BENT LEY GLASS 823 



some 7 per cent of its bonding electron density by transfer to the 

 pyridine portion of the complex. 



^Vhen the charge density and therefore the dipole moment of a 

 molecide is different in the ground and excited states, then so is 

 the dipole-dipole interaction of the solute with the solvent in the 

 two states, and the spectrum shifts. An example is afforded by the 

 NHo free radical, which when trapped in crystalline rare gases under- 

 goes a spectral shift toward the blue in argon, where the "dipole— 

 induced-dijiole" contribution is dominant, but a shift toward the red 

 in krypton and xenon, where it is less than the contribution of the 

 solvent-solute dispersion effect. The effect of dipole-dipole interac- 

 tions is rather larger in n -^ it* transitions than in 77 -» tt* transitions. 

 The former typically show blue shifts in polar as well as non-polar 

 solvents. 



An excited molecule can lose its excess energy by emitting radia- 

 tion, by undergoing a chemical reaction, or by transferring the energy 

 to another molecule during a collision. Three conditions for ready 

 loss of energy during a collision (of the "second kind") are the fol- 

 lowing: (1) The environment should have energy levels as far above 

 the ground state as the excited solute is above its ground state, and 

 spaced quite closely to correspond to the amount of energy lost by 

 the excited molecule when it undergoes transition to a lower energy 

 level. That is to say, many close resonances between the molecules 

 of the environment and the excited molecules should exist in such 

 cases. (2) A molecule will lose excitation energy to the environment 

 more readily if for some configuration of the atoms of the molecule 

 the energy is the same in the more and the less excited states — that 

 is, if their potential energy surfaces (levels) intersect. (3) Some pertur- 

 bation must also be present to "mix" the two states, especially when 

 they have nearly the same energy and geometry, as in the case of an 

 excited singlet and triplet wath the same electronic configuration. 

 The perturbation which mixes the states must be neither too large 

 nor too small, for if it is too large the potential energy surfaces 

 are "pushed apart" and resonance will be lacking; but if the perturba- 

 tion is too small, the probability of any non-radiative transfer of 

 energy is zero. The rate of these non-radiative transfer processes is 

 very sensitive to the environment. In rigid media the rate can be 

 greatly slowed down, but in gases and liquids it is very rapid (10-* 

 to 10-8 sec or less) . Paramagnetic molecules exhibit a well-known 

 quenching effect on phosphorescence from the excited triplet state, 

 which is to say, they catalyze the non-radiative energy transfer. 



