Blue-light Special

PHOTOCHEMISTRY

When Felix N. Castellano displays the digital photograph at conferences, jaws tend to drop. The photo shows a beam of green light from a laser pointer entering a vial of clear liquid and being transformed into a beam of blue light. It's a vivid demonstration that Castellano's group has found a simple, low-power photochemical method to "upconvert" green light to higher energy blue light.

Such upconversions "usually involve extremely high laser powers--in my laser lab, I need three boxes [of electronics] to pull this off," says Daniel G. Nocera, a chemistry professor at MIT.

Red and green light are readily produced in devices such as organic light-emitting diodes, but scientists are still struggling to find an efficient and stable way to generate the third primary color of light--blue--in molecular systems. Hence the interest in Castellano's approach.

Castellano, an associate professor of chemistry at Bowling Green State University, in Ohio, and coworkers Radiy R. Islangulov and Denis V. Kozlov achieve the color transformation using a solution of two simple compounds. The green laser light excites a well-known ruthenium bipyridine complex to the triplet state, and the triplet efficiently transfers its energy to 9,10-diphenylanthracene (DPA), forming a long-lived, excited triplet DPA. When two triplet DPAs encounter each other through diffusion, energy transfer occurs, leading to a higher energy singlet DPA, which fluoresces blue (Chem. Commun. 2005, 3776).

Since the team members submitted their paper to the journal, they have discovered that another metal complex works even better as a so-called triplet sensitizer, Castellano tells C&EN.

The results have surprised many researchers in the field, especially since these types of photochemical reactions have been studied for decades.

"The single biggest challenge" now, Castellano says, is to make this technology practical by developing it into a solid-state system. The upconversion process relies on excited triplets diffusing and reacting, he explains, and diffusion in the solid state is hard to achieve.