Excited-state Processes Research Articles

Computational and data driven molecular material design assisted by low scaling quantum mechanics calculations and machine learning.

Electronic structure methods based on quantum mechanics (QM) are widely employed in the computational predictions of the molecular properties and optoelectronic properties of molecular materials. The computational costs of these QM methods, ranging from density functional theory (DFT) or time-dependent DFT (TDDFT) to wave-function theory (WFT), usually increase sharply with the system size, causing the curse of dimensionality and hindering the QM calculations for large sized systems such as long polymer oligomers and complex molecular aggregates. In such cases, in recent years low scaling QM methods and machine learning (ML) techniques have been adopted to reduce the computational costs and thus assist computational and data driven molecular material design. In this review, we illustrated low scaling ground-state and excited-state QM approaches and their applications to long oligomers, self-assembled supramolecular complexes, stimuli-responsive materials, mechanically interlocked molecules, and excited state processes in molecular aggregates. Variable electrostatic parameters were also introduced in the modified force fields with the polarization model. On the basis of QM computational or experimental datasets, several ML algorithms, including explainable models, deep learning, and on-line learning methods, have been employed to predict the molecular energies, forces, electronic structure properties, and optical or electrical properties of materials. It can be conceived that low scaling algorithms with periodic boundary conditions are expected to be further applicable to functional materials, perhaps in combination with machine learning to fast predict the lattice energy, crystal structures, and spectroscopic properties of periodic functional materials.

Implementation of the interacting quantum atom energy decomposition using the CASPT2 method.

We present an implementation of the interacting quantum atom (IQA) energy decomposition scheme using the complete active space second-order perturbation theory (CASPT2). This combination yields a real-space interpretation tool with a proper account of the static and dynamic correlation that is particularly relevant for the description of processes in electronic excited states. The IQA/CASPT2 approach allows determination of the energy redistribution that takes place along a photophysical/photochemical deactivation path in terms of self- and interatomic contributions. The applicability of the method is illustrated by the description of representative processes spanning different bonding regimes: noble gas excimer and exciplex formation, the reaction of ozone with a chlorine atom, and the photodissociations of formaldehyde and cyclobutane. These examples show the versatility of using CASPT2 with the significant information provided by the IQA partition to describe chemical processes with a large multiconfigurational character.

GW/BSE nonadiabatic dynamics simulations on excited-state relaxation processes of zinc phthalocyanine-fullerene dyads: Roles of bridging chemical bonds

In this work, we employ electronic structure calculations and nonadiabatic dynamics simulations based on many-body Green function and Bethe-Salpeter equation (GW/BSE) methods to study excited-state properties of a zinc phthalocyanine-fullerene (ZnPc-C60) dyad with 6-6 and 5-6 configurations. In the former, the initially populated locally excited (LE) state of ZnPc is the lowest S1 state and thus, its subsequent charge separation is relatively slow. In contrast, in the latter, the S1 state is the LE state of C60 while the LE state of ZnPc is much higher in energy. There also exist several charge-transfer (CT) states between the LE states of ZnPc and C60. Thus, one can see apparent charge separation dynamics during excited-state relaxation dynamics from the LE state of ZnPc to that of C60. These points are verified in dynamics simulations. In the first 200 fs, there is a rapid excitation energy transfer from ZnPc to C60, followed by an ultrafast charge separation to form a CT intermediate state. This process is mainly driven by hole transfer from C60 to ZnPc. The present work demonstrates that different bonding patterns (i.e. 5-6 and 6-6) of the C−N linker can be used to tune excited-state properties and thereto optoelectronic properties of covalently bonded ZnPc-C60 dyads. Methodologically, it is proven that combined GW/BSE nonadiabatic dynamics method is a practical and reliable tool for exploring photoinduced dynamics of nonperiodic dyads, organometallic molecules, quantum dots, nanoclusters, etc.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base.

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn2+. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn2+ ions. The second, with ClO4- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn2+ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al3+ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

Theoretical insights of solvent effect on excited-state proton transfers of 2-aryl-3-hydroxyquinolone

The effect of polar solvents (DMSO, CH3OH, and H2O) on possible conformations, photophysical properties, and excited-state proton transfer (ESPT) processes of 2-aryl-3-hydroxyquinolone (3HQ) has been theoretically investigated using time-dependent density functional theory at B3LYP/TZVP level both static and dynamic calculations. From exploration of potential energy surfaces, two stable conformers with the lowest energy of 3HQ complexing with solvent molecules are found namely Intra-HB and Inter-HB conformers. Both Intra-HB and Inter-HB conformers are attributed to their enol and keto emission peaks depending on type of solvent used. Based on the results of potential energy curve along PT coordinates, reaction energy of PT, and on-the-fly dynamic simulations, excited-state intramolecular PT processes are possible for all Intra-HB conformers while excited-state intermolecular double PT processes are only plausible for 3HQ(CH3OH)-inter and 3HQ(H2O)-inter but not for 3HQ(DMSO)-inter. Moreover, excited-state intermolecular double PT mechanisms of 3HQ(CH3OH)-inter and 3HQ(H2O)-inter conformers are stepwise judged from the time lag between the first and second proton transfers.

Intermolecular interaction on excited-state properties of fluoro-substituted thermally activated delayed fluorescence molecules with aggregation-induced emission: a theoretical perspective

ABSTRACT Thermally activated delayed fluorescence (TADF) molecules have shown promising applications in organic light- emitting diodes. Theoretical investigations to reveal the fluoro-substitution and intermolecular interaction on their photophysical properties especially for aggregation-induced emission (AIE) and TADF mechanisms are highly desired. In this work, two efficient TADF molecules SFDBQPXZ and DFDBQPXZ with fluoro-substitution are selected and studied. The solvent and solid environments are considered by polarisable continuum model and the combined quantum mechanics and molecular mechanics (QM/MM) method, respectively. The frontier molecular orbital properties, heat maps, adiabatic singlet–triplet energy gap (ΔEst ) and intermolecular interactions as well as excited-state dynamic process are analysed in both toluene and solid phase. Results show that the small ΔEst and large spin-orbital coupling constant facilitate the up-conversion process. Moreover, small reorganisation energies and large intermolecular interactions are determined in the solid phase, the non-radiative energy consumption processes of SFDBQPXZ and DFDBQPXZ are significantly suppressed in the aggregation state due to the restricted rotations in low-frequency region, the AIE feature is illustrated, and inner mechanisms for higher luminescence efficiency of SFDBQPXZ than that of DFDBQPXZ ae determined. The experimental measurements are reasonably elucidated and the effect of fluoro-substitution on excited-state dynamics is revealed.

Dual-Emission Zr-MOF-Based Composite Material as a Fluorescence Turn-On Sensor for the Ultrasensitive Detection of Al3.

In this work, a novel zirconium-based metal-organic framework (MOF) composite material, UiO-(OH)2@RhB, has been solvothermally prepared with zirconyl chloride octahydrate, 2,5-dihydroxyterephthalic acid, and rhodamine B (RhB) for ratiometric fluorescence sensing of Al3+ ions in an aqueous medium. The luminescence measurement results showed that, at the single excitation wavelength of 420 nm, the fluorescence intensity of the ligand at 500 nm increased significantly in the case of Al3+, while that of RhB at 583 nm changed slightly, together with an apparent color change. Under optimal conditions, UiO-(OH)2@RhB exhibited an extraordinary sensitivity (10 nM), good selectivity, and a fast response (2 min) for Al3+. As far as we know, the limit of detection is superior to that of the current reported MOF-based Al3+ fluorescence sensors. The response mechanism suggested that -OH could capture Al3+ in water through coordination and high electrostatic affinity and achieved turn-on ratiometric fluorescence through the excited-state intramolecular proton transfer process and stable fluorescence of RhB. In addition, this sensor was also applied to actual food samples (grain beans), with the recoveries ranging from 89.08% to 113.61%. Such a turn-on ratiometric fluorescence sensor will provide a constructive strategy for the ultrasensitive detection of Al3+ in practical applications.

Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Characterization of the Ce,Pr:LuAG phosphor for Co-doped aerosol phosphor thermometry

The Ce,Pr:LuAG phosphor is investigated for use in co-doped aerosol phosphor thermometry (APT). The phosphor was characterized by measuring the emission intensity for each ion as a function of excitation laser fluence and temperature. A simplified three-level model was used to interpret and fit the data. The phosphor was then used for co-doped APT temperature-imaging experiments performed in a heated air jet up to a maximum temperature of 694 K. The three-level model fit was used to analyze sources of bias in the imaging measurements. Both ions’ emission intensities show a nonlinear dependence on excitation fluence that can potentially be explained by temperature-dependent ground- and excited-state absorption processes included in the three-level model. Single-shot temperature precision from the temperature-imaging experiments was measured to be 22, 23, and 32 K for mean temperatures of 497, 603, and 694 K, respectively, at a spatial resolution of ~1.12 lp/mm and average fluence of 33 mJ/cm2. The results extend the measurement range for co-doped APT to at least 700 K. The findings also provide a simple model for phosphor signal non-linearity with excitation fluence for low activator ion concentrations, and provide a method for estimating temperature bias from several sources including laser fluence.

Excited Carrier Dynamics in Two-dimensional Materials

When electrons in materials are excited, they undergo several dynamic processes such as carrier thermalization, transfer, and recombination. These fundamental excited state processes are crucial to understanding the microscopic principles at work in electronic and optoelectronic devices. This article introduces the excited carrier dynamics in a two-dimensional van der Waals material and reveals several interesting phenomena that do not occur in bulk materials. Particularly, the focus will be two dynamic processes: carrier multiplication and ultrafast charge transfer.

Experimentally Observed Reverse Intersystem Crossing-Boosted Lasing.

Thermally activated delayed-fluorescent (TADF) materials are anticipated to overcome triplet-related losses towards electrically driven organic lasers. Thus far, contributions from triplets to lasing have not yet been experimentally demonstrated owing to the limited knowledge about the excited-state processes. Herein, we experimentally achieve reverse intersystem crossing (RISC)-boosted lasing in organic microspheres with uniformly dispersed TADF emitters. In these materials, triplets are continuously converted to radiative singlets through RISC, giving rise to reduced losses in stimulated emission. The involvement of regenerated singlets in population inversion results in a thermally activated lasing; that is, the lasing intensity increases with increasing temperature, accompanied by accelerated depletion of the excited-state population. Benefiting from the suppression of triplet accumulations by RISC processes, a high-repetition-rate microlaser was achieved.

Long-Lived Mixed 2MLCT/MC States in Antiferromagnetically Coupled d3 Vanadium(II) Bipyridine and Phenanthroline Complexes.

Exploration of [V(bpy)3]2+ and [V(phen)3]2+ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4MLCT ← 4GS (ground-state) absorption band (ε400-700nm = 900-8000 M-1 cm-1), the 4MLCT state rapidly (τisc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)3]2+ and [V(phen)3]2+, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)3]3+, which rapidly forms a long-lived doublet metal-centered (2MC) state following photoexcitation and lacks strong visible GS absorption character. 2MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)3]3+, where the GS and 2MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2MLCT state at lower energy than the 4MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.

Photoexcited state properties and antibacterial activities of carbon dots relevant to mechanistic features and implications

Carbon dots (CDots) are strongly absorptive over the visible spectrum, with the effective photon-harvesting driving rich excited state processes and properties. In this work, spectroscopic probing of these processes and properties is coupled with the evaluation of the photoinduced bactericidal function of CDots, aimed toward making correlations among the findings from the former and those from the latter on the inactivation of bacterial pathogens. Within the general mechanistic framework for CDots, the observed effective and efficient antibacterial activities of the CDots under visible light are attributed to major contributions by the initially photo-generated electrons and holes that are trapped at passivated surface defect sites of the dots, in addition to the traditional reactive oxygen species (ROS) produced in the emissive excited states from the recombination of the redox pairs. Such major contributions to the inactivation of the bacteria by the separated redox species in CDots can not be quenched by ROS scavengers commonly used in the study of photodynamic effects with classical molecular photosensitizers, thus making light-activated CDots uniquely potent antimicrobial agents. The characteristic features of photoexcited CDots and their related mechanistic implications are discussed in reference to the similar behaviors and mechanistic model of conventional semiconductor quantum dots.

Determining the Correlation between Excited State Dynamics and Donor and Acceptor Structure in Nonfullerene Acceptors

The design of nonfullerene acceptors (NFAs), which comprise donor and acceptor moieties, has led to significant improvement in organic photovoltaic (OPV) device performance over the past few years. Although there have been many NFAs synthesized and device-optimized, there have been fewer studies on the excited-state photophysics of donor-acceptor-containing NFAs. Using a combination of femtosecond transient absorption spectroscopy and time-resolved photoluminescence (PL), we have investigated the excited-state photophysical processes in three donor-acceptor-containing NFAs, where the chemical structure of the NFAs are systematically modified to allow direct comparison (D-A1-A1-D, D-A1-D, and D-A1-A2, where D represents 9,9-di-n-propylfluorene, A1 represents benzothiadiazole, and A2 represents dicyanovinyl). A large triplet population was observed for the dimer NFA (D-A1-A1-D), whereas for D-A1-D or D-A1-A2, the nonradiative decay rate was significantly decreased, and higher PL quantum yields observed. Furthermore, a singlet exciton relaxation process between S1∗ and S1 on the picosecond timescale was observed for each of the NFAs, although the mechanism was not always the same. Torsional relaxation was found to dominate the picosecond singlet relaxation for D-A1-D and D-A1-A2 in solution and film, and for D-A1-A1-D in solution. In the film, the singlet excited state of D-A1-A1-D predominantly relaxed via resonant excitonic energy transfer. The observation of singlet relaxation on the tens of picosecond timescale also provides a plausible explanation to the reduced VOC loss commonly associated with NFA-based OPV devices.

Spectroscopic studies of the excited-state intramolecular proton and electron transfer processes of methyl benzoate derivatives in cucurbit[7]uril nanocage

The encapsulation of four methyl benzoate derivatives (I-IV) in aqueous solution in the presence of the macrocyclic host molecule cucurbit[7]uril (CB[7]) was studied using steady-state, time-resolved and 1H NMR spectroscopic techniques. The significant changes in the steady-state and time-resolved fluorescence spectra (and calculated from them fluorescence decay times) were attributed to the formation of a well-defined, stable 1:1 inclusion complex. To understand the mechanism of supramolecular solvation dynamics, the time-dependent fluorescence Stokes' shifts, represented by the normalized spectral response function c(t), were studied. Our studies have shown that solvation dynamics of water are significantly inhibited inside a cucurbit cavity. The experimental results were also used to calculate, according to Benesi-Hildebrand plot and nonlinear least-squares regression analysis, equilibrium constants of the formed inclusion complex. For all investigated dyes, the K1 value is significantly larger than this previously calculated for I-IV inclusion into cyclodextrins (CDs), indicating the superior host abilities of CB[7] as compared to CDs. The position of the guest molecule inside the host cavity was investigated by 1H NMR experiment. The nature of the 1H NMR shifts for different protons is qualitatively different for methoxy derivative-CB[7] complex (I-CB[7] and III-CB[7] systems) and hydroxy derivative-CB[7] complex (II-CB[7] and IV-CB[7] systems), which indicates that the inclusion complexes of hydroxy and methoxy derivatives with CB[7] are of a difference in nature. The CB[7]-induced chemical shifts clearly demonstrate that for II-CB[7] and IV-CB[7] complexes, benzene ring is fully embedded in the interior of CB[7], while for I-CB[7] and III-CB[7] complexes, only the part of the benzene ring is located within the CB[7] cavity. Finally, 1H NMR chemical shifts and evolution of the steady-state and time-resolved emission spectra clearly indicate that -NMe2 group attached to the III and IV is located outside of the CB[7] portal.

Photoinduced Dynamics of Charge Carriers in Metal Halide Perovskites from an Atomistic Perspective.

Perovskite solar cells have attracted intense attention over the past decade because of their low cost, abundant raw materials, and rapidly growing power conversion efficiency (PCE). However, nonradiative charge carrier losses still constitute a major factor limiting the PCE to well below the Shockley-Queisser limit. This Perspective summarizes recent atomistic quantum dynamics studies on the photoinduced excited-state processes in metal halide perovskites (MHPs), including both hybrid organic-inorganic and all-inorganic MHPs and three- and two-dimensional MHPs. The simulations, performed using a combination of time-domain ab initio density functional theory and nonadiabatic molecular dynamics, allow emphasis on various intrinsic and extrinsic features, such as components, structure, dimensionality and interface engineering, control and exposure to various environmental factors, defects, surfaces, and their passivation. The detailed atomistic simulations advance our understanding of electron-vibrational dynamics in MHPs and provide valuable guidelines for enhancing the performance of perovskite solar cells.

Fluorescence Toggling Mechanism of Photochromic Phenylhydrazones: N–N Single Bond Rotation-Assisting E/Z Photoisomerization Differs from Imine

Photochromic phenylhydrazones are one of the most promising candidates for a photoswitchable fluorescent probe with potential applications in various fields, but mechanistic understanding of the origin of this unique behavior is limited. In this work, we explored the emission nature and switching mechanism of a model phenylhydrazone-based fluorescent photoswitch, DMA-PHA, by means of TD-DFT and CASPT2 calculations. The fluorescence-emitting Z configuration of DMA-PHA does not involve an excited-state intramolecular proton transfer process since the resonance effect between the DMA group and the rest part of the molecule in the excited state strengthens the hydrogen bond and thus stabilizes the emissive state. The light-induced fluorescence toggling results from E↔Z interconversion driven by an out-of-plane C═N bond torsion and assisted by a N-N single bond rotation, which in total lead to a charge separation process losing the fluorescence activity. The N-N bond rotation in phenylhydrazone further enhances the competitive nonradiative decay and reduces the photoisomerization yields. The theoretical results will provide the guidance for the rational design of novel and improved photoswitchable fluorescent probes with desired performances.

Riboflavin: A natural aggregation-induced emission luminogen (AIEgen) with excited-state proton transfer process for bioimaging

In recent years, aggregation-induced emission luminogens (AIEgens) have many applications in bioimaging due to they have high fluorescence quantum yield, perfect biocompatibility, good resistance to photobleaching. However, the practical application of the AIEgens was greatly limited due to their disadvantages of complex synthesis, high cost, environmentally unfriendly and difficult to degrade. Here, the riboflavin (Rf, namely vitamin B2) was found to be a natural active AIEgen with excited-state proton transfer (ESPT), which is widely existed in the foods from both plants and animals. The AIE characteristics of Rf were investigated by using UV–Vis and photoluminescence (PL) spectroscopy. Furthermore, Rf as an AIEgen with ESPT has perfect biocompatibility and has little cytotoxicity, which was successfully used for bioimaging in cellular cytoplasm. These results all indicated that Rf was a potential natural material as promising AIE probes. This work aim to study the AIE-active luminogen of Rf with ESPT process for bio-probe and bioimaging applications.

Simulation of time-resolved x-ray absorption spectroscopy of ultrafast dynamics in particle-hole-excited 4-(2-thienyl)-2,1,3-benzothiadiazole.

To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of π-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen K edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our ab initio molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum.

Relaxation Dynamics of the Triazene Compound Berenil in DNA-Minor-Groove Confinement after Photoexcitation.

The effects of biomolecular embedding on the photoinduced relaxation process of the DNA-minor-groove binder berenil, diminazene aceturate, are studied with quantum mechanics/molecular mechanics, QM/MM, calculations that employ the algebraic diagrammatic construction through second-order, ADC(2), for the quantum mechanical part and an atomistic polarizable embedding for the classical part. The lowest singlet excitation to the S1 state is a bright transition with a ππ* character and a perichromatic red shift, due to the interactions with the solvent and DNA. The excited-state relaxation pathway is a two-step mechanism, an N═N azo-bond stretch followed by a volume-conserving bicycle-pedal twist. The DNA confinement and the coupling to solvent molecules via hydrogen bonds lead, for the excited-state relaxation process, only to small deviations from the ideal bicycle-pedal relaxation. Because of its volume-conserving character, the S1 excited-state relaxation proceeds almost unhindered, even in a fully rigid minor-groove confinement. With a fully frozen DNA minor groove and solvent, the energy gap for deexcitation from S1 to the ground state increased to 2.0 eV compared to 0.16 eV in aqueous solution. When the relaxation of the first solvation shell is included, the relaxation process on the S1 potential energy surface proceeds to a region on the potential energy surface, where only a small gap to the ground-state potential energy surface remains, 0.43 eV. These results show that the solvent relaxation has a significant effect in controlling the energy gap between the ground and S1 electronically excited states, which explains the experimental observations of the fluorescence characteristics of berenil in DNA confinement.