Andreas B. J. Parusel

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.

Nonlinear optics. A semiempirical study of organic chromophores

Abstract Many donor–acceptor substituted organic compounds show internal charge separation upon excitation. Within this class, the following compounds are studied as suitable candidates for molecules with high second-order nonlinear optical (NLO) response: 4- N , N -dimethylamino-benzonitrile (DMABN), 6-propanoyl-2-(dimethylamino)-naphthalene (PRODAN), and various 4-(4- N , N -dimethylamino-phenyl)-1,3-diphenyl-pyrazolo[3,4-b]quinolines (DMA-DPPQ). The configuration interaction method with single and double excitations (CISD) is used in combination with the sum over states (SOS) approach within the semiempirical AM1 parametrization to calculate the vector component of the hyperpolarizability along the dipole axis β μ . The influence of changing molecular geometry on the second-order nonlinear properties is calculated for various intramolecular relaxation coordinates. Solvent effects are explicitly studied for DMABN by a self-consistent reaction field (SCRF) model revealing an increasing NLO-effect with increasing solvent polarity. The hyperpolarizability is discussed in relationship to the charge transfer properties of the excited state. The calculated values are in good agreement with experimental data for both DMABN and PRODAN. DMA-DPPQ is found to be a good NLO chromophore only when increasing either the donor or acceptor strength. Its nonlinear response can be drastically increased by an additional strong electron acceptor group, e.g. a NO 2 -group, at the primary acceptor moiety.

An ab initio study on excited and ground state properties of the organic fluorescence probe PRODAN

Abstract Contbrmational energies of PRODAN (6-Propionyl-2-(dimethylamino)-naphthalene) have been calculated by Hartree-Fock ab initio methods as a function of the twisting and wagging motion of the amino and the rotational motion of the propionyl group. In agreement with experimental data, PRODAN is found planar in its optimum geometry. The CIS method is used to compute molecular orbital energies, excitation energies and dipole moments in dependence of the rotational isomerization of the dimethylamino group. The calculations allow the peculiar fluorescence properties of PRODAN to be explained by the TICT model.

Synthesis, Spectroscopy, and Semiempirical Study of a Novel Porphyrin–Flavin Dyad

Potential photosynthetic models, porphyrin dyads with various acceptors, that are separated by a diazaphenylene spacer, are synthesized by a new strategy. The amidine and vinamidinium salts are coupled by pyrimidine rings. In one example (n=1 in the figure), excitation of a flavin acceptor results in energy transfer to the porphyrin singlet (≈60 %) and population of a charge-separated state (≈40 %), which relaxes to the ground state on a subnanosecond time scale.

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.

Excited state intramolecular charge transfer in N, N-heterocyclic-4-aminobenzonitriles: a DFT study

Abstract The excited state intramolecular charge transfer (ICT) reaction in a series of N , N -heterocyclic 4-aminobenzonitriles is investigated theoretically by a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Experimentally, increasing ICT emission is observed with increasing ring size. Formation of both a planar and twisted ICT (PICT and TICT) state are energetically unfavorable in the small systems due to high inversion barriers. With increasing ring size, the TICT state is more stabilized than the PICT state. A good agreement of the computed TICT state dipole moment is found with experimental values. The red-shifted fluorescence of all systems is explained by the TICT model due to both arguments.

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.

Theoretical description of solvent effects on fluorescence spectra of bulky charge transfer compound DMA-DMPP

Several theoretical models are compared to reproduce the spectroscopic fluorescence shift of 4‐(4′‐N,N‐dimethylaminophenyl)‐3,5‐dimethyl‐1,7‐diphenyl‐bis‐pyrazolo‐[3,4‐b;4′,3′e]‐pyridine (DMA‐DMPP) in different solvents. DMA‐DMPP is used as a model compound because it shows a large shift in emission energy for solvents of various polarities and dual fluorescence in polar protic solvents. Although the simple Onsager model is not able to reproduce the experimental results, the self‐consistent reaction field (SCRF) model with extension to excited states based on the AM1 Hamiltonian yields excellent agreement. According to the latter model, the red‐shifted emission band can be related to a highly polar charge transfer state without geometrical rearrangements, whereas the normal (short wavelength) emission is attributed to emission from an excited state with increased conjugation in a flattened geometry. A supramolecular approach with six molecules of water surrounding the solute can explain satisfactorily the two distinct fluorescence bands. In protic solvents, the emitting CT state shows additional stabilization of the locally excited state with a planar conformation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1584–1595, 1998

Theoretical investigations of the ground-state properties of stereoselectively substituted aminobenzonitriles

Abstract 4-( N , N -Dimethylamino)-benzonitrile (DMABN) is the classical model compound showing solvent-induced dual fluorescence, a phenomenon attributed to formation of an intramolecular charge-transfer state and concomitant rotational relaxation of the dimetylamino group about the amine–aryl single bond. This mechanism is supported by the fluorescence properties of various stereoselectively substituted derivatives of DMABN and, therefore, the ground-state properties of such a set of model compounds are studied as a basis for a detailed examination of the influence of substitution on excited-state relaxation pathways. The factors examined in this study are torsion about the single bond connecting the donor amino group to the aryl moiety, the amine wagging vibration and the bending of the cyano group (rehybridization). Ground-state geometries and electrostatic properties as well as the energetics of these modes are obtained from semiempirical AM1 calculations and are compared with Hartree–Fock ab initio results using 3-21G* and 6-21G** split valence basis sets and those obtained by the Becke3 density functional theory method with the Lee–Young–Parr correlation function and the D95** basis set.

Enzyme-mimetic model compounds: conformational analysis and far-IR study of Cu(TAAB)2+.

Many enzymes occurring in nature like superoxide dismutase are systems rather too big to be accessible for vibrational and quantum chemical investigations. Thus, enzyme-mimetic model compounds consisting of a biological active metal centre surrounded by a macrocyclic ligand are used to shed light on binding properties of the active metal centre. Far- and mid-range IR spectroscopic investigations and a conformational analysis with the semi-empirical ZINDO/1 method of superoxide dismutase-mimetic complex Cu[TAAB]2+ are performed (TAAB = [b,f,j,n][1,5,9,13]tetra-aza-cyclohexadecine (tetra-anhydroamino benzaldehyde)). A distorted tetrahedral copper(II) centre with slightly twisted phenyl subunits is determined as the most stable conformation. Calculated mid- and far-IR spectra are in good agreement with the experimental data and confirm the proposed structure. A harmonic normal-coordinate analysis is used to assign the vibrational modes of the observed spectra.