[teiwaz]

Hey all,

I'm working on a research concept for my PhD and although I've read many paper there are still some questions. I'm not a physical chemist so my background isn't the best one for this.

My concept to get into this field is to study the influences of substituents on the optical properties of the excimers of benzene.

If you have 2 Chromophores and bring them close together they form a new excited state, the excimer state, which has no stable gorundstate. This excimer state has a lower energy than the S1 state of the chromophore monomer so you get a red-shift in the fluorescence for an emitting excimer.

Benzene is commonly not a chromophore (because of a large HOMO-LUMO gap) but you can turn it into a chromophore via excimer formation because the gap is smaller and you get a blue or near UV-fluorescence. I want to study the effect of electron withdrawing/donating groups (esters, nitriles, amines...) on the optical properties of the benzene excimers. With those substituents you normally change the HOMO-LUMO gap of a chromophore so there must be a effect on the excimer too?!

My questions: Does it make sense to study substituents or are  structural properties more important? Is benzene a good system because there is no fluorescence as a monomer?


This was just a short intodruction, feel free to ask me anything about this!


Thanks

[Corribus]

First, there are a few inaccuracies in your post of which you should be aware:

1. Typically excimer formation occurs via the excitation of one chromophore, which then is already close proximity to, or diffuse to within the excited state lifetime of, another nearby monomer. I.e., the excimer is not usually formed automatically when two monomers happen to be close to each other. You need energy to make this happen.

2. I guess you can call benzene "not a chromophore". However, benzene does have fluorescence. It's just very weak, mostly due to the molecule's high degree of symmetry.

http://www.nature.com/nature/journal/v75/n1943/abs/075295b0.html

This is certainly not a bad area to study, but you should be aware that there is already a lot of work on the effect of molecular structure (physical and electronic) on excimer formation/properties and other related phenomena. See, for example (from a very brief google search):

http://www.sciencedirect.com/science/article/pii/S0039602809003069
http://pubs.acs.org/doi/abs/10.1021/jp100517n
http://onlinelibrary.wiley.com/doi/10.1002/chem.200601498/full
http://pubs.acs.org/doi/abs/10.1021/jp0630828
http://pubs.acs.org/doi/abs/10.1021/ja00428a037

Substitued benzene excimers have also been studied:

http://scitation.aip.org/content/aip/journal/jcp/51/5/10.1063/1.1672282
http://rspa.royalsocietypublishing.org/content/283/1392/83.short
http://www.sciencedirect.com/science/article/pii/0009261481851056
http://pubs.acs.org/doi/abs/10.1021/j100095a001

Therefore, you may need to narrow down your research topic to some degree. What you really need to do is figure out: What specific question are you going to try to understand, what is your central hypothesis, and how is the knowledge gained from your study going to be useful? As always, the best place to start is a deep literature search.

[teiwaz]

Thank you for your answer!

Sorry for the inaccuracies, I will try my best.

I also found more or less old publications for the benzene excimers but they all used concentrated solutions, solutions with high viscosity or solid state measurements. As derivatives there are examples of alkyl substituted benzene derivatives but none with elecetron withdrawing groups.   


Here is a short overview of my concept:



I want to use rigid spacers (which are commonly used in excimer studies) to get specific structures and doing measurements in diluted solutions to prevent intermolecular interactions. As far as I know there are no detailed studies with more than a few derivatives of specific chromophores. And this is making me wonder if this kind of study isn't worth enough and everybody is knowing that or just nobody did this.

The big idea behind this to transfer the results of the benzene system to design new excimer chromophores with special properties (long lifetime and quantum yields).

[Enthalpy]

Good fluorescent materials absorbing and emitting hard UV would be nice for photochemistry
http://www.chemicalforums.com/index.php?topic=81910.0
and you wouldn't need to shift their fluorescence to the blue. But shift your research topic a bit, yes.

A very useful fluorescent material (organic or not!) would absorb UV around 365-385nm and emit a blue agreable to the human eye to improve white Leds. Presently, 405nm is used as a very bad blue and partly converted to green and red, just because we had efficient Leds at 405nm, but the resulting white is horrible, so as Leds at 365-385nm become mature, it would be time to emit the blue from a phosphor too.