Jerzy Karpiuk

Photochromism and proton transfer reaction cycle of new internally H-bonded Schiff bases

Abstract A family of new Schiff bases, derivatives of hydroquinone, has been synthesized. Their synthesis, spectroscopy and photophysics are reported. Two molecules, having one or two internal H-bonds, reveal photochromic properties. Two transients absorbing in the visible have been detected and kinetically separated: one, with lifetime ≈ 300 ps, λ max =570 nm, was assigned as the S 1 → S n absorption of the primary product of single proton transfer, the second, with lifetime 1.2 ms, λ max =540 nm, was interpreted as the photochromic modification of the molecule.

Mechanism and kinetics of fluorescence quenching of aromatic hydrocarbons by a stable nitroxyl radical

Abstract The fluorescence quenching of 1,2-benzanthracene and phenanthrene by the nitroxyl radical in ethanol solution at room temperature has been studied by measuring the transient absorption spectra and build-up curves of the triplet-triplet absorption in the presence of the radical. Using a simple kinetic model the rate constant for the catalyzed intersystem crossing reaction has been calculated and compared with the total quenching rate constant obtained from a Stern-Volmer treatment of fluorescence lifetime data. For both hydrocarbons the rate constants for the catalyzed ISC process is found to be lower, above the error limits, than the total quenching rate constant. Since radical hydrocarbon ions are not formed, an energy transfer or a catalyzed internal conversion are the most probable mechanisms for the minor contribution to the quenching kinetics.

Modification of the intramolecular electron transfer by hydrogen bonding : 4-(dialkylamino)pyrimidines

Abstract The dual fluorescence ( a and b ) and the picosecond kinetics of the TICT state formation in 4-(dialkylamino)pyrimidines are studied in polar solvents. In nitriles as solvents, two emitting states of 4-(N,N-dimethylamino)pyrimidine or 4-(N,N-diethylamino) pyrimidine were shown to reach equilibrium within 20 ps. In alcohols the TICT emission a is attributed to the species hydrogen-bonded at the ring nitrogen atom N1 . The decay of the short wave fluorescence b is clearly multi- or non-exponential; it is assigned to several other complexes, some of which decay fast, not being simply kinetically coupled with the emitting form a . The nature of the fast deactivation channel appearing in alcohols as solvents is discussed taking into account the Mataga-Kakitani discrimination of the Marcus inverted regions for the charge recombination and charge shift electron transfer.

Tuneable white fluorescence from intramolecular exciplexes

Crystal violet lactone (CVL) in solution displays unusually broad (FWHM > 9100 cm(-1)) dual fluorescence with the characteristics of white light. The emission combines a blue CT band from a local chromophore with an orange CT band from an intramolecular exciplex formed adiabatically at appropriate medium polarity. The fluorescence spectrum can be controlled by solvent polarity to yield tuneable emission colours in a broad range of coordinates in the CIE chromaticity diagram including the white region. We show that such dual emission is a general property of CVL-like D-A structures built on sp(3) carbon atoms. The dependence of excited state energetics on molecular structure allows the prediction of width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores. The photophysics-structure relationship found for CVL and its analogues can be generalized into a novel concept of white light generation by small molecules. These D-A systems are studied as a template basis for design and development of white fluorophores.

The Key Role of Solvation Dynamics in Intramolecular Electron Transfer : Time-Resolved Photophysics of Crystal Violet Lactone

The intramolecular electron-transfer reaction in crystal violet lactone in polar aprotic solvents is studied with femtosecond transient absorption spectroscopy. The initially excited charge transfer state (1)CT A is rapidly converted into a highly polar charge transfer state (1)CT B. This ultrafast electron transfer is seen as a solvent-dependent dual fluorescence in steady-state spectra. We find that the electron-transfer process can be followed by a change from a double-peaked transient absorption spectrum to a single-peak one in the low picosecond range. The transient absorption kinetic curves are multiexponential, and the fitted time constants are solvent dependent but do not reproduce the known solvation times. For 6-dimethylaminophthalide, the optically active constituent of crystal violet lactone, only a small temporal evolution of the spectra is found. To explain these findings, we present a model that invokes a time-dependent electron-transfer rate. The rate is determined by the instantaneous separation of the two charge-transfer states. Because of their differing dipole moments, they are dynamically lowered to a different extent by the solvation. When they temporarily become isoenergetic, equal forward and backward transfer rates are reached. The intrinsic electron-transfer ( (1)CT A --> (1)CT B) reaction is probably as fast as that in the structurally analogous malachite green lactone (on the 100 fs time scale). The key element for the dynamics is therefore its control by the solvent, which changes the relative energetics of the two states during the solvation process. With further stabilization of the more polar state, the final equilibrium in state population is reached.

Photoactive Eu(III) and Tb(III) complexes of calix[4]arenes with pyridine-N-oxide pendant groups

Two calix[4]arenes with four 2-pyridyhnethyl-l-oxide pendant groups at the lower rim have been synthesized, and their Tb(III) and Eu(III) complexes are fluorescent upon UV light excitation at 312 nm. The complexes are not stable in aqueous solution, completely losing their luminescent properties.

Determination of triplet formation efficiency from kinetic profiles of the ground state recoveryDedicated to Professor Jean Kossanyi on the occasion of his 70th birthday.

A procedure is presented which allows determination of the triplet formation efficiency from the analysis of transient ground state bleaching curves obtained after short laser pulse excitation. Crucial factors that control the applicability and the accuracy of the method are discussed, such as a proper choice of the detection wavelength, which allows avoidance of interference from transient absorption. The technique is tested on porphycene and octaethylporphyrin and applied for several porphyrin derivatives used as photodynamic therapy agents.

Nanosecond transient absorption spectrophotometer with dye laser probing and computer control

A computer controlled apparatus for laser flash spectroscopy has been constructed. Nitrogen laser (energy 1 mJ, pulse width 1 ns) is used both for excitation of the sample and for pumping a tunable dye laser which serves as a source of monitoring light. Two different scan modes are possible: (i) a scan with a fixed wavelength of the monitoring light: the temporal evolution of the transient absorption in the range from −2 up to 100 ns can be observed by changing the length of the variable self‐compensating optical delay line, or (ii) a scan with a fixed time delay: the corresponding transient absorption in the range 380–800 nm can be measured. For illustration, the transient absorption spectrum of benzophenone in methylcyclohexane, decay curves of the singlet state of benzanthracene in ethanol in the presence of a quencher, and a ‘‘negative absorption’’ of rhodamine B in nitriles are reported.

Photoinduced electron transfer in malachite green lactone

The spectroscopy and photophysics of malachite green lactone (MGL), a lactonic form of the well-know malachite green dye, have been studied as functions of solvent polarity in aprotic and protic solvents at different temperatures in solution and in glass. It has been found that MGL photophysics substantially differs from that of other malachite green leucoderivatives (e.g. leuconitrile, leucohydroxide or halides). In aprotic solvents ultrafast intramolecular electron transfer (estimated τET ≤ 130 fs) between MGL structural components (from initially excited insulated dimethylaniline, DMA, to phthalide, Pd) results in formation of a highly polar (25.0 D) intramolecular CT state which then emits fluorescence. The dynamics of the primary ET are determined mainly by intramolecular vibrational motions and not by the solvation process. The polar nature of the emitting state was verified with the solvatochromic method, and Sn ← S1 transient absorption spectra prove that charge separation results in formation of DMA radical cation in MGL. The separation of charges is maintained in the charge transfer triplet state, which lies below the local triplet levels of MGL structural components, making MGL a unique candidate for studying both CT triplet state and CT-singlet–triplet dynamics. The electronic structure of the 1CT state stabilizes charge separation in moderately polar solvents (e.g.Φfl and τfl in butyl acetate (BA) are 0.12 and 23.8 ns, respectively) and no evidence has been found for photodissociation of C–O bond in lactone ring in aprotic solvents, which is the dominant photoprocess in malachite green leuconitrile or halides. Further increase of solvent polarity results in dramatic enhancement of MGL nonradiative deactivation from the 1CT state (increase in knr by two orders of magnitude on going from BA to ACN) pointing to a solvent polarity-driven deactivation channel. The strong dependence of CT fluorescence quantum yield on CT transition energy found in supercooled and in glassy butyronitrile (BTN), where conformational motions are restricted, suggests that the nonradiative decay is (i) not related to conformational changes and (ii) consists in enhancement of nonradiative return electron transfer (direct radiationless charge recombination), which is proven by the good linear correlation between ln knr and fl in more polar solvents (energy gap law). The large temperature-dependent blue shift of the fluorescence maximum in BTN below the melting point (rigidochromism) makes MGL a very sensitive probe for studying reorientation polarization in rigid and semi-rigid supercooled media.

Luminescent lanthanide complexes with macrocyclic N-oxides

Abstract Bicyclic ligand incorporating 3,3′-biisoquinoline-2,2′-dioxide unit fused with 1,10-diaza-18-crown-6 and 6,6′-dicarboxy-2,2′-bipyridine forms luminescent mononuclear and binuclear europium(III) complexes, respectively, that are stable in water solution. The presence of interfering ions, such as phosphate and Ca(II), caused a slight increase in the quantum yield of Eu(III) emission in the case of bicyclic ligand, and some loss of luminescence efficiency in the case of acyclic Eu(III) complex upon UV irradiation. This feature makes the bicyclic complex very attractive for practical applications in time-resolved fluorimetric immunoassays.