Density Functional Study of Excited Charge Transfer State Formation in 4-(N,N-Dimethylamino)benzonitrile

Molecular torsion controls the excited state relaxation pathways of multibranched tetraphenylpyrazines: effect of substitution of morpholine vs. phenoxazine

Multibranched donor-acceptor derivatives exhibiting desirable photophysical properties are efficiently used in optoelectronic devices, in which the excited state relaxation dynamics of the derivatives control the efficiency of the devices. Here, the effect of intramolecular torsion on the excited state relaxation dynamics of tetraphenylpyrazine (TPP) derivatives in non-polar (toluene) and polar (THF) solvents is investigated by substituting the electron donor of morpholine (TPP-4MOP) and phenoxazine (TPP-4PHO) leading to the planar and twisted configurations, respectively, using femtosecond and nanosecond transient absorption spectroscopy. In the steady state, TPP-4MOP showed feeble emission (ΦF ∼0.03) due to the weak donor by the delocalization of electron density supported by theoretical optimization. The TPP-4PHO exhibited strong emission (ΦF ∼0.18) in toluene compared to that in THF, in which it showed a large Stokes shift (∼9691 cm-1) with low fluorescence quantum yield (ΦF ∼0.01). The observation of large Stokes shifts, inherent nature and theoretical calculations of TPP-4PHO suggest the twisting of the dihedral angle between tetraphenylpyrazine and phenoxazine in the excited state leading to the twisted intramolecular charge transfer state (TICT). The femtosecond and nanosecond transient absorption and picosecond time-resolved emission spectra of TPP-4PHO revealed the signature of the existence of both the partial TICT and TICT states in THF leading to the triplet state. Whereas in the case of TPP-4MOP, the transient absorption spectra showed the formation of the triplet state from the local excited state without the involvement of the TICT state. Aggregation studies of TPP-4PHO in a THF and water mixture reflect the elimination of the TICT state by the restriction of intramolecular torsion in the aggregates leading to an increase of 12-fold of the fluorescence intensity along with shifting of the maximum towards the blue region. These studies revealed that the excited state relaxation pathways of the derivatives are controlled by polarity-dependent torsional motion.

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.

Visual detection of viscosity through activatable molecular rotor with aggregation-induced emission

Food safety has become one of the urgent affairs in the global public health studies, and irregular viscosity is closely associated with the food spoilage extent. In this study, one kind of activatable molecular rotor (TPA-PBZ) based on triphenylamine derivates has been synthesized via the Schiff base condensation reaction. This rotor is comprised by donor-accepter conjugated structure, with aggregation induced-emission feature and a large Stokes shift of 160 nm in water. The rotation of aromatic rings in TPA-PBZ is restricted in high-viscosity microenvironment, with the gradually increasing fluorescence emission signal at 568 nm. Significantly, this rotor TPA-PBZ has successfully been applied not only in the determination of thickening effects of food gum, but also in the detection of viscosity enhancement during the liquid food spoilage process. This molecular rotor can be utilized as an intelligent monitor platform for food quality and safety inspection in viscosity-related conditions.

Exploration of Noncovalent Interactions, Chemical Reactivity, and Nonlinear Optical Properties of Piperidone Derivatives: A Concise Theoretical Approach

The organic compounds with a π-bond system lead to electric charge delocalization which enables them to reveal fascinating nonlinear optical properties. Mono-carbonyl curcuminoids also have an appealing skeleton from the conjugation view point. Interesting chemical structures of the 3,5-bis(arylidene)-N-benzenesulfonyl-4-piperidone derivatives motivated us to perform density functional theory (DFT)-based studies. Therefore, computations using the B3LYP/6-311G(d,p) functional of DFT were executed to explore geometric parameters, highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energies, and natural bond orbital (NBO) analyses. Moreover, three different functionals such as HF, B3LYP, and M06 with the 6-311G(d,p) basis set were used to investigate the average polarizability ⟨α⟩ and first hyperpolarizability (β_tot)-based properties of all compounds. A good concurrence among calculated and experimental parameters was obtained through root mean square error calculations. The molecular stability of piperidone derivatives was examined using the Hirshfeld surface and NBO analyses. Natural population analysis was also performed to obtain insights about atomic charges. Calculated HOMO-LUMO energies showed that charge transfer interactions take place within the molecules. Moreover, global reactivity parameters including electronegativity, chemical hardness, softness, ionization potential, and electrophilicity were calculated using the HOMO and LUMO energies. The average polarizability ⟨α⟩ and first hyperpolarizability (β_tot) values of all compounds were observed to be larger in magnitude at the aforesaid functional than the standard compound.

Influence of charge transfer on the isomerisation of stilbene derivatives for application in cancer therapy

The photoisomerisation of non-toxic trans-combretastatin CA4 to its cytotoxic cis isomer demonstrates the high potential of this and similar compounds for localised cancer therapy. The introduction of intramolecular charge-transfer character by altering the substituents of combretastatin systems opens up possibilities to tailor these stilbene derivatives to the special demands of anticancer drugs. In this TDDFT study we explore how absorption wavelengths for both the trans and cis isomers can be red shifted to enable deeper light penetration into tissue and how the trans → cis and cis → trans isomerisations are affected by charge transfer effects to different degrees.

On the dual emission of p-dimethylaminobenzonitrile and its photophysical implications

A solvatochromic analysis of available absorption and LE and TICT emission data in p-dimethylaminobenzonitrile (DMABN) was conducted. Applying the Abe method to the results of a thermochromic analysis of DMABN in 1-chlorobutane (ClB) allowed us to determine the polarizability and dipole moment of the excited electronic states involved in the absorption and emission transitions. As shown herein, the LE → TICT excited state reaction for DMABN is triggered by the solvent polarity but is additionally influenced by the viscosity. The experimental evidence negates the assumption that the radiative constant for the TICT fluorescence of DMABN is temperature-dependent.

A THEORETICAL STUDY ON DUAL FLUORESCENCE OF 4-DIMETHYLAMINOPYRIDINE BY POLARIZABLE CONTINUUM MODEL

Dual fluorescence spectra of 4-dimethylaminopyridine (DMAP) is investigated using time-dependent density functional theory and complete active space self-consistent field methods. Electronic absorption and emission spectra of DMAP have been investigated in three solvents, that is, cyclohexane, chloroform, and acetonitrile. The present study reveals that the dual fluorescence phenomena of DMAP appear in the cases of acetonitrile and chloroform, but not in cyclohexane. The electronic structures of the ground state and the intramolecular charge transfer states are, therefore, studied in order to reveal the insight of dual fluorescence. Our theoretical results suggest that the twisting of dimethylamino moiety in DMAP is necessary for the intramolecular charge transfer. The mechanism of the dual fluorescence of DMAP is discussed based on the twisted intramolecular charge transfer model and the dual fluorescence phenomenon is explained theoretically.

Influence of prototropic reactions on the absorption and fluorescence spectra of methyl p-dimethylaminobenzoate and its two ortho derivatives

The influence of prototropic reactions on the spectral characteristics of methyl p-dimethylaminobenzoate (I) and its o-methoxy (II) and o-hydroxy (III) derivatives has been studied using steady-state spectroscopic technique and quantum-chemical calculations. This study concerns the solvent-induced shift of the absorption, locally excited (LE) and intramolecular charge transfer (ICT) fluorescence bands in the neat tetrahydrofuran (THF) and its hydrochloric acid solutions at different HCl concentrations. On the basis of the experimental results and quantum-chemical calculations, it was shown that in a hydrochloric acid solution the studied molecules exist as a mixture of neutral, mono-, and dicationic forms. Additionally, the results of spectroscopic measurements were used to calculate, according to the Benesi-Hildebrand method, the equilibrium constants of protopropic reactions in the ground, S(0), and excited, S(1), states. Our findings predestine molecules I and II to be used as acid fluorescence probes in a region of 0-2.5 M of [H(+)] concentrations.

Development of Bacteriorhodopsin Analogues and Studies of Charge Separated Excited States in the Photoprocesses of Linear Polyenes†

Development of bacteriorhodopsin (bR) analogues employing chromophore substitution technique for the purpose of characterizing the binding site of bR and generating bR analogues with novel opto-electronic properties for applications as photoactive element in nanotechnical devices are described. Additionally, the photophysical and photochemical properties of variously substituted diarylpolyenes as models of photobiologically relevant linear polyenes are discussed. The role of charge separated dipolar excited states in the photoprocesses of linear polyenes is highlighted.

Long-range corrected time-dependent density functional study on fluorescence of 4,4′-dimethylaminobenzonitrile

Dual fluorescence of 4,4(')-dimethylaminobenzonitrile (DMABN) was theoretically investigated on the basis of long-range corrected time-dependent density functional theory. Excited-state geometry optimization states and single-point energy calculations with and without solvent effect were carried out. It has been explained that DMABN emits dual fluorescence only in polar solvents through locally excited (LE) and charge transfer (CT) states. It was, however, concluded from this study that although the main spectrum of dual fluorescence in acetonitrile solvent is clearly due to twisted intramolecular CT fluorescence, small secondary fluorescence in acetonitrile may also emanate from CT fluorescence during the DMABN twisting process. This conclusion is supported by an experimental interpretation on polarization spectroscopy. It was also found that the optimized DMABN geometries have certain wagging angles for the CT state and no wagging angle for the LE state. This may support an early experimental hypothesis that the dual fluorescence of DMABN is induced by the wagging mode due to vibronic coupling between LE and CT states. Consequently, the authors propose a fluorescence mechanism of DMABN in gas phase and in acetonitrile solvent: the main absorption proceeds to the CT state in both situations. In gas phase, single fluorescence is chiefly emitted from the LE state through the internal conversion from CT to LE states. Dual fluorescence in acetonitrile solvent may only be emitted from the CT state.

Role of Excited State Intramolecular Charge Transfer in the Photophysical Properties of Norfloxacin and Its Derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.

Influence of Coulomb-attenuation on exchange–correlation functional quality

The dependence of functional quality on the attenuation parameters--which control the limiting (r12-->0, infinity) values and the rate of attenuation--is investigated for a Coulomb-attenuated exchange-correlation functional. For the attenuation and functional form considered, satisfaction of an exact long-range condition is detrimental for properties such as atomisation energies and bond lengths, but does improve classical reaction barriers and small molecule electronic excitation energies. The functionals considered can provide high quality valence, Rydberg, intramolecular and asymptotic intermolecular charge transfer (CT) excitations, but none are able to provide a simultaneously optimal description of all classes; CT excitations are not necessarily improved compared to those from conventional functionals. The study highlights the need for further development of Coulomb-attenuated functionals.

Intramolecular Charge-Transfer State Formation of 4-(N,N-Dimethylamino)benzonitrile in Acetonitrile Solution: RISM-SCF Study

Intramolecular charge-transfer (ICT) state formation of 4-(N,N-dimethylamino)benzonitrile in acetonitrile solution is studied by the reference interaction site model self-consistent field (RISM-SCF) method. Geometry optimizations are performed for each electronic state in solution with the complete-active-space SCF wave functions. Dynamic electron correlation effects are taken into account by using the multiconfigurational quasidegenerate perturbation theory. Two-dimensional free energy surfaces are constructed as the function of the twisting and wagging angles of the dimethylamino group for the ground and locally excited (LE) states. The calculated absorption and fluorescence energies are in good agreement with experiments. The validity of the twisted ICT (TICT) model is confirmed in explaining the dual fluorescence, and the possibility of the planar ICT model is ruled out. To examine the mechanism of the TICT state formation, a "crossing" seam between the LE and charge-transfer (CT) state surfaces is determined. The inversion of two electronic states occurs at a relatively small twisting angle. The effect of solvent reorganization is also examined. It is concluded that the intramolecular twisting coordinate is more important than the solvent fluctuation for the TICT state formation, because the energy difference between the two states is minimally dependent on the solvent configuration.

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.

Photoinduced intramolecular charge transfer to meta position of benzene ring in 6-aminophthalides

Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.

Photoinduced Intramolecular Charge Transfer in 4-(Dimethyl)aminobenzonitrile − A Theoretical Perspective

Recent advances in time-dependent density functional theory (TDDFT) have led to computational methods that can predict properties of photoexcited molecules with satisfactory accuracy at comparably moderate cost. We apply these methods to study the photophysics and photochemistry of 4-(dimethyl)aminobenzonitrile (DMABN). DMABN is considered the paradigm of photoinduced intramolecular charge transfer (ICT), leading to dual fluorescence in polar solvents. By comparison of calculated emission energies, dipole moments, and vibrational frequencies with recent results from transient spectroscopy measurements, a definitive assignment of the electronic and geometric structure of the two lowest singlet excited states of DMABN is possible for the first time. We investigate the mechanism of the ICT reaction by means of minimum energy path calculations. The results confirm existing state-crossing models of dual fluorescence. Our study suggests that analytical TDDFT derivative methods will be useful to predict and classify emissive properties of other donor-acceptor systems as well.

The role of πσ[sup ∗] state in intramolecular electron-transfer dynamics of 4-dimethylaminobenzonitrile and related molecules

Evidence is presented which indicates that the photoinduced intramolecular charge transfer (ICT) in 4-dimethylaminobenzonitrile proceeds by a new mechanism in which pi sigma(C triple bond N) (*) state is the intermediate of a consecutive process that takes the initially excited pi pi(*) state to the fully charge-separated ICT state. The absence of the ICT-state formation in 4-aminobenzonitrile is attributed to the smaller electron-donor strength of the amino group relative to the dimethylamino group, which hinders the pi sigma(*)-->ICT charge-shift reaction.

Excited states of 4-dimethylaminopyridines: magnetic circular dichroism and computational studies

The ultraviolet transitions of 4-(dimethylamino)pyridine and its derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine were investigated by electronic absorption and magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The ortho-methylation in the pyridine ring creates a steric hindrance to coplanarity. The resulting changes in the strength of the dimethylamino substitutent are reflected in the MCD spectra. Near-UV experimental data indicate a presence of three low-energy transitions, assigned to 1Lb, 1La and 1(n,pi*) excited states. These results are corroborated by TD-DFT and INDO/S calculations.

Time-dependent density functional theory (TDDFT) study of the excited charge-transfer state formation of a series of aromatic donor-acceptor systems

Singlet excitation energy calculations for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4-(N,N-dimethylamino)benzonitrile, 4DMAB-CN, 4-(N,N-dimethylamino)benzaldhyde, 4DMAB-CHO, 1-methyl-7-cyano-2,3,4,5-tetrahydro-1H-1-benzazepine, NMC7) and nondual (4-aminobenzonitrile, 4AB-CN, 3-(N,N-dimethylamino)benzonitrile, 3DMAB-CN, and 4-nitro(N,N-dimethyl) aniline, 4DMAB-NO(2)) fluorescent have been performed using time-dependent density functional theory (TDDFT). The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) (Bg) basis set have been used to compute excitation energies. Ground-state geometries were optimized using density functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For most of the molecules presented in this study, potential energy surfaces have been computed according to the coordinates related to the three following mechanisms proposed in the literature: twisting, wagging, and planar intramolecular charge transfer (ICT). Comparison of the three models for the different molecules leads to the conclusion that only the twisting ICT model is able to explain the low frequency, strongly solvent-dependent energy band present in the fluorescence spectra. According to this model, the 4AB-CN molecule is calculated to be nondual fluorescent in agreement with the experimental spectra. The single band observed in the fluorescence spectra of TMAB-CN (4-(N,N-dimethylamino)-3,5-(dimethyl)benzonitrile) is due to a large stabilization of the charge-transfer excited state along the twisting coordinate. The nondual fluorescence of the 4DMAB-NO(2) molecule is explained by the same mechanism. In the case of 3DMAB-CN, the single observed emission, which is solvent-dependent, has been assigned to the lowest charge-transfer excited state. The dual fluorescence of 4DMAB-CN and 4DMAB-CHO is explained within the twisting ICT model by a double mechanism (already proposed by Serrano et al.: Serrano-Andrés, L.; Merchán, M.; Roos, B. J.; Lindh, R. J. Am. Chem. Soc. 1995, 117, 3189) that involves the presence of two low-lying states close enough in energy. The observation of dual fluorescence in NMC7, that has been one of the origins of the planar ICT model put forward by Zachariasse et al. (Zachariasse, K.; van der Haar, T.; Hebecker, A.; Leinhos, U.; Kühnle, W. Pure Appl. Chem. 1993, 65, 1745), could be fully understood by a double mechanism within the twisting ICT model. Within the set of investigated molecules, our calculations confirm that the twisting ICT model is the only mechanism acceptable to explain the dual and nondual fluorescence phenomenon. Our calculations are in complete agreement with experimental data.

Ultrafast charge transfer via a conical intersection in dimethylaminobenzonitrile

The L(a)-like S2 state (2A) of 4-(dimethylamino)benzonitrile was pumped at 267 nm in the gas phase at 130 degrees C. Nonresonant multiphoton ionization at 800 nm with mass-selective detection then probed the subsequent processes. Whereas ionization at the Franck-Condon geometry only gave rise to the parent ion, fragmentation increased on motion towards the charge-transfer (CT) state. This useful difference is ascribed to a geometry-dependent resonance in the ion. The time constants found are interpreted by ultrafast (approximately 68 fs) relaxation through a conical intersection to both the CT and the L(b)-type S1 state (1B). Then the population equilibrates between these two states within 1 ps. From there the molecule relaxes within 90 ps to a lower excited state which can only be a triplet state (T(n)) and then decomposes within 300 ps. Previous experiments either investigated only 1B --> CT relaxation-which does not take place in the gas phase or nonpolar solvents for energetic reasons--or, starting from S2 excitation, typically had insufficient time resolution (>1 ps) to detect the temporary charge transfer. Only recently temporary population of the CT state was found in a nonpolar solvent (Kwok et al., J. Phys. Chem. A. 2000, 104, 4188), a result fully consistent with our mechanism. We also show that S2 --> S1 relaxation does not occur vertically but involves an intermediate strong geometrical distortion, passing through a conical intersection.

An experimental test of C-N bond twisting in the TICT state: syn-anti photoisomerization in 2-(N-methyl-N-isopropylamino)-5-cyanopyridine

A previously untested essential consequence of the "TICT" (twisted internal charge transfer) interpretation of the nature of the "anomalous" excited state of p-dimethylaminobenzonitrile (1) and related compounds has now been verified: at -90 degreesC, the dual fluorescence of a desymmetrized analogue, 2-(N-methyl-N-isopropylamino)-5-cyanopyridine (2), in methanol (MeOH) is accompanied by syn-anti isomerization around the C-N bond, whereas its ordinary fluorescence in tetrahydrofuran is not.