Subject: Thanks and Summary was: Fluorescence and phosphorescence

From: "Thoms, Travis" <[email-masked]>
Date: Wed, 4 Apr 2001 16:50:24 -0700

Thank you all for your responses to my inquiry.  They were all very helpful.
I have included the original question and responses below, as suggested by
the rules of CCL.

Thanks again,

Travis Thoms

Greetings:
	I hope this question is not too far off-topic; it does actually
relate to my feeble attempts at modeling fluorescent and phosporescent
materials.	
	I have reviewed a couple of physical chemistry books concerning
these two topics, and to be quite honest, either they have not explained it
well, or I am missing the big picture.  If you don't mind, I would like to
explain to you what I think is going on, and then you could correct me, or
refer me to some good sources to set me straight.

Fluorescence:
A photon comes in and excites a molecule in a ground state (are most ground
states "singlet states"?) to an excited state (I am assuming that one cannot
excite to a triplet state). There is a percentage or probability that this
excited state will relax back down to the ground state by emissive means,
emitting a photon of light that is smaller than the homo-lumo band gap of
the ground state molecule.  The molecule can also lose its energy through
non-emissive means such as heat.  The energy cannot move from the excited
singlet state to a lower energy triplet state due to the need to conserve
momentum.

Phosphorescence:  Exitation occurs as above, except energy in this case is
allowed to move from the excited singlet state to a lower energy triplet
state through intersystem crossing (which was forbidden in the previous
scenario).  The triplet state in turn can relax to the (singlet?) ground
state via emission because of spin orbital coupleing with the heavy
nucleus).

Does this sound right?  Have I left something out which is important?

Thanks.

Travis Thoms

Reply 1:
That's right.  Experimentally, the difference between phosphorescence &
fluorescence is that the decay time of the triplet is orders of magnitude
longer than the decay time of a singlet because decay from the excited
triplet to the ground state singlet is also a forbidden process.

Irene Newhouse

Reply 2:
 You seem to have the general idea.  Most but not all ground states
are singlet states.  The percentage or probability of a specific process is
usually referred to as a quantum yield, denote by a subscripted Greek phi
(e.g.,
phi_sub_f for the probability that an absorbed photon will produce a
fluorescent
photon).  A number of non-radiative relaxation pathways can be important
including collisional quenching and static quenching in addition to 'heat'.

            I usually recommend R. Lakowicz's text to my students interseted
in
biological systems, but C.A. Parker's much older Photoluminescence in
Solution
may be closer to what you are looking for.

            First-principle modeling of fluorescence yields is more complex
than
the modeling of absorption probabilities (which is the first step).  The
actual
quantum efficiency depends on the relative size of the different relaxation
rates, which in turn are solvent and temperature dependent.  Fluorescence
yield
(and wavelength dependence) is much more dependent on solvent composition
and
temperature than absorbance, and is also subject to quenching by solutes.
Many
(most?) fluorescent modellers use a more empirical approach based on model
compounds.

Reply 3:
Dear Dr. Thoms;
I recommend the book titled "Excited States and Photochemistry of Organic
Molecules" which seems to be me very good in terms of understanding
excited state phenomena.
For fluorescence, only internal conversion is occurred(Singlet-Singlet
transition).
For phosphorescence, intersystem crossing is important(singlet-triplet
crossing) which takes more time than fluorescence.
I hope it will be something helpful for you.

Best regards,

Ohyun Kwon

Reply 4:

Travis Thoms,
in principle you meet the point;
in addition to that, what you are writing:

* F. and P.: light emission (luminescence), 
  induced by absorbed photons.

* F.: emission from the state, into which 
  the molecule was excited; spin-allowed; 
  short lifetimes; coupled vibrations reduce 
  the energy (progressions ...);

* P.: "intersystem crossing": spin-state changes 
  from absorption to emission; longer lifetimes;
  (-> name "phosphorescence"); 

* singulett: -´||,-  is  the ground 
  state in case of closed shell spin paired situations

* triplett (excited state): 
  -´|-
  
  -´|- 
  at lower energies than 
  singulett (excited state):
   -´| -

   -|,-

Regards,
Claudia