Since I was taught evolution I've been wondering why plants did not evolve pigments absorbing the entire electo-magnetic spectrum that arrives on the earth's surface. Then they would appear quite black. But as we all can see they are green, often even light green, which implies even worse energy absorption.

Why?

Is the energy too great? Are leaves whiter when they are adapted to a sunny environment? Are leaves very dark only if the plant has adapted to shade? Is the green colour just one of the possibilities to bleed off excess energy?

Is the answer maybe found within the biochemistry of photosynthesis? Are specific wavelengths and thereby energy levels better at driving endothermic reactions?

This is a very good question. Chlorophyll is green because it absorbs light in the blue and red spectra, but not green light which actually more the the sun's light. 

Evolution is not capable of thinking like an engineer however. An engineer might design a molecule that absorbs as large a spectrum as possible. Evolution works with what it has, so if the ancestors of modern plants used chlorophyll then modern plants will too. It's probably very difficult to evolve another light absorbing molecule that can work as well as chlorophyll, although at least one exists: retinal.

Retinal is used by some species of archeae to get energy from light in the green part of the spectrum. Some scientists have theorized that retinal using organisms may have dominated early life. When organisms evolved using chlorophyll it may be because chlorophyll absorbed light in the part of the specrum "missed" by rentinal and therefore still available. The organisms using chlorophyll found a new niche absorbing the light that other species didn't, subsequently they gave rise to the modern plant, and cyanobacteria lineages. 

That's just one idea, it's very hard to figure out exactly what evolutionary pressures were occurring a few million years ago, let alone billions! 

http://www.livescience.com/environment/ … earth.html