Multiphoton photosensitization system

Details Multiphoton photosensitization system Multiphoton photosensitization system

2001-12-28

2004-06-15

Berman, Susan (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S007000, C522S014000, C522S016000, C522S022000, C522S025000, C522S026000, C522S027000, C522S028000, C522S029000, C522S182000, C522S170000, C522S117000, C430S270100, C430S280100, C430S281100

Reexamination Certificate

active

06750266

ABSTRACT:

FIELD
This invention relates to multiphoton-activatable, photoreactive compositions and to multiphoton methods of photo-inducing chemical reactions.
BACKGROUND
Molecular two-photon absorption was predicted by Goppert-Mayer in 1931. Upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became a reality. Subsequently, two-photon excitation has found application in biology and optical data storage, as well as in other fields.
There are two key differences between two-photon induced photoprocesses and single-photon induced processes. Whereas single-photon absorption scales linearly with the intensity of the incident radiation, two-photon absorption scales quadratically. Higher-order absorptions scale with a related higher power of incident intensity. As a result, it is possible to perform multiphoton processes with three-dimensional spatial resolution. Also, because multiphoton processes involve the simultaneous absorption of two or more photons, the absorbing chromophore is excited with a number of photons whose total energy approximates the energy of an electronic excited state of the multiphoton photosensitizer that is utilized. Because the exciting light is not attenuated by single-photon absorption within a curable matrix or material, it is possible to selectively excite molecules at a greater depth within a material than would be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two phenomena also apply, for example, to excitation within tissue or other biological materials.
Major benefits have been achieved by applying multiphoton absorption to the areas of photocuring and microfabrication. For example, in multiphoton lithography or stereolithography, the nonlinear scaling of multiphoton absorption with intensity has provided the ability to write features having a size that is less than the diffraction limit of the light utilized, as well as the ability to write features in three dimensions (which is also of interest for holography). Such work has been limited, however, to slow writing speeds and high laser powers, due to the low photosensitivities of current multiphoton-activatable, photoreactive compositions.
SUMMARY
Thus, we recognize that there is a need for multiphoton-activatable, photoreactive compositions having improved photosensitivities that will enable faster writing speeds and the use of lower intensity light sources. The present invention provides such a multiphoton-activatable, photoreactive composition. The composition comprises: (a) at least one reactive species that is capable of undergoing an acid- or radical-initiated chemical reaction (preferably, a curable species; more preferably, a curable species selected from the group consisting of monomers, oligomers, and reactive polymers); (b) a photochemically-effective amount of a multiphoton photosensitizer comprising at least one multiphoton up-converting inorganic phosphor; and (c) a photochemically-effective amount of a one-photon photoinitiator system that is capable of being photosensitized by the multiphoton photosensitizer.
The composition of the invention exhibits enhanced multiphoton photosensitivity by using up-converting inorganic phosphors as multiphoton photosensitizers. The phosphors absorb two long-wavelength visible or near-infrared (NIR) photons of light to populate an excited state that re-emits one photon of visible or ultraviolet (UV) light (the “up-converted emission”), which can be used to induce chemical reaction through formation of reaction-initiating species (radicals, acid, etc.). Unlike the organic dyes commonly used as multiphoton photosensitizers, the phosphors can be excited at selected wavelengths to undergo sequential, rather than simultaneous, multiphoton absorption and (due to the long lifetime of the energy transfer state populated through absorption of a first photon) can be activated using compact, inexpensive laser diodes rather than expensive femtosecond or picosecond pulsed lasers. Thus, the composition of the invention satisfies the need that we have recognized for compositions that allow rapid fabrication of three-dimensional structures and that permit the use of lower intensity light sources for exposure.
In another aspect, this invention also provides a method of multiphoton photosensitizing a photoreactive composition. The method comprises (a) preparing the above-described multiphoton-activatable, photoreactive composition; and (b) irradiating the composition with light sufficient to cause sequential or simultaneous absorption of at least two photons, thereby inducing at least one acid- or radical-initiated chemical reaction where the composition is exposed to the light.
DETAILED DESCRIPTION
Definitions
As used in this patent application:
“multiphoton absorption” means the sequential or simultaneous absorption of two or more photons of electromagnetic radiation to reach a reactive, electronic excited state that is energetically inaccessible by the absorption of a single photon of the same energy;
“simultaneous” means two events that occur within the period of 10
−14
seconds or less;
“multiphoton up-converting” means capable of undergoing multiphoton absorption followed by emission of a single photon of higher energy (shorter wavelength) than the photons absorbed;
“electronic excited state” means an electronic state of a molecule or ion that is higher in energy than its electronic ground state, that is accessible via absorption of electromagnetic radiation, and that has a lifetime greater than 10
−13
seconds;
“cure” means to effect polymerization and/or to effect crosslinking;
“optical system” means a system for controlling light, the system including at least one element chosen from refractive optical elements such as lenses, reflective optical elements such as mirrors, and diffractive optical elements such as gratings. Optical elements shall also include diffusers, waveguides, and other elements known in the optical arts;
“three-dimensional light pattern” means an optical image wherein the light energy distribution resides in a volume or in multiple planes and not in a single plane;
“exposure system” means an optical system plus a light source;
“sufficient light” means light of sufficient intensity and appropriate wavelength to effect multiphoton absorption;
“photosensitizer” means a species that lowers the energy required to activate a photoinitiator system by absorbing light of lower energy than is required by the photoinitiator system for activation and interacting with the photoinitiator system (which is thereby “photosensitized”) to produce a photoinitiating species therefrom; and
“photochemically effective amounts” (of, for example, the components of the photoinitiator system) means amounts sufficient to enable the reactive species to undergo at least partial reaction under the selected exposure conditions (as evidenced, for example, by a change in density, viscosity, color, pH, refractive index, or other physical or chemical property).
Reactive Species
Reactive species suitable for use in the photoreactive compositions include both curable and non-curable species. Curable species are generally preferred and include, for example, addition-polymerizable monomers and oligomers and addition-crosslinkable polymers (such as free-radically polymerizable or crosslinkable ethylenically-unsaturated species including, for example, acrylates, methacrylates, and certain vinyl compounds such as styrenes), as well as cationically-polymerizable monomers and oligomers and cationically-crosslinkable polymers (which species are most commonly acid-initiated and which include, for example, epoxies, vinyl ethers, cyanate esters, etc.), and the like, and mixtures thereof.
Suitable ethylenically-unsaturated species are described, for example, by Palazzotto et al. in U.S. Pat. No. 5,545,676 at column 1, line 65, through column 2, line 26, and include mono-, di-, and poly-acrylates and methacrylates (for example, methyl acrylate, methyl methacrylate, ethyl acrylat

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