Question

What transition is responsible for soret band in porphyrin?

Answer 1

In the UV-visible spectrum of porphyrin, there is also a vibronic band, the β band, that appears at slightly lower wavelengths than the α band. The β band is due to transitions to higher vibrational levels in the S1 state and serves as a "normalization band" in porphyrin absorption spectra.

 They have a universal biological distribution and were involved in the oldest metabolic phenomena on earth. Some of the best examples are the iron-containing porphyrins found as heme (of haemoglobin) and the magnesium-containing reduced porphyrin (or chlorine) found in chlorophyll.