There has been significant effort over the past two decades in the treatment of malignancies of epithelial origin,
including some of the most devastating of cancers, such as colorectal cancer (CRC), squamous call carcinoma of the
head and neck (HNSCC), and carcinomas of the pancreas, lungs, (both Small Cell and Non-Small Cell), renal cell,
prostate, bladder and breast. Recurring, refractory HNSCC is a particularly difficult cancer to treat once the tumors
recur due to mutations that are resistant to repeat chemotherapy and radiation. In addition, repeat surgery is often
difficult due to the requirement of significant surgical margins that may not be possible due to the attending potential
functional deficits (e.g., salivary glands, nerves and major blood vessels in confined areas). In this study FaDu HNSCC
xenograft tumors in SCID mice were imaged, and “optical”, as opposed to “surgical” margins defined for the tumor
being treated. The subsequent two-photon treatment irradiation was computer-controlled to carry out the tumor
treatment by rastering the laser beam throughout the tumor volume plus the defined optical margins simultaneously. In
our initial studies, up to 85% regression in tumor volume was observed in 5 days post PDT, with complete tumor
regression in 18 days. No re-growth was observed up to 41 days post-PDT, with little or no scarring and complete hair
re-growth. However, competition between imaging and PDT moieties was also observed in some mouse models,
possibly favoring tumor re-growth. Strategies to selectively optimize the PDT effect will be discussed.
Synthesis of new PDT triads that incorporate a tumor-killing porphyrin with large two-photon cross-section for 150 fs
laser pulses (2000 GM) in the Near-infrared (NIR) at 840 nm, a NIR imaging agent, and a small peptide that targets
over-expressed EGF receptors on the tumor surface. This triad formulation has been optimized over the past year to
treat FADU Head and Neck SCC xenograft tumors in SCID mice. Effective PDT triad dose (1-10 mg/Kg) and laser
operating parameters (840 nm, 15-45 min, 900 mW) have been established. Light, dark and PDT treatment toxicities
were determined, showing no adverse effects. Previous experiments in phantom and mouse models indicate that tumors
can be treated directly through the skin to effective depths between 2 and 5 cm. Treated mice demonstrated rapid tumor
regression with some complete cures in as little as 15-20 days. No adverse effects were observed in any healthy tissue
through which the focused laser beam passed before reaching the tumor site, and excellent healing occurred post
treatment including rapid hair re-growth. Not all irradiation protocols lead to complete cures. Since two-photon PDT is
carried out by rastering focused irradiation throughout the tumor, there is the possibility that as the treatment depth
increases, some parts of the tumor may escape irradiation due to increased scattering, thus raising the possibility that
tumor re-growth could be triggered by small islands of untreated cells, especially at the rapidly growing tumor margins,
a problem we hope to alleviate by using image-guided two-photon PDT.
We compare the experimental absolute two-photon absorption (2PA) cross sections and spectra of various organic
molecules with those obtained from the few (two or three) essential states model, assuming certain parameter values
that are either previously calculated theoretically or measured. We study conditions under which the two- or three-level
models are applicable for quantitative description of 2PA, and estimate the corresponding maximum realistic attainable
peak 2PA cross sections and spectral widths. Based on our observations we formulate the steps required to increase the
intensity and broaden the spectral coverage of instantaneous 2PA by optimizing intrinsic molecular parameters such as
transition dipole moments, permanent dipole moments and excited states energies.
New porphyrin-based photo-sensitizers have been designed, synthesized and characterized that exhibit greatly enhanced
intrinsic two-photon absorption. These new photo-sensitizers have been incorporated into triad formulations that also
incorporate Near-infrared (NIR) imaging agents, and small-molecule targeting agents that direct the triads to cancerous
tumors' over-expressed receptor sites. PDT can be initiated deep into the tissue transparency window at 780-800 nm
utilizing a regeneratively amplified Ti:sapphire laser using 100-150 fs pulses of 600-800 mW. Human tumor xenografts
of human breast cancer (MDA-MB-231) and both small SCLC (NCI-H69) and NSCLC (A-459) have been successfully
treated using octreotate targeting of over-expressed SST2 receptors. In particular, the lung cancer xenografts can be
successfully treated by irradiating from the side of the mouse opposite the implanted tumor, thereby passing through ca.
2 cm of mouse skin, tissue and organs with no discernible damage to healthy tissue while causing regression in the
tumors. These results suggest a new PDT paradigm for the noninvasive treatment of subcutaneous tumors, including the
possibility that the targeting moiety could be matched to individual patient genetic profiles (patient-specific
therapeutics).
Over the past three years we have described the rationale for using new photosensitizers (PS) with greatly enhanced
multi-photon absorption. In particular, we have synthesized new porphyrin-based photosensitizers that also incorporate
small molecule targeting agents that direct the ensemble to over-expressed tumor receptor sites, as well as Near-infrared
imaging agents that will allow practical image-guided two-photon PDT in the tissue transparency window (750-1000
nm) at laser fluences that are harmless to surrounding healthy tissue. We have previously shown (PW2006) successful
treatment of human breast cancer models (MDA-MB-231) in SCID mice, and have recently extended these studies to the
treatment of both human small cell (SC) (NCI-H69) and non-small cell (NSC) (A-459) models in SCID mice. We have
demonstrated that lung cancer xenografts can be successfully treated by irradiating from the side of the mouse opposite
the implanted tumor, thereby passing through ca. 2 cm of mouse skin, tissue and organs before encountering the bulk
tumor. These results suggest that this technology can be used to treat deep subcutaneous spontaneous tumors in larger
animal models (e.g. canine). We would also emphasize that the synthetic route to these triads attaches the targeting
moiety in the last step of the synthesis, and can be easily changed, thus allowing a myriad of targeting agents to be
employed, opening the door to the possibility of patient-specific PDT.
A novel class of porphyrin-based near-infrared photodynamic therapy (PDT) sensitizers is studied. We achieve regressions of human small cell lung cancer (NCI-H69), non-small cell lung cancer (A 459) and breast cancer (MDAMB- 231) xenografts in SCID mice at significant tissue depth by irradiation with an amplified femtosecond pulsed laser at 800 nm wavelength. Significant tumor regressions were observed during the first 10-14 days post treatment. Tumor histopathology was consistent with known PDT effects, while no significant changes were noted in irradiated normal tissues. In vivo imaging studies using intravenous injections of fluorescent dextran demonstrated an early loss of tumor blood flow. RNA was isolated from NCI-H69 PDT treated SCID mouse xenografts and paired untreated xenografts at 4 hours post laser irradiation. Similarly RNA was isolated from PDT treated and untreated Lewis lung carcinomas growing in C57/Bl6 mice. Expression profiling was carried out using AffymetrixTM human and mouse GeneChips®. Cluster analysis of microarray expression profiling results demonstrated reproducible increases in transcripts associated with apoptosis, stress, oxygen transport and gene regulation in the PDT treated NCI-H69 samples. In addition, PDT treated Lewis lung carcinomas showed reproducible increases in transcripts associated with immune response and lipid biosynthesis. PDT treated C57/Bl6 mice developed cytotoxic T cell activity towards this tumor, while untreated tumor bearing mice failed to do so.
We study two-photon absorption (2PA) spectra and absolute 2PA cross sections, as well as fluorescence emission- and
excitation spectra along with time resolved fluorescence in a series of new dendrimers, where a single
porphyrin/phthalocyanine core is decorated with electron-donating/two-photon absorbing groups. We show that if the
combined system has strong π-conjugation/multipolar charge transfer between the core and the attached group(s), then
the observed 2PA undergoes cooperative enhancement, where the peak 2PA cross section reaches σ2=600-2000 GM in
a broad transition wavelength region 375-500 nm (laser wavelengths 750-1000 nm). In the systems with less
conjugation, the 2PA is less enhanced, and in the limit of very weak conjugation maximum σ2 remains essentially the
sum of the cross sections of the constituents. We show that the conjugation strength correlates with the fluorescence
emission properties. In particular, in the strongly linked systems the fluorescence originates mostly from the core
porphyrin, whereas in the weakly linked systems the attached chromophores emit independently from the porphyrin. In
intermediate conjugation strength case we observe non-exponential fluorescence decays and fluorescence rise times,
which indicated Forster resonant energy transfer from the side groups to the core.
Photodynamic therapy (PDT) continues to evolve into a mature clinical treatment of a variety of cancer types as well as
age-related macular degeneration of the eye. However, there are still aspects of PDT that need to be improved in order
for greater clinical acceptance. While a number of new PDT photo-sensitizers, sometimes referred to as second- or third-
generation therapeutic agents, are currently under clinical investigation, the direct treatment through the skin of
subcutaneous tumors deeper than 5 mm remains problematic. Currently approved PDT porphyrin photo-sensitizers, as
well as several modified porphyrins (e.g. chlorins, bacteriochlorins, etc.) that are under clinical investigation can be
activated at 630-730 nm, but none above 800 nm. It would be highly desirable if new PDT paradigms could be
developed that would allow photo-activation deep in the tissue transparency window in the Near-infrared (NIR) above
800 nm to reduce scattering and absorption phenomena that reduce deep tissue PDT efficacy. Rasiris and MPA
Technologies have developed new porphyrins that have greatly enhanced two-photon absorption ( P A ) cross-sections
and can be activated deep in the NIR (ca. 780-850 nm). These porphyrins can be incorporated into a therapeutic triad that
also employs an small molecule targeting agent that directs the triad to over-expressed tumor receptor sites, and a NIR
onephoton imaging agent that allows tracking the delivery of the triad to the tumor site, as well as clearance of excess
triad from healthy tissue prior to the start of PDT treatment. We are currently using these new triads in efficacy studies
with a breast cancer cell line that has been transfected with luciferase genes that allow implanted tumor growth and post-
PDT treatment efficacy studies in SCID mouse models by following the rise and decay of the bioluminescence signal.
We have also designed highly absorbing and scattering collagen breast cancer phantoms in which we have demonstrated
dramatic cell kill to a depth of at least 4 cm. We have also demonstrated that at the wavelength and laser fluences used in
the treatment of implanted tumors in the mouse mammary fat pads, there is little, if any, damage to the skin or internal
mouse organs. In addition, we have also demonstrated that the implanted tumors can be treated to a depth of more than 1
cm by direct radiation through the dorsal side of the mouse.
We demonstrate high contrast interference fringes when near-IR frequencies and UV frequency 3ω excite fluorescence via two alternative but in practice indistinguishable quantum pathways. We use specially synthesized organic dendrimers with an exceptionally large three-photon cross section, σ3=1.5 10-79 cm6 s2 based on bisdiphenylamino stilbene core and bis-diphenylamino di-styryl benzene repeat units. We show for the first time that quantum interference signal can be strong enough for being observed with an unaided eye.
In this paper we measure three-photon absorption spectra and cross sections of two new series of molecules, linear bis-difluorenylamine-substituted oligofluorenylene-vinylenes (N-Fl3 series) and triphenylamine-branched phenylacetylene dendrimers (N-PA3 series). Using our model described before, we evaluate the size of coherent domains in these two series and compare them to those in dendrimers with nitrogen branching center and stilbene branches. This allows us to draw the conclusions about the effect of dimensionality (linear oligomers versus dendrimers), topology of the dendrimer core (3-arm versus 4-arm core), and branch structure on the size of coherent domains, and, hence the strength of cooperative enhancement of multiphoton absorption in macromolecules.
Photodynamic therapy (PDT) has developed into a mature technology over the past several years, and is currently being exploited for the treatment of a variety of cancerous tumors, and more recently for age-related wet macular degeneration of the eye. However, there are still some unresolved problems with PDT that are retarding a more general acceptance in clinical settings, and thus, for the most part, the treatment of most cancerous rumors still involves some combination of invasive surgery, chemotherapy and radiation treatment, particularly subcutaneous tumors. Currently approved PDT agents are activated in the Visible portion of the spectrum below 700 nm, Laser light in this spectral region cannot penetrate the skin more than a few millimeters, and it would be more desirable if PDT could be initiated deep in the Near-infrared (NIR) in the tissue transparency window (700-1000 nm). MPA Technologies, Inc. and Rasiris, Inc. have been co-developing new porphyrin PDT designed to have greatly enhanced intrinsic two-photon cross-sections (>800 GM units) whose two-photon absorption maxima lie deep in the tissue transparency window (ca. 780-850 nm), and have solubility characteristics that would allow for direct IV injection into animal models. Classical PDT also suffers from the lengthy time necessary for accumulation at the tumor site, a relative lack of discrimination between healthy and diseased tissue, particularly at the tumor margins, and difficulty in clearing from the system in a reasonable amount of time post-PDT. We have recently discovered a new design paradigm for the delivery of our two-photon activated PDT agents by incorporating the porphyrins into a triad ensemble that includes a small molecule targeting agent that directs the triad to over-expressed tumor receptor sites, and a NIR one-photon imaging agent that allows the tracking of the triad in terms of accumulation and clearance rates. We are currently using these new two-photon PDT triads in efficacy studies with two breast cancer cell lines, both in vitro and in vivo. Both of these cell lines have been transfected with luciferase genes that allow implanted tumor growth and PDT efficacy to be monitored in living mouse models over time by following the rise and decay of the bioluminescence signals.
Two-photon absorption (TPA) is a promising technique for high density 3D-addressing for writing and read-out of data, provided that suitable two-photon sensitive materials facilitating fast recording and read-out will be developed.
Free-base porphyrins and other metal-free tetrapyrroles, such as phthalocyanies and naphthalocyanines possess a unique fast intrinsic photo-tautomerization mechanism, which consists in switching the position of a pair of protons in the core of the molecule. In the past photo-tautomerization was used for holographic storage, but can be also applied for bit-oriented volumetric information storage using laser-excited fluorescence for readout. However, the utility of the photo-tautomerization for two-photon storage was severely restricted so far by the fact that all known tetrapyrroles have rather low TPA cross section, with values not exceeding 1 - 10 GM (1GM = 10-50 cm4 s photon-1).
Recently we have discovered a new class of porphyrins, where TPA cross section is dramatically amplified by certain chemical modification of the chemical structure, and that some of the new porphyrins have the ability of photo-tautomerization by simultaneous absorption of two photons. In this paper we discuss the photophysics and nonlinear optics of the new porphyrins that can lead to fast volumetric re-writable optical storage. We present a quantitative comparison of the new compounds with previously known TPA chromophores and introduce a merit figure, which takes into account both TPA cross-section as well as the efficiency of light-induced changes. We show the combination of high cross sections of two-photon absorption, up to 1000 GM in near-IR range of wavelength, with the fast photo-tautomerization, offers, for the first time, a sufficiently high merit figure needed for implementation of high-density, high speed volumetric two-photon re-writable optical storage.
There has been a dramatic increase in the application of new technologies for the treatment of cancerous tumors over the past decade, but for the most part, the treatment of most tumors still involves some combination of invasive surgery, chemotherapy and radiation treatments. Photodynamic therapy (PDT), which involves the activation of an administered compound with laser light followed by a series of events leading to programmed cell death of the tumor, has been proposed as a noninvasive alternative treatment to replace the standard surgery/chemotherapy/radiation protocol. However, currently approved PDT agents operate in the Visible portion of the spectrum, and laser light in this region cannot penetrate the skin more than a few millimeters. Two-photon irradiation using more highly penetrating Near-infrared (NIR) light in the tissue transparency window (700-1000 nm) has been proposed for the treatment of subcutaneous tumors, but most porphyrins exhibit extremely small two-photon cross-sections. Classical PDT also suffers from the lengthy time necessary for accumulation at the tumor site, a relative lack of discrimination between healthy and diseased tissue, particularly at the tumor margins, and difficulty in clearing from the system in a reasonable amount of time. We have recently discovered a new design paradigm for porphyrins with greatly enhanced two-photon cross-sections, and are now proposing a nano-ensemble that would also incorporate small molecule targeting agents, and possibly one-photon NIR imaging agents along with these porphyrins in one therapeutic agent. Thus these ensembles would incorporate targeting/imaging/PDT functions in one therapeutic agent, and hold the promise of single-session outpatient treatment of a large variety of subcutaneous tumors.
At present a special attention is concentrated on increasing of the efficiency of multi-photon absorption of organic systems because of new emerging applications based on this effect. In our experiments we use strong two-photon absorbing chromophore, 4,4’-bis(diphenylamino)stilbene (BDPAS), to design new dendrimer molecules, in such a way that the branching center allows for pi-electronic conjugation between branches. Here we present, for the first time, unambiguous spectroscopic evidence of strong cooperative enhancement of two-photon and three-photon absorption in a series of these dendritic macromolecules. Maximum two-photon cross section increases in proportion to N2, where N = 2, 4, 6 is the number of constituent identical chromophore units in the parent BDPAS and lowest, G-0 dendrimer generation. Almost the same scaling law is observed for three-photon absorption. For higher generations, G-1 and G-2, comprising N = 14 and 30 chromophores, respectively, the cooperativity in multiphoton response starts to saturate. We show that three-photon absorption provides important complementary information, which we use for evaluation of the size of domains where chromophores are coherently coupled.
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.
Dendrimers have unique highly branched repeating structures that display intriguing processing and photonic properties. We have recently synthesized several generations of a new dendrimer series based on bis-(diphenylamino)-E-stilbene repeat units which have proven to be highly processible in common organic solvents, and which can be designed and synthesized in either three-arm or four-arm structural motifs. We have measured the dependence of the two-photon absorption (TPA) on excitation wavelength for the G-0, G-1 and G-2 generations of these monodisperse macromolecules, and have shown that the maximum value for the intrinsic TPA cross-section for femtosecond pulses inceases in proportion to the total number of stilbene chromophores, and yields a record high cross-sectrion for the G-2 dendrimer (11,000 GM units). We have now been able to incorporate a variety of electron donor and acceptor substituents in the three-arm dendrimer G-0 system to establish structure-property relatiuonships for the further enhancement of the intrinsic two-photon cross-sections. We have found a dramatic enhancement of the two-photon cross-sections for these dendrimers compared to the parent bis-(diphenylamino)-E-stilbene (BDPAS). In the 3-arm G-0 series, the intrinsic TPA cross-section, measured at the TPA maximum, varies from 1,400 to 1,900 GM units compared to 130 GM units for BDPAS, a more than 10-fold enhancement, while the number of BDPAS repeat units in the G-0 dendrimer is only 3 times the parent BDPAS structure.
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.
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).
With the advent of efficient and relatively inexpensive pulsed and CW laser systems for both civilian and military applications, the need for adequate eye and sensor protection is becoming increasingly important. While it is possible to filter out harmful wavelengths if the laser frequency is known, the proliferation of frequency agile laser sources underscores the need for smart passive materials that can sense the incident wavelength and provide protection. There has been considerable progress made in recent years in the design of optical power limiting (OPL) materials that can function by a variety of mechanisms, most of which derive their limiting behavior from some type of nonlinear absorption cross-sections of the photo-generated transient species are much greater than the original S0 to S1 transition. In this presentation we will discuss the efficacy of charge transfer species for optical limiting, and the need for more and better electron acceptor species.
The design of new organic chromophores with enhanced nonlinear optical (NLO) response over the past several years has focused on several structure-property relationship paradigms. The extension of the conjugation sequence leads to increases in both the second order and third order hyperpolarizabilities. Electron-donating or withdrawing substituents also affect the electron distribution along the conjugation sequence, and such substitution has been utilized in concert with the conjugation length dependence to tailor the molecular absorptivities and polarizabilities. In addition, previous studies of the NLO properties of organic chromophores have shown enhancement of the NLO response when second row elements replace first row elements in the structure. Various substituted amino groups have been used extensively in the design of both second and third order chromophores as strong donor groups. However, to date there have been no systematic studies of the effect of replacing P for N in various chromophore functionalities. In this presentation we will discuss the syntheses of several new chromophores with diphenylphosphino substituents, and compare their structure-property relationships to the equivalent, and more familiar, diphenylamino-substituted chromophores. In particular we will focus on their respective absorption properties in the visible portion of the spectrum, and the possible consequences for NLO applications.
Bis-(diphenylamino)diphenylpolyenes have been shown to form exceptionally stable, highly absorbing bipolaronic dications in solution and thin film. Replacement of one diphenylamino substituent with a N-(hydroxyethyl), N-ethylaminophenyl moiety yields a polyene series that also form stable bipolarons, and are intensely fluorescent. These new chromophores are also easily attached to either a PMMA backbone or to 3,5-dihydroxybenzyl alcohol to yield functionalized dendrons capable of attachment to various core molecules to yield functionalized dendrimers. Diphenylamino-substituted PPV oligomers can also be obtained with similar functionality. These new materials all possess large two-photon cross-sections and display optical limiting for nanosecond pulses with peak activity extending into the visible portion of the spectrum. In this presentation we will discuss the synthesis of these new materials and preliminary characterization as two-photon absorbers, photoluminescent materials suitable for organic light- emitting diodes, and as dendrimers capable of 3D charge delocalization and exceptionally large third order hyperpolarizabilities.
The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) 2, was measured by Z-scan experiments. Strong two photon absorption is indicated by high values for (sigma) 2. Solvent has strong influence in the measurement of (sigma) 2 values.
There has been considerable recent interest in the design of new organic chromophores, oligomers and polymers with potentially large two-photon cross-sections for a variety of applications that span such diverse areas as photo-dynamic therapy to optical power limiting of nanosecond and picosecond laser pulses. One particularly attractive system is based on poly[p-phenylene vinylene] (PPV) oligomers containing electron-donating substituents. We have recently designed and synthesized several PPV dimers with bis- diphenylamino) donor groups attached to the terminal phenyl rings, and have demonstrated that these materials have very large two-photon cross-sections for nanosecond pulses. It is probable that these enhanced cross-sections are due to excited state absorption following the initial two-photon absorption. We have also examined bis- (diphenylamino)diphenylpolyenes, and more recently extended our design concept to dendrimer structures based on bis- (diphenylamino)stilbene repeat units. Initial studies on the dendrimer structures and bis-(diphenylamino)-PPV dimer reveal extremely large two-photon cross-sections which we have also ascribed to probable excited-state absorption. The efficacy of this design approach will be discussed, as well as projected future design paradigms for even greater TPA enhancement.
To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3',4,4'-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtained as a function of the number of double bonds (n equals 1 - 5). Trends in the data reflected the ordering, energy gap between and mixing of 1Bu* and 1Ag* excited state configurations. We investigated the solvatochromism of a series of (alpha) ,(omega) -di(2- dithienyl 3,3',4,4'-butyl) polyenes. Absorption (n equals 1 - 5 double bonds) were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity, giving evidence for a polar 1Bu* absorbing state and a nonpolar 1Ag* emitting state. We observed sensitivity of the absorbing and emitting states to solute-solvent dipole-dipole interactions, suggesting the dithienyl polyenes had a polar syn ground state conformation.
The photophysics of a series (alpha) ,(omega) -dithienyl polyenes was studied. Both one photon excited fluorescence and fluorescence originating from two photon absorption states were investigated. There is strong evidence of two photon absorption from fluorescence studies in agreement with our earlier studies involving nonlinear absorption.
KEYWORDS: Absorption, Polymers, Molecules, Composites, Information operations, Laser applications, Doping, Semiconductor lasers, Solids, Human vision and color perception
Optical power limiting can be accomplished by a variety ofmaterials whose spectral and temporal utility is determined
by a number ofparameters. The most important determinant ofhow a particular material will perform is defmed by
the mechanism(s) by which optical power limiting is achieved. This paper discusses the use of complementary materials
to obtain the greatest spectral and temporal range. One new class of materials has strongly absorbing charge states
(cationic and dicationic) which make them highly desirable for consideration. Several materials are discussed along
with their optical characterization. These processes can be carried out in solutions containing various compounds
including electron acceptors. Other materials such as two photon absorbing materials exhibit optical power limiting with
shorter response times and/or over different spectral regions. Recent progress in the development and characterization
oftwo photon absorbing compounds is discussed with particular attention to how they can be used in a complementary
fashion with other compounds operating through other mechanisms to deliver broad optical power limiting responses.
The absorption and emission spectroscopy and nonlinear
optical absorption of a series of a,o-dithienyl polyenes were studied
in chloroform and n-octane solutions. Dithienyl polyenes containing
butyl substituents were also studied. Steady state fluorescence, time
resolved emission, fluorescence lifetime and quantum yield
measurements were made. The fluorescence quantum yields were
significantly lower than those of the corresponding a,w-diphenyl
polyenes. Among the dithienyl polyenes, 3,3'-substituted polyenes
exhibited stronger fluorescence than those of the 2,2'-substituted.
Nonlinear optical absorption experiments showed evidence of two
photon absorption. In some polyenes, two photon absorption was
followed by excited state absorption.
During the past five years, (pi) -conjugated organic polymers, such as poly[p-phenylene vinylene] (PPV) and polythiophenes, have become attractive alternatives to semiconducting materials as light-emitting diodes. Various approaches to tuning emission wavelength have been proposed, such as controlling the conjugation length by employing polymer oligomers or via steric effects in the fully conjugated polymers, or by using substituent effects to fine-tune the band gap. In the present study, we demonstrate that these features can be designed into copolymeric structures in which (pi) -conjugated emitters of carefully controlled length alternate with various non-emitting flexible spacers which improve solubility and thus processibility. These copolymers display predictable emission characteristics which can also be fine-tuned by incorporation of electronic substituent effects.
We measure the magnitude and phase of the degenerate third-order nonlinear optical susceptibility (chi) (3)llll of solutions of various bis-thienyl polyenes (n-BTP) with the number n of the conjugated double bonds ranging from 3 to 9. We study both neutral and bipolaronic (i.e., doubly ionized) forms of n-BTP. We find that, within experimental error, (chi) (3)llll is proportional to nb where b-5.5 at 532 nm for our neutral n-BTP samples which have 3 <EQ n <EQ 9, and b-14 at 1.06 micrometers for the bipolaron state samples which have 6 <EQ n <EQ 9 where the probing laser wavelength is close to an absorption band. We calculate (chi) (3)llll of the bipolaronic n-BTP assuming it is associated with this absorption band acting as a two-level system and find good agreement with experiment.
Third order nonlinear optical susceptibility of solutions of a triphenodithiazine derivative in dichloromethane has been measured at several different wavelengths utilizing sub-picosecond degenerate four-wave mixing. The values of the second hyperpolarizability, (gamma) , for the neutral and for the oxidized form (dication) are reported. The measurements performed at the wavelengths corresponding to the absorption maxima of the neutral and the dication forms (578 nm and 800 nm, respectively) indicate a twofold increase of the (gamma) /(sigma) figure of merit (where (sigma) is the absorption cross section) for the dication molecule. Time resolved experiments reveal different dynamics of photoexcitation relaxation in the neutral and the oxidized molecules.
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.
Several electroactive polymers, such as polyacetylene, polythlophene, poly (p-phenylene vinylene), and poly (2, 5-thienylene vinylene) have shown promise as NLO-active materials over the past few years. Extended conjugation pi-electron polymers, however, suffer from solubility and processability problems. Several recent experimental and theoretical studies have indicated that long conjugation sequences may not be necessary for enhanced (chi) (3) activity. During the past year we have utilized a suggestion of Wudl and coworkers and discovered that long, branched-chain alkyloxy groups can greatly enhance the solubility of various oligomers related to the above polymers. In this paper we present studies directed toward the incorporation of oligomeric subunits in formal copolymers and how stable bipolaronic charge states can be formed in NLO-active repeat units.
In the design of new organic materials with enhanced nonlinear optical activity, it has been predicted that (gamma) , the third order hyperpolarizability (second hyperpolarizability) will not increase indefinitely in a regular fashion as the conjugation length increases. In regular polyenes, for example, (gamma) has been predicted to level off after circa 10 - 15 repeat units. However, very few experimental systems have been investigated in which the number of repeat units approach these limits. Synthetic difficulties and the lack of solubility of longer oligomers has precluded accurate measures of (gamma) in polyenylic series of well- characterized compounds. In the present study, we discuss how long, and branched-chain alkyl groups attached via alkoxy-substituted phenyl rings can greatly increase the solubility of long polyene oligomers for D,D- and D,A- substituent patterns. This allows, for the first time, a comprehensive study of how (gamma) varies with conjugation length and symmetrical versus asymmetrical substitution.
(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.
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.
During the past three years it has become more evident that long conjugation sequences in electroactive materials may
not be a stringent requirement for high third order nonlinear optical (NLO) activity. Since long conjugation lengths in these
materials often make them difficult to process, the resulting insolubility often precludes the formation of optical quality films
for device applications. The incorporation of shorter electroactive segments alternating with flexible non-active spacers may
allow high NLO activity coupled with good optical film forming capability. In this paper we would like to present several
approaches to copolymer design which incorporate various electroactive oligomer segments with well-defined conjugation
lengths. The control one obtains in this appmach allows the design of sharp optical windows, and the ability to tailor
absorption characteristics to particular frequencies.
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