Excited Donor Research Articles

Lifting the Spectral Crosstalk in Multifluorophore Assemblies

A general strategy for measuring the energy-transfer efficiencies in multifluorophore assemblies is demonstrated. The present method is based on spectral shaping of the excitation light with molecular solutions representing donor and acceptor fluorophores, which causes a suppressed excitation of the respective donor and acceptor molecules in the sample. The changes in the acceptor emission resulting from spectral shaping of the excitation light are then used to determine the energy-transfer efficiencies (Ex) associated with all participating donor–acceptor pairs. Here, the technique is demonstrated through energy-transfer (ET) measurements in a 4-fluorophore construct featuring a DNA supported assembly of three donor/donor-relay (Cy3, Cy3.5, and Cy5) and one acceptor (Cy5.5) molecules. The resulting Ex were validated using the standard photoluminescence (PL) quenching approach as well as measurements of partial 2- and 3-dye assemblies. The present work highlights general benefits of the spectrally shaped ...

Mediating Reductive Charge Shift Reactions in Electron Transport Chains.

We report the synthesis of a full-fledged family of covalent electron donor-acceptor1-acceptor2 conjugates and their charge-transfer characterization by means of advanced photophysical assays. By virtue of variable excited state energies and electron donor strengths, either Zn(II)Porphyrins or Zn(II)Phthalocyanines were linked to different electron-transport chains featuring pairs of electron accepting fullerenes, that is, C60 and C70. In this way, a fine-tuned redox gradient is established to power a unidirectional, long-range charge transport from the excited-state electron donor via a transient C60•- toward C70•-. This strategy helps minimize energy losses in the reductive, short-range charge shift from C60 to C70. At the forefront of our investigations are excited-state dynamics deduced from femtosecond transient absorption spectroscopic measurements and subsequent computational deconvolution of the transient absorption spectra. These provide evidence for cascades of short-range charge-transfer processes, including reductive charge shift reactions between the two electron-accepting fullerenes, and for kinetics that are influenced by the nature and length of the respective spacer. Of key importance is the postulate of a mediating state in the charge-shift reaction at weak electronic couplings. Our results point to an intimate relationship between triplet-triplet energy transfer and charge transfer.

Effect of alkali/mixed alkali metal ions on the thermal and spectral characteristics of Dy3 +:B2O3-PbO-Al2O3-ZnO glasses

Thermal and spectroscopic features of 50 B2O3–10 PbO–10 Al2O3–10 ZnO–(x) Li2O–(y) Na2O–(z) K2O–1.0 Dy2O3 (mol %) (x=19, y=0, and z=0; x=0, y=19, and z=0; x=0, y=0, and z=19; x=9.5, y=9.5, and z=0; x=9.5, y=0, and z=9.5; x=0, y=9.5, and z=9.5) glasses, that were fabricated by utilizing melt-quenching approach, are investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), optical absorption, photoluminescence excitation (PLE), photoluminescence (PL), and PL decay lifetimes. PL spectra for all the Dy3+-doped samples show emission bands at 453nm (blue), 482nm (blue), 573nm (yellow), 662nm (red), and 752nm (red) corresponding to the 4I15/2→6H15/2, 4F9/2→6H15/2, 4F9/2→6H13/2, 4F9/2→6H11/2, and 4F9/2→6H9/2 transitions, respectively, upon excitation at 350nm. Here, Dy3+: Li–Na glass shows the highest PL intensity for all identified emissions. The yellow-to-blue (Y/B) emission intensity ratio (varied within the range 1.257–1.376), CIE chromaticity coordinates (x,y) (slight variation between (0.3410, 0.3802) and (0.3495, 0.3872), and correlated color temperatures (CCTs) (changed from 4953K to 5212K) are calculated following the PL spectra. Dy3+: 4F9/2 decay curves show non-exponential behavior and are fitted by the Inokuti-Hirayama (I–H) model, where S=6 shows best fit, indicating dipole-dipole (d-d) interactions for Dy3+ excited (donor) and ground state (acceptor) ions.

Plasmon-activated intermolecular nonradiative energy transfer in spherical nanoreactors

The effect of silver nanoparticles upon intermolecular radiationless energy transfer between the dye molecules confined in the nanopores of silica was studied both experimentally and by theoretical modeling. Silver nanoparticles were synthesized directly inside the nanopores of silica from AgNO3 using aqueous solution of NaBH4 as a reducing agent. Silica samples with or without nanoparticles were soaked with ethanol solutions of dyes, Acridine Orange (AO) as a donor of electronic excitation energy, and Nile Blue as the acceptor. Studies of fluorescence spectra of samples at different acceptor concentration have shown that addition of nanoparticles decreases the relative quantum yield of fluorescence of donor accompanied by the increase of that of acceptor. This was interpreted as a plasmonic contribution to the rate of nonradiative energy transfer. A theoretical model was proposed, which describes the quenching kinetics of the excited donor molecules in the presence of accepting molecules and metal nanoparticles in a polarized spherical nanopore, as well as the changes in spectral bands of the system. Besides, simulation of the interaction of a point dipole source field with plasmonic silver nanoparticles in a silica cavity was carried out using finite-difference time-domain (FDTD) method. The results of field calculation with the two models are in a good qualitative agreement between each other and predict the regions of local electric field enhancement in the gap between silver nanoparticles and walls of pores that is supposed to account for the experimentally observed increased efficiency of the nonradiative intermolecular singlet–singlet energy transfer in porous silica matrix with silver nanoparticles. The results obtained can be of interest for FRET SNOM.

A minimal double quantum dot

Double quantum dots (DQDs) are a versatile platform for solid-state physics, quantum computation and nanotechnology. The micro-fabrication techniques commonly used to fabricate DQDs are difficult to extend to the atomic scale. Using an alternative approach, which relies on scanning tunneling microscopy and spectroscopy, we prepared a minimal DQD in a wide band-gap semiconductor matrix. It is comprised of a pair of strongly coupled donor atoms that can each be doubly charged. The donor excitation diagram of this system mimicks the charge stability diagram observed in transport measurements of DQDs. We furthermore illustrate how the charge and spin degrees of freedom of the minimal DQD may be used to obtain a single quantum bit and to prepare a Bell state. The results open an intriguing perspective for quantum electronics with atomic-scale structures.

Manipulating the Energetics and Rates of Electron Transfer in Rhodobacter capsulatus Reaction Centers with Asymmetric Pigment Content

Seemingly redundant parallel pathways for electron transfer (ET), composed of identical sets of cofactors, are a cornerstone feature of photosynthetic reaction centers (RCs) involved in light-energy conversion. In native bacterial RCs, both A and B branches house one bacteriochlorophyll (BChl) and one bacteriopheophytin (BPh), but the A branch is used exclusively. Described herein are the results obtained for two Rhodobacter capsulatus RCs with an unnaturally high degree of cofactor asymmetry, two BPh on the RC's B side and two BChl on the A side. These pigment changes derive, respectively, from the His(M180)Leu mutation [a BPh (ΦB) replaces the B-side BChl (BB)], and the Leu(M212)His mutation [a BChl (βA) replaces the A-side BPh (HA)]. Additionally, Tyr(M208)Phe was employed to disfavor ET to the A branch; in one mutant, Val(M131)Glu creates a hydrogen bond to HB to enhance ET to HB. In both ΦB mutants, the decay kinetics of the excited primary ET donor (P*) resolve three populations with lifetimes of ∼9 ps (50-60%), ∼40 ps (10-20%), and ∼200 ps (20-30%), with P+ΦB- formed predominantly from the 9 ps fraction. The 50-60% yield of P+ΦB- is the highest yet observed for a ΦB-containing RC. The results provide insight into factors needed for efficient multistep ET.

Energy transfer mechanism and optoelectronic properties of (PFO/TiO2)/Fluorol 7GA nanocomposite thin films

Energy transfer between poly (9,9'-di-n-octylfluorenyl-2,7-diyl) (PFO) as a donor in presence of TiO2 nanoparticles (NPs) and Fluorol 7GA as an acceptor with different weight ratios has been investigated by steady-state emission measurements. Based on the absorption and fluorescence measurements, the energy transfer properties, such as quenching rate constant (kSV), energy transfer rate constant (kET), quantum yield (ϕDA), and lifetime (τDA), of the donor in the presence of the acceptor, energy transfer probability (PDA), energy transfer efficiency (η), energy transfer time (τET), and critical distance of the energy transfer (Ro) were calculated. Förster-type energy transfer between the excited donor and ground-state acceptor molecules was the dominant mechanism responsible for the energy transfer as evidenced by large values of kSV, kET, and Ro. Moreover, these composite materials were employed as an emissive layer in organic light-emitting diodes (OLEDs). Additionally, the optoelectronic properties of OLEDs were investigated in terms of current density–voltage characteristics and electroluminescence spectra.

Injection-induced terahertz electroluminescence from silicon p–n structures

Injection-induced terahertz electroluminescence from silicon p +–n structures is observed at helium temperatures. Structures fabricated by the diffusion of boron into a phosphorus-doped n-Si substrate are studied. Relatively narrow luminescence lines are observed in the luminescence spectra against a broad smooth background. The spectral position of a number of emission lines corresponds to optical transitions between excited donor states and the ground state of phosphorus donors. The intracenter optical transitions of electrons at phosphorus donors are excited as a result of recombination processes occurring in the n-type region of the structure under the injection of nonequilibrium holes. A number of other lines in the terahertz emission spectra are associated with intracenter transitions at acceptor centers, which are also excited as a result of injection. The structureless background in the electroluminescence spectra may be due to terahertz emission upon the intraband energy relaxation of “hot” carriers having an effective temperature exceeding the lattice temperature. These “hot” carriers appear in the structure under injection conditions.

Verification of Nonequilibrium Mechanism of Ultrafast Charge Recombination in Excited Donor–Acceptor Complexes

Control of charge transfer requires knowledge of its detailed mechanism. Due to the large number of known mechanisms, the identification of the mechanism in specific systems is a challenge so far. In this article we propose the idea of how to distinguish between thermal and nonequilibrium modes of charge recombination in excited donor-acceptor complexes. Simulations of the effect of solvent relaxation time scale on ultrafast charge recombination kinetics in photoexcited donor-acceptor complexes within the framework of the multichannel stochastic model have shown that a series of regularities inherent to the thermal and nonequilibrium charge transfer can strongly differ. Among them there are opposite regularities, for example, the dependence of the dynamic solvent effect on the free energy gap. In particular, theory predicts that in ultrafast charge recombination of excited donor-acceptor complexes the dynamic solvent effect is weak in the area of weak exergonicity and becomes stronger in the area of stronger exergonicity whereas for the thermal reactions an opposite trend is expected. Comparison of such trends with experimental data implemented in this article allowed establishing the regime in which the reaction proceeds. It is shown that observation of dynamic solvent effect in the region of strong exergonicity for ultrafast charge recombination is decisive evidence in favor of nonequilibrium mechanism.

Insights into Er3+↔Yb3+ energy transfer dynamics upon infrared ~1550 nm excitation in a low phonon fluoro-tellurite glass system

Highly upconverting, monolithic transparent inorganic glass co-doped with Er3+/Yb3+ ions have been explored in view of their easy integration with Si-PV cell. A ~4 fold enhancement in the photo-current of mc-Si-PV cell has been observed using a Er3+/Yb3+ co-doped sample compared to Er3+ ions singly doped glass. The role of Yb3+ ions on the enhancement of photo-current has been discussed in light of the Er3+↔Yb3+ energy transfer mechanism involved from IR to NIR and VIS upconversion process upon IR ~1550nm excitation. The influence of excitation pump power and donor Er3+ ion concentration on the energy transfer upconversion (ETU) as well as excited state absorption (ESA) energy transfer mechanisms and its effect on the upconversion emission properties have been described in detail. The prominence of ETU or ESA process were elaborated considering the decay dynamics of NIR upconversion emission upon ~1550nm excitation.

Butanedithiol Solvent Additive Extracting Fullerenes from Donor Phase To Improve Performance and Photostability in Polymer Solar Cells.

In this work, we demonstrated that the excited poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2,6-diyl)] (PTB7-Th) will be degraded by [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) or photolysis fragment of 1,8-diiodooctane (DIO) in the presence of oxygen and under irradiation of red light. From the previous reports, the fragment of DIO may be involved in the reaction directly. Our work indicates the PC71BM is not directly involved in the reaction, but is acting as a catalyst to promote the reaction of excited donors with oxygen. Thus, PTB7-Th urgently needs a kind of nonresidual iodine-free additive to replace DIO and remove the fullerene from the donor phase at the same time. Taking into consideration PC71BM solubility and boiling point difference between solvent additives and host solvents, 1,4-butanedithiol solvent was selected to fabricate PTB7-Th:PC71BM-based solar cells achieving a best power conversion efficiency (PCE) of 10.2% (8.5% for PTB7:PC71BM). Iodine-free butanedithiol can not only avoid excited polymer reacting with the photolysis fragment of DIO but also suppress the degradation of the excited PTB7-Th caused by synergistic effect between the fullerene and oxygen via extracting the free/trapped PC71BM from the donor phase. Eventually, the film prepared with 1,4-butanedithiol shows higher stability than the film prepared without any additives and much better than the film with DIO in macro-/micromorphology, light absorption, and device performance.

Estimating FRET Efficiency using Excitation-Scanning Hyperspectral Imaging

Forster Resonance Energy Transfer, or FRET, is an indispensable tool in the biological sciences for measuring intermolecular distances. Current tools that measure FRET suffer from limited signal strength, making it difficult to observe rapid cell-signaling events. We have developed a unique mode of hyperspectral imaging which filters the excitation wavelengths and collects total fluorescence emission. Our prior work has shown that excitation-scanning is an effective tool for detecting fluorophores in cell samples. The purpose of this study was to assess the feasibility of using this excitation-based technique for FRET measurements. In traditional spectral imaging FRET studies, the emission spectra of donor and acceptor are resolved from the emission spectrum of the sample using linear unmixing However, since excitation scanning collects the entire sample emission without filtering wavelengths, a new approach is needed to utilize our method. Here we present data indicating that excitation scanning can be successfully adapted for FRET studies. Fluorescence excitation-scanning hyperspectral images were acquired by sequentially exciting samples with an 18-20 nm bandwidth (FWHM) tunable excitation source, provided by a 300W Xe arc lamp coupled to a Sutter VF-5 tunable filter. Images were acquired from 380-490nm in 5nm increments. Donor and acceptor excitation signatures were then separated from the total signal using a spectral library. Two long-pass dichroic filters were employed, at 458nm and 495nm. FRET efficiencies were estimated by comparing donor excitation signal between scans with different dichroic filters. Results indicate that excitation-scanning techniques provided accurate estimates of FRET efficiency, and that this method could be a useful tool in FRET-based cell signaling studies. This work was supported by NIH grants P01HL066299, S10RR027535, and the Abraham Mitchell Cancer Research Fund.

The Long Road to Micro-Dynamic Presynaptic FRET Measurements

I will review the history and lead-up to our attempt to measure changes in FRET in assembled SNARE complexes in a very small space and a very short time, where only a tiny fraction of each of the labeled SNARE proteins is part of a SNARE complex. After recounting the saga of grant support and technical development, I will summarize our main findings: Using donor dequenching by receptor bleach as well as FLIM, we find a resting FRET indicating a number of assembled SNARE complexes exceeding the expected fraction of synaptobrevins (or VAMPs) that are in assembled SNAREs of docked and primed vesicles, likely reflecting the existence of still-assembled “orphan SNAREs” left over from prior bouts of secretion. We can detect the dispersion of all three SNARE proteins – VAMP, SNAP-25, and syntaxin – as well as the FRETting complex of assembled SNAREs itself, from the center of the presynaptic active zone to the periphery during and after a train of action potentials. Using sensitized acceptor emission on donor excitation, we also detect the disassembly of N-terminally labeled SNAREs prior to endocytosis of vesicle membrane and proteins, and the assembly of new SNAREs as replacement vesicles dock and prime, and we occasionally see a transient FRET increase while vesicles fuse during the train. By donor quenching, we also detect the rearrangement of C-terminally labeled SNAREs when vesicles fuse with the plasma membrane, and their subsequent dispersion, disassembly, and re-assembly. A variety of control experiments and statistical tests rule out most sources of artefact and alternative interpretations. I will also speculate on why SNARES disperse and are disassembled in the periphery, rather than in the central active zone where exocytosis occurs, as we had originally expected.

電荷移動錯体の選択励起による光不斉合成

The use of photoreactions such as photoredox catalysts in asymmetric synthesis has been expansively recognized as expedient procedure over the past decade. In this paper, a potential application of photoreaction of Charge-Transfer (CT) complex for asymmetric synthesis is described. In chiral donor-acceptor systems, excitation wavelength, along with other environmental factors such as solvent polarity and temperature, plays more substantial role in determining the stereoselectivity of the photochemical outcomes. Accordingly, the excited CT complex produced upon the selective CT-band excitation behaves completely different from the conventional exciplex, which is formed by the direct excitation of donor or acceptor. Such unique wavelength effect, together with other entropic controls, may possibly come to be valuable means in asymmetric photochemical synthesis for the future.

Resonance Energy Transfer in Upconversion Nanoplatforms for Selective Biodetection.

Resonance energy transfer (RET) describes the process that energy is transferred from an excited donor to an acceptor molecule, leading to a reduction in the fluorescence emission intensity of the donor and an increase in that of the acceptor. By this technique, measurements with the good sensitivity can be made about distance within 1 to 10 nm under physiological conditions. For this reason, the RET technique has been widely used in polymer science, biochemistry, and structural biology. Recently, a number of RET systems incorporated with nanoparticles, such as quantum dots, gold nanoparticles, and upconversion nanoparticles, have been developed. These nanocrystals retain their optical superiority and can act as either a donor or a quencher, thereby enhancing the performance of RET systems and providing more opportunities in excitation wavelength selection. Notably, lanthanide-doped upconversion nanophosphors (UCNPs) have attracted considerable attention due to their inherent advantages of large anti-Stoke shifts, long luminescence lifetimes, and absence of autofluorescence under low energy near-infrared (NIR) light excitation. These nanoparticles are promising for the biodetection of various types of analytes. Undoubtedly, the developments of those applications usually rely on resonance energy transfer, which could be regarded as a flexible technology to mediate energy transfer from upconversion phosphor to acceptor for the design of luminescent functional nanoplatforms. Currently, researchers have developed many RET-based upconversion nanosystems (RET-UCNP) that respond to specific changes in the biological environments. Specifically, small organic molecules, biological molecules, metal-organic complexes, or inorganic nanoparticles were carefully selected and bound to the surface of upconversion nanoparticles for the preparation of RET-UCNP nanosystems. Benefiting from the advantage and versatility offered by this technology, the research of RET-based upconversion nanomaterials should have significant implications for advanced biomedical applications. It should be noted that energy transfer in a UCNP based nanosystem is most often related to resonance energy transfer but that reabsorption (and maybe other energy transfer processes) may also play an important role and that more studies regarding the fundamental aspects for energy transfer with UCNPs is necessary. In this Account, we present an overview of recent advances in RET-based upconversion nanocomposites for biodetection with a particular focus on our own work. We have designed a series of upconversion nanoplatforms with remarkably high versatility for different applications. The experience gained from our strategic design and experimental investigations will allow for the construction of next-generation luminescent nanoplatform with marked improvements in their performance. The key aspects of this Account include fundamental principles, design and preparation strategies, biodetection in vitro and in vivo, future opportunities, and challenges of RET-UCNP nanosystems.

Impurity-assisted terahertz luminescence in quantum well nanostructures under interband photoexсitation

The paper presents the results of an experimental study of impurity-assisted photoluminescence in the far- (terahertz) and near-infrared spectral ranges in n-GaAs/AlGaAs quantum well structures with different well widths under interband photoexcitation of electron–hole pairs. The optical electron transitions between the first electron subband and donor ground state as well as between excited and ground donor states were revealed in the far-infrared photoluminescence spectra. Observation of these optical electron transitions became possible because of the depopulation of the donor ground state in the quantum well due to the non-equilibrium charge carrier radiative transitions from the donor ground state to the first heavy hole subband. The opportunity to tune the terahertz radiation wavelength in structures with doped quantum wells by changing the quantum well width was demonstrated experimentally.

Electron Transfer in Bacterial Reaction Centers with the Photoactive Bacteriopheophytin Replaced by a Bacteriochlorophyll through Coordinating Ligand Substitution

The influence of amino acid substitutions at position M214 (M-subunit, residue 214) on the rate and pathway of electron transfer involving the bacteriopheophytin cofactor, HA, in a bacterial photosynthetic reaction center has been explored in a series of Rhodobacter sphaeroides mutants. The M214 leucine (L) residue of the wild type was replaced with histidine (H), glutamine (Q), and asparagine (N), creating the mutants M214LH, M214LQ, and M214LN, respectively. As has been reported previously for M214LH, each of these mutations resulted in a bacteriochlorophyll molecule in place of a bacteriopheophytin in the HA pocket, forming so-called β-type mutants (in which the HA cofactor is called βA). In addition, these mutations changed the properties of the surrounding protein environment in terms of charge distribution and the amino acid side chain volume. Electron transfer reactions from the excited primary donor P to the acceptor QA were characterized using ultrafast transient absorption spectroscopic techniques. Similar to that of the previously characterized M214LH (β mutant), the strong energetic mixing of the P(+)BA(-) and P(+)βA(-) states (the mixed anion is denoted I(-)) increased the rate of charge recombination between P(+) and I(-) in competition with the I(-) → QA forward reaction. This reduced the overall yield of charge separation forming the P(+)QA(-) state. While the kinetics of the primary electron transfer forming P(+)I(-) were essentially identical in all three β mutants, the rates of the βA(-) (I(-)) → QA electron transfer in M214LQ and M214LH were very similar but quite different from that of the M214LN mutant. The observed yield changes and the differences in kinetics are correlated more closely with the volume of the mutated amino acid than with their charge characteristics. These results are consistent with those of previous studies of a series of M214 mutants with different sizes of amino acid side chains that did not alter the HA cofactor composition [Pan, J., et al. (2013) J. Phys. Chem. B 117, 7179-7189]. Both studies indicate that protein relaxation in this region of the reaction center plays a key role in stabilizing charge-separated states involving the HA or βA cofactor. The effect is particularly pronounced for reactions occurring on time scales of tens and hundreds of picoseconds (forward transfer to the QA and charge recombination).

Identifying the Sources and Sinks of CDOM/FDOM across the Mauritanian Shelf and Their Potential Role in the Decomposition of Superoxide (O2-)

Superoxide (O2-) is a short lived reactive oxygen species (ROS) formed in seawater by photochemical or biological sources, it is important in the redox cycling of trace elements and organic matter in the ocean. The photoproduction of O2- is now thought to involve reactions between O2 and reactive reducing (radical) intermediates formed from dissolved organic matter (DOM) via intramolecular reactions between excited singlet state donors and ground-state acceptors (Zhang et al., 2012). In seawater the main pathways identified for the decomposition of O2- into H2O2 and O2, involve reactions with Cu, Mn and DOM. In productive regions of the ocean, the reaction between DOM and O2- can be a significant sink for O2-. Thus DOM is a key component of both the formation and decomposition of O2- and formation of H2O2. In the present work we examined the relationships between O2- decay rates and parameters associated with chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) by using the thermal O2- source SOTS-1. Filtered samples (0.2 µm) were run both in the presence, and absence, of the metal chelator diethylenetriaminepentaacetic acid (DTPA) to determine the contribution from DOM. Samples were collected along a transect across the continental shelf of the Mauritanian continental shelf during a period of upwelling. In this region we found that reactions with DOM, are a significant sink for O2- in the Mauritanian Upwelling, constituting on average 58 ± 13 % of the O2- loss rates. Superoxide reactivity with organic matter showed no clear correlation with bulk CDOM or FDOM properties (as assessed by PARAFAC analysis) suggesting that future work should concentrate at the functional group level to clearly elucidate which molecular species are involved as bulk properties represent a wide spread of chemical moieties with different O2- reactivities. Analysis of FDOM parameters indicates that many of the markers used previously for terrestrial sources of DOM and FDOM are called into question as marine sources exist. In particular recent work (Rico et al., 2013) indicates that algal species may also produce syringic, vanillic and cinnamic acids, which had previously been ascribed solely to terrestrial vegetation.

Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells.

In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.

Theoretical Investigation of the Optical Spectra of Organic Compounds in Natural Surrounding

The hybrid multiscale approximation based on molecular dynamics, quantum mechanics, and statistical theory is used to generate profiles of electronic vibrational absorption and fluorescence bands of some organic compounds and biological objects whose photophysical properties specifically depend on external conditions. A temperature dependence of the spectrum width and intensity of transition to the long-wavelength band of benzene surrounded by cyclohexane molecules is demonstrated. Statistical broadband absorption spectra for estradiol in ethanol, hexane, and dimethyl sulfoxide have been obtained and analyzed at room temperature together with a wide spectrum of transitions to numerous excited states of Trp-cage miniprotein. The absorption and emission spectra of 9-cyan anthracene have been generated under various thermodynamic conditions. This allows changes in the spectral profile with increasing temperatures and pressure to be detected. A dependence of the tryptophan spectra on the protein microsurrounding is investigated. The possibility of charge transfer from tryptophan residue to the eupatorin molecule trapped by human serum albumin is analyzed. Spectral properties and charge transfer from the excited donor to acceptor states are calculated using the polarizable embedding approach for modeling of surrounding protein structure.