hatcher content part 9

what is internal conversion?

radiationless (vibrational) relaxation within the states of the same multiplicity(S2 → S1)

what is intersystem crossing?

radiationless transition between the states of different multiplicity (S1 to T1)

what is fluorescence?

radiative transition between the states of the same multiplicity (spin allowed, S-S)

what is phosphorescence?

radiative transition between the states of different multiplicity (spin forbidden, S-T)

what are the timescales of the different uv/vis transitions?

absorption - femto seconds (10^-15)

internal conversion - pico seconds (10^-12)

intersystem crossing - molecule dependent

fluorescence - 1-1000 nanoseconds (short)

phosphorescence - 1-100 milliseconds (long)

what is stokes shift?

a small shift between absorption and emission spectra

Eem < E abs

atoms always moving so reaction coordinate not ever perfectly aligned

absorption and emission spectra tend to be mirror images

only true if geometries of ground and excited state are very similar

how do you calculate the lifetime for emission spectroscopy in seconds?

1/(kr + knr)

kr is the rate constant of radiative decay

knr is the sum of rate constants of all non-radiative decay processes

what are some excitation sources for emission spectroscopy?

light induced luminescence (absorption)

thermoluminescence

electroluminescence

triboluminescebce (from mechanical energy - crushing crystals)

chemiluminescence

what are some examples of non radiative decay (quenching)?

vibrational deactivation (photons/heat)

deactivation through non-emossive states (mc states for metal containing chromophores)

static quenching - ground state aggregate formation (conc dependent)

dynamic quenching (collisionsl, diffusion limited)

dissociative states - photochemical reaction occurs e.g. bond breaking reactions

photoinduced energy or electron transfer (PEnT and P ET repsectively)

what is an example of dissociative states formation?

spirooxazine and spiropyran

colour change due to increased conjugation in open form = absorption shifts to lower energy

used in reaction lenses that go dark in the sun