4/30/2007

The Big Bright Green Fluorescence Machine

Here at CBC, it is little surprise to anyone that we love conjugated polymers. Why do we love them, you ask? What is not to love? Their extended π-conjugation aids to warm our hearts, their structural rigidity brings us comfort, and their soft luminescence energizes our spirits. Conjugated polymers are the heart and soul of all that we love- they are the paradigm of the things we treasure in Nature. Haha, just kidding. We just like them cause they're pretty when they light up. In this way, they appeal to our basest instincts- I have basically picked a graduate program centered around things that look pretty on paper or in a flask. My long-wave UV lamp has, in this way, given me much satisfaction over these lonely years. I am little more than a john for the lipstick-wearing whore that is... solid-state fluorescence. Heidi Fleiss, eat your heart out.


Anyway. Science. My personal favorite conjugated polymer, one near and dear to my heart, is poly(p-phenylene ethynylene) (PPE)- a highly linear, rigid, conjugated polymer that crazy fluoresces pretty bluish-green in solution and in the solid state. Anything that makes pretty things fluoresce even better than normal is thus fair game for coverage here, and a group in Japan!* has managed to turn PPE into a luminatin' machine. But first, for the non-fluorescence croud, let me explain first fluorescence quantum yield.

It sounds like something straight out of, well, Quantum Leap, but quantum yield is a pretty simple idea. In happy ideal chemistry world, when one photon excites a molecule to a singlet excited state, one photon is released as the molecule relaxes to the ground state. That's fluorescence. Of course, as any of the students in my o-chem lab will tell you, happy ideal chemistry land is a crock of shit. Electrons can relax via many pathways, most non-radiative. In particular, conjugated polymers and other systems that have large pi-systems can form excited-state dimers (excimers- tee hee) or complexes (exciplexes) that cause electrons to prefer the non-radiative relaxation. The quantum yield Φ is simply a ratio of how many photons excite a molecule to how many photons are emitted. (Measuring Φ is trickier.) It's always less than 1. In the case of most conjugated polymers, it's waaaaaay less than 1.

Fortunately, the Yamaguchi group has found a way to greatly increase the fluorescence of PPEs, by adding π-acceptors to the polymer backbone. They've shown that boron functionalities, with their empty p orbitals, conjugate well with the π* orbital of the polymer and increase fluorescence activity. By alternating these π-acceptors with bulky functionalities with π-donors such as amines, fluorenes or carbazoles, the fluorescence quantum yield increases dramatically, both in solution and in the solid state.



If you read the paper, many of the conjugated polymers have quantum yields close to 1 in solution and close to 0.7 as films. (Since that fiasco with Shelley Batts, I am limiting the amount of crap I pull from the paper to pretty pictures. I don't need ACS's lawyers breathing down my necks right now. Maybe later, AFTER finals week at the very least.) It's not often you find quantum yields of 0.9 for conjugated polymers, so that's mightily impressive on paper.

Stay tuned next time, when I compare my interests in photonics to, I dunno, the firebombing of Dresden or something similarly inappropriate.

*I think it should be mandatory that when something cool comes out of Japan to refer to the country with at least one exclamation point, if not more (with some ones and eleventies thrown in for 1337ness) for example, when they started making robots: Japan!!!11!oneone!!111eleventy!11

9 comments:

Anonymous said...

I would worry about oxidative unstability of these boranes if it is to be used for anything pracical. I worked with triphenyl borane a lot at one time and it can be handled on air for a brief time (unlike alkyl boranes) but it was supplied as sodium hydroxide adduct for the long-term storage.

I wonder if they could make a similar shiny polymer with boronic acids or diol-derived cyclic esters thereof - because these are air-stable

Milkshake

Excimer said...

The paper says they are air- and water-stable. I imagine that the bulky aryl groups help protect the boron. As for redox conditions as in OLEDs... no, probably not very stable, but it does show how one can use pi-acceptors to increase fluorescence activity, so one could feasibly use other pi-acceptors in use like cyanoethylenes or the like.

I imagine a major problem with using boronic acids would be aggregation in solution leading to poor solubility and thus making it hard to spin-cast.

Matt Jenks said...

An S&G reference AND Quantum Leap. Excimer, you're tickling my obscure cultural fancy this fine afternoon.

Uncle Al said...

You sport a techno-woodie for photon-belching conjugation but hang limp for Little's ambient temp exciton supercon polyacetylenes. If HOMO-LUMO are a Bose-condensed supercon they can't do chemistry. Electric current aside, markets (DARPA) want unbreakable thread for soft armor, sails, sheets (hawsers), and composite hulls; airframes, rocket skins.

You cannot catch up to the literature. Survival is the literature catching up to you. Do one little ethylene-extrusive ADMET for Uncle Al.

kiwi said...

that acs copyright crapola is insane. fair review anyone?

Excimer said...

Actually that whole thing with her was with Wiley, not ACS. Hopefully ACS will be reasonable.

sushi said...
This comment has been removed by a blog administrator.
Ψ*Ψ said...

How soluble are those?

Lakshmi said...

Hi,
The Science Blog Carnival: Panta Rei edition #5 is specific to all topics related to CHEMICAL SCIENCES. It will be hosted at nOnoscience on May 14, 2007. Please do send in your entries.
You can use this contact form or the blog carnival page for submission