Two-photon Induced Chemistry and Various Applications

Dominik Koszelewski

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• The simultaneous absorption of two photons was analyzed theoretically in the 1930s by Goppert-Mayer (Nobel Prize in 1963), first demonstrated experimentally in 1961 soon after invention of the laser.

• Two-photon absorption became easier to investigate as sub-picosecond pulsed lasers became more readily available in the 1990s (Ti:sapphire laser)

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INTRODUCTION

•The main difference between one-photon absorption (1PA) and two-photon absorption (2PA) is that 2PA involves the simultaneous interaction of two photons, and so it increases with the square of the light intensity, whereas 1PA depends linearly on the intensity.

•2PA is only observed in intense laser beams, particularly focused pulsed lasers, which generate a very high instantaneous photon density.

Anderson, H.L. et al. Angew. Chem. Int. Ed. 2009, 48, 3244-3266

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• Applications

• microscopy• micro-fabrications• three-dimensional data-storage• optical power limiting• up-converted lasing• photodynamic therapy• localized release of bio-active species

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• OPTICAL PRINCIPLES

Differences in confocal fluorescence:

1. one-photon - excitation density in the focal region is proportional to the local intensity, whereas the density of two-photon excitation is proportional to the square of intensity and so falls off rapidly away from the focus

2. The 2PA excitation volume is smaller and increases the resolution of the microscope (phenomenon used in all applications)

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• THE FOCAL VOLUME

• The excitation of a fluorescent dye (fluorescein) by 1PA (at 488 nm) generates a stream of emission along the beam path.

• 2PA (at 960 nm) gives a sharp point (voxel) of emission at the focus.• The concentration ofdye is so high that most of the 1PA occurs before the light reaches

the focus (3D-storage)• Under more dilute conditions the one-photon excited emission would be hour-glass

shaped.

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• Advantages of 2PA

• The size of the focal volume is proportional to the wavelength and, because the contraction in the excitation volume achieved by going from 1PA to 2PA is less than a factor of two, but…..;

• 1) the sample will invariably have much larger absorption and scattering losses at the shorter wavelength, and short-wavelength irradiation often causes photochemical damage in biological samples

• 2) the much sharper contrast in the excitation density prevents the occurrence of parasitic emission or photochemical conversion outside the focal volume.

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• TRACERS, PROBES, and SENSORS

• The most important parameter is brightness (ɸFδ): product of the2PA cross-section (δ) and fluorescence quantum yield (ɸF ) (defined as the ratio of the number of photons emitted to the number of photons absorbed).

• Probe to visualize lipid rafts in live tissues by using two-photon microscopy.

• Kim, H.M., et al. J. Am. Chem. Soc. 2008, 130, 42468

• Metal kations and pH value probes

• Fahrni, C.J., et al. J. Am. Chem. Soc. 2007, 111, 11888 9

• PHOTOACTIVATION And DRUG DELIVERY

• two-photo initiated release (uncaging, deprotection)• localized release of neurotransmitters, proteins and ions

• Photolabile masked glutamate based on coumarin or stilbene

• Calcium uncaging

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• two-photo initiated release of nitric oxide NO (vasodilatation, immune response, and neurotransmission)

• porphyrin as an intramolecular antenna to sensitize a metal cluster

• Ford, P.C., et al. J. Am. Chem. Soc. 2004, 126, 13566 11

• controled relise drug from micelles and liposomes in diseased tissues

• Frechet, J.M.J. et al. J. Am. Chem. Soc. 2005, 127, 9952 12

• PHOTODYNAMIC THERAPY (PDT)

• one-photon used to treat cancers of the skin, hollow organs, and eye disease• phototoxicity due to formation of the singlet oxygen, generated by energy transfer

from excited state of photosensitizer

• Brown, E.A. et al.. Lancet Oncol. 2004, 5, 497. 13

• TWO-PHOTON PDA

• first reported in 1997 by Watcher and co-workers• two-photon PDT has been proposed as a means to improve treatment depth and

targeting

Advantages:1. focal volume2. longer-wavelength (near-IR) penetrates deeper in living tissues then visible light3. lower phototoxicity

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• resonance energy transfer (FRET)• designed to recognize somastatin receptors over-expressed in human cancer cells

• Spangler, C.W., et al.. Clin. Cancer Res. 2008, 14, 6564 15

• selective obturation of arteries

• Anderson, H.L. et al.. Nat. Photonics 2008, 2, 420. 16

• MICROFABRICATION

• three-dimensional microscopic structures with sub-micrometer resolution • photoinitiated polymerization or depolymerization• one-photon outside the focal volume responsible for a loss of contrast and definition

• the acidic-catalyzed cationic polymerizaion of epoxides (higer resolution)• The radical polymerization of acrylates • The polymerization of As4S6 glasses to give a cross-linked insoluble inorganic framework

• LaFratta, C.N., et al.. Angew. Chem. Int. Ed. 2007, 46, 623817

• Polymerization then washing

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• Three-Dimensional Optical Data Storage

• CDs and DVDs writing based on one-photon processes• tightly localized 2PA excitation volume, and high discrimination against the

surrounding background leading to a huge increase in data density (1 Tbyte !!!)

• data are written not on the material surface but within the three-dimensional volume of it.

• two different forms having different absorption spectra, in response to irradiation by light, changing in refraction

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• OPTICAL POWER LIMITING

• material that exhibits intensity-dependent absorption, transparent to light at low intensities but opaque to intense light (behaves like ultrafast shutter)

• protection of eye (tissue) or delicate sensors from lasers (military applications) • in optical telecomunications for removing intensity spikes• „frequency-agile” protection

• Reports abouth blinding : United Air Lines, Boeing 737 near Ontario, CA (green flash), November 1998 Bosnia, two US helicopter pilots recived minor corneal burns, store robberies in UK

• At the current time no universal frequancy agile OPL exist (exeption is carbon black suspensions in carbon disulfide but is cumbersome)

• Tandem limiter employing both filters and smart materials

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• the main mechanismsfor optical limiting are excited-state absorption (ESA) and, nonlinear scattering

• intersystem crossing (ISC) – indantrones, metal clusters compunds, porphyrins, phatlocyanines and fullerenes.

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• effective optical limiting limiting reqires: large 2PA cross-section and small 1PA (lost in transparency), high triplet quantum yield, strong ESA, and long triplet lifetime.

• it is important that the dye can be used at high concentrations in a solvent or solid-phase host material

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Thank you for your attention