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ESIPT study in the higher excited states by fluorescent methods

Tomin V., Jaworski R.

Optica Applicata

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2010

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Vol. 40, nr 3

587--599

EN

Photophysical and photochemical processes in organic molecules are directly related to the singlet electronic S1 state and studied usually without taking into account highest excited states. At the same time the role of high-lying singlet states is not reduced only to processes of intramolecular conversion, they can participate in different intra- and intermolecular processes. Here we consider analytically, in general, how the properties of dually fluorescing molecules are affected in the case where their second fluorescence band is formed as a result of excited state photoreaction and where fluorescence is excited in the range of the S 1 and S n electronic absorption bands. It is shown that, upon excitation of molecular objects via high-lying singlet states, the yield of reaction products can be increased in a number of cases. Then, if fluorescence of initial molecules and their photoproducts is detectable, the probabilities of reactions can be determined via high-lying singlet states. Presented experimental data demonstrate the role of high-lying singlet states of molecules from 3-hydroxyflavone family, in the excited state of which reactions of formation of tautomers as a result of internal proton transfer (ESIPT) can take place. This fact reveals a new opportunity for enlargement yield of photoreactions excited via the Sn states, and in some cases this may be explored practically.

2

Multi-Pulse Excitation Reveals a Novel Excited State Pathway in a Peryleneimide-Pentaphenylene Dyad

Fron F., Schweitzer G., Jacob J., Müllen K., Van der Auweraer M.

Polish Journal of Chemistry

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2008

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Vol. 82, nr 4

701-713

EN

The excited state properties of a peryleneimide-pentaphenylenedyad were in vestigated in detail using pump-probe, single photon timing and three-beam pulsed experiments. Upon excitation with 395 nm pulses ultrafast energy transfer was found to occur from the pentaphenylene to the perylene subunit. In polar solvents this excitation energy transfer is followed by charge transfer from the electron donor pentaphenylene to the electron acceptor perylene. A three-beam femtosecond transient absorption technique revealed the relaxation dynamics of a higher lying singlet state of the peryleneimide subunit to occur on a picoseconds time scale. These results demonstrate the potential use of the threebeam femtosecond technique in manipulating excited states processes.

3

Rotation of PRODAN in excited state

Tomin V I

Optica Applicata

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2006

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Vol. 36, nr 4

s. 523--528

EN

Instant spectra and anisotropy kinetics of PRODAN luminescence in different ranges of spectra has been studied upon excitation by picosecond pulses. Instant spectra of luminescence reveal time dependent Stokes shift having intermolecular nature as its characteristic time correlates with dielectric relaxation time of solvent. The mean Brownian rotation time for the curves obtained shows complex character of spectral dependence with maximum ~39 ns near maximum of luminescence intensity and drops down to 15 ns on the edges of the spectrum. An explanation is based on a treatment of intermolecular energy conversion into thermal movement of molecules during relaxation in excited state. Calculations using hydrodynamic model show “stick” boundary conditions for solutes: rotational volumes correspond to solute together with molecules of the first coordinative sphere filled by solvent.

4

The role of triplet excited state of hydrocarbons in catalysis by transition-metal species

Minaev B. F.

Bulletin of the Polish Academy of Sciences. Chemistry

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2001

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Vol. 49, No. 1

27-56

EN

Reactions of the second-row transition metal (TM) atoms, clusters and complexes with small hydrocarbons have been considered by a simple valence bond (VB) approximation together with conventional molecular orbital approaches. Correlation diagram obtained from VB treatment of the C-H bond activation by TM atom demonstrates the involvement of the triplet excited state of hydrocarbon into the total wave function at the transition state of the catalytic process. VB approximation indicates that the activation barrier for TM atom insertion into C-H bond of a number of alkanes and alkenes is strongly determined by the "singlet-triplet" avoided crossing when the total spin of the reacting system is fixed by the low-spin term of the TM-catalyst. The energy of the triplet state of hydrocarbon determines the barrier in a simple VB approach. The same general features of spin uncoupling are typical of other TM-species and catalytic processes. The nature of barrier in chemical reaction between two closed shell reagents is often determined by avoided grossing of the ground and doubly-excited singlet states; the latter consists of two triplet excitations coupled to the singlet pairing. In order to diminish the barrier one has to activate in some way the triplet state of the reactants. The catalyst supplies its nonpaired electrons in order to activate the "singlet-triplet" avoided crossing in hydrocarbon.