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I've passed on Scott's letter to another Scott - Scott Kable who
is a physical chemist who specialises in fluorescence.  He agrees
with all this except that he was heard to mutter 'vibronic coupling
isn't as simple as that'.  Expect a comment from him (relayed through
me since he isn't a member of this list) in due course.

                                                        Guy

Scott Snyder wrote:

>        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


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