At many European universities (I don't know how it is in other countries), you still have to write a proper PhD thesis, while others allow for writing a summary and attach three published articles. For the former case, it is still best to publish the research before submitting the thesis, and to reference the published work within the thesis accordingly.
The question is: Is it possible to make it the other way around? That is to publish the research with the thesis and later submit a proper article to a journal? The journal might reject it, because the research is already published and publicly available, albeit not peer reviewed in a journal. It could thus be comparable to a preprint, which most journals accept.
Edit: I work in theoretical chemistry
Edit 2: There is already a similar question (Can I publish parts of the Ph.D thesis as a paper in a journal?), which is however about publishing the thesis as a proper book instead of just making it available online in line with the universities requirements.
Edit 3: Another question (Paper was rejected for high similarity with parts of my own dissertation) is also close, but it is about steps to take, if the article was already rejected due to the previous publication within a thesis. It proves, that this question is indeed valid and there are some journals, which might object pre-publication in a thesis.
I identified three other related questions: Publicly available extended summary of PhD thesis and manuscript submission, Chapter in printed thesis vs. journal article, Paper from my already published PhD thesis -- How do I cite it?