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This paper reports on the investigations of the synthesized di-(o-hydroxyaryl ketoimine) compound by the steady state absorption and emission techniques as well as picosecond time resolved emission and femtosecond transient absorption methods in different solvents. The results of the experimental observation have been supported by the theoretical DFT and TD-DFT calculations. The theoretical data have revealed the completed influence of the environmental polarity on particular conformers of studied compound. Dependencies between the activation rate constant and polarizability function as well as Kamlet-Abbond-Taft hydrogen-bonding parameter have been obtained in different solvent. The mechanism of photodynamic changes of di-(o-hydroxyaryl ketoimine) is presented. [ABSTRACT FROM AUTHOR]

Abstract: The temperature effects on coumarin 102 steady-state absorption and fluorescence, and on the decay of its excited state are presented. Our results have shown that similar to coumarin 153, hydrogen bonding between coumarin 102 and protic solvents has an important impact on the deactivation process of the excited state of this dye. But the nature of this hydrogen bonding and especially its temperature dependence is clearly different for these coumarins. This difference leads in consequence to totally different temperature dependencies of the fluorescence lifetime and fluorescence quantum yield of both solutes. Direct experimental evidence is presented that different hydrogen bonding abilities of structurally similar coumarins may lead to distinct photophysical properties both in the ground, excited Franck–Condon and relaxed states, as well as at various temperatures. To obtain a better understanding of the differences between hydrogen bonds formed by coumarin 102 and by coumarin 153 in the excited state, fluorescence anisotropy kinetic measurements as a function of temperature were performed for both dyes as well in one highly protic solvent. Fluorescence anisotropy decays provided experimental evidence that rotation of coumarin 102 is slower than that of coumarin 153 due to stronger hydrogen bonds formed by the former dye, and that for the same reason in coumarin 102 rotation slows down more quickly than in coumarin 153 with decreasing temperature. [Copyright &y& Elsevier]

In a recent paper (J Fluoresc (2011) 21:1547-1557) a temperature induced modulation of Coumarin 153 (C153) fluorescence lifetime and quantum yield for the probe dissolved in the polar, nonspecifically interacting 1-chloropropane was reported. This modulation was also observed in temperature dependencies of the radiative and nonradiative rates. Here, we show that the modulation is also observed in another 1-chloroalkane-1-chlorohexane, as well as in hydrogen bonding propionitrile, ethanol and trifluoroethanol. Change in the equilibrium distance between S an S potential energies surfaces was identified as the source of this modulation. This change is driven by temperature changes. It leads to a modulation of the fluorescence transition dipole moment and it is the primary source of the experimental effects observed. Additionally, we have found that proticity of the solvent induces a rise in the fluorescence transition dipole moment, which leads to a shortening of the fluorescence lifetime. Hydrogen bonds are formed by C153 also with hydrogen accepting solvents like propionitrile. We show that while such bonds do not affect the transition probability, they do change the S an S energy gap which in turn implies a change in non-radiative transition rate in a similar way as in protic solvents, as well as in the fluorescence spectrum position. Finally, the influence of temperature on the energies of hydrogen bonds formed by C153 when acting as hydrogen donor or acceptor is reported. [ABSTRACT FROM AUTHOR]

Absorption, steady-state and timed-resolved fluorescence spectra of nine N-p-bromobenzyl substituted (E)-4′-hydroxy-4-stilbazolium bromides and N-p-bromo- (or nitro-) benzyl substituted (E)-4′-hydroxy-3′methoxy-4-stilbazolium bromides, belonging to the hemicyanine class of compounds, were studied in dry and water-containing polar solvents and in water. All of the studied compounds displayed negative solvatochromism. In solvents with small amounts of water the solutions of each of the compounds change color to red, blue-green or blue, while in extra dry solvents they are all yellow. The new band causing the change in color is interpreted as belonging to the deprotonated form of the respective compound. The absorption and emission spectra of protonated and deprotonated forms of hemicyanines in solvents are presented in comparison with those of selected, isolated deprotonated forms (merocyanines) of the same compounds. The discrimination between the quinoid and zwitterionic deprotonated forms was achieved based on the absorption band location. Time-resolved fluorescence measurements in selected dry and water-containing solvents were also performed. [ABSTRACT FROM AUTHOR]

Abstract: The photophysical properties of N-Boc-3-[2-(9-anthryl)benzoxazol-5-yl]-l-alanine methyl ester (BoxAnt) and N-Boc-3-[2-[4-(9′-(10′-butyl)anthryl)phenyl]benzoxazol-5-yl]-l-alanine methyl ester (BoxPhAnt) were studied in a series of solvents. Their absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show a pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the empirical solvent polarity parameter , a large change of the dipole moment on excitation for BoxPhAnt has been found. From an analysis of the solvatochromic behaviour of the absorption and fluorescence spectra in terms of bulk solvent polarity functions, f(ɛ r, n) and g(n), a larger excited-state dipole moment (about 8 D, ψ = 56) was obtained for BoxPhAnt than for BoxAnt (about 3 D, ψ = 0). Both applied methods gave similar values of the excited-state dipole moments for both compounds studied. [Copyright &y& Elsevier]

Porphyrazines possessing isophthaloxyalkylsulfanyl substituents in the periphery were synthesized and subjected to various photophysical studies, including optical absorption and emission measurements. The fluorescence lifetimes of four porphyrazine derivatives were measured in different organic solvents. The results of the steady-state and time-resolved measurements of magnesium porphyrazines with different alkyl groups between sulfanyl substituent and isophthaloxy group were compared with those of demetallated porphyrazine. The porphyrazine macrocycles exhibit interesting solvatochromic effects, which are described by Δ f solvent polarity scale and four-parameter scale proposed by Catalán. Transient absorption spectroscopy was used to obtain the values of triplet lifetime for magnesium porphyrazine with 2-(3,5-dibutoxycarbonylphenoxy)ethylsulfanyl substituents. Values of singlet oxygen generation quantum yields were obtained in different organic solvents using various techniques, including the characteristic emission of the singlet oxygen at 1270 nm with the steady-state and time-resolved approaches. [ABSTRACT FROM AUTHOR]

Photophysical properties of (E)-1-(4-bromobenzyl)-4-(4-hydroxy-3-methoxystyryl) pyridinium bromide existing in equilibrium of its protonated (BSHB) and deprotonated (BS) forms at different pH were characterized by UV–Vis spectroscopy, and steady-state, and time-resolved fluorescence techniques. Absorption, steady-state, and time-resolved emission spectra, and synchronous scanning spectra were used to explain the observed phenomena. The pK a of BSHB in the ground state was determined. We also found that the deprotonated form – BS, being a weak base, is protonated upon addition of water. The pK a* of BSHB in the excited state was determined in two different ways giving different results. This inspires the discussion on the applicability of the use of the thermodynamic Förster cycle for calculation of the excited-state pK a* for hydroxystilbazolium dyes. Solvatochromism of both BSHB and BS in polar aprotic and protic solvents was analyzed using the four-parameter Catalán solvent scale and the Δf scale. This analysis indicates that both compounds in their emissive states have benzenoid-like (or zwitterionic for BS) structure in protic solvents. However, in aprotic polar solvents the contribution of the zwitterionic structure to their emissive state is reduced in favor of the quinoid structure. Importantly, the Catalán scale shows a strong influence of solvent basicity (hydrogen-bond accepting ability) on the absorption properties of BSHB, indicating strong stabilization of its ground and vertical excited state by hydrogen-bonded complexes. Contrary to BSHB, the absorption spectra of BS are affected by solvent acidity. Hydrogen – bonded complexes stabilize the ground state of this compound, although their energy decreases strongly upon excitation. The hydrogen-bonded complexes influenced the structures of both compounds in their excited states. [ABSTRACT FROM AUTHOR]

Abstract: N-(tert-butoxycarbonyl)-3-(benzoxazol-5-yl)alanine methyl ester derivatives substituted in position 2 by heterocyclic group were synthesized and their photophysical properties in methanol, acetonitrile and methylcyclohexane were studied by means of absorption and fluorescence spectroscopies. The positions of their emission bands depend on the solvent polarity and are shifted to longer wavelengths in more polar solvents as a result of a charge transfer from a substituent to the benzoxazole moiety. High molar absorption coefficient values and fluorescence quantum yields are characteristic for most of the compounds studied making them efficient fluorescent probes. Moreover, the presence of additional heteroatom in the substituent enables those compounds to act as chemosensors for metal ions or protons. [Copyright &y& Elsevier]

Abstract: A series of N-(tert-butoxycarbonyl)-3-[2-(phenyl)benzoxazol-5-yl]alanine methyl ester derivatives possessing electron-donor and electron-acceptor substitutents in the phenyl ring were synthesized and their photophysical properties in methanol, acetonitrile and methylcyclohexane were studied by means of absorption and fluorescence spectroscopies as well as theoretical calculations. The structures of compounds studied in the ground and excited state were calculated using the MOPAC 2002 package with COSMO model of the solvent whereas the positions of vertical absorption and emission transitions were calculated using a INDO/S or CNDO/Param method within a SCRF approximation for the solvent but they are not in good agreement with the experimental values. High molar absorption coefficient values and fluorescence quantum yields are characteristic for all compounds studied. Moreover, in the case of derivatives possessing electron-donor substituents in position 2 of the phenyl ring the fluorescence quantum yield increases with the solvent polarity whereas the opposite effect is observed for derivatives substituted by electron-acceptor groups. Also, the position of the absorption and emission band depends on the solvent polarity and is shifted to longer wavelengths in more polar solvents as a result of a charge transfer from a substituent to the benzoxazole moiety. In the case of derivatives substituted by 2-hydroxyphenyl, the intramolecular proton transfer process is observed. However, the character of the group in position 4 of the phenyl ring influences on it. An electron-acceptor substituent facilitates the proton transfer whereas the opposite is true for an electron-donor group. [Copyright &y& Elsevier]