Sequential Photon Absorption Research Articles

Degradation of Pharmaceuticals through Sequential Photon Absorption and Photoionization in Amiloride Derivatives

Summary Haloaromatic drug molecules of the amiloride family are plagued by photodegradation with associated toxicity. Herein, we report on the photodegradation of analogs of amiloride, which are known to undergo photosubstitution in water. Model compounds built on the same scaffold undergo clean photosubstitution also in alcoholic solvent, where a certain amount of photodehalogenation is normally expected. Available evidence points to a mechanism starting with photoexcitation followed by photoionization to give a radical cation intermediate. Subsequent substitution reaction with the protic solvent is assisted by a general base, possibly strengthened by the proximal solvated electron. Recombination with the solvated electron generates the observed product. Quantum chemical computations reveal that excited state antiaromaticity is relieved when an electron is ejected from the photoexcited molecule by the second photon. The mechanism indicated here could have wide applicability to photoinduced degradation of similar heteroaromatic compounds in the environment, as well as to a class of increasingly popular synthetic photoredox methods.

Influence of Cu content on the physical characteristics of CuxGaCr0.1S2 thin films for intermediate band solar cells

Intermediate band semiconductors can increase the efficiency of photovoltaic solar cells via sequential absorption of photons in a wider range of the solar spectrum. The introduction of Cr substituting Ga atoms in the chalcopyrite CuGaS2 is proposed to create a suitable intermediate band, investigating the influence of the Cu/Ga ratio on the structural, morphological, optical, and electrical characteristics. For this purpose, CuxGaS2 and CuxGaCr0.1S2 thin films (with x ranging from 0.8 to 1.2) are prepared by modulated evaporation from elemental sources and analyzed comparatively by X-ray diffraction, Raman spectroscopy, atomic force microscopy, spectrophotometry, and Hall effect measurements. The incorporation of Cr produces an in-gap absorption that is independent of the Cu/Ga ratio. The mean crystallite size, surface roughness, and electrical conductivity increase with the Cu content in both CuxGaS2 and CuxGaCr0.1S2.

Delocalized Two-Exciton States in DNA Scaffolded Cyanine Dimers.

The engineering and manipulation of delocalized molecular exciton states is a key component for artificial biomimetic light harvesting complexes as well as alternative circuitry platforms based on exciton propagation. Here we examine the consequences of strong electronic coupling in cyanine homodimers on DNA duplex scaffolds. The most closely spaced dyes, attached to positions directly across the double-helix from one another, exhibit pronounced Davydov splitting due to strong electronic coupling. We demonstrate that the DNA scaffold is sufficiently robust to support observation of the transition from the lowest energy (J-like) one-exciton state to the nonlocal two-exciton state, where each cyanine dye is in the excited state. This transition proceeds via sequential photon absorption and persists for the lifetime of the exciton, establishing this as a controlled method for creating two-exciton states. Our observations suggest that DNA-organized dye networks have potential as platforms for molecular logic gates and entangled photon emission based on delocalized two-exciton states.

(Invited) Lanthanide Doped Nanoparticles and Their Applications

Since approximately 2000, lanthanide doped nanoparticles have received significant attention due to their interesting luminescent properties. At the core of this interest is their ability to convert near-infrared excitation light (typically 980 or 800 nm) to higher energies spanning the ultraviolet, visible and near-infrared regions of the spectrum through a multiphoton process known as upconversion. Upconversion differs from conventional multiphoton excitation in other materials where no real intermediate states are present necessitating the use of ultrafast lasers (in the femtosecond regime) for simultaneous excitation to the upper emitting state. The lanthanide ions possess a multitude of 4f electronic states that have long lifetimes (micro- to millisecond) thus act as population reservoirs in the upconversion process. Hence, upconversion occurs through real, long-lived intermediate states through a sequential photon absorption process. This eliminates the need for ultrafast excitation and as a result, upconversion can be observed using inexpensive, continuous wave diode lasers. Upconversion luminescence can be exploited for a number of applications in nanomedicine, theranostics, photovoltaics, photocatalysis, as well as many others.While their upconversion luminescence has been studied in great detail, lanthanide doped nanoparticles can also emit in the near-infrared through a direct Stokes (down-shifted) luminescence process. Of particular importance is that these near-infrared emissions lie within the biological windows where biological tissues are optically transparent. For obvious reasons, a great deal of the work on the nanomedicine and theranostics applications of lanthanide doped nanoparticles has shifted to their near-infrared luminescence properties.In this presentation, we will demonstrate the synthesis of these nanoparticles, establish how changing their nanoscale architecture can affect their luminescence properties (both upconversion and near-infrared), and discuss their potential applications, particularly in nanomedicine.

Upconversion Luminescence Imaging of Tumors with EGFR-Affibody Conjugated Nanophosphors

ABSTRACTNear infrared (NIR) optical imaging has demonstrated significant potential as an effective modality for cancer molecular imaging. Among various NIR probes currently under investigation, upconversion nanophosphors (UCNPs) possess great promise due to their anti-Stokes emission and sequential photon absorption which result in superior detection sensitivity and a simple imaging setup, respectively. Here we investigated the utility of this imaging modality to detect tumor cells expressing the epidermal growth factor receptor (EGFR) using affibody functionalized nanophosphors and a custom built imaging system. Initially, aqueous dispersible NaYF4: Tm+3, Yb+3 UCNPs were synthesized and their photophysical properties were characterized. Then, their luminescence response as a function of concentration and their depth resolving capability in a tissue-simulating phantom were examined. Finally, we demonstrated the use of bioconjugated UCNPs for imaging EGFR-expressing tumors both in vitro and in vivo. Our data suggests that NIR imaging with UCNPs may be useful for noninvasive imaging of tumors.

Estudio comparativo de disociación de iones moleculares inducida por absorción secuencial de fotones infrarrojos

La disociación de iones moleculares de acetofenonas sustituidas inducidas por absorción multifotónica secuencial de fotones infrarrojos ha sido estudiada en un espectrómetro de resonancia ciclotrónica de iones por transformada de Fourier con el objetivo de obtener datos termoquímicos. Dos métodos diferentes se han usado para este fin: 1) la radiación emitida por un filamento de tungsteno incandescente situado en la propia celda del espectrómetro, y 2) un láser continuo de CO2. En el primer caso, es posible obtener una pseudo energía de activación a partir de los valores de la constante de disociación en función de la temperatura. En el segundo caso, el estudio de la constante de disociación en función de la intensidad de la radiación del láser ofrece una alternativa para obtener la energía de activación. El uso de una ecuación cinética maestra se ha utilizado para obtener valores aproximados de las energías de disociación y se discuten las limitaciones de los métodos.

Ocular Photoreception for Circadian Rhythm Entrainment in Mammals.

Circadian rhythms are self-sustained, approximately 24-h rhythms of physiology and behavior. These rhythms are entrained to an exactly 24-h period by the daily light-dark cycle. Remarkably, mice lacking all rod and cone photoreceptors still demonstrate photic entrainment, an effect mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells utilize melanopsin (OPN4) as their photopigment. Distinct from the ciliary rod and cone opsins, melanopsin appears to function as a stable photopigment utilizing sequential photon absorption for its photocycle; this photocycle, in turn, confers properties on ipRGCs such as sustained signaling and resistance from photic bleaching critical for an irradiance detection system. The retina itself also functions as a circadian pacemaker that can be autonomously entrained to light-dark cycles. Recent experiments have demonstrated that another novel opsin, neuropsin (OPN5), is required for this entrainment, which appears to be mediated by a separate population of ipRGCs. Surprisingly, the circadian clock of the mammalian cornea is also light entrainable and is also neuropsin-dependent for this effect. The retina thus utilizes a surprisingly broad array of opsins for mediation of different light-detection tasks.

Photoactive high explosives: linear and nonlinear photochemistry of petrin tetrazine chloride.

Pentaerythritol tetranitrate (PETN), a high explosive, initiates with traditional shock and thermal mechanisms. In this study, the tetrazine-substituted derivative of PETN, pentaerythritol trinitrate chlorotetrazine (PetrinTzCl), is being investigated for a photochemical initiation mechanism that could allow control over the chemistry contributing to decomposition leading to initiation. PetrinTzCl exhibits a photochemical quantum yield (QYPC) at 532 nm not evident with PETN. Using static spectroscopic methods, we observe energy absorption on the tetrazine (Tz) ring that results in photodissociation yielding N2, Cl-CN, and Petrin-CN as the major photoproducts. The QYPC was enhanced with increasing irradiation intensity. Experiment and theoretical calculations imply this excitation mechanism follows sequential photon absorption. Dynamic simulations demonstrate that the relaxation mechanism leading to the observed photochemistry in PetrinTzCl is due to vibrational excitation during internal conversion. PetrinTzCl's single photon stability and intensity dependence suggest this material could be stable in ambient lighting, yet possible to initiate with short-pulsed lasers.

Progress Toward Realizing an Intermediate Band Solar Cell—Sequential Absorption of Photons in a Quantum Well Solar Cell

In order to realize an intermediate band solar cell, which promises high photovoltaic energy conversion efficiency, achieving higher photocurrent while maintaining the cell voltage is essential. We report on a transient photocurrent due to the sequential absorption of photons in a single quantum well by continuously pumping to stimulate interband transitions (from a valence band to an intermediate band) and showing an intersubband transition (from an intermediate band to a conduction band) with a pulsed infrared laser. We demonstrate the extent to which multiple-photon absorption can be achieved in quantum well devices and propose that a quantum well is a suitable candidate for an intermediate band solar cell. From the combination of this and other sequential absorption results, it is clear that enhancing the short lifetime of a carrier in the intermediate band is the next step toward achieving a working intermediate band solar cell. In light of this, we enhance our previous suggestion, the photon ratchet intermediate band solar cell, as a means of increasing the electron lifetime.

IOFF Generally Extends Fluorophore Longevity in the Presence of an Optical Trap

The combination of optical tweezers force microscopy and single molecule fluorescence has previously been complicated by trap-induced photobleaching. Recent studies have suggested that this effect is caused by a sequential absorption of photons, leading to ionization of the fluorescent singlet state. In this work, we show the range of effects of optical trapping radiation on common fluorescent dyes. Using the interlaced optical force fluorescence (IOFF) laser modulation technique, we show that the removal of simultaneous near infrared radiation dramatically reduces photobleaching effects. However, these studies show that the sequential addition of near infrared radiation in some cases extends photobleaching longevity beyond the natural intrinsic decay. We suggest a refined photoelectronic mechanism that accounts for the possibility of reverse intersystem crossing from a reactive triplet state and explains the nature of trap-induced photobleaching.

Wavelength dependence of UV-laser induced emission of neutral and ionic species from single crystal NaNO 3

We report time-resolved, quadrupole mass-selected measurements of neutral and ion emission from single crystal sodium nitrate exposed to ns pulse excimer laser radiation at 157 nm (F 2 excimer, 7.9 eV photons), 193 nm (ArF excimer, 6.4 eV photons) and 248 nm (KrF excimer, 5.0 eV photons). Neutral emissions, including NO, O 2, N 2, and Na are observed at all three wavelengths. At 193 nm, intense atomic N and O emissions are also observed, possibly due to a 1+1 excitation involving the π ∗← π transition in the nitrate ion (centered at 6 eV) followed by excitation to a higher excited state. This transition is not efficiently excited at the other two wavelengths. At 248 nm, much of the emission is attributed to thermally assisted, dissociative electron/hole attachment. Although 157 nm photons do not excite the π ∗← π transition efficiently, the resulting NO emission is found to be quite intense: on a per unit energy (or per photon) basis, 157 nm photons are much more efficient in decomposing nitrate anions than 193 nm photons. Intense ion emission (principally Na + and NO +) is observed at 193 and 157 nm, with weaker Na + emission at 248 nm. The ion intensities show high-order fluence dependence, consistent with photoelectronic emission involving sequential photon absorption as described earlier for 248 nm irradiation [J. Appl. Phys. (80) (1996) 6452].

Upconversion properties of Er 3+ doped lead silicate glasses

We report the spectroscopic behaviour of the Er 3+ ion in two lead silicate glasses containing different concentrations of PbO. Evidence is given for blue and green upconversion fluorescence in the two Er 3+ doped lead silicate glasses with the use of 650 nm, 800 nm and 1.5 µm excitation. Analysis of the results shows that upconversion occurs via the sequential absorption of photons from the metastable 4I 13/2 state. Furthermore, we show that upconversion is facilitated due to the low radiative transition probabilities and to the inefficient multiphonon relaxation.

Multiphoton ionization–fragmentation patterns of tertiary amines

Multiphoton ionization (MPI)–fragmentation patterns are reported for a series of normal and caged tertiary amines. Ionization is enhanced by two-photon resonance with the 3s and 3p Rydberg states of trimethylamine, triethylamine, and the caged amines quinuclidine and triethylenediamine. Over the wavelength region λ = 400–530 nm, N(CH3)3 ionizes to the parent ion (P) and fragments only by the loss of a H atom to yield the P–H daughter ion; N(C2H5)3 ionizes to its parent ion and fragments by the loss of a methyl to form the P–CH3 ion. The branching ratio of daughter to parent ions is found to be essentially independent of laser intensity but strongly dependent on laser wavelength. The caged amines quinuclidine [N(C2H4)3CH, or ABCO] and triethylenediamine [N(C2H4)3N, or DABCO] fragment extensively over this λ range in a manner dependent on both laser wavelength and intensity. The extent of daughter ion formation in N(CH3)3 and N(C2H5)3 can be understood by consideration of the wavelength regions in which the total available energy from the initial three- or four-photon ionization event exceeds the appearance potential of the given daughter ion. For the caged amines direct observation of this mechanism is masked by fragmentation due to sequential absorption of photons (during the ∼5 ns pulse duration) by the parent and/or daughter ions. The present results show that even for molecules with broad, unstructured UV absorption and MPI spectra such as N(CH3)3 and N(C2H5)3, considerable information on photon–molecule and photon–ion interactions can still be gained by the MPI mass spectrometry technique.

Vibrational relaxation of DF (v=1–4) in D2, H2, N2, HF, and CO2

The deactivation of the upper vibrational levels of DF by H2, D2, N2, HF, and CO2 has been studied with the technique of laser-induced fluorescence. The upper vibrational levels were produced by sequential photon absorption in which DF (v=0) was pumped first to DF (v=1) and subsequently to DF (v=2), DF (v=3), and DF (v=4) by photons from a pulsed multiline DF chemical laser. The deactivation rates (V−V+V−R,T) for all the collision partners except D2 scaled with vibrational level as vn with n=1.9 to 2.0 for v=1 to 4. Similar studies have indicated the somewhat larger value of n=2.7±0.2 for HF (v) deactivation by diatomic molecules.