The interactions of regular DNA and abasic site-containing DNA (AP-DNA) with quinolizinium (1a), the linearly fused benzo[b]quinolizinium (2a), the angularly fused benzo[a]quinolizinium (3a), benzo[c]quinolizinium (4a), and dibenzo[a,f]quinolizinium (5a) as well as derivatives thereof were studied with photometric and viscosimetric titrations (regular DNA), fluorimetric titrations and thermal DNA denaturation experiments (regular DNA and AP-DNA). Whereas the parent quinolizinium ion (1a) and the benzo-annelated derivatives 2a, 3a and 4a exhibit no significant affinity to AP-DNA, additional benzo-annelation in 5a leads to an increased selective stabilization of AP-DNA by this ligand. Hence, the latter compound represents the first example of a ligand that does not require ancillary substituents for efficient AP-DNA stabilization. In addition, studies of derivatives with varied substitution patterns revealed an impact of substituents on the stabilization of the AP-DNA. We discovered that a chloro substituent affects the propensity of a ligand to bind to AP-DNA in a similar way as the methyl substituent and may be employed complementary to the known methyl effect to increase the binding affinity of a ligand.

Biaryl derivatives that consist of one DNA-intercalating unit and a sterically demanding component exhibit a specific behavior towards abasic site-containing DNA (AP-DNA) as determined by thermal DNA denaturation experiments, spectrometric titrations and CD spectroscopic analysis. Specifically, these ligands strongly stabilize AP-DNA towards dissociation, whereas they do not or only marginally affect the melting temperature of regular duplex DNA.

8-Styryl-substituted coralyne derivatives bind to duplex and quadruplex DNA and may be used for fluorimetric staining of nucleoli in cells.

SummaryCationic biaryl derivatives were synthesized by Suzuki–Miyaura coupling of 3-bromonaphtho[1,2-b]quinolizinium bromide with arylboronic acids. The resulting cationic biaryl derivatives exhibit pronounced fluorosolvatochromic properties. First photophysical studies in different solvents showed that the emission energy of the biaryl derivatives decreases with increasing solvent polarity. This red-shifted emission in polar solvents is explained by a charge shift (CS) in the excited state and subsequent solvent relaxation. Furthermore, the polarity of protic polar and aprotic polar solvents affects the emission energy to different extent, which indicates a major influence of hydrogen bonding on the stabilization of the ground and excited states.

A series of new 5‐mono‐ and 5,5′‐bisamino‐substituted azothiazole derivatives was synthesized from the readily available diethyl azothiazole‐4,4′‐dicarboxylate. This reaction most likely comprises an initial Michael‐type addition by the respective primary alkyl and aromatic amines at the carbon atom C5 of the substrate. Subsequently, the resulting intermediates are readily oxidized by molecular oxygen to afford the amino‐substituted azothiazole derivatives. The latter exhibit remarkably red‐shifted absorption bands (λabs=507–661 nm) with high molar extinction coefficients and show a strong positive solvatochromism. As revealed by spectrometric titrations and circular and linear dichroism studies, the water‐soluble, bis‐(dimethylaminopropylamino)‐substituted azo dye associates with duplex DNA by formation of aggregates along the phosphate backbone at high ligand–DNA ratios (LDR) and by intercalation at low LDR, which also leads to a significant increase of the otherwise low emission intensity at 671 nm.

It is demonstrated with four representative examples that the dimers of benzo[b]quinolizinium, dibenzo[b,g]quinolizinium and anthracene derivatives may be used as a source to release the DNA‐binding monomers upon irradiation in the presence of DNA, as determined by monitoring the reaction with absorption and CD spectroscopy. Specifically, the dimers of the benzo[b]quinolizinium turned out to be suitable for this purpose as the ligand is formed rapidly in the presence of DNA and subsequently intercalates into the nucleic acid. In combination with the promising biological properties and bioanalytical value of benzo[b]quinolizinium derivatives, this approach may be used as a general method to generate these functional dyes with spatio‐temporal control.

3-Hydroxynaphtho[1,2-b]quinolizinium was synthesized by cyclodehydration route and its optical properties in different media were investigated. The absorption and emission spectra of this compound depend on the pH of the solution. Thus, at higher pH values the deprotonation yields a merocyanine-type dye that exhibits significantly red-shifted absorption bands and causes a dual emisson, i.e., a combination of emission bands of the hydroxyquinolizinium and its deprotonated form. Whereas this compound is a weak acid in the ground state (pK a = 7.9), it has a strongly increased acidity in the excited state (pK a * = 0.4). As a result, the blue-shifted fluorescence of the hydroxyquinolizinium becomes dominant only under strongly acidic conditions. In addition, it is shown that 3-hydroxynaphtho[1,2-b]quinolizinium binds to cucurbit[7]uril (CB[7]) with moderate affinity (K b = 1.8 × 104 M−1, pH 5) and that the pK a and pK a * values of this ligand increase by about two to three orders of magnitude, respectively, when bound to CB[7].

The photoreactivity of four 8‐styryl‐substituted coralyne derivatives was examined by UV/VIS‐ and 1H‐NMR‐spectroscopy. Except for the dimethylamino‐substituted derivative, these cationic azoniaheterocyclic dyes undergo photocyclization that most likely proceeds through an initial E‐Z‐isomerization of the double bond. Subsequent oxidation of the intermediates under aerobic conditions gave the pyrrolo‐annelated quinolizinium derivatives as final products, thus providing a useful synthetic route to polycyclic azoniahetarene derivatives. The 6‐(4‐chlorophenyl)‐substituted pyrroloquinolizinium derivative was isolated on a preparative scale, and the investigation of its photophysical properties revealed significantly red‐shifted absorption (λabs = 465–480 nm) and emission bands (λfl = 550–562 nm) in comparison to the parent coralyne and the styryl‐substituted derivatives.