5/2 Flashcards
for electronic transitions in organic chromophores, what are the two types of excitation
π to π* for pi electrons or n to π* for lonepairs
when comparing the n–> π* and π –> π* transition, which requires more energy
the π to π* requires more energy and the n-π* transition requires less
in terms of intensity, does the n–> π* or π –> π* have higher intensity?
the π –> π* transition is easier because the orbitals are in the same plane, this makes the peaks more intense
the n–> π* transition is more difficult because the orbitals are not in the same plane, this makes the peaks less intense
in a electronic transition spectra, if we see two peaks, which is the n–> π* and π –> π* transition
the n–> π* transition will be lower energy and have less intensity
the π –> π* transition will have higher energy and more intensity
which organic molecules have electronic transitions
molecules with a double bond and adjacent lone pair
what types of electronic transitions can double bonds and alkynes undergo?
only π –> π*
what are the two modes of relaxation that are non-radiative and occur without photon-emission
internal conversion and dissociation
what are the two modes of relaxation that are radiative and occur with photon-emission
fluorescence and phosphorescence
what is internal conversion
the excited electronic energy is converted to thermal energy and is given off to the surroundings
what is dissociation?
when a vertical transition intersects the excited state above the dissociation limit, the electron will dissociate
what does the spectrum for dissociation look like
it has a very broad and intense absorption peak
in terms of singlets and triplets, how does a fluorescence transition exist?
it remains a singlet throughout the whole excitation and relaxation process
fluorescence and phosphorescence are:
radiative and emit an a photon when they get to the ground state
describe the relaxation of fluorescence
an electron is promoted to the excited state (remains a singlet) and then falls down sequentially by colliding with other molecules to give off small amounts of vibrational energy
then it falls to the ground state and emits a photon
in terms of singlets and triplets, how does a phosphorescence transition exist?
it starts as a singlet, is promoted as a singlet, then it becomes a triplet in the intersystem cross and then falls back down as a singlet
for both fluorescence and phosphorescence, how does the energy of the photon compare to the light absorbed
the energy of the photon emitted is less than the light absorbed since some was lost to collisions
what is intersystem crossing
it is where an electron in an excited state crosses to another excited state close in energy before coming back down to the ground state
in an intersystem cross, what happens to the singlet electron
it becomes a triplet
which is faster, phosphorescence or fluorescence
fluorescence is faster because it remains a singlet the whole time
phosphorescence takes longer because it becomes a triplet and must flip back to a singlet
