The effect of aggregation on the photophysical properties of three cationic poly{9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR3(6(I),12(Br),100(Br))) has been studied by steady-state and time-resolved fluorescence techniques. Conjugated polyelectrolytes are known to aggregate in solution and for these PFP-NR3 polymers this causes a decrease in the fluorescence quantum yield. The use of acetonitrile as a cosolvent leads to the breakup of aggregates of PFP-NR3 in water; for PFP-NR3(6(I)), this results in an ∼10-fold increase in fluorescence quantum yield, a ca. 2-fold increase in the molar extinction coefficient at 380 nm, and an increase in the emission lifetime, as compared with polymer behavior in water. Fluorescence anisotropy also decreases with increasing aggregation, and this is attributed to increased fluorescence depolarization by interchain energy transfer in aggregate PFP-NR3 clusters. Förster resonance energy transfer along the polymer chain is expected to be very fast, with a calculated FRET rate constant of 7.3 × 10(12) s(-1) and a Förster distance of 2.83 nm (cf. the polymer repeat unit separation of 0.840 nm) for PFP-NR3(100(Br)). The complex polymer excited-state decay kinetics in aggregated PFP-NR3 systems have been successfully modeled in terms of intrachain energy transfer via migration and trapping at interchain aggregate trap sites, with model parameters in good agreement with data from picosecond time-resolved studies and the calculated theoretical Förster energy-transfer rates.

We have investigated the energy transfer dynamics in mixed lanthanide open-framework silicates, known as Ln-AV-20 materials, with the stoichiometric formula Na 1.08 K 0.5 Ln 1.14 Si 3 O 8.5 ‚1.78H 2 O (Ln) Gd 3+ , Tb 3+ , Eu 3+), using steady-state and time-resolved luminescence spectroscopy. Energy transfer between donor and acceptor Ln 3+ ions is extremely efficient, even at low molar ratios of the acceptor Ln 3+ (<5%). The presence of two different Ln 3+ environments makes the Ln-AV-20 intralayer structure intermediate between purely onedimensional (1D) and two-dimensional (2D). The unusual dimensionality of the Ln-AV-20 layers prevents modeling of energy transfer kinetics by conventional kinetic models. We have developed a computer modeling program for the analysis of energy transfer kinetics in systems of unusual dimensions and show how it may be applied successfully to the AV-20 system. Using the program, nearest neighbor energy transfer rate constants are calculated as (5.30 (0.07) × 10 6 and (6.00 (0.13) × 10 6 s-1 , respectively, for Gd/Tb-and Tb/Eu-AV-20 at 300 K. With increasing acceptor concentration, the energy transfer dynamics tend toward purely one-dimensional behavior, and thus, with careful selection of the ratio of individual Ln 3+ ions, it is possible to tune the energy transfer dimensionality of the AV-20 layers from pure 1D to something intermediate between 1D and 2D.

The interaction of three cationic poly {9,9-bis[N,N-(trimethylammonium)hexyl]fluorene-co-1,4-phenylene} polymers with average chain lengths of ∼6, 12, and 100 repeat units (PFP-NR36(I),12(Br),100(Br)) with both double and single stranded, short and long, DNA and DNA bases have been studied by steady state and time-resolved fluorescence techniques. Fluorescence of PFP-NR3 polymers is quenched with high efficiency by DNA (both double and single stranded) and DNA bases. The resulting quenching plots are sigmoidal and are not accurately described by using a Stern-Volmer quenching mechanism. Here, the quenching mechanism is well modeled in terms of an equilibrium in which a PFP-NR3/DNA aggregate complex is formed which brings polymer chains into close enough proximity to allow interchain excitation energy migration and quenching at aggregate or DNA base traps. Such an analysis gives equilibrium constants of 8.4 × 10(6) (±1.2 × 10(6)) M(-1) for short-dsDNA and 8.6 × 10(6) (±1.7 × 10(6)) M(-1) for short-ssDNA with PFP-NR36(I).

DNA and guanine are efficient fluorescence quenchers of the cationic conjugated polymer, poly {9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} (CCP). Studies with CCPs, of average chain length ~6, 12 and 100 repeat units, with single strand (ss) DNA, double strand (ds) DNA, and guanine, in 25/75 acetonitrile/water (v/v) mixtures result in Stern-Volmer quenching plots that show upward curvature. Initial Stern-Volmer constants, k SV , are in the range ≈ 3-20 x 10 7 M-1 which is much higher than possible by diffusional encounter quenching. Aggregation studies in acetonitrile/water mixtures show that aggregation is also an effective quencher of CCP fluorescence, and we note that both aggregation and quenching by DNA or guanine is accompanied by a reduction in solution absorbance at 380 nm. Comparison of the relationship between changes in absorbance and changes in emission intensity suggest that both solvent and chemical induced fluorescence quenching are due to aggregation. We interpret the correlated changes in absorption and emission, high quenching constants, and upward curving Stern-Volmer plots as evidence that the dominant mechanism for fluorescence quenching by DNA or guanine is via induced aggregation of the polymer. The upward curvature of Stern-Volmer plots and high k SV values for DNA and guanine are indicative of "aggregate energy migration quenching" in which CCP aggregates around a DNA or guanine molecule to form an aggregate complex in which excitation energy migrates between and along the polymer chains until it is quenched at an aggregate trap.

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