Photoinduced intramolecular charge transfer (ICT) in a series of N-bonded donor−acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile, or various dicyanobenzenes as an electron acceptor has been studied in solutions. The latter group of compounds, contrary to benzonitrile and nicotinonitrile derivatives, shows a well-separated low-energy CT absorption band which undergoes a distinct blue shift with increasing solvent polarity. Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all of the compounds studied both in a polar and a nonpolar environment. An analysis of the CT fluorescence and absorption band shapes leads to the quantities relevant for the electron transfer in the Marcus inverted region. The values of the fluorescence rate constants (k f) and corresponding transition dipole moments (M) and their solvent polarity dependence indicate that the electronic coupling between the emitting 1CT state and the ground state is a governing factor of the radiative transitions. The relatively large values of M indicate a nonorthogonal geometry of the donor and acceptor subunits in the fluorescent states. It is shown that Marcus theory can be applied for the quantitative description of the radiationless charge recombination processes in the cases when an intersystem crossing to the excited triplet states can be neglected.

Donor (D)-acceptor (A) compounds containing aromatic amine as an electron donor and acridine as an acceptor show a low-energy CT absorption band which undergoes a red shift with increasing solvent polarity. Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all the compounds studied. A band-shape analysis of the CT absorption and emission spectra leads to the quantities relevant for the electron transfer in the Marcus inverted region. The comparative determination of the electronic transition dipole moments corresponding to the 1 CT r S 0 absorption and the radiative charge recombination 1 CT f S 0 (M abs and M flu , respectively) made possible the estimation at the electronic coupling elements V 0 and V 1 between the 1 CT state and the ground state S 0 or the locally excited 1 LE state lying most closely in energy, respectively. To describe the properties of the excited 1 CT state of aryl derivatives of aromatic amines, the significant contributions of both of the above interactions together with the solvent induced changes of V 0 and V 1 have to be taken into account. In low polarity solvents the conformation of these compounds in the fluorescent 1 CT(f) state is more planar than that in the ground state (and in the unrelaxed Franck-Condon 1 CT(a) excited state), whereas in highly polar environment the compounds do not undergo any significant conformational changes accompanying the excited-state charge separation. The experimental and computational results led us to propose a simple model which allows one to predict the photophysical behavior of a particular D-A compound from the properties of its donor and acceptor moieties.

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Recent development of the phosphorescent cyclometalated iridium(III) chelates has enabled, due to their advantageous electrochemical and photo-physical properties, important breakthroughs in many photonic applications. This particular class of 5d6 ion complexes has attracted increasing interest because of their potential application in electroluminescence devices with a nearly 100 % internal quantum efficiency for the conversion of electric energy to photons. Similar to electroluminescence, the cyclometalated iridium(III) chelates have been successfully applied in the electricity-to-light conversion by means of the electrochemiluminescence (ECL) processes. The already reported ECL systems utilizing the title compounds exhibit extremely large ECL efficiencies that allow one to envisage many potential application for them, especially in further development of ECL-based analytical techniques. This review, based on recently published papers, focuses on the ECL properties of this very exciting class of organometallic luminophores. The reported work, describing results from fundamental as well as application-oriented investigations, will be surveyed and briefly discussed.Graphical abstractDepending on the chemical nature of the cyclometalated irdium(III) chelate different colours of the emitted light can be produced during electrochemical excitation.