Picosecond Transient Absorption Spectra and the Twisted Intramolecular Charge Transfer Phenomenon

Dobkowski, Jacek; Grabowski, Zbigniew R.; Jasny, J.; Zieliński, Zbigniew

doi:10.12693/aphyspola.88.455

Cited by 14 publications

(12 citation statements)

References 24 publications

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“…Energies of CT Emission Maxima in Terms of the Redox Potentials of D and A Subunits. The energy h ν max (CT) of the CT emission maximum of dual fluorescent molecules such as DMABN has been discussed as a source of information on the structure of the CT state . In the TICT model, DMABN is treated as a D − A structure, in which the electron donor ( D ) and acceptor ( A ) parts are considered to be linked together by a single bond .…”

Section: Resultsmentioning

confidence: 99%

Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the Size of the Amino and Benzonitrile Subunits by Alkyl Substituents

1998

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The intramolecular charge transfer (ICT) reaction from the locally excited state (LE) to the charge transfer state (CT) in the singlet excited state is investigated for the dual fluorescent 4-(dialkylamino)benzonitriles DRABN and 4-dialkylamino-2,6-dimethyl-benzonitriles RDB (R = methyl, ethyl, n-propyl) in toluene as a function of temperature by photostationary and time-resolved experiments. The efficiency of the ICT reaction, as expressed by the CT/LE fluorescence quantum yield ratio Φ‘(CT)/Φ(LE), is enhanced by the increase in the size of the dialkylamino group as well as by the presence of the two extra methyls in the phenyl ring. This increase is mainly brought about by slowing down the thermal back reaction (k d) due to a larger activation energy E d or a smaller preexponential factor k d o, respectively. The free enthalpy change ΔG becomes more negative (∼6 kJ/mol) for 4-(diethylamino)benzonitrile (DEABN) and 4-(di(n-propyl)amino)benzonitrile (DPrABN) as compared with 4-(dimethylamino)benzonitrile (DMABN) and also for the 4-dialkylamino-2,6-dimethylbenzonitriles EDB (ethyl) and PrDB (n-propyl) relative to MDB (methyl). The ΔG is about 2 kJ/mol more negative for RDB than for DRABN. In both series DRABN and RDB, the ICT stabilization enthalpy, −ΔH, increases from R = methyl to n-propyl. The increase in the ICT rate constant k a for the pairs DEABN/DMABN and EDB/MDB is primarily caused by a larger preexponential factor k a o counteracted but not reversed by a likewise larger activation energy E a. For the dependence of the energy E(CT) of the CT state on the difference between the redox potentials of the dialkylamino and benzonitrile moieties in DRABN and RDB a substantially smaller correlation coefficient (0.29) is found than the value of 1.0 expected for the TICT model, showing that the amino and benzonitrile groups in the CT state are not electronically decoupled. The energy δE rep of the Franck−Condon ground state reached upon CT emission decreases with increasing size of the dialkylamino group in DRABN and RDB.

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Section: Resultsmentioning

confidence: 99%

Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the Size of the Amino and Benzonitrile Subunits by Alkyl Substituents

1998

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“…Transient absorption spectra were measured by means of a home-built picosecond spectrometer, described in detail in ref 22. Pulses of 25 ps fwhm from a mode-locked Nd 3+ :YAG laser (EKSMA/EKSPLA) were amplified, and the third harmonic (355 nm) was used to excite the sample.…”

Section: Methodsmentioning

confidence: 99%

On the Nature of the Fluorescent State in β-Nitrotetraarylporphyrins

et al. 1998

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2-NO 2 -5,10,15,20-tetraphenylporphyrin (H 2 TPP-NO 2 ) is shown to exist in solution as an equilibrium mixture of two NH tautomers with different spectral and photophysical properties. At 77 K in a rigid glass solution the fluorescence spectra of the tautomers contain two well-resolved narrow bands that are slightly (∼300 cm -1 ) Stokes-shifted with respect to the corresponding Q X00 absorption bands, with the spectrum of the more stable tautomer being ∼500 cm -1 red-shifted as compared to the spectrum of the less stable tautomer. At room temperature, the tautomers show almost identical broad and structureless fluorescence spectra, markedly red-shifted relative to the longest wavelength absorption band. The Stokes shift increases with an increase of solvent polarity, being as large as ∼2000 cm -1 in N,N-dimethylformamide. Fluorescence lifetimes of the tautomers are found to be markedly (about a factor of 2) different, with both decreasing with an increase of solvent polarity. The reasons for these peculiar fluorescence properties can be understood on the basis of semiempirical quantum chemical calculations. Charge-transfer (CT) states are found to be located between the porphyrinic Q and B states for both tautomers, and a different energy of the CT states gives rise to different fluorescence lifetimes of the tautomers. The calculations also predict a lowering of the Q X state energy and an increase of its dipole moment µ with a decrease of the angle θ between the plane of the NO 2 group and the porphyrin plane (relative to the ground-state equilibrium geometry with θ ) 80°). We suggest that, in solution, such an increase in the µ value should result in an increase of the stabilization interaction between the polar porphyrin and solvent molecules and, consequently, in flattening of the Q X state energy curve as a function of θ and a shift of the minimum of this curve to θ values less than 80°. Both quantum chemical calculations and picosecond transient absorption measurements of the S 1 f S n absorption show that the fluorescent Q X state of H 2 TPP-NO 2 has mainly 1 (π,π*) character with a relatively small charge-transfer admixture.

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“…58 This is rather surprising, as among the D/A molecules having a C͑vO͒R substituent, 5-formyl-1-methyl-indoline ͑FMI͒ in MeCN ͑no ICT͒, has a transient absorption spectrum with a maximum at about 610 nm. 7,59,60 Also 5-N , N-dimethylamino-1-indanone ͑DMI͒ in 1-propanol has a pronounced transient absorption maximum in this spectral range ͑shifting from 635 to 620 nm due to relaxation of the polar solvent͒. These bands, of molecules that do not contain a cyano group, have been attributed to the LE state.…”

Section: K Transient Absorption Spectramentioning

confidence: 99%

Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a πσ∗ reaction intermediate

Zachariasse

Druzhinin

Kovalenko

et al. 2009

The Journal of Chemical Physics

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For the double exponential fluorescence decays of the locally excited (LE) and intramolecular charge transfer (ICT) states of 4-(dimethylamino)benzonitrile (DMABN) in acetonitrile (MeCN) the same times tau(1) and tau(2) are observed. This means that the reversible LE<==>ICT reaction, starting from the initially excited LE state, can be adequately described by a two state mechanism. The most important factor responsible for the sometimes experimentally observed differences in the nanosecond decay time, with tau(1)(LE)

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Picosecond Transient Absorption Spectra and the Twisted Intramolecular Charge Transfer Phenomenon

Cited by 14 publications

References 24 publications

Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the Size of the Amino and Benzonitrile Subunits by Alkyl Substituents

Intramolecular Charge Transfer in Dual Fluorescent 4-(Dialkylamino)benzonitriles. Reaction Efficiency Enhancement by Increasing the Size of the Amino and Benzonitrile Subunits by Alkyl Substituents

On the Nature of the Fluorescent State in β-Nitrotetraarylporphyrins

Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a πσ∗ reaction intermediate

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