This is a part answer to your question, based on some recent research of the photochemical behaviour modelled and observed for Titan's tholin haze and modelling of Titan's stratosphere.
The process appears to begin, according to the paper Ice condensation layers in Titan’s Stratosphere (Barth, 2012) (Abstract only - paywalled), with
Photochemical destruction of methane along with the destruction of nitrogen molecules from energetic electrons in Titan’s upper atmosphere result in the production of a number of hydrocarbon and nitrile compounds which may be capable of condensing at the colder temperatures of Titan’s lower stratosphere.
then, according to the paper Laboratory experiments of Titan tholin formed in cold plasma at various
pressures: implications for nitrogen-containing polycyclic aromatic
compounds in Titan haze (Imanaka et al. 2004), in particular in reference to Titan's stratosphere,
. In the stratosphere (100–
300 km), further chemical reactions are induced by the catalytic
$CH_4$ dissociation by such molecules as $C_2H_2$ and
$C_4H_2$ absorbing the long UV (> 155 nm) irradiation
The significance of these UV absorbing molecules is explained in the article Photochemical activity of Titan’s low-altitude condensed haze (Gudipati et al. 2013) (Abstract only - paywalled), they state that tholin haze could form on condensed aerosols in Titan's atmosphere, demonstrating that, at least part of Titan's atmosphere is photochemically active. Through modelling, they found that
Detected in Titan’s atmosphere, dicyanoacetylene ($C_4N_2$) is used in our laboratory simulations as a model system for other larger unsaturated condensing compounds. We show that $C_4N_2$ ices undergo condensed-phase photopolymerization (tholin formation) at wavelengths as long as 355 nm pertinent to solar radiation reaching a large portion of Titan’s atmosphere, almost close to the surface.
and evidence of these ices is suggested in the article Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS (Anderson and Samuelson, 2011) stating that the ices and aerosols
appear to be located over a narrow altitude range in the stratosphere centered at ∼90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.