Travelling from Plovdiv, Bulgaria to Dubrovnik
 

Taking a 3 week trip in June starting in Istanbul to Sofia/Plovdiv and then on to Dubrovnik. My husband thinks we can drive from Plovdiv to Dubrovnik in 1 day, I think it will be very difficult. Has anyone done this drive? No train service in Dubrovnik and flights are too pricy (there are 4 of us). We are also traveling up to Trieste and then from Venice to Rome. It's the Bulgaria to Dubrovnik part that seems the most daunting. Any help is appreciated!

"Difficult", & perhaps dangerous at night on some of the roads you shall encounter enroute. Lovely places enroute you will miss in your rush.
A slower route to consider would take you across western Serbia (Mokra Gora, Tara National Park to Sarajevo and Mostar,

Just a very quick look at google maps tells me it is at best 873km (divide by 1.6 to get approximate miles) and will take 12 hours 6 minutes.

This is not allowing for any stops. This is not allowing for getting lost or taking wrong turns as many of the road signs will be in the Cyrillic alphabet. This is not allowing for any delays or hold ups.

I have not driven this route but I have driven in Croatia and Bosnia & Herzegovina, and I travel to Bulgaria to ski in the winter when I am driven by local drivers. The roads are not wide highways, they twist and turn round the mountains, they can involve big climbs and scary descents and have lots of hairpin bends. Other drivers, the Bulgarians in particular, drive like lunatics, overtaking on bends and racing each other on flat stretches. There will be speed limits which you have to keep to to avoid fines (and there are a lot of police around just waiting to catch you out!)

My advice? Don't even think of attempting to do it in one day.
Even Plovdiv to Sarajevo will take something like 10 hours driving plus time for stops. You need to get a really good map with the largest scale possible and use viamichelin.com rather than google to plan out a route and where to stop along the way.

Thanks Parmenter and Julia! I appreciate your advice. I wish we could just fly from Istanbul direct to Dubrovnik but hubby really wants to see Plovdiv so hopefully this will all work out. As I read more about Sofija I think this will be a very interesting city to visit also. I think relaxing and taking this journey bit by bit make for a much better vacation experience so it is nice to have your imput and experience.

Sofia is an interesting city. If you are looking for a fascinating day trip, consider the Rila Monastery complex, 2 hours south by bus, taxi or local tour group. Lyuba Tours in SOF is a good choice for a private tour. I have not been as impressed with Plodiv as I have with Veliko Turnovo and Arbanassi to the north.
Also, true you can't fly to Dubrovnik from IST but Turkish Airlines does serve Zagreb from IST