Accidental triplet harvesting in donor-acceptor dyads with low spin-orbit coupling.

We present an accidental mechanism for efficient intersystem crossing (ISC) between singlet and triplet states with low spin-orbit coupling (SOC) in molecules having donor-acceptor (D-A) moieties separated by a Sigma bond. Our study shows that SOC between the lowest singlet excited state and the higher-lying triplet states, together with nuclear motion-driven coupling of this triplet state with lower-lying triplet states during the free rotation about a Sigma bond, is one of the possible ways to achieve the experimentally observed ISC rate for a class of D-A type photoredox catalysts. This mechanism is found to be the dominant contributor to the ISC process with the corresponding rate reaching a maximum at a dihedral angle in the range of 72°-78° between the D-A moieties of 10-(naphthalen-1-yl)-3,7-diphenyl-10H-phenoxazine and other molecules included in the study. We have further demonstrated that the same mechanism is operative in a specific spirobis[anthracene]dione molecule, where the D and A moieties are interlocked near to the optimal dihedral angle, indicating the plausible effectiveness of the proposed mechanism. The present finding is expected to have implications in strategies for the synthesis of new generations of triplet-harvesting organic molecules.

Photophysics of uracil: an explicit time-dependent generating function-based method combining both nonadiabatic and spin-orbit coupling effects.

We present a composite framework for calculating the rates of non-radiative deactivation processes, namely internal conversion (IC) and intersystem crossing (ISC), on an equal footing by explicitly computing the non-adiabatic coupling (NAC) and spin-orbit coupling (SOC) constants, respectively. The stationary-state approach uses a time-dependent generating function based on Fermi's golden rule. We validate the applicability of the framework by computing the rate of IC for azulene, obtaining comparable rates to experimental and previous theoretical results. Next, we investigate the photophysics associated with the complex photodynamics of the uracil molecule. Interestingly, our simulated rates corroborate experimental observations. Detailed analyses using Duschinsky rotation matrices, displacement vectors and NAC matrix elements are presented to interpret the findings alongside testing the suitability of the approach for such molecular systems. The suitability of the Fermi's golden rule based method is explained qualitatively in terms of single-mode potential energy surfaces.

Spin-vibronic interaction induced reverse intersystem crossing: A case study with TXO-TPA and TXO-PhCz molecules.

We highlight the important roles the direct spin-orbit (DSO) coupling, the spin-vibronic (SV) coupling, and the dielectric constant of the medium play on the reverse intersystem crossing (RISC) mechanism of TXO-TPA and TXO-PhCz molecules. To understand this complex phenomenon, we have calculated the RISC rate constant, kRISC, using a time-dependent correlation function-based method within the framework of second-order perturbation theory. Our computed kRISC in two different solvents, toluene and chloroform, suggests that in addition to the DSO, a dielectric medium-dependent SV mechanism may also have a significant impact on the net enhancement of the rate of RISC from the lowest triplet state to the first excited singlet state. Whereas we have found that kRISC of TXO-TPA is mostly determined by the DSO contribution independent of the choice of the solvent, the SV mechanism contributes more than 30% to the overall kRISC of TXO-PhCz in chloroform. In toluene, however, the SV mechanism is less important for the RISC process of TXO-PhCz. An analysis of mode-specific nonadiabatic coupling (NAC) between T2 and T1 of TXO-PhCz and TXO-TPA suggests that the NAC values in certain normal modes of TXO-PhCz are much higher than those of TXO-TPA, and it is more pronounced with chloroform as a solvent. The findings demonstrate the role of the solvent-assisted SV mechanism toward the net RISC rate constant, which in turn maximizes the efficiency of thermally activated delayed fluorescence.

Behind the scenes of spin-forbidden decay pathways in transition metal complexes.

The interpretation of the ultrafast photophysics of transition metal complexes following photo-absorption is quite involved as the heavy metal center leads to a complicated and entangled singlet-triplet manifold. This opens up multiple pathways for deactivation, often with competitive rates. As a result, intersystem crossing (ISC) and phosphorescence are commonly observed in transition metal complexes. A detailed understanding of such an excited-state structure and dynamics calls for state-of-the-art experimental and theoretical methodologies. In this review, we delve into the inability of non-relativistic quantum theory to describe spin-forbidden transitions, which can be overcome by taking into account spin-orbit coupling, whose importance grows with increasing atomic number. We present the quantum chemical theory of phosphorescence and ISC together with illustrative examples. Finally, a few applications are highlighted, bridging the gap between theoretical studies and experimental applications, such as photofunctional materials.

The influence of spin-orbit coupling, Duschinsky rotation and displacement vector on the rate of intersystem crossing of benzophenone and its fused analog fluorenone: a time dependent correlation function based approach.

To understand the effect of structural rigidity or flexibility on the intersystem crossing rate, herein we have adopted a time dependent correlation function based approach, an appropriate method for a harmonic oscillator under Condon approximation. Following this technique, we have developed generalized codes for calculating the rate of intersystem crossing (ISC) both at 0 K and at finite temperature. Since the rate of ISC is a measurable quantity, we have separated the real and imaginary parts of the complex correlation function carefully and eliminated the imaginary part by exploiting the odd nature of this function. Using this simplified method, we have calculated the ISC rate constant (kISC) of two molecules, namely, benzophenone and its fused analog, fluorenone. The calculations clearly elucidate that kISC of benzophenone is 103 times larger compared to that of fluorenone. Interestingly, our analyses reveal that the combined effect of spin-orbit coupling and the number of normal modes could increase the rate of ISC of benzophenone by three orders in comparison to that of fluorenone. Furthermore, the Duschinsky rotation matrix (J) and displacement vectors (D) could influence the rate of ISC by one order each, indicating that the overall rate of ISC of benzophenone could have been 105 times higher than that of fluorenone if the latter two factors, namely, J and D have practically no impact on the rate of ISC of fluorenone. However, it has been found that albeit J can't alter the rate of ISC of fluorenone, D indeed can change the rate by two orders, thereby keeping the overall ratio of the rate of ISC of benzophenone and fluorenone as 103. The present study elucidates that none of the above mentioned factors alone can explain the relative rate of ISC of the studied systems; rather a complex interplay between all these factors makes the rate of ISC of benzophenone 103 times higher than that of fluorenone.

Large Negative Differential Resistance and Rectification from a Donor-σ-Acceptor Molecule in the Presence of Dissimilar Electrodes.

A multifunctional spin quantum device obtained by sandwiching 11-mercaptoundeca-2,4,8,10-tetraenenitrile, a donor-σ-acceptor molecule, between gold and iron electrodes is proposed. The device can act as a spin rectifier at lower bias and also exhibits negative differential resistance (NDR) after attaining a bias of 1.3 V. The rectification feature is quite prominent in the spin-up channel, with an appreciable rectification ratio of 68, whereas the NDR indicator, that is, the peak to valley ratio (≈10) of the current-voltage characteristics after 1.3 V, is also quite significant. To understand the origin of this in silico observation, nonequilibrium green's function based DFT calculations have been performed. Analyses reveal that both properties originate from the bias-independent energy offset between the frontier orbitals and electrode Fermi levels, popularly known as Fermi-level pinning. More precisely, rectification results from the Fermi-level pinning of the HOMO and LUMO with the gold and iron electrodes, respectively; the Fermi-level pinning forces a HOMO-LUMO crossover that helps to explain the origin of the NDR.

Intersystem crossing rate dependent dual emission and phosphorescence from cyclometalated platinum complexes: a second order cumulant expansion based approach.

Rates of intersystem crossing (kISC) of two platinum(ii) complexes containing acetylacetonate (acac) and extended cyclometalated ppy (Hppy = 2-phenylpyridine) (1) and thpy (Hthpy = 2-(2' thienyl)pyridine) (2) ligands are calculated using the Condon approximation to the Golden Rule and employing the second-order cumulant expansion method. The emission wavelengths obtained at the RI-CC2 level for the lowest excited singlet (S1) and triplet (T1) states of the two complexes are well in agreement with the experimental results. Our analysis based on kISC evinces that the major pathway involved with the phosphorescence process in complex 1 arises from the S1 → T2 intersystem crossing while the S1 → T1 intersystem crossing is the key step towards the commencement of dual emission in complex 2. Furthermore, it is found that the different pathways are mostly guided by two factors namely, the energy gap and the spin-orbit interaction between the concerned states. Interestingly, the calculated kISC for complex 1 is found to be 107 times larger than that of complex 2, which suggests a rapid depletion of the S1 state population vis-à-vis radiative emission only by phosphorescence from the internally converted lowest excited triplet state while for complex 2, the relatively lower kISC is attributed to the dual emission from this complex.

Eye-Catching Dual-Fluorescent Dynamic Metal-Organic Framework Senses Traces of Water: Experimental Findings and Theoretical Correlation.

A guest-dependent dynamic fivefold interpenetrated 3D porous metal-organic framework (MOF) of ZnII ions has been synthesized that exhibits selective carbon dioxide adsorption. Furthermore, the MOF shows excellent luminescence behavior, which is supported by a systematic study on the guest-responsive multicolor emission of a suspension of the MOF. The dual-emission behavior arises from the excited-state intramolecular proton transfer (ESIPT), and the compound also shows remarkable potential to detect traces of water in various organic solvents. The experimental observations were also painstakingly authenticated by using time-dependent density-functional-theory (DFT) calculations.

Fe(100)-(borazine)n=1-4-Fe(100): a multifunctional spin diode with spin valve action.

Herein, we report that borazine (B3N3H6), alternatively known as inorganic benzene, has the tantalizing potential to act as a multifunctional molecular spin diode with a significantly large spin valve action. The present computational foray into the multifunctionality of (B3N3H6)n=1-4 as a simultaneous spin diode and spin valve has been rationalized by the current rectification ratio with a maximum value of 34 for the tetramer and large tunneling magneto-resistance in the range of 50-100%, respectively. Remarkably, both the properties are evolved due to a single parameter, namely, the electrodes [Fe(100)] surface spin orientation induced charge density localization/delocalization in the singly occupied highest molecular orbital in the Fe(100)-(borazine)n=1-4-Fe(100) system.

Pseudomonas sp. CL7 from Sludge Removed 2,3,4,6-Tetrachlorophenol in Vivo and in Vitro Condition.

The present research focused on 2,3,4,6-Tetrachlorophenol (2,3,4,6-TeCP) mineralizing bacterium from the sludge of pulp and paper industry and identified as Pseudomonas sp. CL7 by 16s rRNA gene sequences analysis. This isolate degraded 2,3,4,6-TeCP as indicated by stoichiometric release of chloride and biomass formation. High pressure liquid chromatography (HPLC) analysis showed that Pseudomonas sp. (CL7) was able to mineralize a higher concentration of 2,3,4,6-TeCP (600 mg/l or 2.5 mM) than any previously reported 2,3,4,6-TeCP degrading bacteria. As the concentration of 2,3,4,6-TeCP increased from 50 (0.21 mM) to 600 mg/l (2.5 mM), the reduction in the cell growth was observed and the 2,3,4,6-TeCP degradation was more than 85% in all the concentrations in the present study. CL7 was able to remove 100% of 2,3,4,6-TeCP from the sludge (in Vitro condition) when supplemented with 100 mg/l (0.42 mM) of 2,3,4,6-TeCP and grown for two weeks. This study showed that CL7 can be used for bioremediation of 2,3,4,6-TeCP.

Chemical control of channel interference in two-photon absorption processes.

The two-photon absorption (TPA) process is the simplest and hence the most studied nonlinear optical phenomenon, and various aspects of this process have been explored in the past few decades, experimentally as well as theoretically. Previous investigations have shown that the two-photon (TP) activity of a molecular system can be tuned, and at present, performance-tailored TP active materials are easy to develop by monitoring factors such as length of conjugation, dimensionality of charge-transfer network, strength of donor-acceptor groups, polarity of solvents, self-aggregation, H-bonding, and micellar encapsulation to mention but a few. One of the most intriguing phenomena affecting the TP activity of a molecule is channel interference. The phrase "channel interference" implies that if the TP transition from one electronic state to another involves more than one optical pathway or channel, characterized by the corresponding transition dipole moment (TDM) vectors, the channels may interfere with each other depending upon the angles between the TDM vectors and hence can either increase (constructive interference) or decrease (destructive interference) the overall TP activity of a system to a significant extent. This phenomenon was first pointed out by Cronstrand, Luo, and Ågren [Chem. Phys. Lett. 2002, 352, 262-269] in two-dimensional systems (i.e., only involving two components of the transition moment vectors). For three-dimensional molecules, an extended version of this idea was required. In order to fill this gap, we developed a generalized model for describing and exploring channel interference, valid for systems of any dimensionality. We have in particular applied it to through-bond (TB) and through-space (TS) charge-transfer systems both in gas phase and in solvents with different polarities. In this Account, we will, in addition to briefly describing the concept of channel interference, discuss two key findings of our recent work: (1) how to control the channel interference by chemical means, and (2) the role of channel interference in the anomalous solvent dependence of certain TP chromophores. For example, we will show that simple structurally induced changes in certain dihedral angles of the well-known betaine dye (TB type) will help fine-tune the constructive channel interference and hence increase the overall TP activity of molecules with this general TP channel structure. Another intriguing result we will discuss is observed for a tweezer-trinitrofluorinone complex (TS type) where, on moving from polar to essentially nonpolar solvents, the nature of the channel interference switches from destructive to constructive, leading to a net abnormal solvent dependence of the TP activity of the system. The present Account highlights the usefulness of the channel interference effect and establishes it as a new and unique way of controlling the TP transition probability in different types of three-dimensional molecules.

Graphitic silicon nitride: a metal-free ferromagnet with charge and spin current rectification.

As a first example, herein we show that g-Si(4)N(3) is expected to act as a metal-free ferromagnet featuring both charge and spin current rectification simultaneously. Such rectification is crucial for envisioning devices that contain both logic and memory functionality on a single chip. The spin coherent quantum-transport calculations on g-Si(4)N(3) reveal that the chosen system is a unique molecular spin filter, the current-voltage characteristics of which is asymmetric in nature, which can create a perfect background for synchronous charge and spin current rectification. To shed light on this highly unusual in-silico observation, we have meticulously inspected the bias-dependent modulation of the spin-polarized eigenstates. The results indicate that, whereas only the localized 2p orbitals of the outer-ring (OR) Si atoms participate in the transmission process in the positive bias, both OR Si and N atoms contribute in the reverse bias. Furthermore, we have evaluated the spin-polarized electron-transfer rate in the tunneling regime, and the results demonstrate that the transfer rates are unequal in the positive and negative bias range, leading to the possible realization of a simultaneous logic-memory device.

Electrically controlled eight-spin-qubit entangled-state generation in a molecular break junction.

The generation of spin-based multi-qubit entangled states in the presence of an electric field is one of the most challenging tasks in current quantum-computing research. Such examples are still elusive. By using non-equilibrium Green's function-based quantum-transport calculations in combination with non-collinear spin density functional theory, we report that an eight-spin-qubit entangled state can be generated with the high-spin state of a dinuclear Fe(II) complex when the system is placed in a molecular break junction. The possible gate operation scheme, gating time, and decoherence issues have been carefully addressed. Furthermore, our calculations reveal that the preservation of the high spin state of this complex is possible if the experimentalists keep the electric-field strength below 0.78 V nm(-1). In brief, the present study offers a unique way to realize the first example of a multi-qubit entangled state by electrical means only.

On the origin of the very strong two-photon activity of squaraine dyes--a standard/damped response theory study.

In the present work, we report the mechanism of a very large increase in the two-photon (TP) activity of squaraine based molecules upon changing the substituents. The replacement of a specific fused ring by ethylene or ethyne moieties enhances the TP transition strength of these molecules up to the order of 10(13) au (∼10(10) GM), both in the gas phase as well as in dichloromethane solvent. Our calculations decisively establish that the reason for this large enhancement in the TP activity of the studied systems is the severe decrease in the corresponding detuning energies. We explain this fact using damped response theory calculations and provide a novel design strategy to control the detuning energy of such molecules. The results are benchmarked against the available experimental findings.

Reverse Intersystem Crossing Dynamics in Vibronically Modulated Inverted Singlet-Triplet Gap System: A Wigner Phase Space Study.

We inspect the origin of the inverted singlet-triplet gap (INVEST) and slow change in the reverse intersystem crossing (rISC) rate with temperature, as recently observed. A Wigner phase space study reveals that, though INVEST is found at equilibrium geometry, variation in the exchange interaction and the doubles-excitation for other geometries in the harmonic region leads to non-INVEST behavior. This highlights the importance of nuclear degrees of freedom for the INVEST phenomenon, and in this case, geometric puckering of the studied molecule determines INVEST and the associated rISC dynamics.

Effect of donor-acceptor orientation on solvent-dependent three-photon activity in through-space charge-transfer systems--case study of [2,2]-paracyclophane derivatives.

We study the effect of donor-acceptor orientation on solvent-dependent three-photon transition probabilities (δ(3PA)) of representative through-space charge-transfer (TSCT) systems, namely, doubly positively charged [2,2]-paracyclophane derivatives. Our cubic response calculations reveal that the value of δ(3PA) may be as high as 10(6) a.u., which can further be increased by a specific orientation of the donor-acceptor moieties. To explain the origin of the solvent cum orientation dependency of δ(3PA), we have calculated different three-photon tensor components using a two-state model, noting that only a few tensor elements contribute significantly to the overall δ(3PA) value. We show that this dependence is due to the large dipole moment difference between the ground and excited states of the systems. The dominance of a few tensor elements indicates a synergistic involvement of π-conjugation and TSCT in the large δ(3PA) of these systems.

Electrostatic spin crossover and concomitant electrically operated spin switch action in a ti-based endohedral metallofullerene polymer.

Herein, we predict that a 1D chain of Ti@C(32) - C(2) - Ti@C(32) (TEMF) will act as a spin switch in the presence of an electric field. The spin resolved density of states analyses reveal that, surprisingly, both the low- and high-spin states of TEMF are half-metal; however, the metallic density of states comes from the opposite spin channels of the two spin states. More remarkably, it is found that the electric field driven spin crossover between the low and high state in TEMF is achievable at field strength 1.04 V/nm, which eventually leads to the realization of the first ever electrically operated spin switch device.

On the microscopic origin of bending of graphene nanoribbons in the presence of a perpendicular electric field.

Herein, we predict that graphene nanoribbons will be nonplanar under the influence of a critical perpendicular field. Our investigation demonstrates that the perpendicular field induces mixing of σ and π orbitals in graphene nanoribbons through the second order Stark effect which eventually modulates the electron-nuclear interaction strongly in favor of a bent structure.

Solvent induced channel interference in the two-photon absorption process--a theoretical study with a generalized few-state-model in three dimensions.

For the first time, we report the effect of interference between different optical channels on the two-photon absorption (TPA) process in three dimensions. We have employed response theory as well as a sum-over-states (SOS) approach involving few intermediate states to calculate the TPA parameters like transition probabilities (δ(TP)) and TPA tensor elements. In order to use the limited SOS approach, we have derived a new formula for a generalized few-state-model (GFSM) in three dimensions. Due to the presence of additional terms related to the angle between different transition moment vectors, the channel interference associated with the TPA process in 3D is significantly different and much more complicated than that in 1D and 2D cases. The entire study has been carried out on the two simplest Reichardt's dyes, namely 2- and 4-(pyridinium-1-yl)-phenolate (ortho- and para-betain) in gas phase, THF, CH(3)CN and water solvents. We have meticulously inspected the effect of the additional angle related terms on the overall TPA transition probabilities of the two 3D isomeric molecules studied and found that the interfering terms involved in the δ(TP) expression contribute both constructively and destructively as well to the overall δ(TP) value. Moreover, the interfering term has a more conspicuous role in determining the net δ(TP) associated with charge transfer transition in comparison to that of π-π* transition of the studied systems. Interestingly, our model calculations suggest that, for o- and p-betain, the quenching of destructive interference associated with a particular two-photon process can be done with high polarity solvents while the enhancement of constructive interference will be achieved in solvents having relatively small polarity. All the one- and two-photon parameters are evaluated using a range separated CAMB3LYP functional.

Enhancement of twist angle dependent two-photon activity through the proper alignment of ground to excited state and excited state dipole moment vectors.

Herein, we show that the two-photon (TP) transition probability (δTP) of o-betaine system will reach its maximum value at a twist angle around 65°. However, the potential energy scan with respect to the twist angle between its two rings indicates that the molecule in its ground state is quite unstable at this twist angle. Out of the different possibilities, the one having a single methyl group at the ortho position of the pyridinium ring is found to attain the optimum twist angle between the two rings, and interestingly, this particular substituted o-betaine has larger δTP value than any other substituted or pristine o-betaine. The twist angle dependent variation of δTP has been explained by employing the generalized-few-state-model formula for 3D molecules. The results clearly reveal that the magnitude of ground to excited state and excited state dipole moment vectors as well as the angle between them are strongly in favor of maximizing the overall δTP values at the optimum twist angle. The constructive interference between the optical channels at the optimum twist angle also plays an important role to achieve the maximum δTP value. Furthermore, to give proper judgment on our findings, we have also performed solvent phase calculations on all the model systems in nonpolar solvents, namely, cyclohexane and n-hexane, and the results are quite consistent with the gas phase findings. The present study will definitely offer a new way to synthesize novel two-photon active material based on o-betaine.