I have a 1st in Chemistry. I am applying for a molecular science PhD. I would like to pursue an academic career. How important is the PhD topic when considering possible academia in the future?

Choice of PhD topic is important. Unfortunately you and most other PhD candidates are not in a position to judge whether the project is a suitable project for a PhD (i.e. will it get you enough results for a thesis). The factors you can take into account are the quality of the department (what is their RAE rating). The reputation of the supervisor is also important. Ask around and have a look at their publication record.