Excited State Behavior Of Single Strand And Bulk P3ht In Contact With A Au-Nanowire Array

Pump-probe transient absorption and single molecule fluorescence microscopy reveal multifaceted excited state behavior of regioregular (RR) P3HT interacting with a hexagonal array of Au nanowires. An average 5-fold increase of fluorescence intensity has been detected for single strands of P3HT in the presence of the nanowire array, with the strongest emitters exceeding the brightness of their counterparts on glass by a factor of 40. The single molecule trajectories show that the same polymer strand is capable of exhibiting a range of enhancement levels. The "off" periods on the Au substrate are exceptionally long and erratic. This behavior is attributed to sporadic electron transfer events between the strand and the metal. Bulk P3HT deposited on the same Au-nanowire array shows a dramatic reduction of the yield and lifetime of singlet excitons in comparison with the glass support, indicating that the proximity of the metal greatly accelerates both exciton dissociation and charge recombination. Interestingly, a large increase of the triplet exciton population is observed in the presence of the metal, indicating a much faster loss of the spin spin correlation. The bulk P3HT results suggest that the large fluorescence enhancement observed in the single molecule experiments occurs only for a few emitters which have been fortuitously placed at the optimum distance and orientation with respect to the metal.