Is it problematic to use a Wikipedia article I wrote in my PhD thesis?
No, but make sure you cite it and mention it, to avoid being accused of copying and pasting from Wikipedia.
In any case, it is great that you have contributed to Wikipedia and I wish more people considered using pieces of their introductory chapters for Wikipedia. Just make sure that people know that you copied to, not from, Wikipedia. (I mean, the order is irrelevant, i.e. whether you first wrote text for Wikipedia or your thesis, but the authorship is.)
The other answers address the question of "once I post this on Wikipedia, can I include it in my thesis?" I would like to address another angle - I am not sure that you should be posting this on Wikipedia at all.
Wikipedia has a policy banning the inclusion of original research. This means "research that is not exclusively based on a summary, review or synthesis of earlier publications on the subject of research." (See English Wikipedia's policy on original research here)That is, basically anything that would be publishable as a thesis. This is because Wikipedia is an encyclopedia - that is, a tertiary source that aggregates information from secondary (and sometimes primary) sources - it cannot accept primary-source material like original research.
You say that you want to share your "bibliographical and synthesis work" on your subject. Bibliographical work may be useful for Wikipedia, but any synthesis you did to add your own ideas and/or discuss them may well be considered original research and be prohibited. The fact that no article exists on the topic leads me to think that it may be a theory or idea of your own and may be considered original research if posted.
That said, as Piotr Migdal said, it's great that you are thinking of contributing to Wikipedia. Wikipedia definitely needs more academics and people who are experts in various areas to contribute their knowledge - it just has to be knowledge that's already accepted in the field, rather than original, non-yet-peer-reviewed research.
I think you can do this safely, but you will want to take some precautions like Piotr mentioned. However, from a practical standpoint, I don't recommend anything that might create problems with your dissertation work. You haven't finished your PhD thesis, and your efforts on that project are far more important than writing a Wikipedia entry for a topic that isn't covered. Even if you cover all your bases, you still have a risk of somebody raising the issue of plagiarism. I suggest minimizing such distractions and side projects and focus exclusively on your thesis. You can then devote your time to these projects after it is defended.