Role of conformational isomerism in solvent-mediated charge transfer in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM): condensed-phase to jet-cooled spectroscopic studies

Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro-3-isoquinoline methanol (THIQM) has been investigated in the condensed phase and in jet-cooled conditions by means of laser-induced fluorescence, dispersed emission, resonance-enhanced two-photon ionization, and IR-UV double resonance experiments, as well as quantum chemical calculations. In the condensed phase, THIQM only shows local emission in nonpolar and protic solvents and dual emission in aprotic polar solvents, where the solvent-polarity dependent Stokes shifted emission is ascribed to a state involving charge transfer from the nitrogen lone pair to the benzene π-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions. In the most stable conformer, THIQM(I), the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position, while the second most stable conformer, THIQM(II), corresponds to the opposite NH···O hydrogen bond, with the nitrogen lone pair in the axial position. The two low-energy jet-cooled conformers of THIQM evidenced from the laser-induced fluorescence and dispersed emission spectra only show structured local emission. Complexes with usual solvents reproduce the condensed phase properties. The jet-cooled complex with aprotic polar solvent acetonitrile shows both local emission and charge transfer emission as observed in solution. The jet-cooled hydrate mainly shows local emission due to the unavailability of the nitrogen lone pair through intermolecular hydrogen bonding.     

ZIBgridfree: efficient conformational analysis by partition-of-unity coupling

Obtaining a sufficient sampling of conformational space is a common problem in molecular simulation. We present the implementation of an umbrella-like adaptive sampling approach based on function-based meshless discretization of conformational space that is compatible with state of the art molecular dynamics code and that integrates an eigenvector-based clustering approach for conformational analysis and the computation of inter-conformational transition rates. The approach is applied to three example systems, namely $n$ -pentane, alanine dipeptide, and a small synthetic host-guest system, the latter two including explicitly modeled solvent.     

New insights into the geometry of resorc[4]arenes: solvent-mediated supramolecular conformational and chiroptical control

The conformations of inherently chiral resorc[4]arenes were studied by circular dichroism (CD) spectroscopy. Whereas in aprotic solvents the crown conformation (C4) is preferred, protic solvents favor the boat conformation (C2). As a result of electronic coupling of the lowest L(b) state of the resorcinol unit in the resorc[4]arene, the CD spectra show a strong dependence on the conformation of the macrocycle. For the first time the solvent dependence of the CD spectra was qualitatively analyzed and simulated by using theoretical methods. We have thus demonstrated not only that the conformation of the calixarene is dramatically manipulated by the solvent but also that the joint use of chiroptical measurements and theoretical calculations is a powerful and versatile tool for elucidating structural variations in supramolecular chemistry.     

Hole transfer in DNA: DNA as a scaffold for hole transfer between two organic molecules

Hole transfer process in ODNs conjugated with two organic molecules, pyrene (Py) and phenothiazine (Ptz) was investigated with the pulse radiolysis measurements. Monitoring the transient absorption of Py⁺ and Ptz⁺, it was shown that the hole transfer rate was dependent on the distance and sequence between Py and Ptz.     

Synthesis and conformational analysis of fructose-derived scaffolds: molecular diversity from a single molecule

Bi- and tricyclic compounds were synthesized starting from fructose. The different hydroxyl groups present in fructose were exploited in the formation of a number of conformationally constrained sugar-based scaffolds, including azido acids. Introduction of an azido group and carboxy terminus into different bicyclic iodo ethers, allowed the synthesis of different conformationally constrained azido acids. Conformational analysis of compounds 10, 11, 17, and 20 by NMR experiments assisted by molecular mechanics, allowed the determination of the distances between the relevant functional groups, that is the azido and carboxy functionalities.     

Chiral and achiral subsets of the conformational manifold of a molecule

Institute of Heteroorganic Compounds, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 2, pp. 197–199, March–April, 1989.     

'Objective' determination of coupling constants and conformational equilibria by solvent variation: a re-evaluation

An 'objective' method for determining conformational equilibria in substituted ethanes, proposed by Abraham et al., has been evaluated by computational methods. Abraham's method involves measuring vicinal couplings, such as (3)J(H,H) and (3)J(H,F), between methine and methylene protons with methine, methylene protons and fluorine in a range of solvents, on the assumption that the underlying coupling constants of the individual conformers are constant, but the fractions of each conformer in each solvent are different and unknown. Abraham posited that this would produce an 'over-determined' data set with more equations than unknowns would. Abraham's procedure is re-evaluated, and it is demonstrated that the type of system being considered here, where there are more equations than unknowns, is not necessarily over-determined. A computer equation solver and Monte Carlo-type procedures were employed to demonstrate that multiple numerical solutions exist for a representative 'over-determined' data set provided by Abraham. A statistical method was also developed to determine precisely which parameter sets constitute plausible solutions.     

'Objective' determination of coupling constants and conformational equilibria by solvent variation: a comment

Following the preceding paper, the 'objective' method of Abraham et al. for determining the conformational equilibrium between molecules rapidly interconverting between a number of different conformers has been re-evaluated. The method relies on the acquisition of a sufficient data set of nuclear couplings in different solvents to obtain an over-determined system with more equations than unknowns. The example considered by Deutsch and Roberts, of 1,2-dichloro-1-fluoroethane, is re-evaluated here. A simple but mathematically rigorous computer scanning process is used to analyse the data set instead of the iterative but not mathematically rigorous process used previously and the results compared with those of Deutsch and Roberts. It is shown that both treatments give similar solutions. However, it is emphasized that in these multi-dimensional analyses the probable errors in the derived parameters (in this case the conformer couplings) may be much larger than the r.m.s. (observed versus calculated) errors and this error analysis is essential for a considered appraisal of the method.     

Dynamic nature of the intramolecular electronic coupling mediated by a solvent molecule: a computational study

We present a combined Molecular Dynamics/Quantum Chemical study of the solvent-mediated electronic coupling between an electron donor and acceptor in a C-clamp molecule. We characterize the coupling fluctuations due to the solvent motion for different solvents (acetonitrile, benzene, 1,3-diisopropyl-benzene) for the charge separation and the charge recombination processes. The time scale for solvent-induced coupling fluctuation is approximately 0.1 ps. The effect of these fluctuations on the observed rate is discussed using a recently developed theoretical model. We show that, while the microscopic charge transfer process is very complicated and its computational modeling very subtle, the macroscopic phenomenology can be captured by the standard models. Analyzing the contribution to the coupling given by different solvent orbitals, we find that many solvent orbitals mediate the electron transfer and that paths through different solvent orbitals can interfere constructively or destructively. A relatively small subset of substrate-solvent configurations dominate contributions to solvent-mediated coupling. This subset of configurations is related to the electronic structure of the C-clamp molecule.     

Mediation of resonance energy transfer by a third molecule

The influence of a third molecule on the rate of resonance energy transfer is studied using diagrammatic perturbation theory within the framework of molecular quantum electrodynamics. Two distinct mechanisms are identified. One corresponds to direct transfer between donor and acceptor while the other involves relay of energy by the third species. Fermi Golden rule transition rates valid for all separation distances beyond wave function overlap are evaluated for these two processes as well as for the interference term between direct and indirect exchange, thereby extending previous work which was limited to the near-zone only. Short- and long-range limits are also obtained in each case. It is found that in the near-zone the indirect rate contribution exhibits inverse sixth power dependence on relative distances of emitter and absorber relative to the third body, in contrast to its far-zone counterpart, which exhibits inverse square behavior. The interference term, however, displays inverse cubic dependence on all three distance vectors at short-range and inverse behavior in the far-zone. Interestingly, for a collinear arrangement of the three molecules in the near-zone, the interference term is negative, reducing the overall rate of energy transfer. The results obtained are interpreted in terms of microscopic and macroscopic pictures of transfer occurring within a surrounding medium.     

Ultrafast energy transfer dynamics of a bioinspired dyad molecule

A caroteno-purpurin dyad molecule was studied by steady-state and pump-probe spectroscopies to resolve the excited-state deactivation dynamics of the different energy levels as well as the connecting energy flow pathways and corresponding rate constants. The data were analyzed with a two-step multi-parameter global fitting procedure that makes use of an evolutionary algorithm. We found that following ultrafast excitation of the donor (carotenoid) chromophore to its S2 state, the energy flows via two channels: energy transfer (70%) and internal conversion (30%) with time constants of 54 and 110 fs, respectively. Additionally, some of the initial excitation is found to populate the hot ground state, revealing another limitation to the functional efficiency. At later times, a back transfer occurs from the purpurin to the carotenoid triplet state in nanosecond timescales. Details of the energy flow within the dyad as well as species associated spectra are disentangled for all excited-state and ground-state species for the first time. We also observe oscillations with the most pronounced peak on the Fourier transform spectrum having a frequency of 530 cm(-1). The dyad mimics the dynamics of the natural light-harvesting complex LH2 from Rhodopseudomonas acidophila and is hence a good model system to be used in studies aimed to further explain previous work in which the branching ratio between the competing pathways of energy loss and energy transfer could be manipulated by adaptive femtosecond pulse shaping.     

Resonance energy transfer from a dye molecule to graphene

We study the distance dependence of the rate of resonance energy transfer from the excited state of a dye to the pi system of graphene. Using the tight-binding model for the pi system and the Dirac cone approximation, we obtain the analytic expression for the rate of energy transfer from an electronically excited dye to graphene. While in traditional fluorescence resonance energy transfer, the rate has a (distance)(-6) dependence, we find that the distance dependence in this case is quite different. Our calculation of rate in the case of the two dyes, pyrene and nile blue, shows that the distance dependence is Yukawa type. We have also studied the effect of doping on energy transfer to graphene. Doping does not modify the rate for electronic excitation energy transfer significantly. However, in the case of vibrational transfer, the rate is found to be increased by an order of magnitude due to doping. This can be attributed to the nonzero density of states at the Fermi level that results from doping.     

Density functional theory-based conformational analysis of a phospholipid molecule (dimyristoyl phosphatidylcholine)

The conformational space of the dimyristoyl phosphatidylcholine (DMPC) molecule has been studied using density functional theory (DFT), augmented with a damped empirical dispersion energy term (DFT-D). Fourteen ground-state isomers have been found with total energies within less than 1 kcal/mol. Despite differences in combinations of their torsion angles, all these conformers share a common geometric profile, which includes a balance of attractive, repulsive, and constraint forces between and within specific groups of atoms. The definition of this profile fits with most of the structural characteristics deduced from measured NMR properties of DMPC solutions. The calculated vibrational spectrum of the molecule is in good agreement with experimental data obtained for DMPC bilayers. These results support the idea that DMPC molecules preserve their individual molecular structures in the various assemblies.     

Photonic switching of photoinduced electron transfer in a dithienylethene-porphyrin-fullerene triad molecule

A dithienylethene (DTE)-porphyrin (P)-fullerene (C(60)) triad molecule in which intramolecular photoinduced electron transfer is controlled by the photochromic DTE moiety has been prepared. Irradiation of the molecule with visible light gives the open form of the dithienylethene (DTEo). Excitation of the porphyrin gives DTEo-(1)P-C(60), which undergoes photoinduced electron transfer with a time constant of 25 ps to generate DTEo-P(.+)-C(60)(.-). Irradiation with ultraviolet light produces the closed form of the dithienylethene (DTEc). Excitation of DTEc-P-C(60) yields DTEc-(1)P-C(60), whose porphyrin first excited singlet state is quenched in 2.3 ps by singlet-singlet energy transfer to DTEc, generating (1)DTEc-P-C(60) and precluding significant photoinduced electron transfer. Such highly reversible photonically controlled intramolecular photoinduced electron transfer may eventually be useful in the design of photonic or optoelectronic devices.     

Electrochromism based on the charge transfer process in a ferrocene-BODIPY molecule

A new type of donor-acceptor molecule DiFc-B combined ferrocene and BODIPY unit has been synthesized, and the NMR spectra, photophysical property, and electrochemical property were studied. The in situ spectro-electrochemical experiment was carried out, a notable absorption change had been observed under oxidative potential, and a recovery could happen under reductive potential and this process could repeat for several times. Such a phenomenon also happened when oxidative metal ion was added into the solution of DiFc-B. In order to study the mechanism of the electrochromism, the theoretical calculation was performed, and the result shows a D (ferrocene)-π-A (BODIPY) system. The energy gap between the HOMO and LUMO had a good consistence with the electrochemical experiment and UV-vis data. The calculation of the frontier orbital belonging to the cation state of DiFc-B shows the directional change of the D-π-A system, which supplied a theoretical basis of the electrochromism.