The phenanthridinium chromophores 5-ethyl-6-phenylphenanthridinium (1), 5-ethyl-6-methylphenanthridinium (2), 3,8-diamino-5-ethyl-6-methylphenanthridinium (3), and 3,8-diamino-5-ethyl-6-(4-N,N-diethylaminophenyl)phenanthridinium (4) were characterized by their optical and redox properties. All dyes were applied in titration experiments with a random-sequence 17mer DNA duplex and their binding affinities were determined. The results were compared to well-known ethidium bromide (E). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of E to be elucidated. Especially, compound 4 was used to compare the weak electron-donating character of the phenyl substituent at position 6 of E with the more electron-donating 4-N,N-diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of 1 and 2, lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with E. In the presence of DNA, a significant fluorescence quenching was observed. Their binding affinity to DNA is reduced by nearly one order of magnitude. The electronic effect of the phenyl group in position 6 on this type of dye is rather small. The properties of the second set, 3 and 4, are similar to E due to the presence of the two strongly electron-donating amino groups in positions 3 and 8. However, in contrast to 1 and 2, the electron-donating character of the substituent in position 6 of 3 and 4 is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron-donating effect of this substituent. Accordingly, compound 4 shows the strongest binding affinity and the strongest fluorescence enhancement. Quantum chemical calculations reveal a general mechanism related to the twisted intramolecular charge transfer (TICT) model. Accordingly, an increase of the twist angle between the phenyl ring in position 6 and the phenanthridinium core opens a nonradiative channel in the excited state that depends on the electron-donating character of the phenyl group. Access to this channel is hindered upon binding to DNA.

Charge transfer processes through the double helix of DNA cover a broad range of mechanistic models ranging from superexchange to hopping mechanisms. Over the last decade, these processes were studied by our group in a photoinduced fashion since (i) the starting time for the charge transfer is clearly defined by the absorption of the photon and (ii) photoexcitation delivers the necessary driving force to the DNA system. It is a prerequisite to modify oligonucleotides synthetically with suitable organic fluorophores that serve as photoinducable charge donors. In the first part of this perspective article we summarize our recent advances in the area of DNA-mediated reductive electron transfer processes over short ranges using synthetic DNA-donor-acceptor systems. The second part of this article focuses on ethidium as the photoinducable charge donor. Ethidium-modified DNA can be used to compare oxidative hole transfer with reductive electron transfer since the type of charge transfer can be controlled by choosing the right charge acceptor. Recent results showed that an efficient charge transfer through DNA using covalently bound ethidium is strongly influenced mainly by DNA dynamics but also by several other parameters that affect the electronic coupling between charge donor and acceptor.