29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

The Generation of Thermal Energy Spikes using Ultrafast Laser Pulses and Photothermal Sensitizers

Abstract-
In this presentation the principles of thermal energy deposition in fluid systems as a result of localized photon absorption will be outlined and some early processes in the deactivation of some potential photothermal sensitizers will be considered. The absorption of a photon of red light (e.g. 700 nm) by a molecule deposits an electronic energy equivalent of 284 zJ (zeptojoule = 10^-21J) in the absorbing unit. This is an extremely significant amount of energy at the molecular level and absorbers in their electronically excited states seek to rapidly dissipate the energy into whatever channels are available. Processes such as fluorescence and phosphorescence can remove a large fraction of the energy in a single radiative event; otherwise non-radiative modes can dissipate the energy via a cascade involving vibrational oscillators of the ground electronic state. This latter process is followed by vibrational energy transfer to nearby oscillators of the thermal bath; as a result the local temperature is raised. Eventually normal dissipative processes bring about the recovery to thermal equilibrium. However, at the instant of the transfer to the local solvent modes the temperature of the sphere of a few hundred picometer radius around the absorber can be raised by over 100⁰C. In a biological milieu such a thermal shock can have significant consequences. Very little is known of the dynamics of the transfer of thermal energy away from such thermal spikes; the few studies that have been undertaken place the dissipative rates in the >10¹¹ s^-1 regime. Thus to ensure maximum local heating it is essential that the internal transfer of electronic to vibrational energy is both extremely rapid and efficient. Chromophores having excited singlet states that deactivate to the electronic ground state non-radiatively within a few picoseconds are therefore potential candidates for sensitizing photothermal events. Our attention has been focused on the transition metal tetrapyrroles such as octabutoxynaphthalocyanato-Ni(II) and Ni(II) and Co(II) phthalocyanines. Ultrafast transient absorption spectrometry studies have shown that the ground state p-surface is reached, via the intervening metal d-orbitals, within a few picoseconds. These and other data will be presented and discussed

Keywords: photothermal, ultrafast, metallotetrapyrroles