Rapid Nonradiative Decay Research Articles

Particle detection through the quantum counter concept in YAG:Er3+

We report on a scheme for particle detection based on the infrared quantum counter concept. Its operation consists of a two-step excitation process of a four level system, which can be realized in rare earth-doped crystals when a cw pump laser is tuned to the transition from the second to the fourth level. The incident particle raises the atoms of the active material into a low lying, metastable energy state, triggering the absorption of the pump laser to a higher level. Following a rapid non-radiative decay to a fluorescent level, an optical signal is observed with a conventional detector. In order to demonstrate the feasibility of such a scheme, we have investigated the emission from the fluorescent level 4S3∕2 (540 nm band) in an Er3+-doped YAG crystal pumped by a tunable titanium sapphire laser when it is irradiated with 60 keV electrons delivered by an electron gun. We have obtained a clear signature that this excitation increases the 4I13∕2 metastable level population that can efficiently be exploited to generate a detectable optical signal.

Mechanism of Co-C bond photolysis in methylcobalamin: influence of axial base.

A mechanism of Co-C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of its axial base on Co-C bond photodissociation has been investigated by time-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base-off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a new channel for rapid nonradiative decay in base-off MeCbl is opened, which competes with bond dissociation. To explain these experimental findings, the corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-O bond distances, and the manifold of low-lying triplets was plotted as a function of Co-C bond length. In contrast to the base-on form of MeCbl in which two possible photodissociation pathways were identified on the basis of whether the Co-C bond (path A) or axial Co-N bond (path B) elongates first, only path B is active in base-off MeCbl. Specifically, path A is inactive because the energy barrier associated with direct dissociation of the methyl ligand is higher than the barrier of intersection between two different electronic states: a metal-to-ligand charge transfer state (MLCT), and a ligand field state (LF) along the Co-O coordinate of the S1 PES. Path B initially involves displacement of the water molecule, followed by the formation of an LF-type intermediate, which possesses a very shallow energy minimum with respect to the Co-C coordinate. This LF-type intermediate on path B may result in either S1/S0 internal conversion or singlet radical pair generation. In addition, intersystem crossing (ISC) resulting in generation of a triplet radical pair is also feasible.

Ligand control of donor-acceptor excited-state lifetimes.

Transient absorption and emission spectroscopic studies on a series of diimineplatinum(II) dichalcogenolenes, LPtL', reveal charge-separated dichalcogenolene → diimine charge-transfer (LL'CT) excited-state lifetimes that display a remarkable and nonperiodic dependence on the heteroatoms of the dichalcogenolene ligand. Namely, there is no linear relationship between the observed lifetimes and the principle quantum number of the E donors. The results are explained in terms of heteroatom-dependent singlet-triplet (S-T) energy gaps and anisotropic covalency contributions to the M-E (E = O, S, Se) bonding scheme that control rates of intersystem crossing. For the dioxolene complex, 1-O,O', E(T2) > E(S1) and rapid nonradiative decay occurs from S1 to S0. However, E(T2) ≤ E(S1) for the heavy-atom congeners, and this provides a mechanism for rapid intersystem crossing. Subsequent internal conversion to T1 in 3-S,S produces a long-lived, emissive triplet. The two LPtL' complexes with mixed chalcogen donors and 5-Se,Se show lifetimes intermediate between those of 1-O,O' and 3-S,S.

Extremely Long Nonradiative Relaxation of Photoexcited Graphane Is Greatly Accelerated by Oxidation: Time-Domain Ab Initio Study

Graphane and its derivatives are stable and extremely thin, wide band gap semiconductors that promise to replace conventional semiconductors in electronics, catalysis, and energy applications, greatly reducing device size and power consumption. In order to be useful, band-gap excitations in these materials should be long lived and nonradiative energy losses to heat should be slow. We use state-of-the-art nonadiabatic molecular dynamics combined with time-dependent density functional theory in order to determine the nonradiative lifetime and radiative line width of the lowest energy singlet excitations in pure and oxidized graphanes. We predict that pure graphane has a very long nonradiative decay time, on the order of 100 ns, while epoxy- and hydroxy-graphanes lose electronic excitation energy to heat 10-20 times faster. The luminescence line width is 1.5 times larger in pristine graphane compared to its oxidized forms, and at room temperature, it is on the order of 50 meV. Hydroxylation lowers graphane's band gap, while epoxidation increases the gap. The nonradiative decay and luminescence line width of pure graphane are governed by electron coupling to the 1200 cm(-1) vibrational mode. In the oxidized forms of graphane, the electronic excitations couple to a broad range of vibrational modes, rationalizing the more rapid nonradiative decay in these systems. The slow electron-phonon energy losses in graphane compared to other graphene derivatives, such as carbon nanotubes and nanoribbons, indicate that graphanes are excellent candidates for semiconductor applications.

Exploring single and double proton transfer processes in the gas phase: A high resolution electronic spectroscopy study of 5-fluorosalicylic acid

Two species that possess different absorption and emission properties have been observed in the low resolution fluorescence excitation spectrum of 5-fluorosalicylic acid (FSA) in the gas phase. The two species were identified as monomer and dimer species using high resolution techniques. Studies of these spectra in the presence of an applied electric field, together with ab initio quantum chemistry calculations, show that the monomer is a "closed" form of FSA exhibiting an intramolecular C = O⋅⋅⋅H-O-C hydrogen bond in the ground state. Absorption of light at ∼344 nm transforms this species into the tautomeric form C-O-H⋅⋅⋅O = C via a barrierless proton transfer process. The large charge rearrangement that accompanies this process results in a significantly red-shifted emission spectrum. The (FSA)(2) dimer exhibits two intermolecular C=O⋯H-O-C hydrogen bonds but in this case the double proton transfer leads to a conical intersection with the ground state and rapid nonradiative decay. The onset of this process and the time scale on which it occurs are revealed by a homogeneous broadening of the dimer's high resolution spectrum.

Photophysics and reductive quenching reactivity of gadusol in solution

The photostability and photophysics of gadusol in aqueous solution has been studied. The photodecomposition quantum yields (ca. 4 × 10(-2) and 1 × 10(-4) at acidic and neutral pH, respectively) confirm the high photostability of the metabolite, independently of the presence of oxygen, under physiological conditions. The nature of the electronic transition of gadusol has been assigned as π→π* on the basis of the solvatochromic shifts of the UV absorption spectrum and the time-dependent density functional theory calculation of the vertical transition energies. The results from the photoacoustic calorimetry point to the rapid non-radiative decay as the dominant relaxation pathway of the excited species at pH 7, which is consistent with the proposed UV-sunscreening role of the molecule in the early atmosphere. Laser flash photolysis experiments probed that the ground state of the enolate form (gadusolate) undergoes electron transfer reactions with some triplet sensitizers in water or methanol solution. A rate constant of 2 × 10(8) M(-1) s(-1) has been determined for the quenching of rose bengal triplet state in water at pH 7. This reductive quenching reactivity may be considered as one of the underlying mechanisms that support the antioxidant capacity of gadusol in biological environments.

Picosecond deactivation of azo dye excited states in solution and in cellulose

Ultrafast time-resolved UV–visible absorption spectroscopy has been used to study four model azo dyes and five commercial azo dyes. All of the dyes have been found to show strong bleaching of their ground-state absorption bands on photoexcitation, with recovery lifetimes of 1–5ps in aqueous solution that are attributed to rapid and efficient non-radiative decay; two commercial azo dyes deposited in cellophane have been found to show an additional longer-lived component with a lifetime of >50ps that is attributed to the effects of intermolecular interactions with the cellulosic environment.

Ultrafast Excited-State Dynamics and Photolysis in Base-Off B12 Coenzymes and Analogues: Absence of the trans-Nitrogenous Ligand Opens a Channel for Rapid Nonradiative Decay

Ultrafast transient absorption spectroscopy was used to investigate the photochemistry of adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), and n-propylcobalamin (PrCbl) at pH 2 where the axial nitrogenous ligand is replaced by a water molecule. The evolution of the difference spectrum reveals the internal conversion process and spectral characteristics of the S(1) excited state. The photolysis yield in the base-off cobalamins is controlled by competition between internal conversion and bond homolysis. This is in direct contrast to the process in most base-on alkylcobalamins where primary photolysis occurs with near unit quantum yield and the photolysis yield is controlled by competition between diffusive separation of the radical pair and geminate recombination. The absence of the axial nitrogenous ligand in the base-off cobalamins modifies the electronic structure and opens a channel for fast nonradiative decay. This channel competes effectively with the channel for bond dissociation, dropping the quantum yield for primary radical pair formation from unity in base-on PrCbl and AdoCbl to 0.2 ± 0.1 and 0.12 ± 0.06 in base-off PrCbl and AdoCbl, respectively. The photolysis of base-off MeCbl is similar to that of base-off AdoCbl and PrCbl with competition between rapid nonradiative decay leading to ground state recovery and formation of a radical pair following bond homolysis.

Photoinduced Phase Transitions in Poly(N-isopropylacrylamide) Microgels

Photoresponsive microgels have been prepared by precipitation polymerization of the thermoresponsive polymer poly(N-isopropylacrylamide) followed by covalent conjugation of the temperature-jump dye malachite green. The photoresponsivity of these dye-labeled microgels was characterized by a pump−probe optical setup. A HeNe laser is used for exciting the dye molecules and a near-IR-diode laser is used to simultaneously measure the turbidity of the colloidal dispersion. Irradiation of malachite green increases the temperature of the sample through rapid nonradiative decay, thereby causing the polymer chains to aggregate. On deswelling, a decrease in the intensity of transmitted light is observed due to scattering. It is also observed that the photoresponsive behavior of the microgels is dependent on the concentration of the dye, intensity of the laser, and bath temperature.

Photophysics of Some Disubstituted Indoles and Their Involvements in Photoinduced Electron Transfer Reactions

Steady state electronic absorption, fluorescence emission, polarized spectra of some disubstituted indoles, 2,3-dimethylindole (23DMI), 2,5-dimethylindole (25DMI), and 1,2-dimethylindole (12DMI), were studied in solvents of different polarity coupled with time resolved measurements. The electrochemical measurements demonstrate that DMIs should act as electron donors in photoinduced electron transfer (PET) reactions with 9Cyanoanthracene (9CNA) which serves as the electron acceptor. The photoexcited 9CNA undergoes fluorescence quenching in the presence of DMI both in nonpolar n-heptane (NH) and highly polar acetonitrile (ACN) solvents though no ground state complex formation was apparent between the reactants. In NH, the observation of the reduction of fluorescence emission intensity of 9CNA accompanied by the formation of emissive exciplex was made only for the N-methyl substituted indole 12DMI. Transient absorption spectra recorded by the laser flash photolysis technique show that photoinduced charge separation reaction proceeds both in the excited singlet and triplet states of 9CNA. Moreover, it is also established that production of the monomeric triplet of 9CNA occurs through the ion recombination mechanism. In NH, the possibility of the formation of a contact neutral radical, through the H-atom transfer reaction, as an additional route for rapid nonradiative decay of the exciplex is hinted at in the case of N−H indoles 23DMI and 25DMI. Nevertheless, it is inferred that in the case of 12DMI the observed emissive exciplex in nonpolar medium is due to its inability to form a contact neutral radical. In ACN, the major nonradiative pathway appears to be due to photoinduced ET rather than formation of exciplex.

Regiosymmetric Dibutyl-Substituted Poly(3,4-propylenedioxythiophene)s as Highly Electron-Rich Electroactive and Luminescent Polymers

We report the synthesis of the dibutyl derivative of poly(3,4-propylenedioxythiophene) as a regiosymmetric and soluble electroactive and photoluminescent conjugated polymer obtained in the neutral form by a Grignard metathesis coupling reaction. Molecular weights have been characterized by gel permeation chromatography (GPC) and matrix-assisted laser desorption/ionization time-of-flight (MALDI−TOF) mass spectrometry, the latter showing end groups due to H/H, H/Br, and Br/Br termini. Solution-cast films of PProDOT-Bu2 are reddish-purple with a λmax value of 544 nm as deposited (576 nm after electrochemical annealing) and an electronic band gap of 1.8 eV. In solution, the polymer displays a strong deep red photoluminescence, and transient absorption studies reveal that direct excitation also affords a long-lived triplet state with low-to-moderate quantum efficiency. The photoluminescence is considerably weaker in the solid state, presumably due to rapid nonradiative decay at interchain trap sites. The polymer is easily oxidized with an E1/2 of ∼0.2 V vs Ag/Ag+ when prepared either by solution casting of the chemically synthesized polymer or by direct electrodeposition and provides a distinct electrochromic switch from a deep red-purple to a highly transmissive sky blue. Similar spectral changes are observed upon solution doping, and conducting films (up to 7 S/cm) have been prepared which exhibit a high level of ambient stability.

Explanation for the Disparity among Absorption and Action Spectra of Eumelanin

The optical properties of eumelanin (from Sepia officinalis) are found to vary with particle size. The absorption spectrum for small eumelanin particles agrees quantitatively with the reported action spectra for photoinduced oxygen consumption and free radical generation by eumelanin. These small particles, unlike the large ones, generate long-lived reactive intermediates upon absorption of UV light. The data presented suggest that the small eumelanin particles may be involved in UV-A-induced photochemical processes believed to lead to DNA damage in skin cells, whereas the large particles efficiently dispose of UV-A energy through rapid nonradiative decay processes. These results provide new insight into the dichotomy that eumelanin is both photoprotective and photosensitizing. This size-dependent photoreactivity may be one of the contributing factors to the observed variations in skin cancer rates among different skin types.

On the mechanism of rapid non-radiative decay in intramolecularly hydrogen-bonded π systems

Reaction paths and potential-energy profiles of excited singlet states of malonaldehyde have been investigated using the ab initio CASSCF method. The results confirm the barrierless nature of the hydrogen-transfer process in the π π ∗ excited singlet state. A hitherto unknown 1 π σ ∗ state has been identified which is strongly repulsive with respect to in-plane detachment of the mobile hydrogen atom, leading to a low-lying conical intersection with the electronic ground state. Vibronic coupling of the 1 π π ∗ with the 1 π σ ∗ state by out-of-plane modes connects the former, via a low barrier, to the conical intersection, providing a mechanism for efficient internal conversion to the ground state. It is argued that these findings reflect general properties of the H-chelate ring in intramolecularly hydrogen-bonded molecules and can explain the photostability of these systems.

Structure, electronic relaxation and vibrational predissociation of the electronically excited SO 2(C̃)Ar complex

Fluorescence excitation spectra of SO 2Ar generated in a supersonic expansion near the SO 2 C ̃ → X ̃ , 0 0 0, 2 0 1, and 3 0 2 bands have been obtained. Three features red-shifted by 82, 60 and 41 cm −1 from monomer bands are assigned to the SO 2Ar complex. The rotational constants of SO 2(C̃)Ar are determined to be A = 0.2769 cm −1, B = 0.0520 cm −1 and C = 0.0488 cm −1. The corresponding complex structure is obtained. Rapid non-radiative decay induced by the Ar atom has been observed for the 0 0 0 level of the C̃ state complex. Vibrational predissociation is identified for the (010) and (002) levels, 377 and 560 cm −1 above the band origin.

Squaraine Photophysics in Polymer Films

Absorption and emission spectra of four squaraine dye~-bis[4-(dimethylamino)phenyl]squaraine (H-Sq), bis[4-(dimethylamino)-2-hydroxyphenyl]squaraine (HO-Sq), bis(trihydroxypheny1)squaraine (3HO-Sq), and bis(azuly1)squaraine (Az-Sq)-are reported in dilute films of poly(methylmethacry1ate) and polystyrene. Obtained for the first time are the fluorescence lifetimes of these dyes in these polymer films. Fluorescence lifetimes in liquid solutions are also reported. These emission kinetics combined with measurements of fluorescence quantum yields allow determination of radiative and nonradiative decay rate constants for three of the dyes. Comparison of determined radiative rate constants with those calculated from the absorption spectrum is consistent with the fluorescence originating from the absorbing species and state. The shortest lived fluorescence observed occurs for bis(trihydroxyphenyl)squaraine, and the short lifetime is attributed to either a rapid excited-state proton-transfer tautomerization or deprotonation. The SI state of bis(azuly1)squaraine is nonfluorescent, presumably due to a rapid nonradiative decay similar to that which occurs in azulene. However, Si' - SO emission is observed for this azulyl-substituted squaraine.

Spectroscopy of jet-cooled benzimidazole and benzotriazole

Laser-induced fluorescence excitation and dispersed fluorescence spectra of jet-cooled benzimidazole and benzotriazole molecules are reported. The first excited singlet state is assigned to 1L b for benzimidazole (origin at 36032 cm −1) and to 1L a for benzotriazole (origin at 34927 cm −1). A rapid nonradiative decay is found to occur for benzotriazole above 1200 cm −1 from S 1 origin. The spectra of benzimidazole show a good Franck—Condon overlap between S 0 and S 1( 1L b) states. For benzotriazole progressions of several in-plane modes (540, 1153, 1242, 1405 and 1447 cm −1) cover about 8000 cm −1 of the dispersed fluorescence spectrum following excitation into the origin of the S 1( 1L a) state.

Electronic spectroscopy and fluorescence decay dynamics of matrix isolated iodine bromide

The spectroscopy and relaxation processes of IBr isolated in a solid Ar matrix have been studied using laser excitation and resolved fluorescence techniques. Excitation wavelengths in the range of 420-630 nm yielded fluorescence from the B(0 + ), A(1), and A'(2) states. Vibrational structure was absent from both the B(0 + )-X(0 + ) excitation and emission spectra. All levels of B(0 + ) were subject to rapid nonradiative decay