Who should I get to write a letter of reference?

This is concerned mostly with applying for postdoc and junior faculty positions.

People who must

-your current supervisor

-your previous supervisor

People who may help your cause

-someone who does not have a vested interest in your success and so is more objective than your Ph.D supervisor

-has a reputation for writing reliable assessments

-someone who is personally known and respected by those reading the letter

People who should not is someone

- who won't get around to sending in the letter

-who has a reputation for writing ridiculous inflated letters ("This is the best student I ever had" for every student!)

-writes generic letters ("Dr. X is a nice person who does good work.")

-who does not really know you

-who is at the same level as the position you are applying for

As you get more senior you need to be getting letters from people less connected to you (e.g., from institutions you have not worked at) and with increasing seniority and reputation.

Some of these ideas are gleaned from John Wilkins one page guides on Vita for the first job after the Ph.D and Writing Letters of Reference.

I did a postdoc with Wilkins and since then he has written some very nice letters on my behalf! Thanks.

Virtual drug design?

To what extent does computational chemistry actually guide drug design?

This came up in the previous post concerning "virtual screening" of candidate molecules for organic photovoltaics.

Seth Olsen brought to my attention a nice article, Virtual Screening: an endless staircase, by Gisbert Schneider that appeared earlier this year in Nature Reviews Drug Discovery. Here is part of the abstract:

Computational chemistry — in particular, virtual screening — can provide valuable contributions in hit- and lead-compound discovery. Numerous software tools have been developed for this purpose. However, despite the applicability of virtual screening technology being well established, it seems that there are relatively few examples of drug discovery projects in which virtual screening has been the key contributor.

Furthermore, the article says:

Substantial progress in virtual screening requires a profound understanding of the forces that govern protein folding and the dynamics of macromolecular complex formation.

The figure above is taken from a this paper about the action of aspirin.

OPV cell efficiency is an emergent property

As discussed in a previous post, the efficiency of organic photovoltaic (OPV) cells appears to be largely determined by solid state (and thus collective) effects such as aggregation, sample morphology, and disorder. A striking example of this is that the efficiency of a cell can be improved significantly by annealing the thin film (i.e., just taking the film and slowly heating and then cooling it). Hence, efficiency is an emergent property and reductionist theoretical approaches that focus on the properties of isolated constituent molecules have debatable value.

At the I2CAM workshop this past week the most disappointing presentation was that from the Harvard clean energy project, led by Alan Aspuru-Gizek . This very ambitious project aims to using the world wide grid of computers (including your own PC) to run quantum chemistry codes to calculate properties of hundreds of thousands of molecules to screen them as candidates for use in OPVs. However, it must be stressed that almost all of these calculations will be on small single and isolated molecules in the gas phase.

It was claimed that one could screen for high charge mobility materials by looking at delocalisation of frontier orbitals and the reorganisation energy associated with ionisation. However, the particularly relevant quantity is the reorganisation energy of the environment of the molecule.

The speaker claimed something like "we are our own harshest critics" and listed possible weaknesses of the project. These were most concerned with whether approaches based on density functional theory (DFT) are adequate for calculating the relevant properties of these molecules. (Many people would say they are not). However, I contend that even if one could calculate exactly the properties of single molecules in the gas phase one would be a long way from being about to determine which molecules will be the best candidates for OPVs.

I asked for a specific example of where such a computational approach has been successful for any area of science and technology. It was stated that drug companies do this all the time when screening. However, I contend the physics and chemistry of that problem is much simpler and more well defined. One knows a specific active site of a protein that ones want to find a small molecule to bind to the hinder the activity at that site. This is a ground state and very local property. In contrast, for photovoltaics excited states, dynamics, and collective effects are involved, and the relevant large scale structures are not well defined. Exactly how the properties of OPVs are related to the properties of the constituent molecules is so poorly understood I am skeptical that a brute force computational approach is going to lead to much progress.

Splitting singlets to save the planet

A really nice talk at the I2CAM workshop on organic photovoltaics this week was the one given by Christopher Bardeen, Exciton Fission and Electronic Delocalisation in Organic Semiconductors.

Here, I will just explain some of the beautiful physics of how you can split a singlet exciton into two triplet excitons.

First, the group theory. The product of two triplet states can be written

Presumably, a similar identity holds for l=1 spherical harmonics.

How might this happen in a conjugated organic molecule?

The Figure below is taken from a seminal paper by Tavan and Schulten.

The singlet Ag- state shown above is "dark" and also plays a key role in photosynthetic systems, as discussed here.

Understanding properties of dye-sensitizers

At the conference today, my colleague Seth Olsen gave a talk Stucture-Property Relationships for Conjugated Organic Dyes.

The goal is to provide a rigorous quantum chemical justification for empirical relationships such as that shown in the curve above, which shows how the absorption wavelength of a conjugated dyes varies with different substitutions at the point R in the molecule.

Much of the talk is based on his recent paper in Journal Chemical Theory and Computation.

Deconstructing excited state dynamics in conjugated polymers

Joseph Shinar (Ames Lab, Iowa State) gave a nice talk today about using optically detected magnetic resonance (ODMR) to elucidate the dynamics of optically excited states in conjugated polymers.

First, two "human interest" asides.

1. The results of this research led to share prices of some companies going up and down.

2. Shinar said he build his first ODMR spectrometer using an ESR spectrometer he got from a dumpster outside the chemistry building at the Technion in Israel.

Basically what one does in this experiment is to monitor an optical property (such as photoluminescence) at the same time that one applies a microwave field and magnetic field. When the microwave field frequency is on resonance with that required to induce transitions between different Zeeman levels on sees changes in the optical property.

Seeing any detectable change may be surprising because one does not necessarily expect optical properties to be so spin dependent. It turns out understanding why one gets a signal at all and the physical mechanism took almost 20 years.

There are many possible competing processes following photoexcitation of a singlet (exciton) state.

S1 --> P+ + P-

or T1 + T1

where P+ is a positive polaron (this is just a cation with a significant bond relaxation

in the neighbourhood of the charge)

and T1 denotes a triplet state.

The polarons have unpaired spins and will produce an ESR signal. But how does flipping these spins enhance the photoluminescence?

Some of the key results to understanding what goes on are in this PRL.

Shinar considered 2 scenarios [note the method of multiple hypotheses].

The one that is consistent with experiment is

Enhanced spin-dependent annihilation of Triplet excitons by polarons

T1 + P+ --> P+ (note this conserves charge and spin)

TPQ [triplet polaron quenching] model

(This is discussed in the classic book by Pope and Swenberg)

Shinar claims TPQ is one of the most important interactions in organic photonic materials and devices.

The picture that emerges of the photoexcitation dynamics is that in the "steady state" the number of triplet excitons and the number of polarons is about 10,000 times larger than the number of singlet excitons.

Eventually the polaron pairs recombine into a singlet exciton which decays radiatively.

Charge transport in organic photovoltaic materials

Today I am chairing a session on charge transport at an I2CAM Exploratory Workshop, Complex Interactions and Mechanisms in Organic Photovoltaics being held here at UQ.

I have written quite a few posts on this topic before. I believe some of the key questions concerning charge transport in molecular materials are:

• What is the mechanism of charge transport?
• What is the origin of the observed electric field dependent mobility?
• What determines the relative magnitude of electron and hole mobilities?
• How does mobility depend on the intermolecular separation and relative orientation?
• Would thermopower measurements be helpful in determining the charge transport mechanism?