Question

Why does pyrene give mirror imaged abs and emm spectra?

Answer 1

Explanation:

pyrene give mirror imaged abs and emm spectra because,

(a) mirror image spectrum is only observed in the condensed phase. This is because solvent collisions cause vibrational relaxation from the vibrational level excited and removes∗ energy to the solvent much faster than fluorescence can occur, thus all emission occurs from v=0. This vibrational relaxation occurs with a rate constant of ≈1013 s−1.(This is not true in the vapour phase where if the pressure is low enough no collisions may occur within the excited state lifetime and fluorescence occurs from the level excited)

∗(The collisions preferentially remove energy as generally the excited state is at a higher 'temperature' than the surrounding solvent, i.e. it has more vibrational energy that the average 3kBT/2 of the solvent)

(b)The geometry of the excited state has to be similar to that of the ground state, thus mirror image spectra are observed for 'rigid'∗∗ molecules such as anthracene (3 benzene rings attached side by side) and not for flexible molecules such as cis or trans stilbene (phenyl=phenyl, where = is a double bond and this molecule isomerizes in the excited singlet state).

hope it will help you dear☺☺☺☺.....