Karl Rechthaler

Excited state properties and deactivation pathways of 7-aminocoumarins

Solvent polarity effects on the photophysical properties and spectral shifts of some 7-aminocoumarins were investigated in a combined experimental and theoretical study. Solvatochromic shifts were found in accordance with Onsagers coatinuum model and excited state dipole moments were estimated and compared to semiempirically calculated values. In solvents of a dielectric constant D < 10 the fluorescence quantum yield is generally unity with the only exception of the primary 7-aminocoumarins C120 and C151 in alkane solution. Ground and/or excited state complexation with polar solvent molecules enhances fluorescence. The onset of a nonradiative decay pathway for flexible 7-aminocoumarins with an acceptor substituent on the α-pyrone ring at a solvent D ≈ 10 is in good agreement with a rotatory decay mechanism to a twisted intramolecular charge transfer (TICT) state. This internal quenching is temperature activated, the activation barrier and pre-exponential factor increasing with polarity. TICT formation is discussed on the basis of dielectric solvation effects on the energy-hypersurface describing the twist of the amino group.

Emissive Properties and Intramolecular Charge Transfer of Pyrazoloquinoline Derivatives

The fluorescence properties of several pyrazoloquinoline derivatives were performed by stationary as well as by time-resolved spectrosopy. Non-donor-substituted compounds show a high quantum yield; transition dipole moments of absorption and fluorescence as well as experimental and calculated lifetimes are in excellent agreement. The donor-substituted compound DMA-DPPQ exhibits a charge transfer fluorescence in polar solvents. Additionally, dual fluorescence appears in polar protic solvents. The nature of the charge transfer state is discussed with respect to the the TICT model.

Fluorescence Properties of Donor–Acceptor-Substituted Pyrazoloquinolines

The photophysical properties of three newly synthesized pyrazoloquinolines, composed of N,N-dimethylaniline as donor subunit and various substituted forms of the acceptor pyrazoloquinoline (DPPQ), were investigated by absorption as well as by stationary and time resolved fluorescence spectroscopy. These compounds show generally highly efficient emission in nonpolar and medium polar solvents; the dipole moment of the emitting state increases and the quantum yield decreases with solvent polarity. These results are explained by state reversion in polar solvents: At low polarities emission originates from a state localized on the DPPQ moiety, whereas in the high-polarity regime the next excited state of charge transfer character, in which an electron is promoted from the amino nitrogen lone pair into an excited orbital of the DPPQ moiety, becomes the fluorescent state. This view is corroborated by semiempirical calculations including the solvent reaction field, low-temperature fluorescence measurements, and the observation of effects of protonation on the spectroscopic and photophysical properties.

Time-resolved spectroscopy of DMABN and its cage derivatives 6-cyanobenzquinuclidine (CBQ) and benzquinuclidine (BQ)

Abstract Picosecond transient absorption spectra of 4-( N , N -dimethylamino)-benzonitrile (DMABN), 6-cyanobenzquinuclidine (CBQ), and benzquinuclidine (BQ) in polar and nonpolar solvents were measured for the first time at wavelengths n ←S 1 excitation, are different from those found in hexane solution; they are similar to the absorption spectrum of the benzonitrile radical anion. This finding supports the intramolecular charge transfer character of the relaxed singlet excited state in polar solvents. The S n ←S 1 spectra develop into T n ←T 1 spectra within the excited state lifetime. An extremely weak fluorescence emission was also observed for BQ and can most probably be ascribed to a charge transfer state, too. The S n ←S 1 spectrum of BQ could not be recorded due to its very short lifetime, and only T n ←T 1 absorption was found.

Photophysical properties of a highly fluorescent push-pull stilbene

Abstract A new push-pull stilbene was synthesized, and its photophysical properties investigated by spectroscopic and theoretical methods. Solvatochromic shifts in the fluorescence were in accordance with Onsagers continuum model, whereas shifts in the absorption spectra only correlated with the polarizability of the solvents. Excited state dipole moments were determined and compared to semi-empirically calculated values. With increasing solvent polarity the fluorescence quantum yield increases steeply as the solvent dielectric permittivity exceeds 8. It is concluded that a strongly luminescent and highly dipolar π, π ∗ state becomes the lowest excited state in polar solvents, and radiative deactivation therefore competes effectively with non-radiative pathways. Preventing internal rotation of molecular subunits is thus no prerequisite for obtaining highly fluorescent push-pull stilbenes.

Formation and stabilization of twisted intramolecular charge transfer states in binary mixed solvents

Abstract Spectral and time resolved fluorescence studies were performed for 4-(N,N-dimethylamino)-benzonitrile (DMABNZ) and some derivatives which were designed to model twisted intramolecular charge transfer (TICT) states, in hexane binary solvent mixtures with ethanol. It is attempted to elucidate the mechanism of local interactions between solute and polar solvent molecules and their capacity to induce intramolecular charge transfer. It is shown that crossing to the TICT state occurs in these mixed solvents at low alcohol concentrations but the mechanism of the intermolecular interaction responsible for this effects depends strongly on the structural and photophysical features of the solute. Whereas 2-MODMABN shows effective ground state solvation because of its short fluorescence lifetime, 3-MDMABN is the best example for the appearance of successive excited state association with single alcohol molecules which leads to different solvate cluster sizes and finally to TICT emission. As excited DMABN reacts much slower with ethanol monomers than the sterically hindered derivative, complexation with cyclic alcohol multimers which become the most important structural feature at higher ethanol concentrations is the predominant mechanism in that case. It leads to efficient charge transfer in these solute clusters with several alcohol molecules, and bulk polarity effects on the resulting TICT fluorescence describe the spectral shift in the high alcohol concentration regime.

Solvent effects on excited state relaxation phenomena

Solvent effects on excited state relaxation phenomena in binary solvent mixtures, studied by steady state and time-resolved fluorescence spectroscopy, are discussed in the frame oE electrostatic interactions, preferential solvation, weak associations with defined stoichiometry and structure and strongly bonded ground and excited state complex formation. It is shown, that effects of hydrogen bonded complex formation can be discriminated from unspecific interactions in hydrocarbon - hydroxylic binary solvent mixtures. The structure of such complexes and their role in altering excited state proces~cs is discussed for some hydroxy- and amino- compounds (phenols, anilines, aliphatic tertiary amines, and aminmumarins etc.).

Semiempirical characterization of substituted bis-pyrazolopyridines as new bulky electron donor–acceptor systems in their electronic ground state

Abstract Geometries, conformational energies and activation barriers for rotations about the single bonds connecting the various subunits of a new class of bulky electron donor–acceptor systems derived from the bis-pyrazolopyridine chromophore are characterized for several representatives by semiempirical methods (AM1 and PM3). For the dimethylaminophenyl derivative DMA–DMPP (4-(4′- N , N -dimethylaminophenyl)-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′e]pyridine) dual fluorescence was observed in polar, protic solvents and was discussed in the context of the TICT (twisted intramolecular charge transfer) hypothesis, first formulated to explain dual fluorescence of 4-( N , N -dimethylamino)benzonitrile (DMABN). Besides the parent H–DMPP molecule, derivatives substituted with different donor or acceptor groups were compared with respect to their optimum ground-state geometries.

Acid–base properties of bis-pyrazolopyridine derivatives in nonaqueous solutions

Acid–base properties of four 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b; 4′,3′-e]-pyridine derivatives with different substituents in position 4, i.e. N,N-dimethylaminophenyl, phenyl, nitrophenyl, and methoxyphenyl (DMA-DMPP, H-DMPP, NO2-DMPP, and CH3O-DMPP) were investigated in nonaqueous solutions. Absorption and stationary fluorescence as well as time-resolved spectroscopy were applied as methods. The acidity of the amino group in DMA-DMPP which is preferentially protonated in the ground state, does practically not change in the first excited state. The second protonation step of DMA-DMPP and the protonation of the other compounds occurs at the heterocyclic subunit, in the ground state most probably on the pyridine nitrogen. The basicity of this nitrogen atom increases remarkably in the first excited state. Experimental results were compared with those of semiempirical calculations. The amino group as the preferential site of protonation before the pyridine nitrogen in the ground state was confirmed. According to quantum mechanical computations, one pyrazolo nitrogen is preferentially protonated in the first excited state. The protonation is accompanied by a geometrical relaxation leading to a planarized conformer with a lower total energy.

Dual fluorescence and intramolecular charge transfer in a bulky electron donor-acceptor system. N,N-Dimethylanilino-bis-pyrazolopyridine

The photophysical properties of 4-(4′-N,N′-dimethylaminophenyl)-1,7-diphenyl-3,5-dimethyl-bis-pyrazolo-[3,4-b;4′,3′-e]pyridine (DMA-DMPP), highly emissive in non-polar solvents and composed from a dimethylanilino electron donor and a bulky electron acceptor, have been investigated. The position of the fluorescence maximum as well as the fluorescence quantum yield and the radiative decay rate are strongly solvent polarity dependent; the magnitude of this effect becomes dramatically larger as the solvent polarity exceeds that of 1-chlorobutane. In protic solvents dual fluorescence is observed. In accordance with theoretical calculations the results are explained in terms of a polarity induced change in the energetic ordering of two emissive states of largely different charge transfer character.