Article pubs.acs.org/JPCA
Insight into the Mechanisms of Luminescence of Aminobenzonitrile and Dimethylaminobenzonitrile in Polar Solvents. An ab Initio Study Isabel Gómez, Pedro J. Castro, and Mar Reguero* Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, C. Marcel·lí Domingo 1, 43007, Tarragona , Spain S Supporting Information *
ABSTRACT: The dual fluorescence of 4-(dimethylamino)benzonitrile (DMABN) has been intensively studied in the last decades, but surprisingly there is not any detailed theoretical study of its photochemistry in polar solvents. In this work, we rationalize the different luminescent behavior of 4-aminobenzonitrile (ABN) and DMABN in acetonitrile by a computational study developed at the CASSCF/CASPT2 level and using the Polarized Continuum Model to reproduce the solvent environment. We present here the critical geometries, energies, and connections between the potential energy surfaces of the low-lying excited states: the locally excited state (LE) and several intramolecular charge transfer states (ICT). The computational results show that the topology of the potential energy surfaces (PES) does not change substantially when the effect of a polar solvent is included, in comparison with the gas phase. For DMABN, though, polar solvents stabilize preferentially the ICT states in such a way that the different interplay with the LE state induces strong qualitative changes in the photochemistry of this compound. Specifically, the planar ICT (PICT) species located on the S2 surface in the gas phase is, in acetonitrile, located on the S1 surface. that is, at the geometry of the PICT minimum, the LE state is higher in energy than the ICT. Now LE and PICT minima are practically degenerate and, given that both correspond to first excited state species, emission can take place from both of them. However, the twisted ICT (TICT) species is still the most favored thermodynamically so it is expected that this species would be preferentially populated. On the other hand, for ABN the equilibrium lies in favor of LE, as the TICT species was found at a much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN, even in polar solvents.
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INTRODUCTION The interest in the spectroscopic properties of 4(dimethylamino)benzonitrile (DMABN) dates back to 1959, when Lippert et al. first discovered its dual fluorescence in polar solvents.1 The nonlinear optical properties of these compounds have also given rise to intensive research in the field of organic materials due to their possible application as electrooptical switches, chemical sensors, and fluorescence probes2−5 and in fact some of these π-electron donor/acceptor systems have already been used in the fabrication of molecular switches based on their dual fluorescence. 6 The origin of this dual luminescence, observed in polar solvents, is associated with the existence of two different singlet excited states. Whereas one of the two emitting states is a typical locally excited (LE) state, the other one has intramolecular charge transfer (ICT) character and is usually slightly higher in energy. Because of its large dipole moment, ICT species can be stabilized relative to LE species by solvents of suitable polarity, making the LE and ICT populations comparable. Dual fluorescence then can occur due to emission from the LE (Lb-like) and from the ICT (Lalike) species, both corresponding to minima on the S1 potential energy surface (PES). Characteristically, the emission from the © 2015 American Chemical Society
ICT state appears red-shifted relative to the emission from the LE band (for a review, see ref 7). Much time and effort has been devoted to study ICT systems in general and aminobezonitrile derivatives in particular, what is reflected in the large amount of literature devoted to the subject (to avoid cumbersome citations, a more detailed (although not exhaustive) compilation is included in the Supporting Information). Nevertheless, there are still two main points of controversy: the structure and geometrical characteristics of the species responsible of the anomalous fluorescence, and the mechanism of population of the LE and ICT states. We will address these two aspects separately in what follows. In the lively debate about the molecular structure of the ICT emitting species of DMABN, five different models have been proposed (Scheme 1). Let us mention first the commonly named TICT, for twisted ICT, which supposes a charge separation in the CT state resulting from complete decoupling between the amino and benzonitrile moieties by twisting the amino group into a perpendicular position.8−10 A planar ICT Received: February 11, 2015 Published: February 13, 2015 1983
DOI: 10.1021/acs.jpca.5b01421 J. Phys. Chem. A 2015, 119, 1983−1995
Article
The Journal of Physical Chemistry A
of the ionization potential and electron affinity by the substituents of the donor and acceptor moieties.26,27 In this sense, the N-methyl groups of DMABN decrease the ionization potential of the amino moiety decreasing the energy of the CT state. Given that the π → π* excitations are very similar in both compounds, which have the same bezonitrile moiety, the energy gap between the first and second excited states decreases, favoring the population of the CT state.28,29 Another factor to take into account is the sterical repulsion introduced by the substitution of H atoms by methyl substituents, which induces wagging or twisting of the amino group, deformations that favor the CT species. On top of this, the induced delocalization of the positive charge on the dimethylamino group in DMABN gives place to a larger dipole moment than in ABN. Therefore, polar solvents will stabilize the CT state more in DMABN than in ABN, leading to a noticeable dependence of the fluorescence patterns on the solvent polarity in the former compound, not observed in the latter one. Some time ago, we reported the results of a CASSCF and RASSCF study on ABN and DMABN in the gas phase.30 In view of our results we were able to explain the mechanism of the dual fluorescence of DMABN, and why ABN does not undergo the ICT reaction. Our study showed that for both molecules, ABN and DMABN, the LE state was not only lower in energy than the CT state at the FC region, but it also presented the less energetic structure of all the excited species located on the S1 surface. The TICT, RICT, and PICT structures were also stable (i.e., they correspond to minima on the potential energy surface of the ICT state) but, while the TICT and RICT were first excited state structures (i.e., these minima were located on the S1 surface), the PICT structure only existed on the S2 surface, higher in energy than the LE state, and consequently, following Kasha’s rule and given the large energy gap between both states, the probability of emission from the PICT species is negligible. The RICT minimum was much higher in energy than the TICT one, so we concluded that it could not be populated in an appreciable amount and consequently would not be competitive with the latter one. In that study no pTICT minimum was located. On the basis of these results, we proposed a detailed mechanism to explain the absence or appearance of dual fluorescence in ABN and DMABN. After the excitation to the S2 (CT) state, the system relaxes quickly to a shallow S2-PICT minimum. Given that the ICT emitting structure must be in this case located on S1, the following part of the reaction path must be nonadiabatic. That is, the S1 and S2 surfaces must cross. We located a conical intersection near the S2-PICT minimum where the S2→ S1 internal conversion takes place easily. In fact, the S2/S1CT-LE radiationless decay can occur at any point of the extended conical intersection “seam” that runs almost parallel to the amino torsion coordinate. The lowest energy point on this seam corresponds to a structure where the carbon atom of the −CN(R)2 group is pyramidalized but the amino group is untwisted, so the branching at the conical intersection must favor the population of the LE minimum, that is, the formation of the LE species. However, the seam is accessible for a large range of torsional angles so, if the decay takes place at large torsional angles, the probability of forming the S1-TICT species enlarges. The LE and TICT minima on S1 are also adiabatically linked along the amino torsion reaction coordinate. Thus, the LE-TICT equilibration on S1 and the dual fluorescence will be controlled by (a) the position along the amino-group-twist coordinate where the S2/S1CT-LE
Scheme 1. Models of ICT-Emitting Species for ABN Derivatives: R = H, ABN; R = CH3, DMABN
(PICT) structure was first suggested by Zachariasse,11,12 with the amino group lying in the benzene plane. In the wagged ICT (WICT)13,14 model, almost completely dismissed nowadays, a change from planar sp2 to pyramidal sp3 hybridization of the amino nitrogen is proposed to induce a decoupling of the nitrogen lone pair from the benzene ring. The rehybridized ICT (RICT) model15,16 is characterized by the rehybridation from sp to sp2 of the cyano carbon atom. The RICT species correspond to the minimum energy structure of the so-called πσ* state, where the excitation is localized in the triple bond of the cyano group. While the RICT species is nowadays ruled out as an emitting species, there are still debates on the involvement of the πσ* state in the mechanism of charge transfer.17−19 The partially twisted (pTICT) model20 has been proposed lately to explain the time-resolved transient absorption spectra observed experimentally.21 Here the benzene and amino groups are only partially rotated (twisting angle
