quinoline-N-oxide molecules.

Zinc compounds in the form of inorganic/organic hybrids containing both zinc halides and heterocyclic ligands show various interesting optical and physicochemical properties. Due to these properties, there is a potential for development of various innovative technologies and applications within the life sciences. In this study, experimental and theoretical results on the absorption and emission (steady state and time-resolved) properties of the hybrid ZnCl2(QO)2 complex formed between ZnCl2 and quinoline N-oxide, have been reported. Single crystal X-ray analysis revealed the tetragonal cell (Z = 4) with P41212 space group and a slight crystal distortion. Interestingly, experiments in aprotic solvents show that both absorption and emission spectra peak in the ultraviolet (UV) region suggesting a weak CT character of the emissive S1 state, confirmed by a middle Stokes shift values. The results of the nanosecond time-resolved emission spectroscopy suggest two different structures of the complex described by the two different lifetimes and variable amplitudes dependent on the polarity of the medium. In the solid state, a relatively strong, bright blue luminescence appears at 413 nm (τ = 2.26 ns). Theoretical calculations (DFT and TD DFT) confirm experimental studies and reveal the solvent-dependent chameleon properties of ZnCl2(QO)2 by two different structures in two solvents of a contrast polarity. In apolar cyclohexane (CHX, µ = 5.612 D), the planes of both lateral quinoline N-oxide (QO) rings show to be nearly parallel each to another, resembling the crystal structure, while in a strongly polar acetonitrile (AN, µ = 9.328 D) they are nearly perpendicular. Such parallel arrangement of quinoline rings of ZnCl2(QO)2 complex in weakly polar methylcyclohexane can hinder the process of Photoinduced Electron Transfer, resulting in a stronger emission and significant quantum yield in comparison to more polar media.

Photophysical behavior of a potential drug candidate, trans-[2-(4-methoxystyryl)]quinoline-1-oxide tuned by environment effects.

Styryl and/or quinoline structural fragments, present in a large number of bioactive substances, inspired the design of various new drug candidates. In this paper, we describe the photophysical behavior of trans-[2-(4-methoxystyryl)]quinoline-1-oxide (trans-MSQNO) on the basis of X-ray analysis data, theoretical calculations as well as steady state and time-resolved spectroscopy experiments in various media. The molecule crystallizes in orthorhombic unit cell containing eight molecules of N-oxide, space group Pbca. The NO bond is substantially shorter in comparison with the NO bond in the ZnTPP unit [1.3052(11) Å vs. 1.335(2) Å]. Variation of emission colors from the violet (~450nm) through blue (480nm), green (525nm) and yellow (575nm) is observed in different environments. Comparable values of lifetimes estimated both at ambient temperature and at 77K suggest that excited state dynamics in this case is viscosity independent. DFT and TD DFT B3LYP/6-31G(d, p) calculations performed for four different trans-MSQNO rotamers in the gas phase, as well as nonpolar and polar media (PCM model) suggest that an equilibrium between them can be significantly altered even by a relatively weak interactions with the environment. It is suggested that varying intensity ratios of experimental absorption bands in different media may be due to the dominant share of one or more rotamers of the excited trans-MSQNO molecule. Gas phase calculations show also that the vertical ππ*, S0→S1, transition resulting from the HOMO→LUMO electronic configuration exhibits only a partial CT nature. On the other hand, in polar media, a substantial increment of excited state dipole moment of all rotamers compared to the ground state, its increase with increasing solvent polarity and a significant red shift in the absorption and emission spectra, point to the enhanced CT nature of the S1 excited state. Hence, the trans-MSQNO molecule may be considered a subsequent styrylquinoline drug candidate where the CT drug-receptor interactions are of a high importance.

Ultrafast Excited-State Dynamics of a Cyano-Substituted "Proton Sponge".

The dynamics of a substituted proton sponge-the 1,8-bis(dimethylamino)-4-cyanonaphthalene (DMAN-CN) molecule-was investigated after excitation in the S1 state. Experimental and theoretical information are reported. The former includes absorption, fluorescence, and time-resolved transient absorption spectra, which were recorded in solution. Real-time dynamics measurements were also performed on gas-phase isolated DMAN-CN. TD DFT/6-31G(d,p) level and CIS/6-31G(d,p) excited-state calculations complement these results. This has allowed revisiting the energy transfer process between a locally excited (LE) and a charge transfer (CT) state, which is often invoked with this kind of molecule.

Ultrafast excited state dynamics of trans-[4-(4'-dimethylaminostyryl)] pyridine N-oxide in solution: femtosecond fluorescence up-conversion and theoretical calculations.

The results of the steady-state and time-resolved fluorescence-spectroscopy measurements and DFT calculations for trans-[4-(4'-dimethylaminostyryl)] pyridine N-oxide (trans-DPO) in various solvents are presented. These results are similar to those reported in the literature for trans-4-(dimethylamino)-4'-cyanostilbene (DCS) where the S1 emissive state shows the charge-transfer (CT) nature. Alcohol solvents, however, have aroused our particular interest because hydrogen-bonded complexes are formed between them and the trans-DPO molecule. They demonstrate a stronger CT character of the lower lying excited state and a larger separation between the first and the second absorption band than in the free trans-DPO molecule. The different effects found in time-dependent femtosecond up-conversion (performed for several emission wavelengths in dioxane and methanol solution over the time range 0-40 ps) were assigned to the hydrogen bond assisted charge transfer process in trans-DPO in methanol, to the excited-state solvation dynamics and mainly to solvent relaxation of trans-DPO dissolved in dioxane and methanol.

Impact of micelle ionic electrical double layer structure on the excited state protolytic reaction in the fluorescent probe bound to the colloidal nanoparticles.

The fluorescent probe, 2-hydroxynaphthalene(dodecylo)-6-sulfonamide (NSDA) bound selectively to shear plane of various electrostatic charges was synthesized and its photophysical properties have been investigated by means of steady state fluorescence and nanosecond time-resolved spectroscopy. Our experimental data allowed us to determine the excited state proton transfer (ESPT) rate and equilibrium constants of NSDA bound to micelles and to estimate the electric potential value (Ψ) at the particle surface. The spatial dependence of proton movement velocity through electric double layer (EDL) of micelles has been thoroughly analyzed. In this article, a new approach of estimating the values of the micelle potential (Ψ(R)) from the excited state proton transfer rate constant of the fluorescent probe bound at a certain distance (R) to a micellar surface has been proposed. The Ψ(R) values, obtained in this way, are compared with electrophoretic values of the particle potential (ζ). Our results on electrophoretic potentials and the reaction course of the ESPT in colloidal environment may contribute to a deeper understanding of micellar interactions and behavior of the living cells in contact with various diluted substances such as pharmacological drugs, hormones, proteins, and other colloidal particles.

Excited state processes of 2-butylamino-6-methyl-4-nitropyridine N-oxide in nonpolar solvents. A transient absorption spectroscopy study.

Earlier steady-state fluorescence studies showed that 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) can undergo fast excited-state intramolecular proton transfer (ESIPT). In a nonpolar solvent such as n-octane, both normal and tautomeric fluorescence was observed. Strikingly, the relative ratio of those two emission bands and the fluorescence quantum yield of the normal emission were found to depend on the excitation wavelength in violation of the Kasha-Vavilov rule. In this work, the system was investigated further by means of transient absorption spectroscopy, followed by global and target analysis. Upon excitation at 420 nm, a normal excited singlet state S(1)(N) is reached, which decays in about 12 ps via fluorescence and ESIPT (minor pathways) and to a long-lived "dark" state (major pathway) that is most probably the triplet T(1)(N). Upon 330 nm excitation, however, a more complex pattern emerges and additional decay channels are opened. A set of four excited-state species is required to model the data, including a hot state S(1)(N)* that decays in about 3 ps to the tautomer, to the long-lived "dark" state and to the relaxed S(1)(N) state. A kinetic scheme is presented that can explain the observed transient absorption results as well as the earlier fluorescence data.

Excited-state forms of 2-methylamino-6-methyl-4-nitropyridine N-oxide and 2-butylamino-6-methyl-4-nitropyridine N-oxide.

Excited-state quantum chemical calculations of two 2-alkyloamino-6-methyl-4-nitropyridine N-oxides are presented. Several different calculation methods and different basis sets were used, which all lead to similar results, although the precise values of excited-state energies and excited-state dipole moments differ. All methods used predict that in the S(1) excited state four types of isomers occur. In three cases, these excited-state local energy minima correspond to ground-state isomers, and these all have a pi pi* character. The fourth excited-state minimum, which we denote L*, does not have a corresponding ground-state isomer and has an n pi* character. This isomer is stable and plays an important role in understanding the photophysics of these molecules. In addition, we also calculated barriers between these excited-state minima, using predescribed reaction pathways. The theoretical results derived in this Article are confronted with experimental data from earlier papers.

Cryogenic fluorescence and absorption spectroscopy studies on monomeric and dimeric species of 2-butylamino-6-methyl-4-nitropyridine N-oxide.

2-Butylamino-6-methyl-4-nitropyridine-N-oxide (2B6M) belongs to a group of compounds that can undergo not only excited-state intra-, but also intermolecular proton transfer. The latter of course requires the presence of dimeric species. Previously, we have shown that for 2B6M in aprotic non-polar solvents in the liquid state such dimers play no role. Under these conditions, only one single monomeric species exists, exhibiting anomalous fluorescence behavior, i.e. proton transfer not only starting from the lowest excited electronic singlet state, but also from higher excited states. However, we also noted that under frozen, crystalline matrix conditions more species show up in the spectra. In order to study this multi-species system in more detail, we present absorption and fluorescence experiments on 2B6M, recorded in n-octane at various temperatures between 293 and 5 K. High-resolution spectra are included, not only in fluorescence but also in absorption. We demonstrate that under cryogenic conditions three species can be discerned, two of these providing high-resolution spectra with their main 0-0 lines around 452 and 465 nm, respectively. A detailed vibrational analysis of their emission spectra is included. The third species gives broad-banded spectra, in absorption extending to about 520 nm with its long-wavelength maximum around 460 nm, in emission with a maximum around 535 nm. We tentatively assign the three species to a monomer, a H-bonded dimer and a strongly interacting (pi-pi-stacked) dimer, respectively. We conclude from the excitation spectra that (anomalous) intramolecular proton transfer at higher excited states is still operative under cryogenic conditions. Indications for excited-state intermolecular proton transfer in the stacked dimeric species were not found.

X-ray crystal structure and magnetic and photophysical properties of novel copper(II) N-oxide adduct [(4-MPyO)2(CuCl2)2(H2O)(C2H5OH)] (4-MPyO = 4-(4-methoxystyryl)pyridine N-oxide).

A new mixed adduct, (4-MPyO)2(CuCl2)2(H2O)(C2H5OH) [where 4-MPyO is the 4-(4-methoxystyryl)pyridine N-oxide], was obtained for the first time. It has been characterized by X-ray studies, IR, electronic absorption, and emission spectra, lifetime measurements, and variable-temperature magnetic susceptibility measurements in the range 80-300 K. The single-crystal X-ray diffraction shows that the geometry around both of the copper(II) ions can be described as a tetragonal pyramid with a trapezoidal base at the corners of which are two oxygen atoms of N-oxide and two chlorine atoms. The oxygen atoms of either water or ethanol are at the apex of the pyramid. Besides that, two molecules of the adduct form a double-hydrogen-bonded superdimer in which they are connected to each other through hydrogen bonds of the O-H...Cl type as formed between the chlorine atoms and ethanol molecule (Cl...O 3.22 A). The copper(II) atoms are antiferromagnetically coupled within a dimeric unit, and a singlet-triplet separation of 2 J value (1100 cm(-1)) is greater than the value expected from Hatfield's rule for the bridging angles Cu-O-Cu equal 108.9 degrees and 110.2 degrees . By means of the PM3-calculated values of vertical excitation energies, the ligand-to-metal charge-transfer (LMCT) and the metal-to-ligand charge-transfer transitions in the unresolved experimental absorption spectra of I have been revealed. From the large Stokes shift value of emission spectra in solvents of different polarity (more than 6500 cm(-1) in acetonitrile), the charge-transfer (CT) nature of the emissive (LMCT) state of I has been concluded. Biexponential decay of the excited complex in acetonitrile and frozen propanol suggests that the two different CT conformers (0.8, 4.12 ns and 1.99, 15.2 ns, respectively) are present in the excited state in solution while only one CT form is indicated by a monoexponential decay (9.0 micros) in the solid.

Intramolecular proton-transfer processes starting at higher excited states: a fluorescence study on 2-butylamino-6-methyl-4-nitropyridine N-oxide in nonpolar solutions.

This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations.

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.

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.

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.

Femtosecond excited state studies of the two-center three-electron bond driven twisted internal charge transfer dynamics in 1,8-bis(dimethylamino)naphthalene.

Femtosecond fluorescence upconversion and transient absorption experiments have been performed to monitor the photoinduced electronic, geometry, and solvent relaxation dynamics of 1,8-bis(dimethylamino)naphthalene dissolved in methylcyclohexane or n-hexane, n-dodecane, dichloromethane, and acetonitrile. The data have been analyzed by using a sequential global analysis method that gives rise to species associated difference spectra. The spectral features in these spectra and their dynamic behavior enable us to associate them with specific processes occurring in the molecule. The experiments show that the internal charge-transfer lpi* state is populated after internal conversion from the 1La state. In the lpi state the molecule is concluded to be subject to a large-amplitude motion, thereby confirming our previous predictions that internal charge transfer in this state is accompanied by the formation of a two-center three-electron bond between the two nitrogen atoms. Solvent relaxation and vibrational cooling in the lpi* state cannot be separated in polar solvents, but in apolar solvents a distinct vibrational cooling process in the lpi* state is discerned. The spectral and dynamic characteristics of the final species created in the experiments are shown to correspond well with what has been determined before for the relaxed emissive lpi state.