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Hi-
There's a book set by Cantor and Schimmel called Biophysical Chemistry that has lots of
information about this that may be useful.  Not much about 2-P absorption per se but it
does talk about symmetry, etc. in the second volume (published in 1980- not sure if
there's a newer edition).  It does have math, but not too much...
-Sarah

---------------------------------------------------------------
Sarah Locknar, Ph.D.
Director, Neuroscience COBRE Imaging/Physiology Core
University of Vermont
802-656-0413
---------------------------------------------------------------

Scott Snyder wrote:

> Search the CONFOCAL archive at
> http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal
>
>         I can definitely echo this sentiment.  In the interests of at least getting
> the discussion started, I will bring to bear what little I know about the
> physics of the situation.  Those who know more can correct me as needed.
>         First, as we are talking about electronic excitation of a molecule, there
> are three main selection rules.  The spin selection rule states that
> changing the spin of an electron makes an electronic transition (excitation
> or emission) forbidden (or at least heartily discouraged and slow).  In
> practical terms, this is why a triplet state lasts longer than a singlet
> state.  Having an excited state electron with the same spin as a ground
> state electron in the orbital it needs to go into for emission means the
> spin must change (can't have the same 4 quantum numbers), making the process
> slow.  Similarly, it forbids direct excitation to the triplet state.
>         The second selection rule is the orbital selection rule.  It is more
> complex and a bit harder to explain without resorting to point group theory,
> molecular orbitals and quantum mechanics.  Basically, it says that the
> excited state must have symmetry with one component of the dipole operator
> being changed.  Not sure whether it applies here.
>         A third rule applies if and only if a molecule has a center of symmetry.
> It is known as the parity selection rule and says that the parity of a
> molecule undergoing an electronic transition must change.  What is parity?
> Well, if you remember back to freshman chemistry to when you say atomic p
> and d orbitals and they had little + and - signs?  If you take all the
> points of the orbital and move them through the center  (the center of
> symmetry) and see all the + signs stay plus, they have one symmetry
> (gerade).  If all the + signs become - (and vice versa) they are ungerade.
> You can do similar things with the carbons on an organic fluorophore.
> Looking at the symmetry of the ground state and excited state orbitals then
> allows you to determine which transitions will be allowed and potentially
> after much math, to which high energy orbital an electron is being excited.
> I think this is the selection rule Guy is referring to.  The basic idea is
> that if transition to the excited state in one photon is allowed, it must
> change parity.  Thus, the excited state for a gerade ground state must be
> ungerade.  For two transitions, gerade would go back to gerade producing a
> DIFFERENT excited state in two photon.
>         A way to bypass the parity selection rule is by vibronic coupling.  Though
> this is definitely NOT an option for live cell and has potential
> photobleaching consequences, the fact that bond vibrations in the
> fluorophore can alter symmetry means that one might alter the excitation
> profile of a molecule by altering its temperature.  Increase temperature
> moderately and new excitation bands may be permissable.  Has anyone ever
> tried this?  Has anyone ever tried cooling?
>         I'd like to give some good references but most of the ones I know of aren't
> that good unless you are either a)  a) physicist  or b) a masochist.  A good
> review would be something a great many of us could benefit from.
>
> D. Scott Snyder
> Integrated Microscopy Core
> Baylor College of Medicine
> (713) 798-4952
>
> -
> Dear Guy--
>
> Thank you for your explanation.  Having photobleached the hell out of
> some of my first samples and gotten turned off to it, I'm ready to give
> 2p another try.
>
> However, I realize that I have no idea why a symmetrical molecule should
> be different than an assymetrical molecule.  Nor do I remember what
> selection rules entail.  Do you have any suggestions for references that
> discuss the physical chemistry of 2-photon in "baby-talk"?  It'd help to
> have some pedgogical hand-holding at this stage.
>
> Best regards--
>
> Martin Wessendorf