Photoinduced charge transfer by one and two-photon absorptions: physical mechanisms and applications

A novel multifunctional fluorescent capillary-based sensor for simultaneous monitoring of pH, O2 and CO2

A fluorescent capillary sensor for detecting the bioanalytically and biologically relevant analytes pH, O2, and CO2 has been designed. The sensor is based on single capillary tube with 2 mm inner diameter, which is simultaneous doped with emissive O2-sensitive indicator Ru(dpp)3(PF6)2 (RuDP), pH-sensitive and CO2-sensitive indicator 8-hydroxypyrene-1, 3,6-trisulfonic acid trisodium salt (HPTS). The multifunctional sensor can be excited at the same wavelength and realize multi-parameter optical monitoring at the different sensing position. Studies in phosphate-buffered solutions display the excellent feasibility of the capillary sensor for fluorescence detection of pH, O2, and CO2. Overall, the multifunctional capillary sensor exhibits great potential in real-time blood gas analysis, and clinical application of multi-parameter biological detection.

Charge Transfer and Level Lifetime in Molecular Photon-Absorption upon the Quantum Impedance Lorentz Oscillator

On the strength of the new quantum impedance Lorentz oscillator (QILO) model, a charge-transfer method in molecular photon-absorption is proposed and imaged via the numerical simulations of 1- and 2-photon-absorption (1PA and 2PA) behaviors of the organic compounds LB3 and M4 in this paper. According to the frequencies at the peaks and the full width at half-maximums (FWHMs) of the linear absorptive spectra of the two compounds, we first calculate the effective quantum numbers before and after the electronic transitions. Thus, we obtain the molecular average dipole moments, i.e., 1.8728 × 10^-29 C·m (5.6145 D) for LB3 and 1.9626 × 10^-29 C·m (5.8838 D) for M4 in the ground state in the tetrahydrofuran (THF) solvent. Then, the molecular 2PA cross sections corresponding to wavelength are theoretically inferred and figured out by QILO. As a result, the theoretical cross sections turn out to be in good agreement with the experimental ones. Our results reveal such a charge-transfer image in 1PA near wavelength 425 nm, where an atomic electron of LB3 jumps from the ground-state ellipse orbit with the semimajor axis a_i = 1.2492 × 10^-10m = 1.2492 Å and semiminor axis b_i = 0.4363 Å to the excited-state circle (a_j = b_j = 2.5399 Å). In addition, during its 2PA process, the same transitional electron in the ground state is excited to the elliptic orbit with a_j = 2.5399 Å and b_j =1.3808 Å, in which the molecular dipole moment reaches as high as 3.4109 × 10^-29 C·m (10.2256 D). In addition, we obtain a level-lifetime formula with the microparticle collision idea of thermal motion, which indicates that the level lifetime is proportional (not inverse) to the damping coefficient or FWHM of an absorptive spectrum. The lifetimes of the two compounds at some excited states are calculated and presented. This formula may be used as an experimental method to verify 1PA and 2PA transition selection rules. The QILO model exhibits the advantage of simplifying the calculation complexity and reducing the high cost associated with the first principle in dealing with quantum properties of optoelectronic materials.

Super-Exchange Charge Transfer in One-Photon and Two-Photon Absorption of Multibranched Compounds

In this work, density functional theory is used to study organic molecules in a donor-acceptor (D-A) system centered on phenothiazine with strip and trigonal structures. The transition modes of the one-photon absorption (OPA) and two-photon absorption (TPA) processes of the two molecules are studied. The calculations show that the molar absorption coefficient of OPA for trigonal molecule TPPO and the cross section of TPA are both larger than those for strip molecule M1 due to the increase in the number of branches of the system. A special local excitation-enhanced charge-transfer excitation appears in strip-type molecule M1. In the charge-transfer process of trigonal D-A structure molecule TPPO, there are not only local excitation-enhanced charge-transfer excitation but also super-exchange charge transfer between the three branches that occurs due to the increase in the planarity of the system.

Light-Induced Charge Transfer from Transition-Metal-Doped Aluminum Clusters to Carbon Dioxide

Charge transfer between molecules and catalysts plays a critical role in determining the efficiency and yield of photochemical catalytic processes. In this paper, we study light-induced electron transfer between transition-metal-doped aluminum clusters and CO₂ molecules using first-principles time-dependent density-functional theory. Specifically, we carry out calculations for a range of dopants (Zr, Mn, Fe, Ru, Co, Ni, and Cu) and find that the resulting systems fall into two categories: Cu- and Fe-doped clusters exhibit no ground-state charge transfer, weak CO₂ adsorption, and light-induced electron transfer into the CO₂. In all other systems, we observe ground-state electron transfer into the CO₂ resulting in strong adsorption and predominantly light-induced electron back-transfer from the CO₂ into the cluster. These findings pave the way toward a rational design of atomically precise aluminum photocatalysts.

Physical Mechanism of Photoinduced Charge Transfer in One- and Two-Photon Absorption in D-D-π-A Systems

The photoinduced charge transfer process of a D-π-A molecule (W1) and three D-D-π-A molecules (WS5–WS7) with triphenylamine as a donor was studied theoretically. D-D-π-A molecules are formed by inserting donors between the triphenylamine and π-linker (π-bridge) on the base of the W1 molecule. The results showed that donor insertion resulted in a red shift in the absorption spectrum, and the absorption intensity increased to a certain extent. A visualization method was used to observe the charge transfer of the four molecules in the process of one- and two-photon absorption (TPA). The local excitation enhanced charge transfer excitation in the TPA process was analyzed and discussed, and the insertion of the thiazolo[5,4-d]thiazole donor showed the largest TPA cross-section. This work contributed to the profound understanding of D-D-π-A molecules and the design of large cross-section TPA molecules.

Optical physics on chiral brominated azapirones: Bromophilone A and B

In this work, we use the visualization method to study their intramolecular electric-magnetic interactions and reveal the physical mechanism of their electronic transition to explain the cause of the opposite ECD spectrum orientations. Azaphilone A and B are two chiral molecules, due to their differing chirality, the electronic circular dichroism (ECD) spectra of bromophilone A and B are very different at 431 nm. Based on the two-step transition process, the charge-transfer characteristics of the corresponding two-photon excited states of the two chiral molecules are analysed in detail by calculating the photoinduced charge transfer and electron-hole coherence in the two-photon absorption (TPA) process.

Physical mechanism of concentration-dependent fluorescence resonance energy transfer

We experimentally report fluorescence resonance energy transfer (FRET), using a novel visualization method of excitation-emission mapping. Firstly, both the absorption and fluorescent spectra of donor and acceptor are measured, respectively, under different molecular concentrations for verifying that these two molecules are suitable for exploring FRET. And then, the excitation-emission mappings of FRET are investigated to reveal the internal regular pattern of FRET. Our theoretical calculations strongly support experimental results of FRET. Our experimental results provided a new visualization method to clearly understand the mechanism of FRET.

Donor-length dependent photoninduced charge transfer in two-photon absorption

In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. It is found that donor length can significantly influence the rate of charge transfer. When the donor length is quarter-thiophene, the rate of charge transfer is less than 50%; while with the increase of donor-length to octant-thiophene, the rate of charge transfer can be reach up to 100%. The transition channels of photoinduced charge transfer in TPA are visualized with transition density matrix and charge difference density. Our results promote deeper understanding and rational design for donor-acceptor system with large rate of photoinduced charge transfer in TPA.

One- and two-photon absorption properties of quadrupolar A-π-D-π-A dyes with donors of varying strengths

In this work, the one- and two-photon absorption properties of two quadrupolar A-π-D-π-A dyes, featuring carbazole and triphenylamine as donors of varying strengths, are calculated using density functional theory. The characteristics of two-photon excitation are analysed by using the three-state model closest to the real physical process. By taking the intermediate state as a bridge, the charge transfer characteristics in the two-photon excitation process are studied in detail by using various visualisation methods. Comparing the intramolecular charge transfer (ICT) characteristics shows that increasing the ability of the push-pull electrons of the central substituents, thus increasing the intramolecular charge transfer, is a more efficient method to enhance the molecular σ_tp. Our study promotes a deeper understanding of the design of A-π-D-π-A molecules with large cross-sections in two-photon absorption.

Optical properties of S2 and S3 excited states of protonated schiff-base retinal chromophores in TPA, ECD and ROA

The electronic transitions of the protonated Schiff base of 1l-cis-retinal (PSB11) and protonated Schiff bases of all-trans retinal (PSBT) for the second or higher electronic excited states are hard to be observed experimentally, due to weak intensities of electronic state excitations. In this paper, we propose visualizations method to investigate these weak electronic state transitions of PSB11 and PSBT, using two-photon absorption (TPA), electronic circular dichroism (ECD) and Raman optical activity (ROA) spectra. Because of the resonance excitations of PSB11 and PSB11 in TPA, the transition intensity of the third electronic state is significantly enhanced, which are much larger than that of S₁ and S₂ electronic transitions. The charge transfer and electron-hole coherence of these electronic transitions in each step in TPA are visualized with charge difference density and transition density matrix. Also, the strong absorptions of S₁ and S₂ electronic excited states are observed with ECD spectra, and the physical mechanism of electric and magnetic interactions for these electronic transitions are revealed by visualization method. The large intensity of ROA at S₃ excited state results from transition electric and magnetic dipole interactions, not from transition electric dipole and transition electric quadrupole interactions. Our results provide a new visualization method to study the optical properties of biological system using TPA and ECD spectra.

Solvent viscosity induces twisted intramolecular charge transfer state lifetime tunable of Thioflavin-T

Excited-state deactivation dynamics of Thioflavin-T (ThT) in gradual viscosity solvents were investigated. Femtosecond transient absorption spectra and dynamic decay curves both present significant distinction of ThT in different volume ratios binary mixtures solvents. Dynamics fitting lifetime of twisted intramolecular charge transfer (TICT) state is strongly dependent on solvents viscosity. Compared to rotation corresponding time of ThT in low viscosity solvent (0.6 cp) experimentally coincident well with Stokes-Einstein-Debye (SED) equation, the relation between rotation corresponding time and relatively high viscosity (5.9 cp to 1091.2 cp) is more consistent with fractional SED equation. Combined with optimized geometric structures of ThT by density functional theory and time-dependent density functional theory, further understand TICT state lifetime increases with increasing solvents viscosity. Our work provides a comprehensive understanding of fluorescence molecular rotor (FMR) deactivation process in different viscosity solvents and is helpful to design new FMR.

Photoinduced charge transfer rate of Cy3/C60 blend material

The rates of charge separation and charge recombination of the cyanine dye/C₆₀ heterojunction solar cell in an external electric field were provided using the Marcus and Marcus-Levich-Jortner formalisms. The vibrational mode as another influencing factor was also introduced into the rate expression for the planar heterojunction solar cell. Detailed theoretical analysis of the excited-state of the Cy3/C₆₀ blend was achieved using density functional theory and time-dependent density functional theory. The Gibbs free energy was regulated by an external electric field, while the reorganisation energy presented the opposite conclusion. Frequency analysis was utilised to demonstrate the energy stability of the obtained structures. The rate calculated using the Marcus formalism was greater than that obtained by the Marcus-Levich-Jortner formalism. Consideration of the calculated rates in all vibration modes and at different external electric field strengths indicated that vibrational mode and external electric field played important roles in determining the rates of charge separate and charge recombination, which could provide a more accurate theoretical rate for organic photovoltaic devices.

Reevaluating the effects of reorganization energy on electron transfer rate for quantum dot-molecular acceptor complexes in different solvents

The electron transfer (ET) rate in quantum dot (QD)-molecular acceptor systems is dependent upon system reorganization energy (RE, λ), which comprises contributions from solvent (λ₀) and reactants (λ_i). However, to date, the effect of λ_i on ET rate has been largely ignored. Herein, the ET from CdSe/ZnS QDs to 1-chloroanthraquinone (1-CAQ) in different solvents was investigated using ultrafast transient absorption spectroscopy as a means to evaluate the effect of λ_i on ET rate. The results revealed that ET rate is strongly solvent dependent. Amazingly, the ET rate in carbon disulfide is 300-times higher than that in n-dodecane. Theoretical calculations indicated that the λ_i contribution from 1-CAQ alone accounts for a large proportion of system RE and varies greatly in different solvents. Furthermore, the ET rate increases first and, then, decreases with the λ value in different solvents. This trend was interpreted consistently in terms of Marcus theory by adding λ_i to λ for different solvents. Thus, our present work demonstrates that the RE of the acceptor molecule has a non-negligible effect on ET rate, providing new insight into the mechanisms of ET process and for the development of QD-based devices.

Photoinduced electron transfer in conjugated molecules containing different donor in an external electric field

Photoinduced electron transfers in different conjugated donor polymer containing with PC₆₀BM as the acceptor (X3:PC₆₀BM and X6:PC₆₀BM) were theoretically investigated. A detailed theoretical analysis of Marcus electron transfer rate in an external electric field was also performed. By discussing the effect of electric field on ΔG, λ, V_DA and charge separation rate, we have come to some conclusions. The results reveal that the free energy change and electronic coupling matrix are significantly influence by the vector properties of external electric field. Thus, the electron transfer process appears critically influenced by external electric fields. Our results will provide meaningful information on enhancing the photoelectric conversion efficiencies of BHJ solar cells.

The role played by ethanol in achieving the successive versus simultaneous mechanism of excited-state double proton transfer in dipyrido[2,3-a:3′,2′-i]carbazole

The excited-state double proton transfer (ESDPT) process of dipyrido[2,3-a:3′,2′-i]carbazole (DPC) in ethanol (EtOH) solvent is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT).