Standard Samples

• 1: BDPA in Polystyrene

1 - BDPA in Polystyrene

General Information

BDPA (α,γ-bisphenylene-β-phenylallyl-benzolate) is a well characterized stable radical, often used as a standard in EPR spectroscopy. The radical is highly stable and one of the first organic, stable radicals ever synthesized [1].

BDPA can be obtained from all major chemical vendors such as Sigma-Aldrich (Milipore Sigma). Typically, BDPA is co-crystallized with benzene in a ratio of 1:1. In the following this complex is referred to as BDPA for simplicity.

Name: α,γ-bisphenylene-β-phenylallyl-benzolate (BDPA)
Empirical Formula: C39H27
CAS Number: 35585-94-5
Molecular Weight: 495.63 g/mol

Usage in EPR Spectroscopy

Neat BDPA Crystals

Neat BDPA can be used in EPR spectroscopy. For example, individual crystals, picked from the bottle make a good standard to observe an FID or if only a small spec of an EPR active substance is required for example to map out the B₁ field of a resonator. However, be careful when handling neat BDPA. The smallest contaminations can result in large EPR signals, which will require extensive cleaning.

BDPA Dissolved in Polystyrene

BDPA dissolved in polystyrene (BDPA/PS) makes a stable and very reliable standard sample for EPR spectroscopy at any frequency. BDPA has a very small g-anisotropy, and, unless the sample is prepared from perdeuterated BDPA in fully deuterated polystyrene, the g-tensor asymmetry is not visible at frequencies below 140 GHz.

EPR Properties

Experimentally determined g-values and hyperfine parameters for BDPA are summarized in the table below:

The values for the two hyperfine tensors are obtained for deuterium nuclei and scaled to proton frequencies. Values taken from [2].

Sample Preparation

The general procedure to prepare the BDPA/PS sample is as follows:

1. Prepare two separate solutions of BDPA and polystyrene in toluene.
2. Mix appropriate aliquots of the BDPA and polystyrene solutions to achieve desired concentration (ratio).
3. Pour mixture onto clean glass plate and let toluene evaporate at room temperature.
4. Using a spatula, scrape plastic film off the glass plate
5. Dry sample material in vacuum

Example – Preparation of a 0.1 % BDPA/PS Sample

For EPR spectroscopy a useful concentration ranges between 0.025 to 0.1 % (w/w) BDPA/PS.

For this preparation, all chemicals are lab grade chemicals and used without further purification.

To prepare a 0.1 % of BDPA in PS follow these steps:

1. Make BDPA stock solution by dissolving 10 mg BDPA in 1 ml toluene.
2. In a small beaker dissolve 1 g of polystyrene in a small amount of toluene. Typically, 2 to 3 ml should be sufficient. The more toluene you use, the longer it will take for the toluene to evaporate.
3. Add 100 µl of the BDPA stock solution to the polystyrene solution for a finale concentration of 0.1 % (w/w) BDPA/PS. To prepare a sample with a lower concentration, adjust the amount of BDPA stock solution that gets added to the polystyrene solution.
4. Pour the entire solution onto a clean glass plate. Make sure the glass plate is leveled and large enough. If necessary, pour the solution in several small badges.
   • Alternatively, use non-stick aluminum foil such as Reynolds Wrap (non-stick).
5. Let the toluene evaporate at room temperature in a fume hood. This can take several days. To speed up the process, once the majority of the toluene is evaporated, put the plate into an oven or incubator. Keep the temperature low (35-40ºC). Make sure the area is well vented, toluene is highly flammable.
6. Once the sample is dry, not gooey or sticky anymore, scrape the sample material off the glass plate using a spatula and transfer the sample material into a small (round-bottom) flask that can be evacuated.
   • Optionally, weigh the empty flask, note down the weight and repeat it once the sample material is transferred into the flask to get the initial weight of the sample material.
7. Dry the sample in vacuum.
   • Weight the flask throughout the drying process. Once the weight is stable, all toluene has been removed.
8. Once dry, grind the sample material to a fine powder using a mortar and pestle.

To avoid the formation of small pockets, make sure to pour a thin layer of the toluene solution onto the glass plate.

Once the sample material is transferred to the EPR tube, the sample can be used as is. However, if all oxygen is removed from the sample the longitudinal relaxation time T_1e can be dramatically improved. Typically, an evacuated sample will have a T_1e about factor 200 longer compared to a sample exposed to air. To remove the oxygen, evacuate the sample capillary and flush with argon. Repeat this cycle several times. After the last cycle the sample capillary must be flamed sealed. You can do this in two ways:

• Do not flush the sample with argon after evacuating. That way, the walls of the sample capillary will collapse, once heated by the torch.
• Flush the sample with argon. Don’t overpressure the sample tube with argon.

Both cases need a bit of practice.

Sample Concentration

References

1. C.F. Koelsch, Syntheses with Triarylvinylmagnesium Bromides. α,γ-Bisdiphenylene-β-phenylallyl, a Stable Free Radical, J. Am. Chem. Soc. 79 (1957) 4439–4441. https://pubs.acs.org/doi/10.1021/ja01573a053
2. V. Weis, M. Bennati, M. Rosay, J.A. Bryant, R.G. Griffin, High-field DNP and ENDOR with a novel multiple-frequency resonance structure, J Magn Reson. 140 (1999) 293–9. https://doi.org/10.1006/jmre.1999.1841.