RI-CC2 Methods Research Articles

Why does the orientation of azulene affect the two-photon activity of a porphyrinoid-azulene system?

Attaching a dipolar molecule in a symmetric system induces a major change in the electronic structure, which may be reflected as the enhancement of the optical and charge-transfer properties of the combined system as compared to the pristine ones. Furthermore, the orientation of the dipolar molecule may also affect the said properties. This idea is explored in this work by taking porphyrinoid molecules as the pristine systems. We attached azulene, a dipolar molecule, at various positions of five porphyrinoid cores and studied the effect on charge-transfer and one- and two-photon absorption properties using the state-of-the-art RICC2 method. The attachment of azulene produces two major effects - firstly it introduces asymmetry in the system and, secondly, being dipolar, it makes the resultant molecule dipolar/quadrupolar. Porphyrin, N-confused porphyrin, sub-porphyrin, sapphyrin, and hexaphyrin are used as core porphyrinoid systems. The change in charge-transfer has been studied using the orbital analysis and charge-transfer distance parameter for the first five singlet states of the systems. The effect of orientation of azulene on the said properties is also explored. The insights gained from our observations are explored further at the dipole and transition dipole moment levels using a three-state model.

Benchmarking Density Functional Approximations for Excited-State Properties of Fluorescent Dyes.

This study presents an extensive analysis of the predictive power of time-dependent density functional theory in determining the excited-state properties of two groups of important fluorescent dyes, difluoroboranes and hydroxyphenylimidazo[1,2-a]pyridine derivatives. To ensure statistically meaningful results, the data set is comprised of 85 molecules manifesting diverse photophysical properties. The vertical excitation energies and dipole moments (in the electronic ground and excited states) of the aforementioned dyes were determined using the RI-CC2 method (reference) and with 18 density functional approximations (DFA). The set encompasses DFAs with varying amounts of exact exchange energy (EEX): from 0% (e.g., SVWN, BLYP), through a medium (e.g., TPSSh, B3LYP), up to a major contribution of EEX (e.g., BMK, MN15). It also includes range-separated hybrids (CAM-B3LYP, LC-BLYP). Similar error profiles of vertical energy were obtained for both dye groups, although the errors related to hydroxyphenylimidazopiridines are significantly larger. Overall, functionals including 40–55% of EEX (SOGGA11-X, BMK, M06-2X) ensure satisfactory agreement with the reference vertical excitation energies obtained using the RI-CC2 method; however, MN15 significantly outperforms them, providing a mean absolute error of merely 0.04 eV together with a very high correlation coefficient ( = 0.98). Within the investigated set of functionals, there is no single functional that would equally accurately determine ground- and excited-state dipole moments of difluoroboranes and hydroxyphenylimidazopiridine derivatives. Depending on the chosen set of dyes, the most accurate predictions were delivered by MN15 incorporating a major EEX contribution (difluoroboranes) and by PBE0 containing a minor EEX fraction (hydroxyphenylimidazopiridines). Reverse trends are observed for , i.e., for difluoroboranes the best results were obtained with functionals including a minor fraction of EEX, specifically PBE0, while in the case of hydroxyphenylimidazopiridines, much more accurate predictions were provided by functionals incorporating a major EEX contribution (BMK, MN15).

Theoretical study of linear and non-linear optical activity in dithienylethene-based photo-switch and its derivatives

In this contribution, our aim is to study the linear and non-linear optical activity and their variation with the addition/removal of electron withdrawing and electron donating groups in hexafluorinated dithienylethenes (HFDTE). By varying the terminal groups in the parent HFDTE, we have considered 16 closed-open pairs of substituted HFDTE and studied their electronic structures, one and two-photon absorption spectra and static first hyperpolarizability. All the calculations have been performed using the state-of-the-art linear and quadratic response theory within the framework of TD-DFT methods. RI-CC2 method is used as a reference to validate the applicability of TD-DFT method.

Two-Photon Absorption Activity of BOPHY Derivatives: Insights from Theory

We present a theoretical study of a two-photon absorption (2PA) process in dipolar and quadrupolar systems containing two BF2 units. For this purpose, we considered 13 systems studied by Ponce-Vargas et al. [J. Phys. Chem. B2017, 121, 10850−1085829136383] and performed linear and quadratic response theory calculations based on the RI-CC2 method to obtain the 2PA parameters. Furthermore, using the recently developed generalized few-state model, we provided an in-depth view of the changes in 2PA properties in the molecules considered. Our results clearly indicate that suitable electron-donating group substitution to the core BF2 units results in a large red-shift of the two-photon absorption wavelength, thereby entering into the desired biological window. Furthermore, the corresponding 2PA strength also increases significantly (up to 30-fold). This makes the substituted systems a potential candidate for biological imaging.

Influence of stacking on the ground and excited states of 2-aminopyridine

In the present work, the ground and excited electronic state properties of seven isomers of 2-aminopyridine (2AMP) stacked dimer have been investigated at the estimated CCSD(T)/CBS limit and RI-CC2/def2-TZVP levels of theory, respectively. The interaction energy values for the seven isomers fall in the range of −7.7 to −4.9 kcal/mol. The most stable dimer is stabilized by two weak hydrogen bonds as well as by stacking interaction between the two moieties. The performance of several density functional theoretical methods has been tested for the ground state as well as the excited states. For the ground state, the functionals B3LYP-D3 and B97D were found to perform the best, giving a mean absolute deviation of 0.3 kcal/mol. For the excited states, the use of the functionals M06-2X and CAM-B3LYP led to results in good agreement with those of the RI-CC2 method, giving a mean absolute deviation of 2.3 kcal/mol. In addition, the vertical excitation energy values for a single proton transfer in the hydrogen bonded (planar) 2-aminopyridine dimer and double proton transfer in the stacked dimer of (2AMP)2 are also reported.

Benchmarking the Performance of Exchange-Correlation Functionals for Predicting Two-Photon Absorption Strengths.

The present work investigates the performance of exchange-correlation functionals in the prediction of two-photon absorption (2PA) strengths. For this purpose, we considered six common functionals used for studying 2PA processes and tested these on six organoboron chelates. The set consisted of two semilocal (PBE and BLYP), two hybrid (B3LYP and PBE0), and two range-separated (LC-BLYP and CAM-B3LYP) functionals. The RI-CC2 method was chosen as a reference level and was found to give results consistent with the experimental data that are available for three of the molecules considered. Of the six exchange-correlation functionals studied, only the range-separated functionals predict an ordering of the 2PA strengths that is consistent with experimental and RI-CC2 results. Even though the range-separated functionals predict correct relative trends, the absolute values for the 2PA strengths are underestimated by a factor of 2-6 for the molecules considered. An in-depth analysis, on the basis of the derived generalized few-state model expression for the 2PA strength for a coupled-cluster wave function, reveals that the problem with these functionals can be linked to underestimated excited-state dipole moments and, to a lesser extent, overestimated excitation energies. The semilocal and hybrid functionals exhibit less predictable errors and a variation in the 2PA strengths in disagreement with the reference results. The semilocal and hybrid functionals show smaller average errors than the range-separated functionals, but our analysis reveals that this is due to fortuitous error cancellation between excitation energies and the transition dipole moments. Our results constitute a warning against using currently available exchange-correlation functionals in the prediction of 2PA strengths and highlight the need for functionals that correctly describe the electron density of excited electronic states.

Excitation Energies of Canonical Nucleobases Computed by Multiconfigurational Perturbation Theories.

In this computational work, we assessed the performance of abinitio multireference (MR) methods for the calculation of vertical excitation energies of five nucleobases: adenine, guanine, cytosine, thymine and uracil. In total, we have studied 38 singlet and 30 triplet excited states. Where possible we used the multireference configuration interaction (MRCI) method as a reference for various flavors of multireference perturbation theory to second order. In particular, we have benchmarked CASPT2, NEVPT2 and XMCQDPT2. For CASPT2, we have analyzed the single-state, multistate (MS) and extended MS variants. In addition, we have assessed the effect of the ionization potential electron affinity (IPEA) shift. For NEVPT2, we have used the partially and the strongly contracted variants. Further, we have tested the commonly used RI-CC2, RI-ADC2 and EOM-CCSD methods. Generally, we observe the following trends for singlet excited states: NEVPT2 is the closest MR method to MRCISD+Q, closely followed by CASPT2 with the default IPEA shift. The same trend is observed for triplet states, although NEVPT2 and CASPT2-IPEA are getting closer. Interestingly, the n, π* singlet excited states were described more accurately than π, π* excited states, while for triplet states the trend is inverted except for NEVPT2. This work is an important benchmark for future photochemical investigations.

Photodissociation UV-Vis Spectra of Cold Protonated Azobenzene and 4-(Dimethylamino)azobenzene and Their Benzenediazonium Cation Fragment.

Gas phase photodissociation electronic spectra of protonated azobenzene (ABH(+)) and 4-(dimethylamino)azobenzene (dmaABH(+)) were measured in a cryogenically cooled ion trap at temperatures of a few tens of Kelvin. Experimental results were complemented with electronic structure calculations in the ground state at the MP2/cc-pVDZ level of theory, and in the low lying excited states using the RI-CC2 method. Calculated energies revealed that only the trans isomers of the azonium molecular ions (protonation site on the azo group) will likely exist in the trap at the temperatures achieved in the experiment. The first transition of trans-ABH(+) is π* ← π, and the absorption band in the spectrum appears strongly red-shifted from that of the neutral molecule. The calculations showed that upon excitation the quasi-planar ground state (S0) transforms into a chairlike excited state (S1) by twisting the CNNC dihedral angle about 96°. A 41 cm(-1) active vibrational progression found in the ABH(+) spectrum may be associated with the twisting of the azo bond. Conversely, the electronic spectrum of dmaABH(+) exhibits a steep and unstructured S1 ← S0 absorption corresponding to a less distorted S1 state. The next two quasi-degenerate bands in the ABH(+) spectrum evidence sharper onsets and a charge transfer character. Using a second fragmentation laser and an additional He cooling pulse in the trap, it was possible to measure the UV spectrum of cold benzenediazonium fragments.

Excited State Proton Transfer and Deactivation Mechanism of 2-(4'-Amino-2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine and Its Analogues: A Theoretical Study.

In the present study, the results of comprehensive theoretical exploration on the nonradiative relaxation of three (hydroxyphenyl)imidazole-based organic compounds (abbreviated AHP, HPIP, and HPBI) in the gas phase are presented. Having small structural differences, the selected systems have commonalities in the excited state intramolecular proton transfer (ESIPT) process. The ground and S1 excited state potential energy profiles of titled systems have been determined on the basis of the RI-MP2 and RI-CC2 methods, and the effect of small structural distinctions on their photophysical characters will be extensively addressed. Although, in the presence of solvent, high fluorescence quantum yield is another characteristic of AHP and HPIP, owing to accessible conical intersections between the S1/S0 state potential energy profiles of both systems, nonradiative relaxation can be proposed as the most important feature of these two systems in the gas phase. These conical intersections are responsible for ultrafast deactivation of excited systems via internal conversions to the ground state. The nonradiative deactivation mechanism determined in this work deals with the remarkable photostability of the AHP and HPIP molecules.

Electronically Excited States of Neutral, Protonated α-Naphthol and Their Water Clusters: A Theoretical Study.

The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-naphthol is essentially different from that of its neutral homologue. In protonated α-naphthol, the calculations reveal that (1)σπ* state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the (1)πσ* state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-naphthol, mostly by alteration of the (1)ππ* PE profile, from a bound state in an isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-naphthol and water; with a small barrier; (ΔE< 0.1 eV).

Ultrafast CSpiro–O Dissociation via a Conical Intersection Drives Spiropyran to Merocyanine Photoswitching

The mechanism of the photochemical conversion of spiropyran to merocyanine is investigated theoretically. Calculations were performed at TD-DFT/ωB97XD/cc-pVDZ level of theory, which shows good agreement with the reference RI-CC2 method. A two-dimensional scan of the potential energy surface has been performed along the C-O distance and the central torsion angle in the ground state and in the first excited state, where the reaction takes place. Starting at the Franck-Condon geometry, the energy of the first excited state decreases in the direction of the C-O dissociation while the ground-state energy increases. This leads to a barrierless C-O bond dissociation in the first excited state. While relaxing on the S1 PES toward longer C-O distances, the torsion angle hardly changes, but other coordinates start to vary, leading to a conical intersection of the ground state and the first excited state at a C-O distance of about 3.4 Å. Passing the conical intersection, the reaction continues on the ground-state PES. At these large C-O distances, either barrierless Cspiro-O rebinding occurs that quenches spiropyran isomerization or rotation around the central torsion angle occurs that leads to merocyanine. For the latter an energy barrier of 0.1 eV must be overcome explaining the low quantum yield of spiropyran to merocyanine photoswitching.

Toward Reliable Prediction of the Energy Ladder in Multichromophoric Systems: A Benchmark Study on the FMO Light-Harvesting Complex.

We present an evaluation of the performance of various single-reference QM methods for the prediction of the relative site energies and transition moments of the Q bands in the bacteriochlorophyll a (BChl a) pigments of the Fenna-Matthew-Olson (FMO) complex. We examine the relative merits of ZINDO, CIS, TD-DFT (with the functionals PBE, BLYP, PBE0, B3LYP, and CAM-B3LYP) and RI-CC2 in reproducing the variations across the pigments that occur as a consequence of geometrical and electrostatic effects of the FMO complex by comparison to DFT-BHLYP/MRCI. We find that these pigments are near-multiconfigurational in nature and, thus, constitute critical cases for the RI-CC2 method. The commonly used ZINDO method is fairly reliable for the site energies of the isolated pigments; however, it overestimates the coupling to the environment, thereby leading to variations across the embedded pigments that are too drastic. The overall best performance is provided by the investigated TD-DFT methods, where PBE0 is found to be slightly superior to the other functionals tested.

Quantum chemical simulation of the interaction of membrane fluorescent probe 4-dimethylaminochalcone with hydroxy groups of the environment

The quantum chemical simulation of the ground and electron-excited states of diverse complexes of fluorescent probe 4-dimethylaminochalcone (DMAC) and water in vacuo was performed by the HF/MP2 and RI-CC2 methods. Molecules of the DMAC probe and water can form five types of stable complexes. The geometries corresponding to the potential energy minimum and dipole moments for two lowest singlet and one lowest triplet states were calculated for each type of the complexes. The partial charges on the DMAC atoms and their changes due to the intramolecular charge transfer upon photoexcitation were determined. The coordination of the water molecule at the carbonyl group of DMAC is preferable in vacuo. The formation of hydrogen bonds between the carbonyl group of DMAC and water molecules decreases the energy of the excited state of the complex 1(π, π*), due to which the fluorescence yield increases upon photoexcitation. The calculation results are confirmed by the experimental data on studying the fluorescence of the probe in binary mixtures of benzene and alcohols.

Electronically excited states of protonated phenol and para-substituted phenol

The low-lying electronic excited states of protonated phenol and para-Fluorophenol have been investigated extensively by RI-MP2/RI-CC2 methods. Although, protonation of phenol leads to a small red-shift-effect on the S1–S0 (ππ∗) electronic transition in respect to its neutral homologue, a large red-shift-effect, on the same electronic transitions of para-substituted phenol has been predicted. The ππ∗ excited state of protonated phenol stays in the UV range (4.34eV), while its πσ∗ state lies in the VUV region (8.3eV). The S1 excited-state geometry optimization of protonated phenol predicted unstable S1 state owing to the strong out-of-plane deformation in the benzene ring.

The invalidity of the photo-induced electron transfer mechanism for fluorescein derivatives

The ground and excited state geometries, the excitation and emission energies for a series of fluorescein derivatives in aqueous solutions have been investigated using time-dependent density functional theory (TD-DFT) with B3LYP and a long-range corrected CAM-B3LYP functional. The RI-CC2 method was employed to confirm the CAM-B3LYP results. As far as we know, the excited state geometries for a series of fluorescein derivatives were optimized for the first time, and the conformational changes upon photoexcitation were discussed. Importantly, the previous proposed photo induced electron transfer (PeT) mechanism for dictating the fluorescence quantum yield (Φ(fl)) of fluorescein derivatives was not fully supported by our calculations. Internal conversion may still be the most likely mechanism for dictating the Φ(fl) of fluorescein derivatives, which indicates a need for further experimental and theoretical studies on the excited state dynamics of fluorescein derivatives.

Protonation effect on the electronic structure of small PAHs: Acenaphthylene and Acenaphthene

The low lying singlet and triplet electronic excited states of neutral and protonated Acenaphthylene (C12H8, ACYN) and Acenaphthene (C12H10, ACN) have been investigated extensively by RI–MP2 and RI–CC2 methods. The first and second electronic excited tates (S1,S2) of protonated ACYN and ACN have ππ∗ nature and lie in the visible or UV region. Similar to naphthalene, anthracene, and other linear PAHs, the protonation of ACYN and ACN leads to a strong red shift of the electronic transition as compared to the neutral molecule. The calculations indicate a charged transfer character of S1–S0 transition in protonated ACYN and ACN as well as protonated naphthalene.

Low-lying absorption and emission spectra of pyrene, 1,6-dithiapyrene, and tetrathiafulvalene: A comparison between ab initio and time-dependent density functional methods

The gas-phase and in-solvent absorption and emission spectra of pyrene, 1,6-dithiapyrene, and tetrathiafulvalene are studied theoretically in the visible spectral region with the complete active space self-consistent field method, the complete active space second order perturbation theory method, and the resolution-of-identity second order perturbative corrected coupled cluster doubles (RICC2) method, with basis sets up to augmented polarized triple-zeta quality. The time-dependent density functional theory (TDDFT) formalism is also used employing a series of functionals. The nature of the excited states is discussed. With respect to literature theoretical values of the absorption and emission wavelengths of these three molecules, substantial improvements are achieved and comparison with experiment is favorable. Moreover, theoretical absorption and emission spectra of 1,6-dithiapyrene are presented for the first time. It is also exhibited that in most cases, a TDDFT treatment with hybrid functionals combined with a modest basis set (6-31G( *)) appears to be capable of providing reliable estimates for absorption and emission in all three molecules with relatively low computational cost. Furthermore, the RICC2 method (standalone or in conjunction with TDDFT) provides a satisfactory ab initio alternative, providing a good compromise between accuracy and computational effort.

Excited-State Dynamics of the 2-Hydroxypyridine−Ammonia Complex

The excited-state lifetimes of different vibrational levels of the 2-hydroxypyridine-ammonia complex have been recorded with the picosecond pump probe technique. These lifetimes decrease with increasing energy. The decrease of the lifetime is discussed based on the possible enol keto isomerization computed using RI-CC2 methods.

Nonradiative Deactivation in Benzylidene Malononitriles

Electronic structure calculations are used to examine the nuclear motions responsible for ultrafast internal conversion in the benzylidene malononitriles DMN (4-N,N-dimethylaminobenzylidenemalononitrile) and JDMN (julolidinemalononitrile). Gas-phase B3LYP and RI-CC2 calculations using triple-ζ valence polarized basis sets reproduce the structural features measured in the crystalline state and the solution-phase dipole moments of these molecules in their ground states with reasonable accuracy. Most properties of the vertical S0 → S1 transition, which is well separated from other transitions, are also reasonably reproduced by both types of calculation. The large change in dipole moment (8−9 D) upon excitation is grossly underestimated by TDDFT calculations, despite the fact that such calculations predict the transition energies to within experimental uncertainties. Exploration of the S1 potential energy surface of DMN using DFT, RI-CC2, and CASSCF methods indicates that the internal conversion pathway is do...

Photodynamics Simulations of Thymine: Relaxation into the First Excited Singlet State

Ab initio nonadiabatic dynamics simulations are reported for thymine with focus on the S(2) --> S(1) deactivation using the state-averaged CASSCF method. Supporting calculations have been performed on vertical excitations, S(1) and S(2) minima, and minima on the crossing seam using the MS-CASPT2, RI-CC2, MR-CIS, and MR-CISD methods. The photodynamical process starts with a fast (<100 fs) planar relaxation from the S(2) pipi* state into the pi(O)pi* minimum of the S(2) state. The calculations demonstrate that two pi-excited states (denoted pipi* and pi(O)pi*) are actually involved in this stage. The time in reaching the S(2)/S(1) intersections, through which thymine can deactivate to S(1), is delayed by both the change in character between the states as well as the flatness of the S(2) surface. This deactivation occurs in an average time of 2.6 ps at the lowest-energy region of the crossing seam. After that, thymine relaxes to the npi* minimum of the S(1) state, where it remains until the transfer to the ground state takes place. The present dynamics simulations show that not only the pi(O)pi* S(2) trapping but also the trapping in the npi* S(1) minimum contribute to the elongation of the excited-state lifetime of thymine.