We present studies of fluorescence photomodulation and solvatochromism in nanoparticles of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with a photochromic spirooxazine dye. The fluorescence properties of doped nanoparticles with dyes in the spirooxazine form are identical to those in undoped control nanoparticles. UV irradiation converts some of the dyes to their visibleabsorbing merocyanine form, which is an efficient quencher of MEH-PPV fluorescence. The fluorescence intensity of the nanoparticles drops to less than 10% of its initial value and recovers when the merocyanines undergo thermal reversion to spirooxazines. The fluorescence modulation can be cycled many times without fatigue or photodegradation, and the degree of quenching is linear with merocyanine concentration. The photochromic conversion can also be used as a probe of the environment within the nanoparticles as both the kinetics of the thermal merocyanine-to-spirooxazine conversion and the merocyanine absorption spectrum are sensitive to the dye environment. The kinetics of the thermal dye reversion in the nanoparticles are first order and nearly as fast as those in THF, while those in a MEH-PPV film are biexponential and substantially slower. The position of the merocyanine absorption within the nanoparticles is likewise distinct from that in a MEH-PPV film and implies a liquid-like environment that is more polar than THF. We hypothesize that those dyes that undergo spirooxazine-to-merocyanine conversion are adhered to solution-exposed MEH-PPV segments within the nanoparticles or to the particle surface and thus have ample free volume for the photochromic conversion. These findings will be useful in designing future stimulus-responsive nanoparticle systems.

Binding of a porphyrin carboxylate anion () to tetrathiafulvalene calix[4]pyrrole (TTF-C4P) results in electron transfer from TTF-C4P to Li(+)@C60 to produce the charge-separated state (1/TTF-C4P˙(+)/Li(+)@C60˙(-)) in benzonitrile. Upon photoexcitation of , photoinduced electron transfer from the triplet excited state of to TTF-C4P˙(+) occurs to produce the higher energy charge-separated state (˙(+)/TTF-C4P/Li(+)@C60˙(-)), which decays to the ground state with a lifetime of 4.8 μs.

The effect of ionic species on the binding of fullerenes (C60 and C70) by tetrathiafulvalene-calix[4]pyrrole (TTF-C4P) receptors and the nature of the resulting supramolecular complexes (TTF-C4P + fullerene + halide anion + tetraalkylammonium cation) was studied in the solid state through single crystal X-ray diffraction methods and in dichloromethane solution by means of continuous variation plots and UV-vis spectroscopic titrations. These analyses revealed a 1:1 stoichiometry between the anion-bound TTF-C4Ps and the complexed fullerenes. The latter guests are bound within the bowl-like cup of the C4P in a ball-and-socket binding mode. The interactions between the TTF-C4P receptors and the fullerene guests are highly influenced by both the nature of halide anions and their counter tetraalkylammonium cations. Three halides (F(-), Cl(-), and Br(-)) were studied. All three potentiate the binding of the two test fullerenes by inducing a conformational change from the 1,3-alternate to the cone conformer of the TTF-C4Ps, thus acting as positive heterotropic allosteric effectors. For a particular halide anion, the choice of tetraalkylammonium salts serves to modulate the strength of the TTF-C4P-fullerene host-guest binding interactions and, in conjunction with variations in the halide anion, can be exploited to alter the inherent selectivity of the host for a given fullerene. Differences in binding are reflected in the excited state optical properties. Overall, the present four-component system provides an illustration of how host-guest binding events involving appropriately designed artificial receptors can be fine-tuned via the addition of simple ionic species as allosteric modulators.

A new class of redox-active free base and metalloporphyrins fused with the 1,3-dithiol-2-ylidene subunits present in tetrathiafulvalene, termed MTTFP (M = H2, Cu, Ni, Zn), have been prepared and characterized. The strong electron-donating properties of MTTFP were probed by electrochemical measurement and demonstrated that oxidation potentials can be tuned by metalation of the free base form, H2TTFP. X-ray crystal structures of H2TTFP, ZnTTFP, and CuTTFP revealed that a severe saddle-shape distortion was observed with the dithiole rings bent out of the plane toward one another in the neutral form. In contrast, the structure of the two-electron oxidized species (CuTFFP(2+)) is planar, corresponding to a change from a nonaromatic to aromatic structure upon oxidation. A relatively large two-photon absorption (TPA) cross-section value of H2TTFP(2+) (1200 GM) was obtained for the free base compound, a value that is much higher than those typically seen for porphyrins (<100 GM). Augmented TPA values for the metal complexes were also seen. The strong electron-donating ability of ZnTTFP was further enhanced by binding of Cl(-) and Br(-) as revealed by thermal electron-transfer between ZnTTFP and Li(+)-encapsulated C60 (Li(+)@C60) in benzonitrile, which was "switched on" by the addition of either Cl(-) or Br(-) (as the tetrabutylammonium salts). The X-ray crystal structure of Cl(-)-bound ZnTTFP was determined and provided support for the strong binding between the Cl(-) anion and the Zn(2+) cation present in ZnTTFP.

Fluorescent systems that can undergo intensity photomodulation in aqueous environments are finding increasing applications, particularly in high-resolution imaging of biological samples. We seek to develop conjugated polymer nanoparticles (CPNs) with bright fluorescence that can be modulated with a light signal. Here, we present CPNs, doped with a photochromic diarylethene dye, that exhibit efficient fluorescence photomodulation that is thermally irreversible. In their UVabsorbing open form, the diarylethenes have no effect on the fluorescence properties of the bright CPNs. A brief period of UV irradiation converts the dyes to their visible-absorbing closed form, which is an efficient fluorescence quencher for the CPNs, likely via a fluorescence resonance energy transfer mechanism. Aqueous suspensions of dye-doped CPNs prepared from the homopolymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) or a polyfluorene-phenylenevinylene copolymer (PFPV) exhibit thermally stable bright and dark levels. The dye-doped MEH-PPV CPNs also exhibit photomodulation in single-nanoparticle imaging experiments, which reveal that nearly all CPNs retain a small amount of residual emission in the dark state. Their PFPV counterparts undergo irreversible fluorescence photobleaching rather than photomodulation in singlenanoparticle studies. The photostability of the CPNs under the UV irradiation conditions required for photochromic conversion is investigated on the single-particle level, and PFPV CPNs are found to be particularly susceptible to photobleaching upon 254 nm irradiation. These results will guide the selection of polymers and photochromes for CPNs intended for single-particle photomodulation.

A supramolecular assembly is formed upon mixing millimolar concentrations of a tetrakis-tetrathiafulvalene calix[4]pyrrole (TTF-C4P) and a porphyrin tetraethylammonium carboxylate salt in benzonitrile (PhCN). The TTF-C4P binds to the carboxylate moiety of the porphyrin with a 1:1 stoichiometry and a binding constant of 6.3 × 104 M–1 in this solvent at 298 K. Laser photoexcitation of the supramolecular complex results in formation of the triplet charge-separated (CS) state composed of a radical cation of the TTF-C4P receptor and the radical anion of the porphyrin carboxylate. These processes and the resulting states were characterized by means of transient absorption and electron spin resonance (ESR) spectroscopies. The rate constants corresponding to the forward and backward intramolecular electron-transfer (ET) processes were determined to be 2.1 × 104 and 3.6 × 102 s–1, respectively. The rate constants of intermolecular forward and backward electron transfer were also determined to be 4.4 × 108 and 9.8 × 108 M–1 s–1, respectively. The electronic coupling constant (V), 1.2 × 10–2 cm–1, and the reorganization energy (λ), 0.76 eV, for back electron transfer were evaluated from the temperature dependence of the rate constants of intramolecular electron transfer. The small V value indicates little spin-forbidden interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and substantiates the long-lived CS lifetime. These results were corroborated by density function theory (DFT) calculations, which provided support for the conclusion that the HOMO and LUMO, located on a TTF moiety of the TTF-C4P and the porphyrin core, respectively, have little interaction though space.

A π-extended tetrathiafulvalene-boradiazaindacene chimera, ex-TTF-BODIPY, has been prepared. The resulting system undergoes sequential one-electron oxidations, allowing access to both the mono-oxidized radical cationic and dicationic states. Additionally, ex-TTF-BODIPY displays electrochromic and electrofluorochromic behaviour in the near-IR portion of the electromagnetic spectrum and functions as a redox switched "on-off-on" emissive system.

Tetrathiafulvalenes (TTF)-annulated [28]hexaphyrin affords an electron rich flexible π-conjugated system whose limiting conformations can be controlled through choice of solvents. The conformation-dependent intramolecular charge transfer character, as well as electron reserve capability of the hexakis-TTF annulated hexaphyrin, was analyzed.