Overdriven Pulsed Light Emitting Diodes: An Inexpensive Excitation Source for Time-Resolved Luminescence Lifetime Measurements

Time-resolved luminescence lifetime measurements are an important photophysical technique. However, access to this technique at the undergraduate level is limited by the expense of the instrumentation needed to create the well-defined light pulses required to measure the luminescence decay of an excited state molecule. We present here an inexpensive, improved light emitting diode (LED) circuit that is capable of supplying bright light pulses. Our circuit is designed to overdrive the LED resulting in a bright excitation light source with a well-defined square wave pulse shape. This allows for the facile measurement of luminescent molecules with decay lifetimes in the hundreds of nanoseconds to microsecond range. Data are presented for five UV, violet, and blue LEDs as driven by the improved circuit that can be used as a guide for selecting the best excitation light source for a particular molecule of interest. Our students have studied the quenching of aqueous tris(bipyridine)ruthenium(II) chloride with ferric ion using an overdriven blue LED as an excitation source. While ferric ion is not an efficient quencher compared to molecular oxygen, it is much easier to handle, allowing for the preparation of accurate standard solutions free of dissolved oxygen.