Protonation effects on the photophysical properties of poly(2,5-pyridine diyl)

We have recently shown that poly(2,5-pyridine diyl) (PPY) can be synthesize d to yield a polymer with high photoluminescence quantum yield (PLQY) in th e solid state, and that it is an excellent electron transport material. To explore the photophysical properties of PPY further, we have used a range o f acidic "dopants" to protonate the nitrogen sites on each ring and made ob servations on how this affects the optical properties of the resultant prot onated PPY films. In general, we find that sulphonic acids have the greates t effect, causing perturbations to both the ground-state and excited-state properties of the PPY. These changes occur with only moderate reduction of the PLQY, whereas nonsulphonic acids cause a larger reduction in PLQY witho ut significantly affecting the ground- or excited-state energy levels. Thes e aspects of the photophysics of PPY can be described using a simple ring t orsion argument. This model can also account for the observed shifts betwee n solution state and solid-state emission wavelengths. (C) 1998 American In stitute of Physics. [S0021-9606(98)70147-3].