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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.

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.

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.

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.

The influence of solvation on α-helix formation is studied for a 14-residue homoalanine and a 16-residue alanine-based polypeptide, in aqueous solution. Four variants of Dynamic Monte Carlo (DMC) simulations, using a multiple annealing procedure are performed. The results obtained under in vacuo conditions are compared to simulations considering either contributions to the solvation energy of non-favourable solvent interactions by hydrophobic groups, or the total solvation energy, both calculated by a continuum approximation. The highest average helix lengths are found when the total solvation energy (hydrophilic and hydrophobic contributions) is considered, although solvation counteracts the formation of compact peptide structures. The influence of solvation on helix formation is discussed.

Cyclodextrins (CDx) and their complexes are good model systems to study the structural complexity of macro-molecular systems in general and, in particular, unspecific molecular recognition processes in host-guest systems. In this paper the conformational space of α-CDx, β-CDx and of a 2-naphthol-β-CDx complex are studied by molecular mechanics methods and are discussed in adapted symmetry coordinates. In contrast to highly symmetrical α-CDx, symmetry breaking due to a strong relative tilt of the glucose subunits and to an elliptical squeeze of the macrocycle is demonstrated for β-CDx. The formation of symmetrical fragments within globally unsymmetrical β-CDx is shown. The conformational landscapes of β-CDx and the β-CDx-2-naphthol complex are then converted into correlational maps between individual geometry variables to discuss dynamic aspects of the macromolecules. The severe impact of complexation on the conformational space of the host is demonstrated as correlational maps simplify drastically and the geometrical variables defining the complex geometry become highly correlated. The impact of complexation on β-CDx symmetry is demonstrated, as two distinct types of complex structure can be characterized.