Excited States

How are excited states created

through an electronic transition which is induced by photon absorption.

What are the 3 selection rules

Spin, Orbital and Vibrational

What’s vibrational relaxation

Relaxation from a higher vibrational energy state to the ground state via collisions with surrounding molecules

What’s fluorescence

transition between states of the same multiplicity from an excited state to ground state via an emission of light

What’s internal conversion

transition between states of the same multiplicity from the ground vibrational level of an excited state to a vibrationally excited level of the ground state by transferring electronic energy into vibrational

What’s intersystem crossing

A process where the state is changed through an electron spin-flip

What’s phosphorescence

A transition between states of different multiplicity from an excited state to a ground state via the emission of light

Why is the fluorescence transition in emission lower than it is in absorption?

stokes shift → the solvent rearranges to stabilise the excited state after absorption but before emission thus lowering the energy.

When is Stokes shift more prevalent

in polar systems

Quantum yield=

number of molecules undergoing process/ number of photons absorbed

lifetime =

1/ sum of rates of decay processes

Lifetime is…

time when the concentration of the species in its excited state falls to 1/e of its initial value

The effect of temperature on the rates of photophysical decay

very small effect on rate

affect of decreasing temperature on quantum yields of photophysical processes

decreases the rate of other processes so increases quantum yield of photophysical

Affect of heavy atoms (in molecule or solution) on the quantum yield of photophysical processes

increasesthe rate constant for all spin-flip processes due to spin-orbit coupling

What’s photodissociation

on excitation the molecule dissociates

What’s photopredissociation

Initially creating a bound excited state but internal conversion occurs to a disocciative state

What decreases the quantum yield of photodissociation and photopredissociation?

In solution, the solvent cage can keep the fragments together long enough to recombine

Whats photoisomerisation?

the molecule undergoes a change in structure upon excitation. Usually cis-trans

What’s a photostationary state?

a steady state reached with a mixture of the isomers

What’s photochromism?

change in colour upon excitation which is usually linked to isomerisation

What’s an excimer

an excited state dimer

What are excimers characterised by

the excimer fluoresces at a longer wavelength without vibrational structure when you increase concentration of the compound in its ground state

what’s an exciplex

an excited state complex

What’s different between an exciplex and excimer

exciplexes are polar, charge transfer complexes

What’s electron transfer?

a form of quenching where the excited compound transfers an electron to the quencher

When is electron transfer yield highest?

in polar solvents as they stabilise the products

what’s energy transfer

the electronic excitation energy is transferred from the excited compound (donor) to an acceptor

3 processes of energy transfer

radiative, long range coulombic and short range electron transfer

describe radiative energy transfer

donor emits a photon, acceptor absorbs it

describe long range coulombic energy transfer

occurs by a dipole-dipole interaction between the excited donor and acceptor and is usually between singlet states

describe short range electron transfer (energy transfer)

exchanging electrons between the donor and acceptor which can occur between many combinations of states

What’s photosensitisation

using an energy transfer to create T1 states that can’t be readily created

what does photosensitisation require

a donor with a high quantum yield for intersystem crossing (to form T1 state) and this T1 state needs to have higher energy than the acceptor T1 state

Energy (E) =

hv = hc/ wavelength

wavenumber=

1/ wavelength x 0.01

absorbance (A) =

molar absorption coefficient x c x l

multiplicity=

2S + 1