[Zensation]

This question may be a bit involved for some people, though I figured I would ask. I'm trying to understand the mechanism here.

With regard to a specific reaction: SRN1 http://en.wikipedia.org/wiki/Radical-nucleophilic_aromatic_substitution. This reaction takes places only after initial homolysis of a carbon-halide bond on an aryl halide. This reaction is known to be promoted by electron withdrawing groups, and retarded by electron donating groups, both on the aromatic ring.

Yet, 95% of my google searches turn up that photolysis/photodisassociation, by UV light, is accelerated by electron donating groups, and hindered by electron withdrawing groups. This, of course results in homolytic cleavage most of the time.

Photons during photolysis are allegedly supposed to simulate electrons attacking a particular bond. Non-photocatalyzed SRN1 reactions are initiated into a homolytic state after attack by a nucleophile - which acknowledging this it would make sense that electron withdrawing groups accelerate the process by enhancing the electrophilicity of the ring, and thus enhancing the ease of attack by the nucleophile. With photolysis, the photon directly enters into the halide bond and breaks it.

Why is one process hindered by EDG's and the other is promoted by it, when they both result in the same effect?

My only theory is that: Electron withdrawing groups may promote attack by a nucleophile, yet, since photons can enter the bond and break it so easily, electron withdrawing groups are irrelevant at this step. Instead in addition, there is a second step; once the bond is successfully energized/attacked, it can more easily undergo [homolytic] cleavage depending on how electron rich the ring is, which this step could be promoted by EDG's since maybe the bond can more easily retain its gained energy. This concept would explain why photolytic homolysis is promoted by electron donating groups, yet, homlysis via attack by a nucleophile is hindered by it. Both reactions, regardless of how initiated, would result in radicals, which readily react with nucleophiles in solution regardless.

I was hoping to get some input here. This is the only explanation I can come up with that explains this phenomena.

[Zensation]

Found some interesting documents of relevance.

Photochemistry of Aromatic Molecules

Such a selectivity is more easily rationalized on the basis of a zwitterionic intermediate wherein the electron donating methoxy would prefer to stabilize the positive charge while the electron withdrawing cyano group would stabilize the negative charge.

That makes sense. The radical intermediate is essentially a positively charged, very reactive electrophile. Electron-donating groups may retard attack of the nucleophile, but if the the nucleophilic radical initiator is avoided by using light which can directly energize the target bonds, then the radical can rapidly come to be and then become stabilized by the electron donating groups. If it is stabilized, the reactivity may be decreased ever so slightly, but being a radical it is still very reactive. The net result being the radical existing for a longer period allowing it more of a chance to react.

Nucleophilic substitution of aromatic molecules substituted with three methoxy groups never occurs in the ground state. On the other hand, irradiation of 1,3,5-trimethoxy benzene in alcohol-water mixture containing cyanide ion results in the replacement of C—H by C—CN (Scheme 71). Examination of examples provided in Scheme 84 leads to the following
generalizations: (a) Highly electron rich aromatic molecules undergo nucleophilic substitution reactions in the excited state, (b) in such substitution reactions methoxy group is ortho/para directing, and (c) when there are two groups, one electron donating and the other electron withdrawing (e.g., methoxy vs. nitro, methoxy vs. fluoro), the electron withdrawing group is more readily replaced.

Quite interesting indeed.