Cutting edge

September 11, 1998

The laser and the application of quantum theory created a boom in photochemistry and opened the way for revolutionary medical techniques.

Given the fascination with sunlight that all mankind has had since the earliest times, it might not be too surprising that some scientists with a chemical bent look to light as the driving force for their chemistry.

My first experiences as a research student in Birmingham in the early 1960s used light merely as a kind of Bunsen burner to create reactive species, the subsequent chemistry of which was the main topic of interest.

However, photochemistry was about to take a quantum leap forward, driven by the development of the first laser by Theodore Maiman in 1960 and the applications of quantum theory to photochemistry. From the mid-1960s one was able to understand why some molecules upon excitation with light gave back strong emitted light, whereas others dissociated or performed improbable rearrangements of their constituent atoms, and yet others simply turned the energy gained by absorbing light into heat.

By the mid-1970s, interest turned in a large part towards applications of the fundamental science. I was part of this migration of interest from fundamental research to application, and in time my attention was taken by thoughts of the interactions of light with the human body.

One had always understood the superficial but sometimes deadly effects of light on the skin: tanning, premature ageing and, most recently, a huge increase in skin cancers. But what of more subtle and special effects?

My introduction to what is now my main research came through a collaboration with industry, in which we were attempting to create a "photochemical" bleach to put into a cold-water washing powder that would bleach in sunlight.

A chance meeting with a physician apprised me of recent work in the United States in which such "photochemical sensitisers" had been used for the first time to target tumour tissue in cancer patients so that the application of red laser light was able to destroy the tumour, sparing the surrounding normal tissue. This "magic photon bullet" approach was instantly exciting, and for the past 15 years my group has, in collaboration with University College Hospital, been devising new sensitisers for the treatment of various kinds of cancer.

Photodynamic therapy, as the treatment is called, is now in use throughout the world. Some 100,000 patients having been treated for a variety of tumours, with very encouraging results.

Most recently, attention has been focused on the application of these principles to the destruction of bacteria, responsible for a huge number of disastrous medical conditions.

The transfer of electrons between a molecule that has absorbed light and another substrate is extremely common in nature. The process can take place within a few femtoseconds, or millionths of a billionth of a second.

To study these processes, one needs a pulse of light shorter than the time-scale of the event to take ultra-fast snapshots of what is occurring. Together with colleagues at the Lasers for Science facility at the Rutherford Appleton Laboratories, we have built a system using femtosecond-lasers to study these electron-transfer processes.

This is fundamental, intellectually immensely stimulating research, but it is also not a million miles away from the applications of electron transfer in the "bacterial warfare" with which we also occupy and amuse ourselves.

David Phillips is head of the department of chemistry, Imperial College, London.

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