Eric Nickel

Efficient singlet oxygen generation upon two-photon excitation of new porphyrin with enhanced nonlinear absorption

We demonstrate efficient generation of singlet oxygen upon two-photon excitation with 150-fs 780-nm laser pulses of a new porphyrin photosensitizer molecule whose two-photon absorption cross section has been considerably enhanced by chemical design.

Synthesis and incorporation of ladder polymer subunits in copolyamides, pendant polymers, and composites for enhanced nonlinear optical response

Several electroactive polymers, such as polyacetylene, polythiophene, poly [p-phenylene vinylene] and poly [2,5-thienylene vinylene] have shown promise as NLO-active materials over the past few years. However, as several theoretical and experimental research groups have pointed out in recent publications and symposia, it is not evident that long conjugation lengths are necessary for enhanced (chi) (3) activity. As recently demonstrated, copolyamides which incorporate polyenylic or PTV oligomeric repeat units show (chi) (3)/(alpha) values of ca. 10-13 esu-cm at 532 nm(band-edge). In this paper, the authors discuss how ladder subunits related to the electroactive polymers POL and PTL can be incorporated into polymer films as (a) copolymer repeat units, (b) pendant groups attached to poly [p-hydroxystyrene] and (c) guest-host composites in polycarbonate. Sharp optical absorptions are found in all cases as well as promising (chi) (3) properties.

Strong two-photon absorption in new porphyrins with asymmetrical meso-substitution

Porphyrins and related molecules with strong two-photon absorption (TPA) are extremely called for because of several emerging applications, including 3D optical memory, high-resolution fluorescence microscopy and photodynamic therapy. In this paper we demonstrate for the first time that an asymmetric meso-substitution of porphyrin macrocycle with electron-donating diphenylamino-stilbene or bis-(diphenylamino)-stilbene groups results in a drastic enhancement of intrinsic TPA cross section in the near-IR region. The cross section value amounts to 500 - 900 GM depending on substituent group and link structure, which is about 102 times the corresponding value for the unsubstituted parent molecule. Compared to symmetrical porphyrins, the TPA spectra of this series follow qualitatively the corresponding one-photon spectra. Therefore, we describe the observed TPA spectra and absolute cross section values by taking into account the change of permanent dipole moment upon excitation. A new zeroth-generation dendrimer, consisting of a porphyrin core, symmetrically tertakis-meso-substituted with strong TPA dendrons, reveals 7 times increase of the cross section (740 vs 110 GM) as compared to its mono-meso-substituted analogue. We also demonstrated an efficient singlet oxygen generation upon two-photon excitation of these new molecules, which makes them particularly attractive for photodynamic therapy.

The design of new organic materials with enhanced nonlinear optical properties

Abstract Bis-anthracenyl and bis-thienyl polyenes have been synthesized using Wittig methodology. These polyenes can be oxidatively doped with SbCl5 to yield bipolaron-like charge states that can be cast as thin film composites. X (3) measurements performed by degenerate four wave mixing on neutral and doped polyenes indicate significant enhancement of X (3) by bipolaron charge state generation. In the bis-thienyl polyene series, dramatic increases in solubility and processibility have been achieved by multiple alkyl group substitution thus allowing synthesis and characterization of long polyene sequences as oligomeric models of polyacetylene.

Strong two-photon absorption and singlet oxygen photogeneration in near-IR with new porphyrin molecule

Classical photodynamic therapy (PDT) has a drawback of limited penetration of visible light. It has been proposed that by utilizing two-photon absorption (TPA), where illumination is carried out at near-IR wavelengths falling into tissue transparency window, the PDT can be used for deeper treatment of tumors. Here we introduce new porphyrin photosensitizer 5-(4-diphenylaminostilbene),15-(2,6-dichlorophenyl)-21H,23H- porphine (hereafter referred as DPASP) with greatly enhanced TPA cross-section in near-IR range of wavelengths. The design of DPASP was based on structure-property relationships, empirically known to enhance TPA cross-section in organic (pi) -conjugated chromophores. In our case introduction of a 4-(diphenylaminostilbene)-substituent into the 5-position of the tetrapyrrole ring results in 20-fold enhancement of TPA cross-section at (lambda) exc = 780 nm as compared with parent molecule 5-phenyl,15-(2,6-dichlorophenyl)-21H,23H-porphine (DPP). The high value of TPA cross-section of DPASP enables to reliably detect for the first time an efficient luminescence of singlet oxygen produced upon two-photon excitation of porphyrin. Singlet oxygen luminescence was also measured upon two-photon excitation of several other porphyrins including water-soluble derivative 5,10,15,20-tetrakis-(4-N-methylpyridyl)-21H,23H-porphine (TmpyP).

Substituent effects in the design of new organic NLO materials

Several approaches have been suggested during the past few years for increasing third order nonlinearity in organic compounds. In general, these have focused on manipulation of the effective conjugation length of the pi-electron framework to maximize orbital overlap. It has only recently become apparent that substituent effects, which affect the overall electron density distribution, may be an extremely effective way of enhancing nonlinearity. In this study we review known substituent effects on nonlinearity, and suggest how donor-donor, acceptor- acceptor, and donor-acceptor interaction can be utilized in the design of molecules and polymers with enhanced nonlinearity.

Mechanisms of enhanced two-photon absorption in porphyrins

We study absolute cross section of simultaneous two-photon absorption (TPA) in a series of porphyrins and tetraazaporphyrins by 100-fs-duration pulses in two ranges of laser wavelength, from 1100 to 1500 nm and from 700 to 800 nm. The cross section in Q transition region is, sigma(2) ~ 1-10 GM, and is explained by partial lifting of the parity prohibition rule. In Soret transition region we find enhancement by about an order of magnitude due to Q transition, which acts as a near-resonance intermediate state, and also due to gerade energy levels, which we identify in this spectral region. Further enhancement (up to sigma(2) ~ 1600 GM) is achieved by symmetrical substitution in tetraazaporphyrins with strong electron acceptor groups.

Extended pi-electron systems incorporating stabilized quinoidal bipolarons: anthracenyl polyene-polycarbonate composites

(alpha) , (omega) -Bis(9-anthracenyl) polyenes containing 3, 4, 5, and 6 ene repeat units have been synthesized. These polyenes can be oxidized to extremely stable bipolaronic charge states in solution by SbCl5. There is significant shift in oscillator strength attending the formation of these gap states with (lambda) max for the BP states shifting into the NIR. Stable polycarbonate composites of both the neutral polyenes and the bipolarons can be cast from solution to yield optical quality films for (chi) (3) characterization.

New Copolymers for Nonlinear Optics Application which Incorporate Electroactive Subunits with Well Defined Conjugation Lengths

Over the past five or six years, rapid advances have been made in the field of photonics, or the manipulation of light by light. Although organic polymers have been generally regarded as insulators in the electronics industry, research advances in the area of electroactive materials have shown that polymers such as polyacetylene can achieve metallic conductivity approaching that of copper when oxidized or reduced chemically or electrochemically (doping). More recently, it has been recognized in our laboratory and others that electroactive materials also have exceptionally high optical nonlinearities. Research on these materials holds the promise that they indeed may be suitable for the eventual design of many nonlinear and electro-optic devices, including an all-optical computer.

Formation and stabilization of organic charge states in conjugated systems: Are there limits to the synthesis of conducting molecular weres of known delocalization length?

The chemistry and physics associated with the development of conducting organic materials during the past decade has resulted in a great deal of speculation regarding their possible role as molecular wires in molecular switching. In typical conducting polymers, such as polyacetylene, polythiophene or poly[arylene vinylene], polaronic or bipolaronic charge states are delocalized over 3–6 repeats units (18–36 atoms) as determined by a variety of techniques. In this paper we will explore how these delocalized charge states can be extended and stabilized, and whether there seems to be limits to further delocalization. If inherent limitations are structure related, then more consideration must be given as to how molecular wires need to be visualized.