Photochemical Hole Burning Research Articles

Ultrafast Pump-Repump-Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching.

We demonstrate pump-repump-probe (PRP) transient hole burning as a spectroscopic tool for differentiating reactive from nonreactive deactivation of excited photochemical reactants observed by transient absorption spectroscopy (TAS). This method utilizes a time-delayed, wavelength-tunable ultrafast pulse to alter the excited reactant population, with the impact of "repumping" quantified through depletions in photoproduct absorption. We apply this approach to characterize dynamics affecting the nonadiabatic photocyclization efficiency to form S0 dihydrotriphenylene (DHT) following 266 nm excitation of ortho-terphenyl (OTP). TAS studies revealed bimodal deactivation of OTP*, but neither relaxation time scale (700 fs and 3.0 ps) could be assigned unambiguously to DHT formation due to overlap of excited-state and product spectra. PRP studies reveal that S1 OTP only cyclizes on the slower of these time scales, with the faster process attributable to nonreactive deactivation. We demonstrate that this method offers greater photochemical insights without assuming models to globally fit spectral transients collected by TAS.

The distributions of enhancement factors in close‐packed and nonclose‐packed surface‐enhanced Raman substrates

A method employing photochemical hole burning, previously developed to measure the distribution of Raman enhancement factors on a nanostructured substrate for surface‐enhanced Raman scattering, is used to compare the enhancement distributions of benzenethiol adsorbed on substrates optimized for 532 nm laser excitation consisting of close‐packed (CP) or nonclose‐packed (NCP) nanospheres. The ensemble‐averaged Raman enhancement factor was 2.8 times smaller for the NCP substrate. The measured distributions revealed additional information. For instance, 92% of the molecules on the CP substrate and 93.6% of the molecules on the NCP substrate had Raman enhancements below average. The minimum enhancements on both substrates were ~104, but on the NCP substrate the maximum enhancement was 1.2 × 108, whereas on the CP substrate the maximum was 2 × 1010. The Ag‐coated nanospheres form hemisphere‐on‐cylinder mushroom‐like structures on both lattices, but on the NCP lattice, one third of the molecules are on the flat regions between the mushrooms. The flats on the NCP lattice have enhancements of ~104, showing they are part of a resonant plasmonic structure. The highest NCP enhancements of ~108 are tentatively associated with regions at the bases of the mushrooms, whereas the highest CP enhancements of 2 × 1010 are tentatively associated with gaps between nanospheres where 0.0025% of the molecules reside. Copyright © 2011 John Wiley & Sons, Ltd.

ChemInform Abstract: New Route to Unsymmetrical Phthalocyanine Analogues by the Use of Structurally Distorted Subphthalocyanines.

In addition to traditional uses as dyes and in photocopying devices, phthalocyanines (Pcs) are now rapidly growing in importance in many fields such as chemical sensors, electrochromism, batteries, photodynamic cancer therapy, molecular metals, photochemical hole burning, and liquid crystals. In this communication, the authors present a completely new method for the preparation of monosubstituted type unsymmetrical Pcs and Pc analogues, which utilizes the so-called subphthalocyanines (SubPcs).

On the assignment of PSHB in D1/D2/ cytb559 reaction centers

We re-visit the phenomenon of persistent spectral hole-burning (PSHB) in isolated PS II reaction centers in light of advances in the understanding of the overall electronic structure of PS II. A recent analysis of non-selective photochemical hole-burning and the consequent changes in CD spectra associated with the photo-reduction of PheoD1 (the primary electron acceptor) supports an analysis in which very substantial and non-correlated chlorin site-energy inhomogeneity leads to a range of realizations of the exciton structure of reaction centers. The lowest energy excited state can be described as predominantly involving the dominantly ChlD1 (the primary acceptor) exciton, but in a significant fraction of centers the lowest excited state is a dominantly PheoD2 (the inactive pheophytin) exciton. The narrow linewidth of PSHB reported a decade ago in these systems, with homogeneous holewidths approaching that corresponding to the radiative lifetime, along with our ability to reproduce the published PSHB action spectrum, leads to the suggestion that aspects of hole-burning in these systems can be attributed to centers in which the exciton realizations lead to a predominantly PheoD2 character as the lowest energy exciton. This assignment is discussed in terms of models of the PS II reaction center.

Optical Properties of Functional Hybrid Organic–Inorganic Nanocomposites

AbstractFunctional hybrids are nanocomposite materials lying at the interface of organic and inorganic realms, whose high versatility offers a wide range of possibilities to elaborate tailor‐made materials in terms of chemical and physical properties. Because they present several advantages for designing materials for optical applications (versatile and relatively facile chemistry, easy shaping and patterning, materials having good mechanical integrity and excellent optical quality), numerous silica or/and siloxane based hybrid organic–inorganic materials have been developed in the past few years. The most striking examples of functional hybrids exhibiting emission properties (solid‐state dye lasers, rare‐earth doped hybrids, electroluminescent devices), absorption properties (photochromic), nonlinear optical (NLO) properties (second‐order NLO properties, photochemical hole burning (PHB), photorefractivity), and sensing are summarized in this review.

Nanoscopic Aggregation in Random Copolymer Bulk Films Studied with Photochemical Hole Burning and Transmission Electron Microscopy

Nanoscopic aggregation in random copolymers of methyl methacrylate (MMA) with n-butyl methacrylate (n-BMA) or benzyl methacrylate (BzMA) in bulk films has been studied for the first time at low temperatures by using photochemical hole burning (PHB) spectroscopy for tetraphenylporphine (TPP) dispersed in the copolymers and also with transmission electron microscopy (TEM). Temperature cycle experiments with PHB were carried out for the copolymers with different ratios of comonomer groups. The increases in hole widths due to structural relaxation of butyl groups or benzyl groups in the copolymers of n-BMA or BzMA with MMA were much larger than those expected for hypothetical systems where MMA and comonomer groups with the corresponding molar ratios are randomly dispersed. The results have been attributed to the nanoscopic aggregation of n-BMA or BzMA units with the preferential inclusion of TPP within them in the films. PHB spectroscopy provides a new tool for studying nanoscopic aggregation larger than 2.3 nm in transparent random copolymer bulk films. The nanoscopic aggregation of MMA-rich domains with the diameter of 20−30 nm in a random copolymer bulk film of MMA with BzMA has also been ascertained with TEM measurements. A TEM picture of a polymer blend film of PMMA and PBzMA (1:1) showed phase separation with PBzMA-rich domains of ca. 500 nm diameter.

A model for persistent hole burned spectra and hole growth kinetics that includes photoproduct absorption: Application to free base phthalocyanine in hyperquenched glassy ortho-dichlorobenzene at 5 K

Persistent nonphotochemical and photochemical hole burning of the S0→S1 origin absorption bands of chromophores in amorphous hosts such as glasses, polymers and proteins at low temperatures have been used to address a number of problems that range from structural disorder and configurational tunneling to excitation energy transfer and charge separation in photosynthetic complexes. Often the hole burned spectra are interfered by photoproduct (antihole) absorption. To date there has been no systematic approach to modeling hole burned spectra and the dispersive kinetics of zero-phonon hole growth that accounts for the antihole. A “master” equation that does so is presented. A key ingredient of the equation is a time-dependent, two-dimensional site excitation frequency distribution function (SDF) of the zero-phonon lines. Prior to hole burning (t=0) the SDF is that of the educt sites. For t>0 the SDF describes both educt and photoproduct sites and allows for burning of the latter that revert to the educt sites from which they originate (light-induced hole filling). Our model includes linear electron–phonon coupling and the three distributions that lead to dispersive hole growth kinetics, the most important of which is the distribution for the parameter λ associated with tunneling between the bistable configurations of the chromophore-host system that are interconverted by hole burning. The master equation is successfully applied to free base phthalocyanine (Pc) in hyperquenched glassy ortho-dichlorobenzene (DCB) at 5 K. The mechanism of hole burning is photochemical and involves tautomerization of the two protons at the center of the macrocycle (Pc) that occurs in the S1(Qx) and/or T1(Qx) state of Pc. A single set of parameter values (some of which are determined directly from the hole burned spectra) provides a satisfactory description of the dependence of the hole burned spectra and hole growth kinetics on the location of the burn frequency within the inhomogeneously broadened Qx absorption band. The hole growth kinetics are found to be quite highly dispersive, although to a lesser degree than the kinetics for free base phthalocyanine tetrasulphonate in hyperquenched glassy water [Reinot et al., J. Lumin. 98, 183 (2002)]. The dispersion is attributed to structural heterogeneity of solvent molecules in the inner shell that leads to a distribution of chromophore-host interactions that affect the height of the barrier separating the two tautomers. The new master equation should also prove useful with no additional assumptions or modifications for interpretation of nonphotochemical hole burned spectra.

Fabrication of a protein film by electrospray deposition method and investigation of photochemical properties by persistent spectral hole burning

Transparent protein film of iron-free cytochrome c (Cyt. c) was successfully manufactured by electrospray deposition (ES) method and the properties of the protein film were investigated. Excursion temperature dependence of spectral hole profiles in the photochemical hole burning for the iron-free Cyt. c in protein film and in glassy solution was investigated and compared with that of iron-free porphyrin embedded in a synthetic polymer film to clarify photochemical properties of electrospray deposited protein film. The spectral holes of iron-free porphyrin were thermally more stable in Cyt. c than in polymer, indicating compact packing of the chromophore in the protein. The ES-deposited iron-free Cyt. c in protein film showed less stable spectral hole than that in glassy solution. This difference is attributable to the electrospray deposition process of the protein involving ionization and to subsequent cross-linking of protein molecules.

Function of Eu ion and defect centers in photochemical hole-burning of Eu-doped sodium borate glass

Function of Eu ion and defect centers in photochemical hole-burning of Eu-doped sodium borate glass

Broadband hole burning: a new technique for the measurement of hole burning kinetics, removing the influence of inhomogeneous broadening

Abstract One of the problems associated with measurement of hole burning (HB) kinetics in disordered media is their non-exponential time dependence. There are two sources of non-exponentiality inherent to all glassy systems: random orientation of the chromophores and strong inhomogeneous broadening of their optical spectra. The non-exponentiality caused by the above factors conceals other, more interesting sources of dispersive burning kinetics such as dispersion of burning quantum efficiency and distribution of homogeneous line widths. A modification of the HB method, the broadband hole burning (BHB), is proposed, which eliminates the strongest source of non-exponentiality—the influence of inhomogeneous broadening. The new technique was applied to measurements of BHB kinetics on a simple, well-characterized system: H 2 -tetraphenylporphine (TPP) in polystyrene (PS). Only photochemical HB (i.e., proton tautomerization) with negligible dispersion of burning efficiency was expected in this sample. The BHB technique revealed the existence of a second, previously unknown photoreaction, with quantum efficiency about one order of magnitude lower than proton tautomerization. The ordinary HB technique does not detect this photoreaction because its manifestation in burning kinetics is smeared out by the above-mentioned sources of non-exponentiality.

Photochemical hole-burning in C 60–H 2TPP supramolecule doped polymer: a comparative study with the behaviour of separated H 2TPP and C 60 entities

We present optical spectroscopy measurements of a supramolecular assembly C 60–H 2TPP in poly-vinylbutyral (PVB) and compare its photophysical properties to separated C 60 and H 2TPP in PVB. Optical absorption and emission from the supramolecular assembly mainly appears as resulting from a superposition of the respective properties of the fullerene and porphyrin moieties. However, emission features unique to the supramolecule assembly are found and reported. Spectral hole-burning properties of the supramolecule are similar to those observed for separated C 60 and H 2TPP. This demonstrates that the phototautomerisation induced in the porphyrin moiety and thought to be responsible for the formation of the spectral hole is largely unaffected by the presence of the fullerene.

Non-photochemical versus photochemical hole burning in hyperquenched glassy water and cubic ice

The Shu–Small mechanism (J. Opt. Soc. Am. B 9 (1992) 724) for non-photochemical hole burning (NPHB) of electronic transitions of chromophores in amorphous hosts at low temperatures is based on a hierarchy of configurational tunneling events that begin in the outer shell and terminate in the inner shell that surrounds the chromophore. Connectivity between the intrinsic and extrinsic two level systems that stem from structural disorder and the excess free volume of glasses are key features of this mechanism. This mechanism has recently been modified to take into account multilevel extrinsic systems (J. Chem. Phys. 114 (2001) 9105). An important prediction of the Shu–Small mechanism is that NPHB should cease upon formation of a crystalline phase produced, for example, by warming of a vitrified glass. In this paper data are presented that are consistent with that prediction. The systems studied are aluminum phthalocyanine tetrasulphonate (APT) and free base phthalocyanine tetrasulphonate (PcT) in hyperquenched glassy films of water (HGW) and cubic ice ( I c). The hole-burning mechanisms for APT and PcT are non-photochemical and photochemical, respectively. Surprisingly, the hole-growth kinetics for PcT are as highly dispersive as are the kinetics for APT. This means that the kinetics for photoinduced proton tautomerization in PcT are strongly influenced by its nanoenvironment of water molecules, some of which are involved in H-bonding to the –NH groups of PcT.

Photochemical reactions of samarium ions in sodium borate glasses irradiated with near-infrared femtosecond laser pulses

Photoinduced processes of Sm 3+-doped sodium borate glasses under high-power, 800 nm femtosecond laser irradiation have been studied. Both the photoreduction of Sm 3+ to Sm 2+ and the formation of intrinsic defects are observed at an irradiance well below the threshold for laser-induced damage. We propose that these photoinduced processes arise from the linear and two-photon absorption of the short-wavelength component of the supercontinuum and the subsequent ionization of the glass matrix. Photochemical hole burning (PHB) has been observed for 5 D 0– 7 F 0 transition of Sm 2+ in the sodium borate glasses irradiated with near-infrared femtosecond laser pulses. The efficiency of hole burning at 77 K was about 5 times higher in the irradiated glasses than in thermally reduced glasses. We consider that both the metastable Sm 2+ and the intrinsic defects were responsible for the highly efficient PHB phenomenon.

Relationship between room temperature hole-burning in glasses and doped Eu-ion concentration

Photochemical hole-burning in sodium-borate glass (35Na 2O–65B 2O 3) was investigated as a function of doped Eu-ion concentration from 0.1 to 5 mol% both at room temperature and 12 K. In our preparation, 50–70% of Eu 3+ was reduced to Eu 2+. As the concentration of Eu ion exceeded 1 mol%, the fluorescence was quenched, and meanwhile the photochemical hole started burning. The spectral hole was most efficiently burnt at concentrations between 2 and 3 mol% at room temperature. Low temperature experiment showed that there are three kinds of processes for the hole formation. One is a transient process relaxed within 1–2 min. Another is observed only at low temperatures. The third one is observed persistently even at room temperature. The extent of these components depends on Eu-ion concentration. In view of these concentration dependences, the suggested mechanism of room temperature hole-burning is an energy or electron transfer between Eu 3+ and Eu 2+. Defect centers created by reduction process of the glass may play a role to help this transfer.

Photoionization related phenomena in doped insulators: The role of inversion site symmetry of impurity centers

The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

Irreversible dynamics in incommensurate biphenyl studied by thermocycling of spectral holes

Both photochemical and nonphotochemical spectral hole burning mechanisms were observed in the incommensurate phase III (ICIII) of chlorin-doped biphenyl. Distributions of corresponding energetic barriers for the back reaction were acquired from temperature cycling experiments. The non-photochemical mechanism is characterized by finite (unlike to glasses) but low (compared to the photochemical mechanism) threshold of 6--7 K and is interpreted as a photoinduced depinning-repinning of the incommensurate modulation wave. Holewidth dependence on cycling temperature up to 28 K was measured, crossing at 17 K the phase transition between two incommensurate phases in pure biphenyl. The measured irreversible hole broadening exhibits a fast (close to cubic) increase up to 17 K and then changes abruptly to a linear in the higher-temperature incommensurate phase II, which extrapolates back to zero at $\stackrel{\ensuremath{\rightarrow}}{T}0\mathrm{K}.$ The results support the view that ICIII is a metastable state.

Excitation, Hole-Burning, and Stark Spectroscopy of Free Base Isobacteriochlorin in an n-Octane Matrix at Liquid Helium Temperatures

Excitation spectra of trans-free base isobacteriochlorin in an n-octane Shpol'skii matrix at liquid helium temperatures have been obtained. Upon rapid cooling, the guest isobacteriochlorin molecule inhabits two sites (A and B). In the 0−0 region of the lowest lying optical transition, each site displays a series of spectral peaks, which are investigated by photochemical hole burning. This radiant saturation technique confirms pairings of spectral peaks in site A and higher order connectivity of spectral lines and photophysically induced lines in site B. Application of an electric field to the system, while scanning the excitation spectrum, results in an irreversible reduction in intensity of the photophysically formed lines of site B. Evidence is also presented in which molecules inhabiting site B can interconvert to site A by optical means.

Nanoscopic Aggregation in Amphiphilic Random Copolymer Bulk Films Studied with Photochemical Hole Burning

Nanoscopic aggregation and highly constrained nanoenvironments within bulk films of amphiphilic random copolymers have been studied for the first time by using photochemical hole burning spectroscopy at low temperatures. Films containing sodium sulfonate (A) and cyclododecyl (CD) or lauryl (LA) groups) with tetraphenylporphine (TPP) dispersed in the copolymers, as well as for TPP covalently bound to the copolymer chains, were prepared. Temperature cycle experiments were carried out for the copolymer films with different ratios of the hydrophilic and hydrophobic groups. Holes burned in the copolymer films with CD and sodium sulfonate groups were thermally more stable than those expected for hypothetical systems where the groups are randomly dispersed with the corresponding ratios. The discrepancy can be attributed to the “pinning down” of TPP in constrained configurations of CD aggregation in the films. On the other hand the increase in hole width in the copolymer films of LA groups with sodium sulfonate was enhanced compared to those expected for hypothetical systems where sodium sulfonate and LA groups are randomly dispersed with the corresponding ratios. The results could be attributed to the aggregation of LA groups, with the preferential inclusion of TPP within it, in the films.

Preparation and optical properties of TPPS-doped metal–carboxylate-salts glasses

Li and Na carboxylate salts glasses with different number chains containing molecules of Tetraphenyl-porphine-tetrasulfonic acid (TPPS) were prepared by the melting method. TPPS doped in the glasses had the same form as the TPPS in the aqueous solutions. However the form of TPPS in the glasses changed because TPPS reacted with matrix glass during the melting process. Tetraphenyl-porphine (TPP) could no be incorporated into the Li and Na carboxylate salts glasses by the present melting method. The free base TPPS is important for photochemical hole burning (PHB) properties, and a mixed metal–carboxylate salts glasses containing free-base TPPS, which is the active form for PHB, were prepared by controlling the melting condition. It was found that a preparation condition such as holding time of the melts affects the formation of the complex of the TPPS and that in the mixed metal–carboxylate salts melts the TPPS formed a complex with Li but did not form a complex with Na.

Photochemical Hole Burning of Sm2+ in Sodium Borate Glasses Induced by Near-Infrared Femtosecond-Laser Irradiation.

We report on photochemical hole burning of Sm2+ in sodium borate glasses irradiated with femtosecond-laser pulses of 800nm wavelength. The irradiation of Sm3+-doped sodium borate glasses with femtosecond-laser pulses causes electron-trapped Sm3+, i.e., Sm2+ and point defects such as an oxygen hole center and an electron trapped near Na+. The measurements of spectral hole burning were carried out for the 5D0-7F0 transition of Sm2+ using a DCM dye laser. Photoionization hole burning is observed at 77K to room temperature for the glasses exposed to femtosecond-laser pulses, and also for the glasses prepared by melting under reducing conditions. The hole-burning efficiency of Sm2+ produced by the photoinduced process is about twice higher than that by the thermal process. It is thought that the metastable Sm2+ ions and oxygen hole centers induced by femtosecond-laser irradiation are responsible for the observed phenomenon. We propose that the efficient photoionization hole burning is caused by the photostimulated recombination of the electron trapped by Sm3+ with the oxygen hole center.