Facile Synthesis of a Furan–Arylamine Hole‐Transporting Material for High‐Efficiency,Mesoscopic Perovskite Solar Cells

A novel hole‐transporting molecule (F101) based on a furan core has been synthesized by means of a short, high‐yielding route. When used as the hole‐transporting material (HTM) in mesoporous methylammonium lead halide perovskite solar cells (PSCs) it produced better device performance than the current state‐of‐the‐art HTM 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). The F101‐HTM‐based device exhibited both slightly higher J_sc (19.63 vs. 18.41 mA cm^?2) and V_oc (1.1 vs. 1.05 V) resulting in a marginally higher power conversion efficiency (PCE) (13.1 vs. 13 %). The steady‐state and time‐resolved photoluminescence show that F101 has significant charge extraction ability. The simple molecular structure, short synthesis route with high yield and better performance in devices makes F101 an excellent candidate for replacing the expensive spiro‐OMeTAD as HTM in PSCs.     

Ultrafast heat transfer on nanoscale in thin gold films

Heat transfer processes, induced by ultrashort laser pulses in thin gold films, were studied with a time resolution of 50 fs. It is demonstrated that in thin gold films heat is transmitted by means of electron–phonon and phonon–phonon interactions, and dissipated on nanoscale within 800 fs. Measurements show that the electron–phonon relaxation time varies versus the probe wavelength from 1.6 to 0.8 ps for λ=560–630 nm. The applied mathematical model is a result of transforming the two-temperature model to the hyperbolic heat equation, based on assumptions that the electron gas is heated up instantaneously and applying Cattaneo’s law to the phonon subsystem, agrees well with the experimental results. This model allows us to define time of electron–phonon scattering as the ratio of the heat penetration depth to the speed of sound in the bulk material that, in turn, provides an explanation of experimental results that show the dependence of the electron–phonon relaxation time on the wavelength.     

BASE RELEASE FROM DNA AND POLYNUCLEOTIDES UPON 193 nm LASER EXCITATION

Release of bases form calf thymus DNA and three polynucleotides, induced by 20 ns excitation at 193 nm in aqueous solution at pH 7, was detected by HPLC. The quantum yields of formation of free bases (phi B) from double-stranded DNA (0.4 mM) are independent of intensity, indicating a one-quantum mechanism of N-glycosidic bond cleavage. The phi B values increase in the order guanine, thymine, adenine, cytosine, the latter being phi C approximately 7 x 10(-4) for double-stranded DNA under Ar and O2. The larger phi B values in N2O-saturated solution, e.g., phi C = 1.2 x 10(-3), are ascribed to additional base release via OH-adduct radicals. The phi B values of homopolynucleotides increase in the order poly(G), poly(A) and poly(C), e.g. phi C = 7 x 10(-3) under Ar, as do the efficiencies for base release per radical cation (eta B). A comparison of the eta B values with the efficiencies of single-strand breakage for poly(C), poly(A) and DNA shows a similar trend; both are markedly larger for pyrimidines than for purines. Pathways to undamaged bases, initiated from base radical cations, are proposed.     

Depopulation of Highly Excited Singlet States of DNA Model Compounds: Quantum Yields of 193 and 245 nm Photoproducts of Pyrimidine Monomers and Dinucleoside Monophosphates

Abstract— Formation of uracil and orotic acid photodimers, uridine and 5'-UMP photohydrates, TpT photodimers and (6-4)photoproducts, dCpT photohydrates and (6-4)photo-products and UpU, CpC and CpU photohydrates were studied in neutral deoxygenated aqueous solution at room temperature upon irradiation at either 193 or 254 nm. The photoproducts were identified and quantified and the contribution from photoionization to substrate decomposition, using λ_irr= 193 nm, was separated. The ratio of the quantum yields of respective stable products,η=φ¹⁹³/φ²⁵⁴ is indicative of the yield of internal conversion from the second to the first excited singlet state, S₂→ S₁. For the observed photodimers η decreases from 0.94 for uracil to 0.7 for TpT and further to 0.55 for orotic acid. For the (6-4)photoproducts of TpT and dCpT T| = 0.5-0.8 and for the photohydrates in the cases of UpU, CpC, CpU and dCpT TJ ranges from 0.55 to 1.     

Two-quantum photoprocesses in DNA under picosecond laser UV irradiation at 216 and 270 nm.

It is demonstrated that at high power picosecond laser irradiation of 216 and 270 nm, two-quantum photodestructions of the DNA secondary structure, such as interstrand covalent crosslinks, "weak" crosslinks, and B----C conformational transition, take place. Thermal effects do not contribute to the observed effects.     

Gravity lens critical test for gravity constants and dark sector

The recent study of the strong gravitational lens ESO 325-G004 (Collett et al., Science, 360:1342, 2018) leads to a new possibility for testing General Relativity and its extensions. Such gravity lens observational studies can be instrumental for establishing a limitation on the precision of testing General Relativity in the weak-field regime and on the two gravity constants (the Newtonian and cosmological ones) as described in Gurzadyan and Stepanian (Eur Phys J C 78:632 2018). Namely, we predict a critical value for the involved weak-field parameter \(\gamma _{cr}=0.998\) (for \(M=1.5\,\,10^{11}\, M_{\odot }\) lens mass and \(r=2\, kpc\) light impact distance), which remarkably does not depend on any hypothetical variable but is determined only by well measured quantities. If the critical parameter \(\gamma _{cr}\) will be established at future observations, this will mark the first discrepancy with General Relativity of the conventional weak-field Newtonian limit, directly linked to the nature of the dark sector of the Universe.     

Chromophore/DNA interactions: femto- to nanosecond spectroscopy, NMR structure, and electron transfer theory

The mechanism of photoinduced hole injection into DNA has been studied using an integrated approach that combines NMR structural analysis, time-resolved spectroscopy, and quantum-chemical calculations. A covalently linked acridinium derivative, the protonated 9-amino-6-chloro-2-methoxyacridine (X+), is replacing a thymine and separated from either guanine (G) or the easier to oxidize 7-deazaguanine (Z) by one adenine.thymine (A.T) base pair. The key features of this donor/acceptor system are the following: (i) In more than 95% of the duplexes, X+ is located in a central, coplanar position between the neighboring A.T base pairs with its long axis in parallel showing minimal twist and tilt angles (<15 degrees). The complementary adenine base is turned out into the extrahelical space. In a minority of less than 5%, X+ is found to be still attached to the duplex. X+ is most probably associated with one of the phosphates, since it is neither intercalated between more remote base pairs nor bound to sugars or grooves. This minority characterized by an excited state lifetime >10 ns gives rise to a small background signal in time-resolved measurements and contributes predominantly to steady-state fluorescence spectra. (ii) Although the intercalation mode of X+ is well defined, the NMR structure reveals that there are two conformations of X+ with respect to the arrangement of its methoxy substituent. In one conformation, the methoxy group is in the plane of the chromophore, while, in the other extraplanar conformation, the methoxy group forms an angle of 70 degrees with the acridinium ring. The fluorescence decay of 5'-ZAX and 5'-GAX tracts can be fitted to a biexponential function with similar amplitudes, reflecting the oxidation dynamics of G and Z, with the slower rate being determined by larger thermal activation energy. The attribution of biexponential electron transfer (ET) dynamics to the bimodal orientation of the methoxy group at the acridinium is supported by quantum-chemical calculations. These predict a larger free energy change for hole transfer in the nonplanar conformation as compared to the planar one, whereas the difference in the electronic couplings is negligible. (iii) Kinetic studies of the directionality of the 1(X+)* induced hole injection reveal similarly fast decay components in both directions of the duplex, that is, in 5'-ZAX and 5'-XAZ, with the amplitude of the fast component being significantly reduced in 5'-XAZ. The NMR structure shows that local structural deviations from B-DNA are much more pronounced in the 3'-5' direction than in the 5'-3' direction. According to quantum-chemical calculations, the directionality of charge injection is not a universal feature of the DNA duplex but depends critically on the rotation angle of the aromatic plane of the acridinium within the pi stack. The arrangement of X+ in 5'-ZAX and 5'-XAZ corresponds to a conformation with weak directionality of the electronic couplings. The increased disorder in the 3'-5'direction favors slow hole transfer components at the expense of the fast ones. (iv) A comparison of the hole transfer in 5'-GAX and 5'-ZAG shows that classical Marcus theory can explain the ratio of the charge shift rates of more than 2 orders of magnitude on the basis of a free energy difference between G and Z of 0.3 eV. Both NMR structures and quantum-chemical calculations justify the appreciable neglect of differences of electronic couplings as well as in the reorganization energy in 5'-GAX and 5'-ZAG. Despite the attractive concept for the behavior of floppy DNA oligonucleotides, in this acridinium/DNA system, there is no evidence for conformational gating, that is, for fluctuations in the electronic couplings that permit the ET to occur.