The 10-methylacridinium ion displays an emission associated with a charge-shifted (CSH) species when substituted in the 9-position with a substituent having a relatively low ionization potential (naphthyl, biphenyl). Flash photolysis and time-resolved EPR (TREPR) measurements show that photoexcitation of these donor−acceptor systems generates an acridinium-localized excited (LE) triplet state. While values of zero-field splitting parameters are virtually unaffected by the nature of the substituent, spin polarization patterns observed in the TREPR spectra display a striking dependence on substituent as well as orientation of donor ring system. Flash photolysis and TREPR data show that the LE triplet state is formed from the CSH singlet state. In these directly linked donor−acceptor molecules, in which the aromatic rings are near perpendicular because of steric hindrance, CSH singlet → LE triplet intersystem crossing (isc) is driven by spin−orbit coupling. This mechanism generates a unique dependence of isc spin selectivity on molecular structure which in one case even results in a temperature-dependent spin polarization pattern. The results demonstrate that TREPR can be a valuable source of information on molecules with twisted internal charge-transfer (TICT) states.
The synthesis of several amphiphilic squaraine dyes and a study of their aggregation behavior and photophysics are reported. The several different squaraines are found to give spectrally blue-shifted aggregates in aqueous and mixed aqueous-organic solution and in microheterogeneous media (bilayer vesicles). While in some cases an intermediate dimer can be detected in the monomer to aggregate conversion process, in others direct conversion of monomer to aggregate is observed. The aggregation number can be determined together with the equilibrium constant and thermodynamic parameters for some of the squaraines in different environments. In several cases the aggregation number is found to be ca. 4. The finding of a strong induced circular dichroism signal when the aggregate (but not dimer or monomer) is generated in the presence of a chiral host (or counterion) suggests that the aggregate is chiral. From these results and molecular simulations indicating that an extended monolayer of some of the squaraines adopts a glide or herringbone lattice we propose a chiral "pinwheel" structure for the unit aggregate and suggest that extended aggregate structures or crystals may be a mosaic of these unit aggregates. In contrast to the monomers, which are strongly fluorescent, the squaraine dimers and aggregates are nonfluorescent and have extremely short exciton lifetimes, as indicated by transient spectroscopy.
Synthetic fatty acid and phosphatidylcholine amphiphiles incorporating a trans-stilbene (TS) chromophore in the fatty acid chain have been found to exhibit sharp changes in absorption and fluorescence spectra upon self-assembly in Langmuir−Blodgett films and aqueous dispersions. The spectral changes are readily associated with aggregates in which there is a strong noncovalent interaction between the TS chromophores. In this paper, we report determination of the size, structure, and properties of these “supramolecular” aggregates using both experiments and simulations. Important findings are that the key “unit aggregate” having distinctive spectroscopic properties is a cyclic “pinwheel” tetramer characterized by strong edge−face interactions. These tetramers may be packed together to form an extended aggregate with only small changes in absorption or fluorescence properties. While it was initially expected that aggregation occurred as a consequence of amphiphile self-assembly, studies of films of the stilbene fatty acids at the air−water interface show the predominance of aggregate prior to compression. Similarly in phospholipid dispersions aggregates persist above temperature at which chain melting occurs. Aggregation produces strong effects on the photophysics and photochemistry of the TS chromophore. No photoisomerization occurs; however, a slow photobleaching is observed for certain assemblies which can be attributed to formation of a photodimer. Fluorescence from aggregated TS chromophores is attributed to extended aggregates and dimers (excimers), and the latter is likely responsible for the photodimerization. The supramolecular aggregates observed in this study appear quite general and closely related to those observed with a wide variety of amphiphiles incorporating aromatic chromophores.
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