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.
The occurrence of photoinduced hydrogen atom transfer between two remote spots of a molecule is experimentally demonstrated. This photoprocess involves the intermediacy of an intramolecular "crane". In an experimental case study, 7-hydroxy-4-methylquinoline-8-carbaldehyde monomers isolated in low-temperature Ar matrices are investigated. On UV (lambda>295 nm) irradiation, a hydrogen atom is transferred from the O(7)H group to the N(1) atom of the quinoline ring. Subsequent irradiation with UV (lambda>360 nm) light reveals that the phototransformation is partially photoreversible. In the studied hydrogen-atom-transfer process, the exocyclic carbaldehyde group plays the role of an intramolecular crane. The possible application of systems analogous to 7-hydroxy-4-methylquinoline-8-carbaldehyde as optically driven molecular switches is discussed.
Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by (1)H and (13)C NMR spectroscopy, photostationary and time-resolved UV-vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular "frame" (7-hydroquinolines, 7-HQs) and a proton "crane" (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, OH (7-quinolinol)) and NH (7(1H)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, HMMQC, shows only a trace of the NH form in highly polar and/or protic media). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV-vis spectroscopic properties of 7-HQCs with those of 7-HQs, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted 7-HQCs.
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