NEAR-IR TWO-PHOTON PHOTOINITIATED POLYMERIZATION USING A FLUORONE/AMINE INITIATING SYSTEM

Emerging device technologies such as microelectromechanical systems and integrated sensors are placing increased demands on the development of materials processing and fabrication techniques.1,2 In response, the characteristic three-dimensional (3-D) spatial resolution of the simultaneous two-photon absorption (2PA) process is being harnessed for 3-D photoinitiated polymerization and microlithography. This is facilitated by the unique properties associated with simultaneous absorption of two-photons relative to single-photon mediated processes. There have been a limited number of reports of two-photon photopolymerization of commercial acrylate monomer systems, pre-formulated with UV photoinitiators.3-6 Efficient two-photon absorbing compounds based on phenylethenyl constructs bearing electron-donating and/ or electron-withdrawing moieties have been reported.7 Among these are electron-rich derivatives that have been found to undergo a presumed two-photon induced electron transfer to acrylate monomers8 or proposed fluorescence energy transfer to a photoinitiator,9 initiating polymerization. The reportedly efficient twophoton photoinitiators, although more photosensitive than previously studied UV photoinitiators, are not commercially available and require rather involved syntheses. Thus, the practicality of their broader use is questionable. Herein, we report the near-IR two-photon induced polymerization of (meth)acrylate monomers using a commercially available photoinitiator system based on a visible light-absorbing dye. Multiphoton absorption has been defined as simultaneous absorption of two or more photons via virtual states in a medium.10 The process requires high peak power, which is available from pulsed lasers. A major feature that distinguishes single-photon absorption from two-photon absorption is the rate of energy (light) absorption as a function of incident intensity. In single or onephoton absorption, the rate of light absorption is directly proportional to the incident intensity (dw/dt R I), while in 2PA, the rate of light absorption is proportional to the square of the incident intensity (dw/dt R I2).10,11 The quadratic, or nonlinear, dependence of two-photon absorption as a function of light intensity has substantial implications. For example, in a medium containing one-photon absorbing chromophores, significant absorption occurs all along the path of a focused beam of suitable wavelength light, leading to outof-focus absorption and associated processes. In a two-photon process, however, negligible absorption occurs except in the immediate vicinity of the focal point of a light beam of appropriate energy. This allows spatial resolution about the beam axis, as well as radially, and is the principle basis for two-photon fluorescence imaging.12 In 2PA, the final state reached has the same parity as the initial state, while in single-photon absorption the parity is opposite as given by dipole selection rules. However, in most solids and complicated molecules, the parity can become mixed in “bands”. In such molecules (such as the fluorone dye used in this report) two near-IR photons achieve essentially the same electronically excited singlet state as would be obtained via resonant singlephoton absorption at or near λmax. These molecules are expected to display identical photochemical and photophysical behavior when excited by one or 2PA. If such species return to the ground state via emission, fluorescence results with the energy of emission greater than the energy of the individual photons involved in the 2PA excitation. It is widely believed that a “revolution” in miniaturization, particularly in the field of microelectromechanical systems (MEMS), is underway. It is projected that the design and manufacturing technology that will be developed for MEMS may rival, or even surpass, the far-reaching impact of ICs on society and the world’s economy. At the forefront of techniques being explored for 3-D spatially resolved materials imaging and processing are methods based on 2PA. The use of longer wavelength light as the excitation source leads to deeper penetration depths than possible with conventional UV or visible excitation techniques. Since the absorption/excitation is confined to the focal volume in the 2PA process, there will be virtually no out-of-focus excitation/reaction, facilitating 3-D spatial control of the process.