Temperature Dependence of Local Domain Photophysics

Chris BinghamReid Lab

General Exam Practice

Why do we care?• Mission of CMDITR

– Create new materials for information technologies

• A lot of interest lies in using chromophores and polymers– Chromophores and polymers show a lot of

promise for these new devices• We are generally interested in the

electro-optic (EO) activity of these

Why aren’t they used?• EO activity is not inherent in composite

materials, as it is with some inorganics (like LiNbO3).– EO activity arises from χ(2) processes, so

order within the composite is important. – Rotational studies

• Chromophore robustness (irreversible photo-decomposition)– Fluorescent intermittency studies

What are the objectives of my research?

• Understand the processes behind creating order and photo-decomposition

• Understand the relation between the local environment (domain) and the chromophore (probe)

• Understand the probe-domain interactions that give rise to the population and depopulation of the dark state

• Increase understanding of SM photophysics, which are surprisingly complex

What variables can we control?• Probe

– Size– Shape– Functional groups

• Domain– Glass transition temperature (Tg)– Side groups

• Temperature– How does a change in the thermal energy of the

system affect photophysics?

Experimental setup• Confocal fluorescence microscopy

Experimental conditions

• Violamine R (VR)• Poly(vinyl

alcohol) (PVOH)• Nano-molar

concentrations• Temperature

range from 23C to 85C

Fluorescence traces

Power Law Plots

• Power law (PL) plots are indicative of distributed kinetics

• Roll off on the plots means that it is moving to a more single exponential distribution

Memory Plots• A density of points along the diagonal indicates

memory• Memory is related to domain exchanges• However, due to poor statistics, SM memory plots

Problems with VR• One of the downsides to using VR is

that it comes as a relatively low dye content (60-65%)

• The question is what is the other ~40%?– Is it also fluorescent, particularly at 532

nm?• Also, the fluorescence of VR is solvent

dependent

Solid Walled Pockets• In studies performed by Orrit and

coworkers (Zondervan, 2007), they showed that some polymers exhibit domains that are slow to exchange with the local environment, even above Tg.

• The evidence for a slow exchange is that there is not a drastic jumps in the rotational timescales of the probe molecules

Support for SWPs• PVOH is able to hydrogen bond

– Is known to h-bond with itself– Orrit and coworkers showed SWPs for glycerol

and o-terphenyl, both which are capable of intra-polymer bonding • H-bonds in glycerol• Stacking in o-terphenyl

• PMA does not hydrogen bond, or at least not as easily– Memory plots are very different than those of

PVOH

Domain stress• The free volume of the polymers is

much smaller than that of the Van der Waals radii of the probes

• Free volume expansion by introduction of probe may increase side chain interactions, such as hydrogen bonds

• This may be a cause for SWPs

Differences between domains• PVOH

– Simple structure– Can form hydrogen bonds between itself as well

as the probe• KAP

– Crystal– Domain is constant– Oxygen impermeable

• PMA– Longer side chain– Has a relatively high chain mobility

Reduced temperature scale• Reduced temperature scale is relative

to a polymers Tg

• By looking at T/ Tg, we are able to directly compare chain mobilities, even though the absolute temperatures may be different

Comparing domains in the reduced scale

• The results of VR/PMA (23˚C) compare to the VR/PVOH (85˚C)– T/ Tg is ~1.04

• The overall shape of the PL plots are the same– Both exhibit roll offs at the ends

• However, one big difference is the density of points is much greater in

What do we get out of this?• So, if PL distributions are similar, but

memory is different, what does that tell us?

• The processes that are responsible for the population and depopulation of the dark state are consistent between the two domains, but in PMA, the domain is able to exchange much more quickly than PVOH, resulting in a

Where do we go?• PVOH and PMA share a complication.

– It is difficult to properly span the Tg in both polymers.

– PMA has a relatively low Tg (~9˚C)– PVOH has a relatively high Tg (~72˚C)

• Use a polymer that has a Tg that can be easily covered– Poly(isobutyl methacrylate) (PiBMA)

Why PiBMA?• PiBMA has a mid-range Tg (~55˚C)• The structure is similar to PMA• PiBMA has been used in previous EO/

SHG studies (Dhinojwala, 1993)• PiBMA is a fairly common polymer

Change the probe• Size• Shape• Functional groups• DCM is a common laser dye, which has

also been used in previous studies by our group

DCM• Dye content is 98%, as opposed to VR

which has a dye content of 60%• Hydrogen bonds formed with DCM

may be weaker than those formed with VR

Europium• Europium (Eu) is fluorescent• Adding ligands increases size• Does the size of the Eu complex affect

domain exchanges?• Will an increase in stress on the

polymer result in slower exchanges?

What have we done?• First Act

– Violamine R (VR) in poly(vinyl alcohol) (PVOH)

• Second Act– VR in potassium acid phthalate (KAP)

• Third Act– VR in poly(methyl acrylate) (PMA)

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