New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol(-1) and 450,000 g mol(-1), respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I(E)/I(M) decreases with the increase of pH, oppositely to the increase in the I(E)/I(M) ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK(a) values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

Porphyrins are typically weak emitters, which presents challenges to their optical detection by single-molecule fluorescence microscopy. In this contribution, we explore the enhancement effect of gold nanodimer antennas on the fluorescence of porphyrins in order to enable their single-molecule optical detection. Four meso-substituted free-base porphyrins were evaluated: two cationic, one neutral, and one anionic porphyrin. The gold nanodimer antennas are able to enhance the emission from these porphyrins by a factor of 105–106 increase in the maximum detected photon rates. This extreme enhancement is due to the combination of an antenna effect on the excitation rate that is estimated to be above 104-fold and an emission efficiency that corresponds to an increase of 2–10 times in the porphyrin’s fluorescence quantum yield.