RIP AIM. Had one last chat on Thursday night. I used a web client and it just didn't feel the same.
We use Skype for chatting at work. It's... not very good. If you don't like being bothered by lots of notifications, maybe check it out, since Skype will only tell you about new messages when it feels like it.
I've never used Slack, but maybe that?
With Skype and Slack both, I don't know the difference between free and paid versions, or if you can use free versions at all in a work environment.
I have used Slack and Skype (and Skype for Business) and am now using Google Hangouts at work.
Of the three, Slack is probably the "best" but the hardest to get people to use. Hangouts has the advantage of being able to integrate with Apple Messages (if you (a) care about that and (b) are willing to put up with warnings about how you're compromising security by doing so). I understand that the future of Hangouts isn't guaranteed though.