Helge Lemmetyinen - Academia.edu (original) (raw)
Papers by Helge Lemmetyinen
Free-base porphyrins with two thioacetate terminated linkers in different positions on the porphy... more Free-base porphyrins with two thioacetate terminated linkers in different positions on the porphyrin molecule were synthesized and attached to tetraoctylammonium bromide (TOABr) stabilized gold nanoparticles with a core diameter of 5nm. The different positions of the linkers affect the stability of the functionalized gold nanoparticles and have an effect on the packing density of the molecules on the nanoparticle surface.
European Journal of Organic Chemistry, 2011
Novel bispinacolborane-N,NЈ-bis(octyl)-3,4,9,10-perylenetetracarboxylic acid bisimide (PBI) was s... more Novel bispinacolborane-N,NЈ-bis(octyl)-3,4,9,10-perylenetetracarboxylic acid bisimide (PBI) was synthesized in high yield. The title compound is a key intermediate in the preparation of PBI aryl derivatives through Suzuki cross-coupling by using a large variety of easily accessible aryl halides. The benign effect of a nonpolar solvent, mild base, and high reac-
Journal of controlled release : official journal of the Controlled Release Society, Jan 5, 2011
A large number of different polymers have been developed and studied for application as DNA carri... more A large number of different polymers have been developed and studied for application as DNA carriers for non-viral gene delivery, but the DNA binding properties are not understood. This study describes the efficiency of nanoparticle formation by time-resolved fluorescence measurements for poly(β-amino esters), cationic biodegradable polymers with DNA complexation and transfection capability. From the large library of poly(β-amino esters) ten polymers with different transfection efficacies were chosen for this study. The binding constants for nanoparticle formation were determined and compared to with the same method. Although the DNA binding efficiency of the amine groups are similar for both types of polymers, the overall binding constants are an order of magnitude smaller for poly(β-amino esters) than for 25 kDa polyethylenimines, yet poly(β-amino esters) show comparable DNA transfection efficacy with polyethylenimines. Within this series of polymers the transfection efficacy show...
The Langmuir-Blodgett films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (O... more The Langmuir-Blodgett films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (ODA) inactive matrix were found to maintain an ordered head-to-tail (Z-type) structure up to at least 20 molecular layers. UV-Vis polarized absorption and polarized second-harmonic generation indicate that TSe is oriented in the films with the molecular axis aligned about the film normal. The degree of orientation increases for thick films suggesting that the ordering of the first few layers is compromised by substrate roughness. Improved orientation is obtained also by depositing pure ODA layers before the active layers.
Journal of Luminescence, 1997
ABSTRACT The interlayer excitation energy transfer between 12-(1-pyrenyl)dodecanoic acid (PDA), 1... more ABSTRACT The interlayer excitation energy transfer between 12-(1-pyrenyl)dodecanoic acid (PDA), 12-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-y1)-N-methylamino] dodecanoic acid (NBDDA) in LB films has been examined with picosecond time-resolved fluorescence decay measurements. Four fluorescing species are present in PDA LB films: monomer (M), dimer (D) and two types of excimers (E1 and E2). The fluorescence of PDA monomers and excimers E2 is quenched by energy transfer to NBDDA. The critical transfer distances of 40.5 and 40.3 Å for monomers and excimers respectively were calculated.
The Journal of Physical Chemistry B, 2008
We provide a detailed investigation of the second-order nonlinear optical and structural properti... more We provide a detailed investigation of the second-order nonlinear optical and structural properties of Langmuir-Blodgett (LB) films of nonracemic thiohelicenebisquinone (THBQ). We prepare both X-and Y-type films of different thicknesses and characterize them using optical second-harmonic generation and atomic-force microscopy (AFM). We find that the overall nonlinear properties of the samples are essentially independent of the film thickness and the deposition type and arise from susceptibility tensor components associated with chirality. Both X-and Y-type films can be described by D 2 symmetry, which is a higher symmetry than the previously assumed C 2 of LB films of THBQ and a similar helicenebisquinone (HBQ). However, the two types of films are shown to differ significantly with respect to the orientation of the in-plane axis. For Y type, the axis follows the direction of vertical sample deposition, but for X type, the direction of the axis varies randomly and significantly between different samples. The Y-type samples are therefore more ordered than the X-type samples. This was confirmed by AFM measurements in which the Y type exhibits uniform ordering into columnar structures. Similar structures in X type, on the other hand, are shorter and more randomly oriented, like those earlier observed for racemic samples of HBQ [Verbiest, T., et al. Science 1998, 282, 913]. The common nonlinear properties and different high-level ordering observed here for two different types of nonracemic samples reinforces that the nonlinearity of THBQ (and probably HBQ, as well) originates from the low-level columnar aggregation of the molecules with the higher-level structures playing a lesser role. In addition, within the columns, the molecules likely assume fairly random azimuthal orientations so that the columns themselves exhibit approximate D ∞ symmetry.
Langmuir, 2010
The organization of pi-pi stacking perylenediimide (PDI) derivative, PDI12, was studied in soluti... more The organization of pi-pi stacking perylenediimide (PDI) derivative, PDI12, was studied in solution and in thin films. Films were prepared with the Langmuir-Schaeffer (LS) method and characterized by means of AFM, optical profilometry, steady-state absorption, emission, fluorescence lifetime, and transient photovoltage measurements. The columnar aggregates observed previously in PDI12 solutions and in spin-coated films persist also in LS films. Because of the specific conditions during the preparation of the LS film, i.e., hydrophobic interactions and lateral compression, the columnar aggregates seem to organize with their long axis perpendicular to the layer plane whereas in spin-coated films the columns were oriented parallel to the layer plane. According to AFM and profilometer results, the thickness of LS monolayer of PDI12 is 10 nm, indicating that it consists mainly of aggregates, each containing approximately 30 monomers. Intermolecular photoinduced energy and electron transfer processes in C(60)|PDI12 double layer junction were studied. The fluorescence lifetime of PDI12 film is exceptionally long, but the quenching is very efficient in the presence of C(60). In charge transfer studies, long-lived photovoltage signal was observed for the double layer. Results of this work indicate that PDI12 acts as an electron acceptor and fullerene C(60) as an electron donor.
Journal of Applied Polymer Science, 2001
The dynamic, isothermal, and model-free isoconversional modes of differential scanning calorimetr... more The dynamic, isothermal, and model-free isoconversional modes of differential scanning calorimetry measurement were used to monitor the curing process of the polyurethane system. Conversions obtained from these three methods were in good agreement with one another, indicating that isothermal and model-free isoconversional modes can successfully be used for monitoring the curing process. For the isothermal mode, the highest reaction rate occurred at time zero, and autoacceleration was not observed for this system. From the model-free isoconversional mode, it was possible to calculate the activation energy changes during the curing process.
The Journal of Chemical Physics, 2004
We present a new technique, based on regression analysis, to determine the second-order nonlinear... more We present a new technique, based on regression analysis, to determine the second-order nonlinear optical susceptibility tensor of thin films. The technique does not require the absolute levels or phases of measured signals to be mutually calibrated. In addition it yields indicators that address the quality of theoretical models describing the sample. We use the technique to determine the susceptibility tensor of samples of a nonracemic chiral material which have very low symmetry ͑both chiral and anisotropic͒ and have many independent tensor components. The results show the importance of using detailed theoretical models that account for the linear optical properties of the sample.
Macromolecules, 2000
In this study the curing degree and the shelf life of an epoxy resin were monitored by a fluoresc... more In this study the curing degree and the shelf life of an epoxy resin were monitored by a fluorescence method. Four extrinsic fluorescence probes, so-called TICT compounds, were used. An intensity ratio method was applied, in which ratios of the lowest and highest ...
Journal of Physical Chemistry B, 2006
Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a te... more Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.
Applied Physics A-materials Science & Processing, 2003
Femtosecond time-resolved photoluminescence experiments have been used to study the nonlinear dyn... more Femtosecond time-resolved photoluminescence experiments have been used to study the nonlinear dynamics of novel monolithic GaInNAs/GaAs semiconductor saturable absorber mirrors at 1.08 and 1.55 µm. The mirror structures were grown using molecular-beam epitaxy followed by Ni-ion implantation and thermal annealing. We present photoluminescence measurements showing the critical role of post-growth processing on the response time of GaInNAs/GaAs absorber mirrors.
Macromolecules, 2000
... a) Dousa, P.; Konak, C.; Fidler, V.; Dusek, K. Polym. Bull.1989, 22, 585. ... Daniel Antrim,P... more ... a) Dousa, P.; Konak, C.; Fidler, V.; Dusek, K. Polym. Bull.1989, 22, 585. ... Daniel Antrim,Patrick Bunton, Lydia Lee Lewis, Brian D. Zoltowski, and John A. Pojman. The Journal of Physical Chemistry B 2005 109 (23), 11842-11849. ...
Journal of Physical Chemistry A, 2003
ABSTRACT
Journal of Physical Chemistry B, 2000
... 4) Gregory, P.; Patten, V.; Shreve, AP; Lindsey, JS; Donohoe, RJ J. Phys. Chem. B 1998, 102, ... more ... 4) Gregory, P.; Patten, V.; Shreve, AP; Lindsey, JS; Donohoe, RJ J. Phys. Chem. B 1998, 102, 4209. [ACS Full Text.
Thin Solid Films, 1992
ABSTRACT Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) ... more ABSTRACT Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) matrix were prepared on quartz plates and the photocycle and spectral properties of the films were studied. The transient absorption spectrum of BR LB film was measured by the nanosecond laser flash photolysis method in a wavelength range from 380 to 650 nm. The absorption maxima were at 540 and 415 nm for the L and M states respectively. In water the rate of decay of the L state was 2.3 × 104 s−1 and the rates of formation and decay of the M state were 2.3 × 104 s−1 and 3.0 × 102 s−1 respectively. The photocycle reaction was not observed in nitrogen-dried BR LB films, but when the films were put back in water, even after being stored for 8 months in dry conditions, the phenomenon could be observed. The energy transfer from trytophan to BR retinal takes place in both dry and wet LB films. The films were relatively stable to changing physical conditions as well as to laser excitation and no changes were observed following repeated photocycles.Molecular surface potentials of BR-soya-PC and soya-PC Langmuir films as a function of mean molecular area decreased towards zero with increasing surface pressure.
Journal of The American Chemical Society, 2002
The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their... more The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.
Polymer, 2000
The curing processes of three types of aliphatic polyurethanes (PUs) were monitored through measu... more The curing processes of three types of aliphatic polyurethanes (PUs) were monitored through measuring changes in the IR absorption of the isocyanate group. Two exponential fittings were used to present the decrease of the absorption band of the isocyanate group as a function of the curing time. The curing process contained chemically and diffusionally controlled periods. According to a cross-linking
Journal of Polymer Science Part A-polymer Chemistry, 2009
ABSTRACT Cationic photopolymerization is a convenient in situ polymerization method for organic t... more ABSTRACT Cationic photopolymerization is a convenient in situ polymerization method for organic thin film preparation. In this work, the polymerization mechanisms is applied for highly viscous cross-linking monomers, using tetra-alkylepoxyporphyrin (TAEP) as a case study. By comparing the UV-Vis spectra of the polymerized sample before and after the unreacted monomers have been dissolved, it is possible to estimate the polymerization yield. An IR spectrum of a reference thick film confirms full polymerization. Scanning fluorescence lifetime microscopy and AFM show the uniformity of the polymer. It was shown that photopolymerization is highly dependent on the substrate nature and requires at best case a 10 min illumination at 90 °C. Thermal polymerization of the same sample requires 10 min heating at 150 °C in dark. It was also shown that TAEP works as a self-sensitizer for cationic photopolymerization. The proposed method is a mild and versatile technique for in situ preparation of thin polymeric films directly from chromophore monomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6095–6103, 2009
The Journal of Physical Chemistry, 1989
... (13) Blackwell, M. F.; Gounaris, K.; Zara, S. J.; Barber, J. Biophys. J . 1986, 51, 735. (14)... more ... (13) Blackwell, M. F.; Gounaris, K.; Zara, S. J.; Barber, J. Biophys. J . 1986, 51, 735. (14) Thuren, T.; Vainio, P.; Virtanen, J. A.; Somerharju, P.; Blomqvist, K.; Kinnunen, PKJ Biochemistry 1984, 23, 5129. (15) Thuren, T.; Virtanen, JA; Lalla, M.; Kinnunen, PKJ Clin. Chem. ...
Free-base porphyrins with two thioacetate terminated linkers in different positions on the porphy... more Free-base porphyrins with two thioacetate terminated linkers in different positions on the porphyrin molecule were synthesized and attached to tetraoctylammonium bromide (TOABr) stabilized gold nanoparticles with a core diameter of 5nm. The different positions of the linkers affect the stability of the functionalized gold nanoparticles and have an effect on the packing density of the molecules on the nanoparticle surface.
European Journal of Organic Chemistry, 2011
Novel bispinacolborane-N,NЈ-bis(octyl)-3,4,9,10-perylenetetracarboxylic acid bisimide (PBI) was s... more Novel bispinacolborane-N,NЈ-bis(octyl)-3,4,9,10-perylenetetracarboxylic acid bisimide (PBI) was synthesized in high yield. The title compound is a key intermediate in the preparation of PBI aryl derivatives through Suzuki cross-coupling by using a large variety of easily accessible aryl halides. The benign effect of a nonpolar solvent, mild base, and high reac-
Journal of controlled release : official journal of the Controlled Release Society, Jan 5, 2011
A large number of different polymers have been developed and studied for application as DNA carri... more A large number of different polymers have been developed and studied for application as DNA carriers for non-viral gene delivery, but the DNA binding properties are not understood. This study describes the efficiency of nanoparticle formation by time-resolved fluorescence measurements for poly(β-amino esters), cationic biodegradable polymers with DNA complexation and transfection capability. From the large library of poly(β-amino esters) ten polymers with different transfection efficacies were chosen for this study. The binding constants for nanoparticle formation were determined and compared to with the same method. Although the DNA binding efficiency of the amine groups are similar for both types of polymers, the overall binding constants are an order of magnitude smaller for poly(β-amino esters) than for 25 kDa polyethylenimines, yet poly(β-amino esters) show comparable DNA transfection efficacy with polyethylenimines. Within this series of polymers the transfection efficacy show...
The Langmuir-Blodgett films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (O... more The Langmuir-Blodgett films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (ODA) inactive matrix were found to maintain an ordered head-to-tail (Z-type) structure up to at least 20 molecular layers. UV-Vis polarized absorption and polarized second-harmonic generation indicate that TSe is oriented in the films with the molecular axis aligned about the film normal. The degree of orientation increases for thick films suggesting that the ordering of the first few layers is compromised by substrate roughness. Improved orientation is obtained also by depositing pure ODA layers before the active layers.
Journal of Luminescence, 1997
ABSTRACT The interlayer excitation energy transfer between 12-(1-pyrenyl)dodecanoic acid (PDA), 1... more ABSTRACT The interlayer excitation energy transfer between 12-(1-pyrenyl)dodecanoic acid (PDA), 12-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-y1)-N-methylamino] dodecanoic acid (NBDDA) in LB films has been examined with picosecond time-resolved fluorescence decay measurements. Four fluorescing species are present in PDA LB films: monomer (M), dimer (D) and two types of excimers (E1 and E2). The fluorescence of PDA monomers and excimers E2 is quenched by energy transfer to NBDDA. The critical transfer distances of 40.5 and 40.3 Å for monomers and excimers respectively were calculated.
The Journal of Physical Chemistry B, 2008
We provide a detailed investigation of the second-order nonlinear optical and structural properti... more We provide a detailed investigation of the second-order nonlinear optical and structural properties of Langmuir-Blodgett (LB) films of nonracemic thiohelicenebisquinone (THBQ). We prepare both X-and Y-type films of different thicknesses and characterize them using optical second-harmonic generation and atomic-force microscopy (AFM). We find that the overall nonlinear properties of the samples are essentially independent of the film thickness and the deposition type and arise from susceptibility tensor components associated with chirality. Both X-and Y-type films can be described by D 2 symmetry, which is a higher symmetry than the previously assumed C 2 of LB films of THBQ and a similar helicenebisquinone (HBQ). However, the two types of films are shown to differ significantly with respect to the orientation of the in-plane axis. For Y type, the axis follows the direction of vertical sample deposition, but for X type, the direction of the axis varies randomly and significantly between different samples. The Y-type samples are therefore more ordered than the X-type samples. This was confirmed by AFM measurements in which the Y type exhibits uniform ordering into columnar structures. Similar structures in X type, on the other hand, are shorter and more randomly oriented, like those earlier observed for racemic samples of HBQ [Verbiest, T., et al. Science 1998, 282, 913]. The common nonlinear properties and different high-level ordering observed here for two different types of nonracemic samples reinforces that the nonlinearity of THBQ (and probably HBQ, as well) originates from the low-level columnar aggregation of the molecules with the higher-level structures playing a lesser role. In addition, within the columns, the molecules likely assume fairly random azimuthal orientations so that the columns themselves exhibit approximate D ∞ symmetry.
Langmuir, 2010
The organization of pi-pi stacking perylenediimide (PDI) derivative, PDI12, was studied in soluti... more The organization of pi-pi stacking perylenediimide (PDI) derivative, PDI12, was studied in solution and in thin films. Films were prepared with the Langmuir-Schaeffer (LS) method and characterized by means of AFM, optical profilometry, steady-state absorption, emission, fluorescence lifetime, and transient photovoltage measurements. The columnar aggregates observed previously in PDI12 solutions and in spin-coated films persist also in LS films. Because of the specific conditions during the preparation of the LS film, i.e., hydrophobic interactions and lateral compression, the columnar aggregates seem to organize with their long axis perpendicular to the layer plane whereas in spin-coated films the columns were oriented parallel to the layer plane. According to AFM and profilometer results, the thickness of LS monolayer of PDI12 is 10 nm, indicating that it consists mainly of aggregates, each containing approximately 30 monomers. Intermolecular photoinduced energy and electron transfer processes in C(60)|PDI12 double layer junction were studied. The fluorescence lifetime of PDI12 film is exceptionally long, but the quenching is very efficient in the presence of C(60). In charge transfer studies, long-lived photovoltage signal was observed for the double layer. Results of this work indicate that PDI12 acts as an electron acceptor and fullerene C(60) as an electron donor.
Journal of Applied Polymer Science, 2001
The dynamic, isothermal, and model-free isoconversional modes of differential scanning calorimetr... more The dynamic, isothermal, and model-free isoconversional modes of differential scanning calorimetry measurement were used to monitor the curing process of the polyurethane system. Conversions obtained from these three methods were in good agreement with one another, indicating that isothermal and model-free isoconversional modes can successfully be used for monitoring the curing process. For the isothermal mode, the highest reaction rate occurred at time zero, and autoacceleration was not observed for this system. From the model-free isoconversional mode, it was possible to calculate the activation energy changes during the curing process.
The Journal of Chemical Physics, 2004
We present a new technique, based on regression analysis, to determine the second-order nonlinear... more We present a new technique, based on regression analysis, to determine the second-order nonlinear optical susceptibility tensor of thin films. The technique does not require the absolute levels or phases of measured signals to be mutually calibrated. In addition it yields indicators that address the quality of theoretical models describing the sample. We use the technique to determine the susceptibility tensor of samples of a nonracemic chiral material which have very low symmetry ͑both chiral and anisotropic͒ and have many independent tensor components. The results show the importance of using detailed theoretical models that account for the linear optical properties of the sample.
Macromolecules, 2000
In this study the curing degree and the shelf life of an epoxy resin were monitored by a fluoresc... more In this study the curing degree and the shelf life of an epoxy resin were monitored by a fluorescence method. Four extrinsic fluorescence probes, so-called TICT compounds, were used. An intensity ratio method was applied, in which ratios of the lowest and highest ...
Journal of Physical Chemistry B, 2006
Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a te... more Three porphyrin-fullerene dyads, in which a diyne bridge links C(60) with a beta-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which pi-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H(2)TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C(60) dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C(60) triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP(*+)/C(60)(*-) charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP(*+)/C(60)(*-) in PhCN was approximately 40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k(CS)) to rates for charge recombination (k(CR)), k(CS)/k(CR), is quite small, approximately 7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C(60) to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k(CS)/k(CR) approximately equal to 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k(CR) but not on k(CS). On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the beta-linked dyads.
Applied Physics A-materials Science & Processing, 2003
Femtosecond time-resolved photoluminescence experiments have been used to study the nonlinear dyn... more Femtosecond time-resolved photoluminescence experiments have been used to study the nonlinear dynamics of novel monolithic GaInNAs/GaAs semiconductor saturable absorber mirrors at 1.08 and 1.55 µm. The mirror structures were grown using molecular-beam epitaxy followed by Ni-ion implantation and thermal annealing. We present photoluminescence measurements showing the critical role of post-growth processing on the response time of GaInNAs/GaAs absorber mirrors.
Macromolecules, 2000
... a) Dousa, P.; Konak, C.; Fidler, V.; Dusek, K. Polym. Bull.1989, 22, 585. ... Daniel Antrim,P... more ... a) Dousa, P.; Konak, C.; Fidler, V.; Dusek, K. Polym. Bull.1989, 22, 585. ... Daniel Antrim,Patrick Bunton, Lydia Lee Lewis, Brian D. Zoltowski, and John A. Pojman. The Journal of Physical Chemistry B 2005 109 (23), 11842-11849. ...
Journal of Physical Chemistry A, 2003
ABSTRACT
Journal of Physical Chemistry B, 2000
... 4) Gregory, P.; Patten, V.; Shreve, AP; Lindsey, JS; Donohoe, RJ J. Phys. Chem. B 1998, 102, ... more ... 4) Gregory, P.; Patten, V.; Shreve, AP; Lindsey, JS; Donohoe, RJ J. Phys. Chem. B 1998, 102, 4209. [ACS Full Text.
Thin Solid Films, 1992
ABSTRACT Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) ... more ABSTRACT Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) matrix were prepared on quartz plates and the photocycle and spectral properties of the films were studied. The transient absorption spectrum of BR LB film was measured by the nanosecond laser flash photolysis method in a wavelength range from 380 to 650 nm. The absorption maxima were at 540 and 415 nm for the L and M states respectively. In water the rate of decay of the L state was 2.3 × 104 s−1 and the rates of formation and decay of the M state were 2.3 × 104 s−1 and 3.0 × 102 s−1 respectively. The photocycle reaction was not observed in nitrogen-dried BR LB films, but when the films were put back in water, even after being stored for 8 months in dry conditions, the phenomenon could be observed. The energy transfer from trytophan to BR retinal takes place in both dry and wet LB films. The films were relatively stable to changing physical conditions as well as to laser excitation and no changes were observed following repeated photocycles.Molecular surface potentials of BR-soya-PC and soya-PC Langmuir films as a function of mean molecular area decreased towards zero with increasing surface pressure.
Journal of The American Chemical Society, 2002
The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their... more The photoinduced electron transfer in differently linked zinc porphyrin-fullerene dyads and their free-base porphyrin analogues was studied in polar and nonpolar solvents with femto- to nanosecond absorption and emission spectroscopies. A new intermediate state, different from the locally excited (LE) chromophores and the complete charge-separated (CCS) state, was observed. It was identified as an exciplex. The exciplex preceded the CCS state in polar benzonitrile and the excited singlet state of fullerene in nonpolar toluene. The behavior of the dyads was modeled by using a common kinetic scheme involving equilibria between the exciplex and LE chromophores. The scheme is suitable for all the studied porphyrin-fullerene compounds. The rates of reaction steps depended on the type of linkage between the moieties. The scheme and Marcus theory were applied to calculate electronic couplings for sequential reactions, and consistent results were obtained.
Polymer, 2000
The curing processes of three types of aliphatic polyurethanes (PUs) were monitored through measu... more The curing processes of three types of aliphatic polyurethanes (PUs) were monitored through measuring changes in the IR absorption of the isocyanate group. Two exponential fittings were used to present the decrease of the absorption band of the isocyanate group as a function of the curing time. The curing process contained chemically and diffusionally controlled periods. According to a cross-linking
Journal of Polymer Science Part A-polymer Chemistry, 2009
ABSTRACT Cationic photopolymerization is a convenient in situ polymerization method for organic t... more ABSTRACT Cationic photopolymerization is a convenient in situ polymerization method for organic thin film preparation. In this work, the polymerization mechanisms is applied for highly viscous cross-linking monomers, using tetra-alkylepoxyporphyrin (TAEP) as a case study. By comparing the UV-Vis spectra of the polymerized sample before and after the unreacted monomers have been dissolved, it is possible to estimate the polymerization yield. An IR spectrum of a reference thick film confirms full polymerization. Scanning fluorescence lifetime microscopy and AFM show the uniformity of the polymer. It was shown that photopolymerization is highly dependent on the substrate nature and requires at best case a 10 min illumination at 90 °C. Thermal polymerization of the same sample requires 10 min heating at 150 °C in dark. It was also shown that TAEP works as a self-sensitizer for cationic photopolymerization. The proposed method is a mild and versatile technique for in situ preparation of thin polymeric films directly from chromophore monomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6095–6103, 2009
The Journal of Physical Chemistry, 1989
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