Commonly used fluorescein and rhodamine dyes are very prone to bleaching. Fluorescein in particular bleach in minutes under illumination used in fluorescence microscopes. For organic dyes the Alexa dyes are the benchmark of photostability. Without the optimisation that has been performed on the Alexa dyes, the KU dyes exhibit similar photostability, see below.
KU dyes offers some of the only high brigthness dyes on the blue side of fluorescein. KU450 and KU470 have molar absorption coefficients > 100.000 and high quantum yields. They are small compared to other fluorescent dyes, and are ideal candidates if fluorescein are not giving you the results you are looking for.
Bleaching experiment performed in a laser scanning fluorescence microscope. KU dyes replacement dyes are the full columns on the left, while corresponding Alexa dyes (Alexa488, Alexa 514 and Alexa555) are the striped columns on the right.