I. Deperasińska

12-hydroxy-1-azaperylene-limiting case of the ESIPT system: enol-keto tautomerization in S0 and S1 states.

Absorption, fluorescence, and fluorescence excitation spectra of 12-hydroxy-1-azaperylene (HAP) and 1-azaperylene were studied in n-alkane matrices at 5 K. Two stable tautomers of HAP, each of them in n-nonane embedded in two sites, were identified and attributed to the enol and keto forms. Theoretical calculations of the energy and vibrational structure of the spectra suggest that tautomer A, with the (0, 0) transition energy at 18,980 ± 10 cm(-1) (and 19,060 ± 10 cm(-1) in the high energy site), should be identified as the keto form, whereas tautomer B, with the (0, 0) energy at 19,200 ± 20 cm(-1) (19,290 ± 20 cm(-1)), as the enol form. Observation of absorption and fluorescence of both tautomeric forms and lack of large Stokes shift of fluorescence of the keto form classify HAP as the limiting case of the excited-state intramolecular proton transfer system.

Low-Temperature Spectra of the Analogues of 10-Hydroxybenzo[h]quinoline as an Indication of Barrierless ESIPT

The absorption and fluorescence spectra of two analogues of 10-hydroxybenzo[h]quinoline (10-HBQ), namely, 1-hydroxy-7-methylbenzo[c]acridine (HMBA) and 4-hydroxybenzo[c]phenanthridine (HBPA), were studied in n-alkane matrices at 5 K. Considerable energy separation between the onsets of the spectra and broadening of the bands was an indication that intramolecular proton transfer (ESIPT) takes place at such a low temperature. DFT and ab initio methods were used to calculate the electronic transition energies and oscillator strengths and the vibronic structure of the electronic spectra. Shortcomings in our knowledge of the shape of the potential energy surface for ESIPT systems are highlighted in the context of the discussion of the shape of the electronic spectra. The π-expansion of the 10-HBQ chromophore achieved by adding a benzene moiety at various positions adjacent to the pyridine ring led to compounds possessing diverse photophysical properties, ranging from the non-ESIPT strongly fluorescent molecule of 10-hydroxy-1-azaperylene to weakly emitting (or nonemitting) molecules, where ESIPT occurs very efficiently.

Dipolar Dyes with a Pyrrolo[2,3‐b]quinoxaline Skeleton Containing a Cyano Group and a Bridged Tertiary Amino Group: Synthesis, Solvatofluorochromism, and Bioimaging

Two strongly polarized dipolar chromophores possessing a cyclic tertiary amino group at one terminus of the molecule and a CN group at the opposite terminus were designed and synthesized. Their rigid skeleton contains the rarely studied pyrrolo[2,3-b]quinoxaline ring system. The photophysical properties of these regioisomeric dyes were different owing to differing π conjugation between the CN group and the electron-donor moiety. These dipolar molecules showed very intense emission, strong solvatofluorochromism, and sufficient two-photon brightness for bioimaging. One of these regioisomeric dyes, namely, 11-carbonitrile-2,3,4,5,6,7-hexahydro-1H-3a,8,13,13b-tetraazabenzo[b]cyclohepta[1,2,3-jk]fluorene, was successfully utilized in two-photon imaging of mouse organ tissues and showed distinct tissue morphology with high resolution.

Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2-a]pyridine derivative has been achieved for the first time. In cases in which the most-electron-rich position of the electron-excessive heterocycle was blocked by a naphthalen-1-yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2-de]naphtho[1,8-ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion-radical coupling reaction, which employed dry O2 from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2-a]pyridine in 63% yield. Interestingly, whereas the fluorescence quantum yield (Φ(fl)) of compound 3, despite the freedom of rotation, was close to 50%, the Φ(fl) value of flat naphthalene-imidazo[1,2-a]pyridine was only 5%. Detailed analysis of this compound by using DFT calculations and a low-temperature Shpolskii matrix revealed phosphorescence emission, thus indicating that efficient intersystem-crossing from the lowest-excited S1 level to the triplet manifold was the competing process with fluorescence.

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 Hatfields 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.

Structural and spectral analysis of 3- and 4-methylated nitraminopyridine N-oxides

Abstract The X-ray structures of 3-methyl-2-nitraminopyridine N -oxide (3M) and 4-methyl-2-nitraminopyridine N -oxide (4M) has been determined and experimental electronic absorption spectra investigated. The X-ray results show that both compounds occur as dimers either in the molecular form (3M) or in the proton transferred (PT) form (4M). A ‘driving force’ for protonation depends on the relative position of the two monomers in the crystal lattice (parallel for 3M and perpendicular for 4M) and on the degree of electronic conjugation of –NH–NO 2 substituent with the π-system of the pyridine ring. Position of the methyl group does not affect the direction of the CT process. Quantum-chemical semi-empirical PM3-CI calculations allowed an interpretation of experimental electronic absorption spectra and suggested the prototropic amino–imino equilibria which possible to occur in the solution.

Fluorescence excitation and fluorescence spectra of jet-cooled acridine molecules: acridine dimer formation and structure

Abstract The fluorescence excitation and fluorescence spectra from a supersonic helium jet seeded with acridine molecules have been obtained. The presence of low-frequency vibrational transitions in the fluorescence excitation spectrum, which can be assigned as intermolecular vibrational modes, clearly indicates that acridine dimers are formed under jet-cooling conditions. The acridine dimer ground-state geometry determined by AM1 calculations is a head-to-tail-like structure with an almost collinear arrangement of the short molecular axes of both molecular components. The results of calculations of spectral characteristics predict a large enhancement of the oscillator strength for the transition from the ground to first excited singlet state of the dimer (as compared to acridine molecule). Such enhancement is entirely connected with structure of the dimer (which presumably is also stabilized by the formation of a hydrogen bond).

The analysis of excited-state equilibria of weak electron donor–acceptor systems:: Tetracyanobenzene–toluene complex

Abstract The electron donor–acceptor system of 1,2,4,5-tetracyanobenzene–toluene is a prime example of a weak charge-transfer complex which is stable in the ground state and which is also capable of forming an exciplex in diffusion-controlled reaction in liquid solution (in non- or low-polarity solvents). The potential energy surfaces calculated for the ground and excited state of this system reveal the presence of different orientational isomers of the isolated complex (of charge-transfer and of van der Waals character). An extended photokinetic scheme which includes excitation of these isomers as well as the diffusion-controlled formation of an exciplex has been proposed and analyzed. Computer simulations of the time profiles of emission of the investigated system performed within the framework of such a photokinetic scheme were in satisfactory agreement with experimental observations. This leads to the conclusion about the existence of different Franck–Condon excited states of the TCNB–toluene system in liquid solutions with relaxation pathways strongly controlled by the physical conditions of the solution.

π-Expanded 1,3-diketones – synthesis, optical properties and application in two-photon polymerization

Four π-expanded α,β-unsaturated 1,3-diketones have been prepared via attaching strong electron-donating and electron-withdrawing groups at positions 9 and 10 of the anthracene scaffold. The strategic incorporation of (C12H25)2N groups at the periphery of these D–π–A molecules resulted in dyes with excellent solubility in most organic solvents. These non-planar diketones possess very broad absorption of light and negligible fluorescence. The two-photon absorption cross-section was measured via Z-scan as well as by two-photon excited fluorescence. The results clearly confirm that, in the case of compounds possessing such small fluorescence quantum yields, the Z-scan was a more reliable approach. Depending on the chemical structure, these compounds exhibited two-photon absorption cross-sections (σ2) up to 2500 GM at 725 nm. For the first time an α,β-unsaturated ketone derived from a proton sponge has been synthesized and was shown to possess optical features distinct from its simpler analogs, such as weak emission. All studied ketones possessed two-photon absorption cross-sections ∼200 GM at wavelength of two-photon polymerization (i.e. 800 nm) and broad fabrication windows have been achieved using all these dyes as photoinitiators.

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.