Charge Transfer State Lifetime Research Articles

Heavy-atom-free BODIPY dendrimer: utilizing the spin-vibronic coupling mechanism for two-photon photodynamic therapy in zebrafish.

In this study, the heavy-atom-free BODIPY dendrimer TM4-BDP was synthesized for near-infrared photodynamic therapy, and was composed of a triphenylamine-BODIPY dimer and four 1-(2-morpholinoethyl)-1H-indole-3-ethenyl groups. The TM4-BDP could achieve near-infrared photodynamic therapy through two different photosensitive pathways, which include one-photon excitation at 660 nm and two-photon excitation at 1000 nm. In the one-photon excitation pathway, the TM4-BDP could generate singlet oxygen and superoxide radicals under 660 nm illumination. In addition, the one-photon PDT experiment in human nasopharyngeal carcinoma (CNE-2) cells also indicated that the TM4-BDP could specifically accumulate in lysosomes and show great cell phototoxicity with an IC50 of 22.1 μM. In the two-photon excitation pathway, the two-photon absorption cross-section at 1030 nm of TM4-BDP was determined to be 383 GM, which means that it could generate reactive oxygen species (ROS) under 1000 nm femtosecond laser excitation. Moreover, the two-photon PDT experiment in zebrafish also indicated the TM4-BDP could be used for two-photon fluorescence imaging and two-photon induced ROS generation in biological environments. Furthermore, in terms of the ROS generation mechanism, the TM4-BDP employed a novel spin-vibronic coupling intersystem crossing (SV-ISC) process for the mechanism of ROS generation and the femtosecond transient absorption spectra indicated that this novel SV-ISC mechanism was closely related to its charge transfer state lifetime. These above experiments of TM4-BDP demonstrate that the dendrimer design is an effective strategy for constructing heavy-atom-free BODIPY photosensitizers in the near-infrared region and lay the foundation for two-photon photodynamic therapy in future clinical trials.

Long-Lived Charge-Transfer State in Spiro Compact Electron Donor-Acceptor Dyads Based on Pyromellitimide-Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization.

We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

Long‐Lived Charge‐Transfer State in Spiro Compact Electron Donor–Acceptor Dyads Based on Pyromellitimide‐Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization

AbstractWe observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

Correlating Charge-Transfer State Lifetimes with Material Energetics in Polymer:Non-Fullerene Acceptor Organic Solar Cells.

Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss (Eg/q - VOC) in polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs). In this work, transient absorption spectroscopy is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. The CT state lifetime is observed to show an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing/hybridization between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset ΔECT-S1 is reduced. This study highlights how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

Extending the Lifetimes of Charge Transfer States Generated by Photoinduced Heterolysis of Photochromic Radical Complexes

AbstractNovel fast photochromic phenothiazine‐imidazolyl radical complex (PTIC) derivatives were developed, which form the charge transfer (CT) states and the neutral open‐shell transient species in a stepwise manner upon light irradiation. The lifetimes of the CT states were extended from picoseconds to hundreds of picoseconds by the substitution of a triphenylamine group on the phenothiazine moiety. The CT forms have different optical properties and chemical reactivities from those of the neutral open‐shell transient species. Therefore, the developments of fast T‐type photochromic molecules that generate long‐lived CT forms are important for expanding potential applications of fast photoswitching materials.

Dynamics of Charge Transfer and Multiple Exciton Generation in the Doped Silicon Quantum Dot-Carbon Nanotube System: Density Functional Theory-Based Computation.

We use the Boltzmann transport equation (BE) to study time evolution of a photoexcited state, including phonon-mediated exciton relaxation, multiple exciton generation (MEG), and energy-transfer processes. BE collision integrals are derived using Kadanoff-Baym-Keldysh many-body perturbation theory (MBPT) based on density functional theory (DFT) simulations, including exciton effects. We apply the method to a nanostructured p- n junction composed of a 1 nm hydrogen-terminated Si quantum dot (QD) doped with two phosphorus atoms (Si36P2H42) adjacent to the (6, 2) single-wall carbon nanotube (CNT) with two chlorine atoms per two unit cells adsorbed to the surface. We find that an initial excitation localized on either the QD or CNT evolves into a transient charge-transfer (CT) state where either electron or hole transfer has taken place. The CT state lifetime is about 40 fs. Also, we study MEG in this system by computing internal quantum efficiency (QE), which is the number of excitons generated from an absorbed photon during relaxation. We predict efficient MEG starting at 3 Eg ≃ 1.5 eV and with QE reaching QE = 1.65 at about 5 Eg, where Eg ≃ 0.5 eV is the lowest exciton energy, i.e., the gap. However, we find that including energy transfer and MEG effects suppresses CT state generation.

Unusual photophysics of anticancer azapodophyllotoxin: The collective effect of discrete H-bond motif spills the beans

Abstract Azapodophyllotoxin (AZP) derivatives, the alternative scaffolds to podophyllotoxins, are intriguing class of small molecules with high anti-proliferative activity against several cancer cell lines. The physicochemical factors associated with or responsible for, anticancer activity which change as a function of the intrinsic molecular nature and its interaction with environment encouraged us to study the detailed photophysics of structurally simplest AZP, 4-(2-Hydroxyethyl)-10-phenyl-3,4,6,7,8,10-hexahydro-1H-cyclopenta[g]furo[3,4-b]quinoline-1-one (HPFQ), for the first time for this class, using steady-state absorption, emission and time-resolved fluorescence (TRF) spectroscopy and TRF anisotropy in solvents of diverse polarity. HPFQ exhibits (i) primary emission originating from a mixed contribution of locally excited (LE) and pure charge transfer (CT) states and (ii) a red shifted excimer emission. The presence of more than one emitting species is confirmed by the wavelength dependent TRF studies. HPFQ shares compatible H-bonding motifs with all solvents: the H-bond acceptor solvents interact with −OH functionality while H-bond donor solvents interact with both −OH and carbonyl oxygen in cyclic ester (ring D). It is demonstrated that the H-bond interactions causes a small change in dipole moment (Δμ ∼3.52D) and directly affect the extent of HPFQ fluorescence quantum yield (ϕf). This cartel of substantial H-bond network and the weak electron acceptor nature of ring D serve as protagonist for the photophysical conduct and impressive fluorescence in all solvents, which is a very rare phenomenon and has been difficult to achieve across the entire polarity scale. Additionally, ϕf and fluorescence lifetime of CT state illustrate high sensitivity to solvent polarity, proticity and viscosity. The lower viscosity of water propels a faster deactivation and a sudden decrease in CT state lifetime in aqueous medium. The lower ϕf in water when compared to alcohols is possibly due to entrapment of water solvent molecules within the cavity formed by close proximity of flexible CH2CH2OH and phenyl (ring E) groups. Thus, the knowledge provided herein can prove momentous for decoding the fluorescence behaviour of other compounds in this category, and even the parent podophyllotoxins! Overall, the magnificent emission showcased by the antitumor HPFQ is important as it can act as a dual therapeutic and imaging agent without the use of co-staining for determining the impact of environmental factors in cancer pathophysiology in future.

Charge Transfer States in Dilute Donor-Acceptor Blend Organic Heterojunctions.

We study the charge transfer (CT) states in small-molecule blend heterojunctions comprising the nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70, using electroluminescence and steady-state and time-resolved photoluminescence spectroscopy along with density functional theory calculations. We find that the CT exciton energy blue shifts as the C70 concentration in the blend is either decreased or increased away from 50 vol %. At 20 K, the increase in CT state lifetime is correlated with the increasing diameter of C70 nanocrystallites in the blends. A quantum confinement model is used to quantitatively describe the dependence of both CT energy and lifetime on the C70 or DBP domain size. Two discrete CT emission peaks are observed for blends whose C70 concentration is >65%, at which point C70 nanocrystallites with diameters >4 nm appear in high-resolution transmission electron micrographs. The presence of two CT states is attributed to coexistence of crystalline C70 and amorphous phases in the blends. Furthermore, analysis of CT dissociation efficiency versus photon energy suggests that the >90% dissociation efficiency of delocalized CT2 states from the crystalline phase significantly contributes to surprisingly efficient photogeneration in highly dilute (>80% C70) DBP/C70 heterojunctions.

Photoinduced long-lived charge transfer excited states in AT-DNA strands.

The IR spectrum of a charge transfer (CT) excited electronic state in DNA has been computed for the first time, enabling assignment of the long-lived component of the transient IR spectrum of a d(AT)9 single strand to an A → T CT state. Experimentally, the CT state lifetime is much shorter than in the double strand, and our calculations explain this result using Marcus Theory.

Temperature- and Energy-Dependent Separation of Charge-Transfer States in PTB7-Based Organic Solar Cells

A decisive factor for the performance of organic bulk heterojunction solar cells is the competition between charge separation and geminate electron–hole recombination through a charge-transfer (CT) state after exciton separation across the heterointerface. By time-resolving the near-infrared emission of the high-performance low-bandgap PTB7:PC71BM material system, we selectively study the properties of CT states formed after optical excitation with a femtosecond laser pulse. We observe that the CT emission yield is higher after 705 nm excitation of the polymer rather than after 400 nm excitation of the fullerene, which can be attributed to better charge separation for excitons generated in fullerene aggregates. Additionally, the CT states are weakly bound with their emission not far red-shifted from that of the exciton. Examining the time-resolved CT emission from room temperature to 10 K, we observe changes in CT state lifetime with energy and temperature, indicative of CT state separation effectively co...

High Electron Mobility and Its Role in Charge Carrier Generation in Merocyanine/Fullerene Blends

Charge carrier generation and drift dynamics have been investigated in two types of dye:fullerene heterojunctions: vacuum-deposited merocyanine:C60 and solution-processed merocyanine:PC61BM blends by combining electric-field-induced fluorescence quenching and ultrafast time-resolved carrier drift measurements. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. Interfacial CT states on large C60 domains have low energies, while CT states on PC61BM domains have larger energies, which are weakly dependent on the domain sizes. We distinguish two interfacial CT state dissociation pathways: (i) ultrafast, weakly dependent on the electric field and (ii) slow field-assisted dissociation during entire CT state lifetime. We attribute process i to low-energy, weakly bound CT states on large fullerene domains and process ii to strongly bound CT states on small domains or single fullerene molecules. The electron mobility in films with 50% of C60 is several times higher than in the films with PC61BM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons; thus, fast motion and extraction of electrons are crucial for efficient solar cells.

Influence of surface band gaps on the lifetimes of charge transfer states

Using a first principles electronic structure method, we investigate how a band gap in the surface projected density of states of a noble metal influences the resonant tunneling lifetime of an atomic level near the surface. Comparing Li atoms outside Cu(1 1 1), where the Li 2s level is within the gap, with Cu(0 0 1), where the Li 2s level is below the gap, reveals that the lifetime of the ionization level of Li is longer on Cu(1 1 1). This difference is caused both by the decreased final state phase space for the tunneling electron on Cu(1 1 1) and by a reduction in intra-atomic hybridization in Li outside a surface whose band gap encompasses the ionization level. Our results are consistent with previous experiments and theory that predict a similar trend for the ionization level of Cs above Cu(1 1 1) and Cu(0 0 1).

Lifetimes of charge-transfer states in molecular crystals

Abstract A simple microscopic model is proposed to described the transition rates to, from and within the manifold of charge-transfer states. The results suggest possible reasons for the existense of long-lived CT states in molecular crystals.