On the Nature of the Low-Lying Singlet States of 4-(Dimethyl-amino)benzonitrile

Monitoring Density Redistribution at the Excited State in a Dual Emitting Molecule: An Analysis Based on Real-Time Density Functional Theory and Density Descriptors

In this work, we computed and analyzed, by means of density-based descriptors, the real-time evolution of both the locally excited (LE) and charge-transfer (CT) excited states for the planar and twisted conformations of the DMABN (4-(N,N-dimethylamino)benzonitrile) molecule using real-time time-dependent density functional theory (DFT) and three different exchange-correlation energy functionals (EXC) belonging to the same family (the PBE one). Our results based on the analysis of density-based descriptors show that the underlying EXC modifies the evolution in time of the density. In particular, comparing the frequency of density reorganization computed with the three functionals (PBE, PBE0, and LC-PBE), we found that the frequency of electronic interconversion of the individual determinants involved during the dynamics increases from PBE to PBE0 and to LC-PBE. This allows us to show that there is a correlation between the delocalization of the electronic density and the frequency of reorganization. In particular, the greater the mean hole-electron distance during the dynamics, the lower is the frequency of density reorganization.

Learning from the 4-(dimethylamino)benzonitrile twist: Two-parameter range-separated local hybrid functional with high accuracy for triplet and charge-transfer excitations

The recent ωLH22t range-separated local hybrid (RSLH) is shown to provide outstanding accuracy for the notorious benchmark problem of the two lowest excited-state potential energy curves for the amino group twist in 4-(dimethylamino)benzonitrile (DMABN). However, the design of ωLH22t as a general-purpose functional resulted in less convincing performance for triplet excitations, which is an important advantage of previous LHs. Furthermore, ωLH22t uses 8 empirical parameters to achieve broad accuracy. In this work, the RSLH ωLH23ct-sir is constructed with minimal empiricism by optimizing its local mixing function prefactor and range-separation parameter for only 8 excitation energies. ωLH23ct-sir maintains the excellent performance of ωLH22t for the DMABN twist and charge-transfer benchmarks but significantly improves the errors for triplet excitation energies (0.17 vs 0.24 eV). Additional test calculations for the AE6BH6 thermochemistry test set and large dipole moment and static polarizability test sets confirm that the focus on excitation energies in the optimization of ωLH23ct-sir has not caused any dramatic errors for ground-state properties. Although ωLH23ct-sir cannot replace ωLH22t as a general-purpose functional, it is preferable for problems requiring a universally good description of localized and charge-transfer excitations of both singlet and triplet multiplicity. Current limitations on the application of ωLH23ct-sir and other RSLHs to the study of singlet-triplet gaps of emitters for thermally activated delayed fluorescence are discussed. This work also includes the first systematic analysis of the influence of the local mixing function prefactor and the range-separation parameter in an RSLH on different types of excitations.

Ab Initio Investigation of Intramolecular Charge Transfer States in DMABN by Calculation of Excited State X-ray Absorption Spectra

Dual fluorescence in 4-(dimethylamino)benzonitrile (DMABN) and its derivatives in polar solvents has been studied extensively for the past several decades. An intramolecular charge transfer (ICT) minimum on the excited state potential energy surface, in addition to the localized low-energy (LE) minimum, has been proposed as a mechanism for this dual fluorescence, with large geometric relaxation and molecular orbital reorganization a key feature of the ICT pathway. Herein, we have used both equation-of-motion coupled-cluster with single and double excitations (EOM-CCSD) and time-dependent density functional (TDDFT) methods to investigate the landscape of excited state potential energy surfaces across a number of geometric conformations proposed as ICT structures. In order to correlate these geometries and valence excited states in terms of potential experimental observables, we have calculated the nitrogen K-edge ground and excited state absorption spectra for each of the predicted "signpost" structures and identified several key spectral features that could be used to interpret a future time-resolved X-ray absorption experiment.

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

Micro-Solvated DMABN: Excited State Quantum Dynamics and Dual Fluorescence Spectra

In this work, we report a complete analysis by theoretical and spectroscopic methods of the short-time behaviour of 4-(dimethylamino)benzonitrile (DMABN) in the gas phase as well as in cyclohexane, tetrahydrofuran, acetonitrile, and water solution, after excitation to the La state. The spectroscopic properties of DMABN were investigated experimentally using UV absorption and fluorescence emission spectroscopy. The computational study was developed at different electronic structure levels and using the Polarisable Continuum Model (PCM) and explicit solvent molecules to reproduce the solvent environment. Additionally, excited state quantum dynamics simulations in the diabatic picture using the direct dynamics variational multiconfigurational Gaussian (DD-vMCG) method were performed, the largest quantum dynamics "on-the-fly" simulations performed with this method until now. The comparison with fully converged multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) dynamics on parametrised linear vibronic coupling (LVC) potentials show very similar population decays and evolution of the nuclear wavepacket. The ring C=C stretching and three methyl tilting modes are identified as the responsible motions for the internal conversion from the La to the Lb states. No major differences are observed in the ultrafast initial decay in different solvents, but we show that this effect depends strongly on the level of electronic structure used.

Twisted intramolecular charge transfer (TICT) and twists beyond TICT: from mechanisms to rational designs of bright and sensitive fluorophores

The twisted intramolecular charge transfer (TICT) mechanism has guided the development of numerous bright and sensitive fluorophores. This review briefly overviews the history of establishing the TICT mechanism, and systematically summarizes the molecular design strategies in modulating the TICT tendency of various organic fluorophores towards different applications, along with key milestone studies and representative examples. Additionally, we also succinctly review the twisted intramolecular charge shuttle (TICS) and twists during photoinduced electron transfer (PET), and compare their similarities and differences with TICT, with emphasis on understanding the structure-property relationships between the twisted geometries and how they can directly affect the fluorescence of the molecules. Such structure-property relationships presented herein will greatly aid the rational development of fluorophores that involve molecular twisting in the excited state.

Simulation and Analysis of the Transient Absorption Spectrum of 4-(N,N-Dimethylamino)benzonitrile (DMABN) in Acetonitrile

4-(N,N-Dimethylamino)benzonitrile (DMABN) is a well-known model compound for dual fluorescence-in sufficiently polar solvents, it exhibits two distinct fluorescence emission bands. The interpretation of its transient absorption (TA) spectrum in the visible range is the subject of a long-standing controversy. In the present study, we resolve this issue by calculating the TA spectrum on the basis of nonadiabatic molecular dynamics simulations. An unambiguous assignment of spectral signals to specific excited-state structures is achieved by breaking down the calculated spectrum into contributions from twisted and nontwisted molecular geometries. In particular, the much-discussed excited-state absorption band near 1.7 eV (ca. 700 nm) is attributed to the near-planar locally excited (LE) minimum on the S1 state. On the technical side, our study demonstrates that the second-order approximate coupled cluster singles and doubles (CC2) method can be used successfully to calculate the TA spectra of moderately large organic molecules, provided that the system in question does not approach a crossing between the lowest excited state and the singlet ground state within the time frame of the simulation.

Excited state Born-Oppenheimer molecular dynamics through coupling between time dependent DFT and AMOEBA

We present the implementation of excited state Born-Oppenheimer molecular dynamics (BOMD) using a polarizable QM/MM approach based on a time-dependent density functional theory (TDDFT) formulation and the AMOEBA force field. The implementation relies on an interface between Tinker and Gaussian software and it uses an algorithm for the calculation of QM/MM energy and forces which scales linearly with the number of MM atoms. The resulting code can perform TDDFT/AMOEBA BOMD simulations on real-life systems with standard computational resources. As a test case, the method is applied to the study of the mechanism of locally-excited to charge-transfer conversion in dimethylaminobenzonitrile in a polar solvent. Our simulations confirm that such a conversion is governed by the twisting of the dimethylamino group which is accompanied by an important reorientation of solvent molecules.

A long lasting sunscreen controversy of 4-aminobenzoic acid and 4-dimethylaminobenzaldehyde derivatives resolved by ultrafast spectroscopy combined with density functional theoretical study

4-Aminobenzoic acid (PABA) is one of the earliest patented and most commonly used sunscreen components. There is however a long-lasting controversy on its photo-protective efficacy owing to the lack of information on its protolytic equilibrium and photo-dynamics after absorption of ultraviolet radiation in physiologically relevant aqueous solution. The excitation dynamics in water also remains largely unknown for analogs of PABA such as 4-dimethylaminoacetophenone (DMAAP) and 4-dimethylaminobenzaldehyde (DMABA) which are recognized as prototypes for photo-induced twisted intramolecular charge transfer (TICT). Herein we report a combined application of femtosecond broadband time-resolved fluorescence and transient absorption coupled with density functional theoretical study for PABA, DMAAP, and DMABA under several solvent conditions with representative properties in terms of the pH, polarity and hydrogen bonding capacity. The results we gained demonstrate that, in a neutral aqueous solution, PABA taking the deprotonated anion form in the ground state undergoes rapid protonation after excitation, producing excited state species in the neutral form that may shift effectively by intersystem crossing (ISC) to the long-lasting triplet state capable of damaging nucleic acids. This provides evidence at the molecular level for the detrimental effect of PABA if used as a sunscreen ingredient. In contrast, our investigation on DMAAP and DMABA unveils an unusual solvent controlled deactivation dynamics rendered by the participation of the carbonyl oxygen associated nOπ* state featuring energy and structure strongly responsive to solvent properties. In particular, these molecules in water exhibit solute-solvent hydrogen bonding at the sites of the carbonyl oxygen and the amino nitrogen which is, respectively, weakened and strengthened after the excitation, leading to state reversal and formation of a nOπ* state with a peculiar non-planar structure. This quenches strongly the excitation, eliminates the TICT, suppresses the ISC and opens up the otherwise inaccessible internal conversion (IC) to account for ∼80% of the entire deactivation. The IC, observed to proceed at a rate of ∼2.5 ps, allows the effective recovery of the ground state, providing substantial protection against ultraviolet irradiation. Moreover, the revelation of highly solvent sensitive fluorescence emission from DMABA and DMAAP implies the potential application of these molecules as the functional element in the design of sensory materials for probing the polarity and hydrogen bonding character of the surrounding environment.

Simulating the Nonadiabatic Relaxation Dynamics of 4-(N,N-Dimethylamino)benzonitrile (DMABN) in Polar Solution

The compound 4-(N,N-dimethylamino)benzonitrile (DMABN) represents the archetypal system for dual fluorescence, a rare photophysical phenomenon in which a given fluorophore shows two distinct emission bands. Despite extensive studies, the underlying mechanism remains the subject of debate. In the present contribution, we address this issue by simulating the excited-state relaxation process of DMABN as it occurs in polar solution. The potential energy surfaces for the system are constructed with the use of the additive quantum mechanics/molecular mechanics (QM/MM) method, and the coupled dynamics of the electronic wave function and the nuclei is propagated with the semiclassical fewest switches surface hopping method. The DMABN molecule, which comprises the QM subsystem, is treated with the use of the second-order algebraic diagrammatic construction (ADC(2)) method with the imposition of spin-opposite scaling (SOS). It is verified that this level of theory achieves a realistic description of the excited-state potential energy surfaces of DMABN. The simulation results qualitatively reproduce the main features of the experimentally observed fluorescence spectrum, thus allowing the unambiguous assignment of the two fluorescence bands: the normal band is due to the near-planar locally excited (LE) structure of DMABN, while the so-called "anomalous" second band arises from the twisted intramolecular charge transfer (TICT) structure. The transformation of the LE structure into the TICT structure takes place directly via intramolecular rotation, and is not mediated by another excited-state structure. In particular, the oft-discussed rehybridized intramolecular charge transfer (RICT) structure, which is characterized by a bent nitrile group, does not play a role in the relaxation process.

Using density based indexes to characterize excited states evolution

With the aim of offering new computational tools helping in the description of photochemical reactions and phenomena occurring at the excited state, we present in this work the capability of a density based index (Π) in locating decay channels from higher to lower excited states. The Π index, previously applied to disclose non-radiative decay channels from the first excited state to the ground state, is very simple in its formulation and can be evaluated, practically with no extra computational cost, and coupled to any quantum method able to provide excited states densities. Indeed, this index relies only on the knowledge of energetics and electron densities of the different electronic states involved in the decay. In the present work, we show the proficiency of the Π index in the general case of decay between excited states by applying it to two model systems well characterized both theoretically and experimentally. In both cases, this descriptor was successful in spotting the regions where excited states are more likely to decay, thus suggesting its potential interest for further application in the design of new compounds. © 2018 Wiley Periodicals, Inc.

Revealing the switching mechanisms of an off–on–off fluorescent logic gate system

We showed that TICT and PET were responsible for the off–on–off switching mechanisms of a fluorescent molecular logic gate.

Analytic Excited State Gradients for the QM/MM Polarizable Embedded Second-Order Algebraic Diagrammatic Construction for the Polarization Propagator PE-ADC(2)

An implementation of a QM/MM embedding in a polarizable environment is presented for second-order Møller-Plesset perturbation theory, MP2, for ground state energies and molecular gradients and for the second-order Algebraic Diagrammatic Construction, ADC(2), for excitation energies and excited state molecular gradients. In this implementation of PE-MP2 and PE-ADC(2), the polarizable embedded Hartree-Fock wave function is used as uncorrelated reference state. The polarization-correlation cross terms for the ground and excited states are included in this model via an approximate coupling density. A Lagrangian formulation is used to derive the relaxed electron densities and molecular gradients. The resolution-of-the-identity approximation speeds up the calculation of four-index electron repulsion integrals in the molecular orbital basis. As a first application, the method is used to study the photophysical properties of host-guest complexes where the accuracy and weaknesses of the model are also critically examined. It is demonstrated that the ground state geometries of the full quantum mechanical calculation for the supermolecule can be well reproduced. For excited state geometries, the deviations from the supermolecular calculation are slightly larger, but still the environment effects are captured qualitatively correctly, and energy gaps between the ground and excited states are obtained with sufficient accuracy.

Structure of 4-(Dimethylamino)benzonitrile Using Gas Electron Diffraction: A New Lease of Life for the Only Gas Electron Diffractometer in the U.K.

The continued demand for gas-phase molecular structures has led to the recommissioning of a gas electron diffractometer, formerly housed at the University of Reading. The gas electron diffractometer, now the only one of its kind in the U.K., is currently housed at the University of York, where it is now used routinely to determine directly structures of isolated molecules in the gas phase. The instrument has been fitted with an air-heated nozzle assembly to increase the range of molecules accessible to study in the gas phase; the efficacy of this assembly is demonstrated in this article via the determination of the gas-phase structure of 4-(dimethylamino)benzonitrile (DMABN) at high temperature. A series of complementary theoretical calculations using the B2PLYP DFT functional of Grimme et al. with correlation-consistent basis sets of double, triple, and quadruple-ζ quality are also presented. The agreement between the experimental and theoretical structural parameters attests to the accuracy of the applied theoretical calculations and of our gas-phase structural solution.

Charge-transfer and isomerization reactions of trans-4-(N-arylamino)stilbenes

We studied the excited-state dynamics of trans-4-(N-arylamino)stilbenes with aryl = phenyl (p1H), 4-methoxyphenyl (p1OM), or 4-cyanophenyl (p1CN) in solvents of varied polarity and viscosity by using femtosecond transient absorption and time-correlated single photon counting techniques. In nonpolar solvents the decay is triexponential, in which the rapid component corresponds to vibrational cooling combined with solvation, the intermediate temporal component 41-120 ps to trans-cis isomerization, and the long one ∼1 ns to fluorescence decay of the S₁ state. The S₁ state has a delocalized geometry and charge-transfer characteristics, corresponding to a planar intramolecular charge transfer (PICT) state. In polar solvents, an excited-state absorption band appears near 520 and 480 nm for p1OM and p1CN, respectively but not for p1H. This band has a rise lifetime of 4.3/7.5, 16.3/9.4, and 29.5/16 ps for p1CN/p1OM in acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively and matches the decay of the 600 nm PICT band. This band is thus assigned to the absorption of a singlet twisted intramolecular charge transfer state (TICT). The conversion rate decreases as the solvent viscosity is increased and is consistent with a large structural variation amplitude. Theoretical calculations using density functional theory (DFT), method PEB0, were employed to obtain the optimized structures and energies of those states. The PICT state possesses delocalized π electrons along the molecule. The TICT for p1CN is formed by twisting about the aminostilbene-benzonitrile C-N bond by ∼90°, but it is about the stilbene-aniline C-N bond for p1OM. We observed faster conversion rates for p1CN in alcoholic solvents, in which the lifetimes for both the PICT and TICT states are shortened to 20-99 ps and 120-660 ps, respectively, as a result of solvent-solute H-bonding interactions. In p1OM, the TICT state has an elongated C[double bond, length as m-dash]C bond in the stilbene moiety, which might facilitate the trans-cis isomerization reaction and thus account for the relatively short lifetime of 58-420 ps in polar solvents.

COSMO-RI-ADC(2) excitation energies and excited state gradients

Evaluating vertical excitation energies and excited state analytic gradients in solution at COSMO-ADC(2).

Nonradiative dynamics determined by charge transfer induced hydrogen bonding: a combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water

Hydrogen bonding with water alters nonradiative pathway of a twisted charge transfer state in methyl dimethylaminobenzoate.

Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N-dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations

With 4-fluoro-N,N-dimethylaniline (DMA4F), only a single fluorescence from a locally excited (LE) state is observed, irrespective of solvent polarity, temperature, and excitation wavelength. The relatively small excited state dipole moment μ_e = 7.3 D confirms the identification as LE. The single exponential fluorescence decays in the nonpolar n-hexane (2.04 ns) and in the strongly polar acetonitrile (5.73 ns) are a further support. Similar results are obtained with 4-chloro-N,N-dimethylaniline (DMA4Cl), having a chlorobenzene subgroup, a somewhat better electron acceptor than the fluorobenzene moiety in DMA4F. The absence of intramolecular charge transfer (ICT) with DMA4F is in accord with its large energy gap ΔE(S₁,S₂) of 8300 cm^-1 in n-hexane between the two lowest singlet excited states, which is even larger than that (6300 cm^-1) of N,N-dimethylaniline (DMA), for which an LE → ICT reaction likewise does not occur. The results with DMA4F are in contradiction with a publication by Fujiwara et al. ( Chem. Phys. Lett. 2013 , 586 , 70 ), in which the appearance of dual LE + ICT emission is reported for DMA4F in n-hexane and MeCN at room temperature. The ICT/LE fluorescence quantum yield ratio Φ'(ICT)/Φ(LE) reached a maximum value of ∼2, in n-hexane and surprisingly also in MeCN, as the excitation wavelength approaches the red-edge of the absorption spectrum. These, in our opinion, erroneous observations were supported by time-dependent density functional theory (TDDFT) calculations, which compute a perpendicularly twisted lowest ICT state (TICT) state. This is a further example of the general tendency of computations to find a TICT conformation for the lowest excited singlet state of electron donor/acceptor molecules such as p-substituted anilines.

Intramolecular Charge-Transfer Excited-State Processes in 4-(N,N-Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State

The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S₂(L_a, CT) and S₁(L_b, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S₂ to S₁. Polar solvent effects are not found to have a significant influence on this situation. Applying C_s symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.

The effect of gold(i) coordination on the dual fluorescence of 4-(dimethylamino)pyridine

Coordination of 4-(dimethylamino)pyridine to perhalophenyl gold(i) fragments leads to the formation of novel fluorescent complexes, in which the photophysical properties of the ligand are greatly influenced by the polarity induced by the organometallic fragment.

Excited-state intramolecular hydrogen bonding of compounds based on 2-(2-hydroxyphenyl)-1,3-benzoxazole in solution: a TDDFT study

The excited-state properties of intramolecular hydrogen bonding in the compounds based on 2-(2-hydroxyphenyl)-1,3-benzoxazole (6 and its tautomers 6a and 6b) have been investigated using theoretical methods. According to the geometric optimization and IR spectra in the ground and excited states calculated by density functional theory (DFT) and time-dependent DFT (TD-DFT) methods respectively, the type of intramolecular hydrogen bonding N⋯HO in 6 and 6a is demonstrated to be significantly strengthened, while NH⋯O in the tautomers 6a and 6b are proved to be sharply weakened upon excitation to excited state S1. The calculated absorption peaks of 6 are in good accordance with the experimental results. Moreover, other compounds based on 6 that R1 and R2 are both substituted as well as that only R1 is substituted are investigated to understand the effect of substituent on intramolecular hydrogen bonding. It is found that the hydrogen bond strength can be controlled by the inductive field effect of the substituent. In addition, the intramolecular charge transfers (ICT) of the S1 state for 6 and its tautomers 6a and 6b were theoretically investigated by analyses of molecular orbital.

Quantum-chemical insights into mixed-valence systems: within and beyond the Robin–Day scheme

The application of quantum-chemical methods to both organic and transition-metal mixed-valence systems is reviewed, with particular emphasis on how to describe correctly delocalisation vs. localisation near the borderline between Robin–Day classes II and III.

Intramolecular charge transfer and dual fluorescence of 4-(dimethylamino)benzonitrile: ultrafast branching followed by a two-fold decay mechanism

In this contribution we present new experimental and theoretical results for the intramolecular charge transfer (ICT) reaction underlying the dual fluorescence of 4-(dimethylamino)benzonitrile (DMABN), which indicate that the fully twisted ICT (TICT) state is responsible for the time-resolved transient absorption spectrum while a distinct partially twisted ICT (pTICT) structure is suggested for the fluorescent ICT state.