Monday, February 25, 2013

How do chemical subsitutions change the colour of a dye?

Last week I heard Seth Olsen give a nice talk about his recent paper
Why Bindschedler's Green is redder than Michler's Hydrol Blue

It addresses the important and subtle question of what happens in methine dye molecules when the central carbon atom is replaced by a nitrogen atom:
I was going to write a summary. But, the abstract of the paper is beautifully written, summarising the main results. And so, here it is.
We offer a new physical interpretation of the color shift between diarylmethane dyes and their azomethine analogues. We use an isolobal analogy between state-averaged complete active space self consistent field solutions for corresponding methines and azomethines to show that the shift contains a significant contribution from configuration interaction between a methine-like ππ* excitation and an nπ* excitation out of the azomethine lone pair. The latter does not exist in the corresponding methine systems. This picture is qualitatively inconsistent with traditional models of the shift based on molecular orbital perturbation theory of independent π-electron Hamiltonians. A key prediction is the existence of a dipole-allowed band in the blue/near-UV spectra of the azomethines, which has polarization parallel to the lowest energy band. This forces a revision of past assumptions about the nature of the low-energy spectra of the azomethines. We show that a band at the predicted energies was observed as far back as 1938, but its significance at the time appears to have been unrecognized.

3 comments:

  1. That is a very clear, well written abstract. So much so that I'm bothering to comment. You don't read too many abstracts that are even nearly that helpful.

    The emphasis (put in by Ross, of course) helps, especially on skim-reading. It's a funny thing about clear writing that you oughtn't need emphases such as those (nor smiley faces, for that matter), and that the language itself should highlight key points and phrases.

    However, as physicists and chemists aren't expected to write as well as Seth, nor should they be, and nor are they expected to be as familiar with the language as you need to be to properly recognise the full weight of the word choice and sentence structure etc., it raises the question for me whether it would be worth having more bold-face in abstracts, for example.

    I feel like if I put bold-face in my abstracts they'd be removed by the copy-editor, probably frowned upon by referees, and perhaps even viewed as patronising to the community.

    But then, it would be helpful too, I think.

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  2. My dear wife told me I should use more bold face in my blog posts and so I started doing it, particularly in highlighting parts of long quotes.

    I don't think bold face in abstracts will fly. However, I would have thought a little italics may be acceptable (and would be a good thing).

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