Nanojoules, nanoliters and nanosecond calorimetry

A nano-scale time-resolved calorimetric experimental approach is described using a photoacoustic cell made from narrow-bore flexible fused-silica capi llary tubing. The fused silica capillary permits the transmission of an exc itation source from the deep UV to the near infrared. Methods to couple the excitation energy into the sample and the acoustic energy away from the sa mple to the detector are discussed. To demonstrate the unique capabilities of the fused silica capillary cell, the excited triplet state energy (E-t) and lifetime (tau) of benzophenone (Bp) in the presence and absence of a qu encher was measured. Irradiation of an air-saturated acetonitrile solution of Bp in the nano-scale photoacoustic cell, (i.e., absorption of ca. 100 nJ in a sample volume of 30 nl), yielded E-t = 69.5 +/- 1.4 kcal/mol and a ta u of a few hundred nanoseconds. In the presence of potassium iodide (KI), a Bp*(3) quencher, the lifetime was significantly reduced. The rate of tripl er quenching by KI (k(q) = 3.9 +/- 0.2 x 10(9) M-1 s(-1)) was determined fr om the slope of the Stern-Volmer plot of 1/tau observed versus quencher con centration. (C) 1999 Elsevier Science S.A. All rights reserved.