1 Introduction

Aniline (C6H5NH2) is an important prototypical aromatic amine for both the photochemistry and photophysics behaviors modeling along the amino moiety in the purine-derived DNA bases guanine and adenine [1, 2]. The dissociative πσ* state has been suggested to provide a nonradiative relaxation pathway along its NH2 coordinates via πσ*/S0 conical intersection, leading either to N–H bond dissociation or to the species in the ground state [3].

The photochemistry of isolated aniline has been extensively studied both experimentally [1, 4,5,6,7,8,9,10,11,12,13,14,15,16,17] and theoretically [1, 7, 14,15,16,17,18,19,20,21,22,23]. These studies focused mainly on the role of 1πσ* channel in the excited state dynamics. Two 1πσ* mixed electronic configurations are deduced: Rydberg (1π3s) and valence antibonding character (1πσ*) states. It has been suggested that in the Franck–Condon (FC) region, the mixed 1π3s/πσ* state has a significant Rydberg character and changes into a dissociative valence character at longer N–H bond stretching [4, 8, 18]. Whereas, in the case of phenol, the 1π3s and 1πσ* components are nearly degenerate in the vertical Franck–Condon region, and the entire potential energy surface will be almost totally dissociative [24, 25].

Works by Ashfold and co-workers [1] and Stavros and co-workers [7] concluded that the non-adiabatic coupling that occurs in the Franck–Condon region is responsible for the transfer of the initially populated S1(1ππ*) state to the S2(1πσ*) state, leading to the N–H bond fission and the anilino C6H5NH radical formation. This photoreaction is energetically accessible for λ = 269.5 nm.

In addition, the aniline dimer and trimer structures in the ground and lowest electronically excited states have been previously studied [26,27,28]. These structures are stabilized by hydrogen bonds between the hydrogen of the amino group on one part and the nitrogen atom or with the π electrons of its benzene ring on the other part. To reveal the solvation effects on the photochemistry of aniline, Poterya et al. have investigated the photodissociation dynamics of N–H bonds in aniline in homoclusters (PhNH2)n and mixed clusters with water PhNH2-(H2O)n [15]. For small homoclusters (PhNH2)n≤3 and small mixed PhNH2-(H2O)n≤10 clusters that presented free N–H bonds, the formation of fast H fragments is attributed to the dissociation on the πσ* surface. While on the surface of large water clusters, (H2O)430, they found experimentally that dissociation via πσ* state is quenched [15]. In the same context and more recently, (PhNH2)n≥3 homocluster dynamics along the repulsive 1πσ* state have been detected, while the dimer does not present the N–H dissociative 1πσ* channel [29].

Among the photochemical reactions involved in these types of molecular hydrogen bonding interactions, hydrogen atom transfer (HAT) or detachment and proton-coupled electron transfer (PCET) have received a wide number of studies [30,31,32,33]. However, these mechanisms are different and can be distinguished by the analysis of frontier molecular orbitals, based on whether a proton and an electron are transferred between the same or different sets of molecular orbitals [32], and the potential energy surface characteristics, in terms of the energetic accessibility of the conical intersection between the lowest electronic states [33].

Pyridine is an important biological solvent and is well known as an electron quencher in photochemical reactions. In this study, we have performed investigations of the photochemistry and photophysics processes involved through the N–H bond photodissociation in aniline and the complexes it forms with pyridine PhNH2-(Py)n (n = 1, 2). This system is a model for which the proton donor and acceptor are π electron conjugated system. Therefore, it is convenient to compare the N–H photodissociation in both isolated aniline and clustered with pyridine. Aniline undergoes a fast dissociation of the N–H bond as an isolated molecule, while strong hydrogen bonding affects the photodissociation of the N–H bond upon complexation in the Py clusters. We may ask whether the photochemical and photophysical processes are operative in these kinds of reactions.

To reveal the molecular details of the photochemical and photophysical processes, potential energy profiles of the excited state PCET reactions from PhNH2 to Py as well as hydrogen atom detachment from the NH2 group of PhNH2 were determined with ab initio calculations. The reaction paths connect the Franck − Condon region of the lowest populated excited state vertically above the ground state equilibrium geometries to the accessible S1/S0 conical intersections. Besides, photophysical processes involved in these photochemical reactions were studied to understand and discuss the competition between the radiationless deactivation processes via S1/S0 conical intersections in which the complex relaxes back to the electronic ground state and H-atom ejection.

2 Theoretical methods

All calculations were carried out with the TURBOMOLE program package version 5.8 [34]. Ground state structures of PhNH2·∙∙(Py)1,2 were optimized assuming Cs and C2v point group symmetries (if possible) or without any constraints (C1) at the second-order Moller–Plesset MP2 level within the resolution of identity (RI) approximation for electron repulsion integrals (RI-MP2) [35]. These geometries were used in further calculations of vertical excitation energies. The correlation consistent polarized valence double-ξ basis set of cc-pVDZ quality augmented with diffuse functions (aug-cc-pVDZ) was used for all atoms. The use of diffuse basis functions is required to correctly describe low-lying Rydberg πσ* excited states.

Equilibrium geometries of the complexes in neutral and biradical forms in the lowest excited singlet states were calculated via the second-order approximate coupled cluster (CC2) method employing the resolution of identity (RI) approximation [36, 37] using the aug-cc-pVDZ basis set. For both ground and excited state geometry optimizations, starting geometries were constructed with Cs and C1 symmetry constraints. Within the Cs point group, the aniline molecule lies in the symmetry plane and the excited-state wavefunctions are transformed according to A′ and A′′ irreducible representations. The planar structure confers also a C2v point group and an A1, A2, B1 and B2 electronic states on the excited state. Cs and C2v symmetries were imposed in geometry optimization in some of these calculations due to the near-degeneracy of the lowest excited 1ππ* and 1πσ* states in the Franck–Condon region. To optimize excited state geometries, the minimum energy structure of the ground state was chosen as the starting point for the lowest excited states. Potential energy (PE) profiles were calculated along the minimum energy path (MEP) for elongation of the N–H stretching coordinate in the PhNH2···(Py)1,2 complex; for a given value of the N–H coordinate, all remaining coordinates were optimized.

3 Results and discussion

3.1 Vertical excitation and equilibrium geometries

The Cartesian coordinates of the optimized geometries are shown in the ESI of this article.

  1. 1.

    Isolated aniline

Free aniline (PhNH2) was constrained to be planar with C2v and Cs symmetries constraints. Vertical excitation energies with associated transitions and oscillator strengths (f) into the low-lying singlet excited state of isolated aniline at its ground state equilibrium geometry were calculated at RICC2/aug-cc-pVDZ level. These specifications for the lowest singlet excited states are reported in Table 1 and compared with previous calculations and experimental values. The two lowest excited singlet states are 1πσ* and 1ππ* are quasi-degenerate with calculated vertical excitation energy of 4.49 eV and 4.51 eV, respectively. The 1πσ* state involves the promotion of an electron from the highest π orbital to the lowest diffuse σ* which is antibonding with respect to the NH bonds (see Fig. 2). It can be seen that the first bright state is of 1ππ* character with an oscillator strength (f = 0.037) with the contribution percentage of 75%, both of π and π* orbitals are delocalized over the entire PhNH2 molecule (Fig. 2). However, the second bright 1ππ* state with an energy transition of 5.44 eV owns the highest oscillator strength (f = 0.228) and will dominate the absorption intensity from the ground state in comparison to the first bright 1ππ* state. This result is close to previous work, that excitation to the 21ππ* state has become an open channel at 240 nm 8. From Table 1 it can be seen that our computed results are in good agreement with the experimental observations of Rajasekhar et al. [9].

Table 1 Vertical excitation energies (eV) and oscillator strengths f of the lowest singlet excited states of isolated aniline at the ground state minimum under C2v and Cs symmetries, computed at the RICC2 level with the aug-cc-pVDZ basis set and compared with values from previous ab initio calculations and experimental values
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Starting from ground state minimum geometry, the lowest excited state geometries of A′ and A” representations retaining Cs symmetry were optimized by the use of the RICC2/aug-cc-pVDZ method. The states of aniline considered here are the 1ππ* and the 1πσ* excited states, situated energetically below the vertical excitation by about 0.2 and 0.13 eV, respectively.

  1. 2.

    PhNH2-Py complex

Two conformers of the PhNH2-Py complex that exhibit an intermolecular hydrogen bond between an N–H of PhNH2 moiety and the N atom of the Py moiety have been considered. These two conformers are presented for the mutual orientation of the PhNH2 molecule vis-a-vis the Py molecule. The first (conformer (I)) displays Cs symmetry, all atoms of the complex PhNH2–Py were constrained to be in the same plane. The second conformer (II) displays also Cs symmetry and forming a T-shaped structure in which the pyridine molecule becomes in the plane perpendicular to the molecular plane of aniline. These two conformers in the ground state have no change in equilibrium geometry and energy when constraints were removed (C1 symmetry). Both planar and T-shaped conformers exhibit a strong hydrogen bond NH1···N between aniline and the nitrogen atom of pyridine, in which aniline is an H atom donor. The second hydrogen atom of the amino group NH2 denoted H2 is free hydrogen in both conformers.

Figure 1 shows the ground state (S0) equilibrium geometries of the PhNH2–Py complex with (Cs) and without symmetry constraints ((I) and (II) conformers), which were optimized at the RI-MP2 level using the aug-cc-pVDZ basis set. The hydrogen bond length is increased by more than 0.07 in favor of the conformer (II) when the PhNH2–Py complex is changed from planar (I) to T-shaped (II). Moreover, the hydrogen bond length between the H1 and N atoms is 2.047 and 2.115 Å for the hydrogen-bonded (I) and (II) conformers, respectively, and the N–N distances are 3.066 and 3.088 Å, respectively. Additionally, dissimilarly to conformer (II), the hydrogen bond angle in the conformer (I) is very close to 180°. Although all these values comparison are in favor of more stability of the conformer (I), the RICC2 level of theory predicts the lowest energy to be the conformer (II) with a difference in stability of about 0.1 eV. This can be explained by the existence of CH···π hydrogen bond which certainly contributes to the stabilization of the conformer (II). This interaction involves an aromatic CH donor (aniline) and an aromatic π system as acceptor (pyridine). Therefore, conformer (II) of PhNH2-Py is stabilized by two hydrogen bonds NH1···N and CH···π. Several studies have been interested in the aromatic CH···π hydrogen bonds (T-shaped interactions) [38,39,40]. It has been suggested that aromatic CH···π hydrogen bonds play an important role in the stability of 3D structures of biological macromolecules (proteins, DNA, …). The major source of attraction in the CH···π interaction is the dispersion interaction and the electrostatic contribution is small, while the electrostatic interaction is mainly responsible for the attraction in the conventional H-bonds [41].

Fig. 1
figure 1

Ground and excited states optimized structures of PhNH2-Py and PhNH2-(Py)2 and their corresponding electron/proton transferred complexes (biradicals). Distances are given in Å

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Vertical excitation energies with corresponding transitions and oscillator strengths of the conformers (I) and (II) of the PhNH2···Py complex were calculated. Table 2 shows these parameters for the lowest five singlet excited states in both A′ and A′′ symmetries. At the RICC2/aug-cc-pVDZ level of theory, the calculation shows a quasi-degenerate lowest 1ππ* and 1πσ* states for both conformers (I) and (II), which exhibits a vertical excitation energy difference about 0.02 and 0.03 eV, respectively. The lowest 1A′′ state is of 1πσ* character and 1ππ* nature for conformer (I) and conformer (II), respectively. For conformer (I) in Fig. 2, the 1πσ* excited state arises from the excitation from the 7a"(π) orbital to the diffuse σ* orbital (29a'), while excitation from the 7a"(π) orbital to the 11a"(π*) orbital which are entirely localized on aniline, gives rise to the lowest locally excited 1ππ*(LE) state. For conformer (II) in Fig. 2, the LE and the 1πσ* excited states are attributed both to excitation of the 10a"(π) orbital to the 16a"(π*) orbital and the diffuse σ* orbital (25a'), respectively. The diffuse orbital is extremely sensitive to the NH bond lengths and is localized mainly on the amino group of aniline upon vertical excitation. As a result, when one of the NH bond lengths is slightly extended, this orbital collapses to the 1 s orbital of the departing H-atom as is well demonstrated for acidic aromatic systems [3]. These two states are followed by 1ππ*(CT), where π is located on PhNH2 and π* is located on Py.

Table 2 Vertical excitation energies (eV) and oscillator strengths f of the lowest five singlet excited states for conformers (I) and (II) of PhNH2-Py complex at the ground state minimum under Cs symmetry, computed at the RICC2 level with the aug-cc-pVDZ basis set
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Fig. 2
figure 2

Molecular virtual orbitals involved in vertical electronic transitions in isolated aniline and PhNH2-(Py)n (n = 1,2) complexes

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The vertical energy difference between the lowest 1ππ*(LE) excited state and the lowest CT state is higher for the conformer (I) by about 0.32 eV than the corresponding value for the conformer (II). Indeed, when passing from the conformer (I) to the conformer (II), the excitation energies of the 1πσ* and 1ππ*(LE) are slightly shifted to greater values by 0.02 and 0.07 eV, respectively. Whereas the position of the lowest 1ππ*(CT) state is shifted to significantly lower energy by 0.3 eV. As a result, the energy gap between LE and CT states was reduced to 0.08 eV in the case of the T-shaped conformer. This is mainly the result of the contributions of two acidic sites NH1···N and CH···π on the stability of the conformer (II).

Compared to the isolated aniline, the vertical excitation energy of the 1πσ* and 1ππ* states in conformer (I) are both redshifted by 0.13 eV, while in conformer (II) these states are redshifted by 0.06 and 0.11 eV, respectively.

  1. 3.

    PhNH2-(Py)2 complex

The structures of optimized PhNH2-(Py)2 conformers (I), (II), and (III) are shown in Fig. 1. It should be noted that the structures of these complexes are the most suitable to facilitate the formation of the conical intersection. Conformer (I) with its planar structure (all atoms in the same plane) may be regarded as belonging to both C2v and Cs symmetries and involves pyridine molecules binding to each H atom of the amino group of aniline similarly to PhNH2-Py but with stronger interactions, in which two hydrogen atoms of the amino group interact with two Py molecules. The bond lengths and energies of Cs and C2v optimized geometries are very similar.

T-shaped conformer (II) presents a combined planar and T-shaped structure, in which one Py is in the same plane with PhNH2 and interact through NH···N hydrogen bonding, the second Py forms a CH···π interaction on one side of the Py ring in addition to the NH···N interaction. This latest conformer is calculated to be more stable by about 0.15 eV.

The third conformer (III) is stabilized by two H bonds like conformer (I) and by a π-π interaction between the pyridine molecules, which are parallel and located face-to-face forming a sandwich separated by about 3.5 Å. (see Fig. 1). The calculation indicates that the formation of the sandwich dimer of pyridine in the complex causes an energy redshift by about 0.48 eV compared with the conformer (I) due to π-π stacking interaction between the indicated molecules. This structure of pyridine dimer in this complex is analog to other molecular systems like benzene dimer, benzene-phenol, benzene-toluene [42, 43].

Vertical excitation energies with corresponding transitions and oscillator strengths for all considered conformers of the PhNH2–(Py)2 complex calculated using RICC2/aug-cc-pVDZ method are presented in Table 3. As it is shown, for the planar conformer (I), the two lowest excited states are of the dark 1πσ* and bright 1ππ*(LE) features and are quasi-degenerate at 4.2 eV, and the oscillator strengths of these states are 0.002 and 0.04. The next higher states are two 1ππ*(CT) at 4.4 eV. The molecular orbitals involved in these two electronic excitations are presented in Fig. 2. For T-shaped conformer (II), The excitation spectrum consists of a low-lying dark 1ππ*(CT) at 4.19 eV. The transition into this state from the ground electronic state is unlikely, as indicated by the small oscillator strength (f = 0.000). While the first bright 1ππ*(LE) is at 4.31 eV with oscillator strength 0.035 and is exactly degenerate with the 1πσ* state. In conformer (III), the 1ππ*(LE) state disappears in the five first lowest excited states. However, the S1 and S2 states are of 1ππ*(CT) character and are bright in absorption with excitation energies of 4.45 and 4.56 eV and oscillator strengths of 0.028 and 0.013, respectively. The two lowest 1πσ* excited states lie higher in energy in the vertical excitation spectrum (4.69 and 4.79 eV).

Table 3 Vertical excitation energies (eV) and oscillator strengths f of the lowest five singlet excited states for conformers (I), (II) and (III) of PhNH2-(Py)2 complex at the ground state minimum under C2v, Cs and C1 symmetry, respectively. computed at the RICC2 level with the aug-cc-pVDZ basis set
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3.2 Photophysical and photochemical processes through N–H bond photodissociation

The N–H bond photodissociation channel was recognized as one of the primary radiationless deactivation mechanisms as well as H-atom elimination in a variety of systems in the gas phase. Since, when complexed with pyridine, there are two such bonds in PhNH2, free and hydrogen bonded, both types were considered in each of aniline-pyridine complexes to investigate the photophysical processes and photochemical reactions. The 1ππ* and 1πσ* excited electronic states have been identified to play an important role in photophysical processes for these types of complexes: DNA/RNA nucleobases, aromatic amino acids and their corresponding chromophore subunits [3, 11, 24, 44], malonaldehyde, o-hydroxybenzaldehyde, salicylic acid and 7-hydroxy-1-indanone [45, 46], indole-water clusters [47], and phenol-water and phenol-ammonia clusters [48]. To reveal the existence of conical intersections and the possibility of an internal conversion between the excited 1ππ*and 1πσ* states and the ground state S0, it is required to scan the PES of these states, to estimate the energy barrier between the Franck–Condon region and CI.

3.2.1 Proton Coupled Electron Transfer PCET

  1. 1.

    PhNH2–Py

The potential energies of the lowest excited singlet and ground states along the N-H1 bond length of aniline in the conformers (I) and (II) of the PhNH2–Py complex are explored in this section. Here, for a fixed value of the N–H1 coordinate, all remaining coordinates were optimized. Figure 3a and b depict the potential energy profiles of the S0 and singlet states. The potential energy functions along the reaction coordinate were constructed by stretching the N-H1 bond by steps of 0.1 Å or 0.2 Å. Along the reaction path, the geometry of the lowest 1ππ*(CT) singlet state was optimized, whereas the energies of the electronic ground (S0) and excited singlet (1ππ*(LE) and 1πσ*) states were calculated along the reaction path optimized in the 1ππ*(CT) state. The 1ππ*(LE) and 1πσ* states are energetically too high to play a significant role in the photophysics of the two conformers near the Franck–Condon region.

Fig. 3
figure 3

Potential energy profiles along the N-H1 stretching coordinate of the PhNH2–Py complex, a planar structure (conformer (I)) and b T-shaped (conformer (II)), calculated with the RICC2 method. For RN-H1 = 1.02 Å, at the optimized energy of the electronic ground state S0(open diamond), the lowest singlet 1πσ* (open green circle), 1ππ*(LE) (open red triangle) and 1ππ*(CT) (blue filled square) excited states were calculated at this point. For RN-H1 > 1.02 Å, S0, 1πσ* and 1ππ*(LE) were calculated at optimized geometries of the 1ππ*(CT) state. The (open times) symbol represents the minimum energy of neutral and biradical PhH•···HPy• complexes. The RICC2 ground state energy computed at the corresponding RI-MP2 optimized geometry is used as the reference energy

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As the N-H1 coordinate is elongated, the potential energy of the ππ*(LE) state increases and the energy of the 1ππ*(CT) state in which an electron from the π orbital of PhNH2 is promoted to the π* orbital of Py decreases rapidly. Vertical excitation energies indicate that the CT state is the fourth and third electronic state for planar and T-shaped conformers, respectively. This CT appears upon a slight elongation of the distance between the donor N of the amino group and the acceptor N atom of Py involved in the hydrogen bond. This induces a PCET reaction, which can occur via stepwise mechanisms, with the electron being transferred first, followed by the proton transfer.

For RN-H1 smaller than 1.6 Å, optimization of the structures of conformers (I) and (II) without symmetry constraints leads to the nearly T-shaped structure. Furthermore, when the RN-H1 distance is further stretched, the energy of the CT state decreases, the charge separation exerts a strong force on the proton to follow the electron. The proton then crosses the hydrogen bond transition state and attaches to the N atom of pyridine leading to a strong energetic stabilization radical pair PhH•···HPy•. The photophysics of conformer (I) in Cs symmetry (A′ representation) differ from the second (II) at a larger reaction coordinate (see Fig. 3a). However, in its final structure (biradical), the excited 1ππ*(CT) sate of conformer (I) (planar structure) has an energy about 0.15 eV above the ground state which makes the 1ππ*(CT)/S0 crossing not detected, it may occur at a larger distance. Along this reaction, we also found a 1ππ*(LE)/1ππ*(CT) conical intersection near the Franck–Condon region with a small barrier (0.16 eV). Whereas for conformer (II) (T-shaped), the proton transfer stabilizes the 1ππ*(CT) sate and the S0 energy computed at the 1ππ*(CT) geometries is destabilized. Consequently, the barrierless energy curve of the 1ππ*(CT) state crosses the S0 energy profile at RNH1 = 1.855 Å with an energy of 2.25 eV above the S0 minimum (see Fig. 3b). Since both energy profiles are calculated at the same geometries, the crossing of these curves is a true energy crossing, and it represents a conical intersection. These results show that after crossing the conical intersection between the ground and excited states, the photoreaction can lead to the formation of a stabilized biradical or to internal conversion to the ground state of the complex.

  1. 2.

    PhNH2–(Py)2

The photophysics of the electronically excited and ground states of the PhNH2–(Py)2 complex without any symmetry constraint is linked to the N–H bond photodissociation. It entails investigating the potential energies of the lowest excited singlet and ground states along the N–H reaction coordinate and decaying at a region of excited and ground states degeneracy (near the expected S1/S0 conical intersection). The potential energy profiles of the S0, 1ππ*(LE) and 1ππ*(CT) states leading to the most relevant feature of the deactivation mechanism are summarized in Fig. 4. along the reaction path, the geometry of the lowest 1ππ*(CT) singlet state was optimized, whereas the energies of the electronic ground (S0) and excited singlet ( 1ππ*(LE) and 1πσ*) states were calculated along the reaction path optimized in the 1ππ*(CT) state. Optimization of the planar complex (conformer (I)) without symmetry constraints leads to the T-shaped structure (conformer (II)) for RN-H less than 1.4 Å. As a result, for the planar conformer, we reduced the PE profiles only for RN-H ≥ 1.4 Å. As previously stated, the 1ππ*(LE) and 1πσ* states are higher in energy and do not play a role in photophysics. PE profiles of the T-shaped conformer, as shown in Fig. 4, need not require any crossing between the 1ππ*(LE) and 1ππ*(CT) states. The decay of the T-shaped structure, on the other hand, is attributed to direct relaxation from the first repulsive 1ππ*(CT) state to the S0 state, giving rise to a conical intersection at RNH = 1.57 Å with an energy of 2.03 eV above the S0 minimum. This CI represents a pathway for either ultrafast internal conversation to the ground state or biradical formation. Whereas the planar structure appears only between RNH = 1.2 Å and 1.4 Å. The potential energy of its lowest 1ππ*(CT), where the proton follows the electron and attaches to the N atom of pyridine, leads to the neutralization of ion pair and the formation of the stable PhH•···H(Py)2• biradical complex at RNH = 1.825 Å, decreases significantly. This biradical, which is produced through unidirectional PCET has an energy of roughly 0.19 eV higher than the ground state.

Fig. 4
figure 4

Potential energy profiles along the N–H stretching coordinate of the PhNH2–(Py)2 complex, planar conformer (I) (dashed line) and T-shaped conformer (II) (solid line), calculated with the RICC2 method. For RNH = 1.015 Å, at the optimized energy of the electronic ground state S0(open diamend), the lowest singlet 1πσ* (open cicle), 1ππ*(LE) (open triangle) and 1ππ*(CT) (filled black square) excited states were calculated at this point. For RNH > 1.015 Å, S0, 1πσ* and 1ππ*(LE) were calculated at optimized geometries of the 1ππ*(CT) state. The (open times) symbol represents the minimum energy of biradical PhH•···H(Py)2• complex. The RICC2 ground state energy computed at the corresponding RI-MP2 optimized geometry is used as the reference energy

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3.2.2 H-atom detachment from aniline

  1. 1.

    Isolated PhNH2

Calculated Potential energy (PE) profiles for the H-atom detachment reaction from the amino group NH2 of free aniline are shown in Fig. 5. The energies of the ground S0 state were calculated at geometries which were optimized in the 1πσ* state for fixed N–H bonds. Also, geometries of the lowest 1ππ* excited state were optimized along the reaction coordinate. As it is shown in this figure the two lowest singlet excited state 1ππ* are bound and parallel to the PE function of the electronic S0 state, while the PE profile of the lowest excited state 1πσ* is dissociative with an exit barrier to N–H dissociation of about 0.5 eV with the maximum is located near 1.4 Å. This result provides the possibility of a relatively longer excited state lifetime for the hydrogen detachment mechanism. This barrier separates the Rydberg (diffuse) part of the PE function and the valence (repulsive) part, in which the σ* orbital collapses to the 1 s orbital of the hydrogen atom. In the same figure (Fig. 5), molecular orbitals involved in the lowest excited states of isolated aniline at the starting of the reaction and near the conical intersection, are shown.

Fig. 5
figure 5

Potential energy curves of isolated PhNH2 along the N–H bond stretch coordinate at the planar geometry, obtained at the RICC2/aug-cc-pVDZ level. For RNH = 1.01 Å, at the optimized energy of the electronic ground state S0(open diamend), the lowest singlet 1πσ* (filled black circle), 11ππ* (open square) and 21ππ* (open triangle) excited states were calculated at this point. For RN-H < 1.01 Å and RN-H > 1.01 Å, S0 and 11ππ* and 21ππ*were calculated at optimized geometries of the 1πσ* state. The (open times) symbol represents the minimum energy of the neutral complex. The RICC2 ground state energy computed at the corresponding RIMP2 optimized geometry is used as the reference energy. The top left and top right insets show σ* natural orbital of the 1πσ* state obtained at its equilibrium geometry and at a stretched N–H bond length (RN-H = 2.0 Å) of the amino group, respectively

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The population of the dark 1πσ* state from the bright 1ππ* state occurs easily because the optimized 1πσ* and 1ππ* states are quasi-degenerate in the Franck–Condon region, with an energy difference about 0.02 eV. In this region, the 1πσ* state has shallow local minima. When the N–H bond was stretched to 1.9 Å, a 1πσ*/S0 energy crossing was found at an energy about 4.82 eV above the ground state minimum and 0.33 eV above to the starting point (Franck–Condon region). Furthermore, because the appropriate PE profiles were computed at the same geometry optimization, this 1πσ*/S0 conical intersection is real. As a result, a 1πσ*/S0 conical intersection is highly probable; therefore, detachment of hydrogen from isolated aniline may cause competition between nonradiative decay to the ground state and H atom elimination. This result is qualitatively supported by previous studies [18].

  1. 2.

    PhNH2–Py

It is required to determine the PE profiles of the two lowest 1ππ* and 1πσ* states along the reaction coordinate, to show the presence of a conical intersection, the possibility of an internal conversion between the excited state S1 and the ground state and to estimate the energy barrier between the Franck–Condon region and CI. The electron/proton-transfer detachment over the N-H2 bond length of aniline in the conformers (I) and (II) of the PhOH–Py complex was followed using RICC2/aug-cc-pVDZ computations. Figure 6a and b show potential energy (PE) profiles calculated along the minimum energy path (MEP) for elongation of the N–H2 stretching coordinate in the conformers (I) and (II) in C1 symmetry; all remaining coordinates were optimized for the lowest 1πσ* excited state for a given value of the N–H2 coordinate. Along the N–H2 reaction coordinate, the energy of the ground state and 1ππ* state were calculated on the basis of the S1 optimized geometry.

Fig. 6
figure 6

Potential energy profiles along the N-H2 stretching coordinate of the PhNH2–Py complex, a planar structure (conformer (I)) and b T-shaped (conformer (II)), calculated with the RICC2 method. For RN-H2 = 1.02 Å, at the optimized energy of the electronic ground state S0(open square), the lowest singlet 1πσ* (filled black circle) and 1ππ* (open square) excited states were calculated at this point. For RN-H2 < 1.02 Å and RN-H2 > 1.02 Å, S0 and 1ππ* were calculated at optimized geometries of the 1πσ* state. The ( ⊗) symbol represents the minimum energy of the neutral complex. The RICC2 ground state energy computed at the corresponding RIMP2 optimized geometry is used as the reference energy. The top left and top right insets show σ* natural orbital of the 1πσ* state obtained at its equilibrium geometry and at a stretched N–H bond length (RN-H2 = 2.0 Å) of the amino group, respectively

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As one can see from Fig. 6, the reaction coordinates connect the Franck–Condon region and the 1πσ*/S0 CI geometry. It appears that hydrogen atom detachment takes place on the 1πσ* surface and requires a crossing of a barrier of 0.5 eV and 0.56 eV with respect to the Franck–Condon geometries of conformers (I) and (II), respectively. The energy barrier is associated with the Rydberg (diffuse) to valence transformation of the σ* orbital [3]. The diffuse orbital at the Franck–Condon geometry (equilibrium geometry), and the valence orbital towards the 1 s valence orbital of the hydrogen atom detachment (H2 atom) are presented in Fig. 6. In both conformers, the PES profile of the 1πσ* state along the N–H2 coordinate and after exiting the barrier is dissociative. For planar conformer (I), the stretching of the N–H2 bond of the amino group leads to a 1πσ*/S0 crossing at an N-H2 bond length of 1.87 Å and energy of 4.66 eV above the S0 minimum. While for T-shaped conformer (II), The conical intersection occurs at an N–H2 bond length of 1.87 Å and at an energy of 4.68 eV above its S0 minimum. Following the 1πσ*/S0 crossing, there may be a competition between internal conversion to the ground state and H-atom ejection upon evolution on the dissociative part of the 1πσ* state. In the former mechanism, the electronic energy is converted primarily into the N-H2 stretching vibration. In the latter mechanism, the electronic energy is converted into kinetic energy of the ejected hydrogen atom. As a result, the complex may promote photochemical H2 generation.

3.3 Discussion

Photochemical reactions and photophysical processes induced by the formation of hydrogen bonds have already been studied for a family of heterocyclic compounds that possess both a proton donor (OH, NH, or NH2 group) and the nitrogen (N) atom of pyridine as an acceptor [25, 49,50,51,52,53,54,55,56,57,58,59]. The common feature of all results is based on the arguments of Mataga and coworkers [55]. Fluorescence quenching was commonly generated by intermolecular hydrogen-bonding interaction, especially it consists of a generic model of fluorescence quenching when two conjugate π-electronic systems were interacted. For hydrogen-bonded complex in the excited state, a slight proton shift from donor toward acceptor appears to induce CT followed by a large-scale proton transfer. After the CT/S0 conical crossing, an ultrafast radiationless transition to the vibrational ground state of the hydrogen bond occurs. The possibility of an excited state proton transfer from PhNH2 to the pyridine molecule has been investigated. To reach another minimum for the PhNH2–Py clusters on the 1ππ*(CT) surface, the N–H bond has thus cleaved. This minimum was found for all the studied clusters, and it consists of PhNH•⋯HPy• radicals. The CT state of the radical complex and the 1ππ*(CT)/S0 conical intersection lie below the vertical excitation (Franck–Condon region) of the neutral form by about 2.7 and 2.1 eV, respectively. Adding one more Py molecule lowers the energy difference by more than 0.5 eV for the former one and slightly increases (0.07 eV) the 1ππ*(CT/S0) conical intersection. For all the studied clusters, the energetic barrier calculated for this PCET process is too small or barrierless. Here, when both O − H bonds of NH2 groups are hydrogen bonded to multiple Py molecules, the H-atom elimination channel remains suppressed.

For the H-atom detachment, the 1πσ*/S0 conical intersection lies about 0.3 eV above the Franck–Condon excitation for both isolated aniline and monomer complex. Moreover, for the T-shaped PhNH2-Py conformer this intersection is located at 2.43 eV above the 1ππ*(CT)/S0 one. This result is in favor of the PCET reaction than the H-atom detachment in the same conformer exhibiting both N–H···H hydrogen bonding and free N–H bond.

In this respect, nonradiative decay for the PCET reaction could be described as ultrafast and is assigned to barrierless ππ*(CT) deactivation, whereas relatively slower decay for the H-atom detachment reaction is assigned to 1πσ* deactivation after overcoming an energy barrier about 0.5 eV.

4 Conclusion

In this contribution, a systematic study of the nonradiative decay processes and photochemical pathways has been explored through the RICC2 level of theory. The PE profiles of the coupled electron-proton transfer photoreactions in the hydrogen-bonded complexes of aniline with pyridine have been computed along reaction coordinates. Two main excited-state reaction mechanisms have been identified in PhNH2-(Py)n (n = 1,2) complexes. The first is the excited state proton-coupled electron transfer (PCET) from aniline molecule to pyridine through hydrogen bonding. The calculated PE profiles are governed mainly by a repulsive 1ππ*(CT) state and reveal the existence of a barrierless path for PCET, leading to a low-lying 1ππ*(CT)/S0 conical intersections along hydrogen-bonded reaction paths from PhNH2 to Py which can promote ultrafast excited state deactivation or stabilization to a biradical complex.

Another photophysical reaction mechanism identified is hydrogen atom detachment in complexes exhibiting free hydrogen in the amino group of aniline. The energy profiles of this reaction are the same for the two conformers. The N–H dissociation occurs via the dissociative 1πσ* state, where a conical intersection with the S0 state can be reached after overcoming an energy barrier leading both to nonradiative relaxation to the S0 minimum or generation of H atom radicals. These radicals may subsequently react with each other to produce H2 molecules.