Effect of Chemical Substituents on the Energetical Landscape of a

Oct 18, 2010 - The effect of chemical substitutions on the energetical landscape of an optical molecular switch (Phys. Chem. Chem. Phys. 2008, 10, 124...
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J. Phys. Chem. A 2010, 114, 11879–11889

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Effect of Chemical Substituents on the Energetical Landscape of a Molecular Photoswitch: An Ab Initio Study Michał F. Rode* and Andrzej L. Sobolewski Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland ReceiVed: June 21, 2010; ReVised Manuscript ReceiVed: September 28, 2010

The effect of chemical substitutions on the energetical landscape of an optical molecular switch (Phys. Chem. Chem. Phys. 2008, 10, 1243) was studied with the aid of ab initio electronic structure methods. Series of different chemical moieties were substituted into the molecular frame of 7-hydroxyquinoline as well as into the “molecular crane” at position 8 of the frame. It was shown that the π-electron-donating/withdrawing properties of substituents substantially modify the energetical landscape of the system in the ground as well as in the lowest excited ππ* and nπ* singlet states. 1. Introduction The search for optically switchable molecules is nowadays an important and popular issue (see refs 1-3 and references therein). Optical molecular switches enable the storage of information on a molecular level and route signals in molecular electronic logic circuits.2,4-8 They are considered as the controllers of current flow when linked to conjugated polymer chains.9 In addition to future practical applications, they are also present in the living organisms10 in which a biological process, such as paralysis,11 may reversibly be induced when two different wavelengths of light are used to toggle the molecular switch between its two structural forms. Eventually, the use of light may trigger structural changes in molecular systems that are of great importance in biochemistry and medicine.10 In order to serve as a molecular switch, a molecular system must possess at least two stable forms that may be switched by an external electric filed, electron transfer, chemical reaction,12 or optical excitation.13-15 Most mechanisms for optical switches considered so far are based on light-induced conformational changes, in particular, cis-trans isomerization in azobenzene6,7,16-20 and its derivatives13,20 or ring-opening reactions.5,8,21-27 Organic molecules are considered to be promising candidates for these switching molecular machines due to the variety of potential designs through chemical synthesis. The weak point of bifunctional organic molecules is their susceptibility to photoinduced degradation, which is a major difficulty for the construction of reliable photofunctional systems. Recently, one of us (A.L.S.) has proposed a new class of organic functional molecules28 that very likely are free of these drawbacks. The mechanistic principle of the functionality of these systems involves the transfer of a proton between two remote spots on an aromatic molecule by optical excitation, the so-called excited-state intramolecular proton-transfer (ESIPT) reaction. ESIPT processes are of particular interest, since they appear to play an essential role in so-called photostabilizers,29 which are in widespread technical use for the protection of organic polymers against degradation by the UV components of sunlight.30,31 These compounds exhibit high absorption coefficients in the near-UV region and have a nondegradative * To whom correspondence [email protected].

should

be

addressed.

E-mail:

pathway for rapid return to the electronic ground state. In one of the best known photostabilizers (TINUVIN), the photophysical cycle is completed on a subpicosecond time scale.32 The general mechanistic model for the function of photostabilizers that emerges from these studies involves barrierless enolto-keto proton (or hydrogen) transfer in the electronically excited (S1) state, ultrafast (ca. 100 fs) radiationless decay of the excited state (S1′) keto form, and barrierless proton backtransfer to the electronic ground (S0) state, thus closing the photocycle.33-35 Theoretical explorations of the photophysical dynamics of salicylic acid and 2′-(2′-hydroxyphenyl)benzotriazole in the S1(ππ*) state36,37 showed that a low-lying S1-S0 conical intersection (CI) exists in these compounds which can, after the initial proton-transfer reaction, be reached by a nearly barrierless reaction path involving primarily torsion of the proton-accepting group in combination with its moderate pyramidalization. This theoretical prediction has recently found experimental confirmation.38,39 The optical switch operating within the ESIPT phenomenon is composed of two covalently bound molecular moieties that can be called a molecular “frame” and a proton “crane” (Figure 1). The molecular frame has two proton-donating/accepting (PDA) atoms, X and Z, which is an important modification in comparison with ESIPT molecules.28 In contrast to the molecular frame, the proton crane has only a single proton-accepting (PA) site (Y). The overall photophysical scheme of an optical switch, presented in Figure 1, is divided into three panels. The left and right panels describe the proton transfer reactions, PTab and PTdc, respectively, where the proton is transferred from X or Z donor sites of the molecular frame to the acceptor (Y) site of the proton crane, respectively. Thus, the main reaction coordinate in both panels is the XH or ZH bond stretching distance, respectively. The left panel describing the PTab reaction links two tautomeric forms of the switch: “a”, with the proton-transferred form “b”. Similarly, the right panel describing the PTdc process links another pair of tautomers: “d”, with the proton transferred form “c”. The b and c tautomeric forms are related to each other by rotation of the protonated crane (the central panel in Figure 1). In this case, the torsional angle between two rotating parts of the system is taken as the main reaction coordinate. The molecular system which is schematically outlined in Figure 1 may generally possess four stable minima in the ground state and in the lowest excited singlet state. Moreover, a typical

10.1021/jp105710n  2010 American Chemical Society Published on Web 10/18/2010

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Figure 1. Potential-energy diagram illustrating the photophysics of an optical molecular switch driven by the ESIPT process.

spectroscopic feature of heteroaromatic systems is the presence of low-lying ππ* and nπ* excited states. These states can easily be distinguished by symmetry, if the system is planar. Keeping the system planar in the course of the proton-transfer reaction seems to be justified assumption in view of small mass of proton and though its movement is expected to be very fast in comparison to other vibrational motions. Since the system may also possess four minima in the excited nπ* singlet state, thus the optical switch considered in this work may generally possess 12 photophysically relevant minima which should be considered. The operating principles of such molecular systems working as optical switches can be described as follows (see Figure 1). The S0a f S1a optical excitation induces the S1a f S1b proton transfer (PTab) reaction. The system further evolves to the S0-S1 conical intersection (CI) by twisting around the covalent bond that connects the molecular frame and the proton crane moieties (the central panel of Figure 1). After the S1 f S0 nonadiabatic transition at the CI, the S0b form is populated or the system can continue the torsional motion (leading to the formation of the Y-H · · · Z hydrogen bond) on the ground-state PES (form S0c). The S0b and S0c structures, in which the proton is covalently attached to the Y atom, are generally not expected to be stable on the ground-state PES. The proton initially attached to the atom Y spontaneously transfers along the Y-H · · · X (or Y-H · · · Z) hydrogen bond to atom X (or Z). In this way, either the system restores the doorway structure S0a or the structure S0d is generated. The net effect of the photoreaction is therefore a transfer of the hydrogen atom from atom X to atom Z of the molecular frame via an intramolecular proton crane. The prototype of the optically driven molecular switch based on the ESIPT phenomenon was theoretically characterized in ref 28 for the example 7-hydroxy(8-oxazine-2-one)quinoline (see C12 in Table 3), in which the oxazine moiety acts as a proton crane linked to the 7-hydroxyquinoline (7HQ) as a molecular frame. The photophysically relevant PE landscape of this system

is shown in Figure 2, where, apart from the S0 and ππ* singlet states considered in ref 28, also the potential energy profile of the lowest nπ* singlet state is shown. It is important to notice that PE profiles of the ππ* and nπ* excited state show a typical behavior of these states along the PT reaction coordinate; while the reaction is barrierless in the lowest ππ* state, it possesses a barrier in the lowest nπ* state.40,41 Thus, one of the tasks of the present work is to find such substituents to the molecular system that will result in blue-shift of the “intruding” nπ* state. The photophysics of the first functional system synthesized according to theoretical prescription, 7-hydroxy-8-carbaldehyde4-methylquinoline (see F4 in Table 1) was investigated by Armatrix isolation UV/IR spectroscopy.42 The reversibility of the optical switching mechanism was unequivocally proven in this experiment, although a small yield of the keto-to-enol backreaction was observed. The theoretical ab initio study on the photophysics of the above system has indicated its crucial obstacles: (i) the profound barrier for the excited state PTdc reaction, from the S1d to the S1c form of the system, which was most likely the reason for the small yield of the switching-back process, and (ii) an intruding “nonreactive” nπ* state was found to be lower in energy than the corresponding “reactive” ππ* state. Although, the barrierless ESIPT reaction was shown to occur in the ππ* state in the forward direction, i.e., from S1a to S1b, the intruder nπ* state appeared to be adiabatically below the minimum of both S1a and S1d forms in the ππ* state.42 The aim of this work is the systematic study of the effect of chemical substituents onto the molecular frame of the 7-hydroxyquinoline (7HQ) as well as to the “crane” attached to position 8 of the 7HQ frame on the energetical landscape of the ground and the lowest excited ππ* and nπ* singlet states with respect to its performance as an optical switch. The most important feature of the excited-state PES of a molecular switch operating on the basis of a reversible ESIPT reaction is

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Figure 2. Energy profiles of 7-hydroxy(8-oxazine-2-one)quinoline in the S0 state (circles), in the 1ππ* state (squares), and in the 1nπ* state (triangles), determined at the CC2/cc-pVDZ level along the minimum-energy path (solid) for hydrogen transfer from the enol form (a) toward the proton-transferred form (b) (left panel), for oxazine-ring torsion (central panel) and for hydrogen transfer from the keto form (d) toward the protontransferred form (c). S0(S1) denotes the energy of the S0 state, calculated along the minimum-energy path of the S1(ππ*) state (dashed curves). See Figure 1 for the a, b, c, and d abbreviations.

barrierless and exoergic access from the Franck-Condon area of each tautomeric form to the central conical intersection with the ground state. In order to measure this quantity, we define the enthalpy of the reaction according to the formula dc b(c) ∆Hab ] - E[S1a(d)] r (∆Hr ) ) E[S1

(1)

where E[S1i] (i ) a, b, c, or d) denotes the adiabatic energy of the S1 at the optimized minimum of the respective form. For the case where a given doorway form has no stable minimum on the excited-state PES, the enthalpy of the reaction is defined as follows:

∆Hab,VE (∆Hdc,VE ) ) E[S1b(c)] - E[S1a(d),VE] r r

(2)

Here E[S1a(d),VE] is the S1 excited state energy for the FC geometry of the S0a form (or the S0d form); VE denotes the vertical excitation with no change of the geometry of the system upon excitation, while E[S1b] and E[S1c] are defined as in eq 1. The lack of stable minimum in the excited state is indicated in the Tables by the dash or by the estimated value of the adiabatic energy in S1(ππ*) state (given in the parenthesis) for a given form for the frozen OH/NH distance of 1.7 Å (arbitrary chosen). With aid of these definitions, both types of ESIPT reactions, with and without a barrier on the excited-state PES, can qualitatively be compared. The number of substituents studied in this work prohibits characterization of the reaction in terms of the minimum-energy pathways on the relevant PES. Thus, the quality of a given substituent is described in terms of the exoergicity of the ESIPT process according to eq 1 or in terms of a barrierless reaction, according to eq 2. 2. Computational Methodology The ground-state equilibrium geometries of the systems have been determined with the second-order Møller-Plesset (MP2) method. Excitation energies and response properties have been calculated with the CC2 method,43,44 which can be considered

as the equivalent of MP2 for excited electronic states. The equilibrium geometries in the lowest excited singlet ππ* and nπ* states have been determined at the CC2 level, making use of CC2 analytic gradients.45,46 To allow cost-effective explorations, the standard split-valence double-ζ basis set of TURBOMOLE47 with polarization functions on the heavy atoms (defSV(P))48 has been employed in these MP2 and CC2 geometry optimizations. 3. Results and Discussions Our foremost motivation for this theoretical study was to find the most appropriate molecular system which could effectively work as an optical switch on the basis of the ESIPT phenomenon. This implies the search, either for appropriate molecularframe and/or for proton-crane units. For transparency of effect of chemical modifications, we decided to keep the same molecular frame (7HQ) in all examples studied and only modify the aromatic rings by different substitutions, as well as by changing the molecular crane at position 8 of 7HQ. 3.1. Single Substitutions to the Molecular Frame. In the first step of the search, we concentrated on the chemical modification of the 7HQ frame. In this section, the molecular system (Scheme 1) contains the carbaldehyde moiety attached to position 8 of the 7HQ frame (the system denoted as F1 in Table 1). a. Methyl and Amino Substitutions to F1. To check the scale of the substituent effect and its position-sensitivity, first, the methyl and amino groups have been chosen to be substituted at four possible positions of the 7HQ frame. These substituents are expected to exert quite opposite electronic properties; while CH3 is neither an electron-donor nor -withdrawer, NH2 is both a strong π-electron donor and strong σ-electron withdrawer in the S0. The calculated energies of the stable minima in the S1(nπ*) and S1(ππ*) states of the respective tautomeric forms are collected in Table 1. The F2-F5 systems have a single methyl group in the 2-, 3-, 4-, or 6-position of the 7HQ frame, respectively, while the F6-F10 systems have a single amino group in the respective position. For all the methylated systems, the S1 energetical landscape seems to be similar to the 4-methylated system F442

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TABLE 1: Adiabatic (∆Hrdc) and Vertical (bold) (∆Hrab,VE and ∆Hrdc,VE) Enthalpies of the Two ESIPT Reactions, PTab and PTdc (characterized by the adiabatic and vertical energy of the two respective excited-state forms), in the “Reactive” 1ππ* State (upper row) and the Respective Adiabatic Minima in the Intruding “Nonreactive” 1nπ* State (lower row, only for selected forms) for Mono- and Poly-Derivatives (F) of 7-Hydroxy-8-carbaldehydequinolinea frame F

S0 + EVE EaVE

Ea

Eb

∆Hrab,VE Eb - EaVE -0.38

Ec

Ed

S0 + EVE EdVE

∆Hrdc Ec - E d

∆Hrdc,VE Ec - EdVE

F1

H

4.07 3.86

3.46

3.69 3.34

(3.78) 2.92

2.92 2.39

3.36 3.32

(>+0.8)

(>+0.4)

F2

2-CH3

(>+0.8)

(>+0.4)

F5

6-CH3

3.38 3.36 3.30 3.29 3.29 3.25 3.33 3.33

(>+0.4)

4-CH3

2.96 2.44 2.89 2.38 2.87 2.34 2.91 2.41

(>+0.8)

F4

Monomethyl Substitutions 3.67 -0.40 (3.76) 3.35 2.93 3.64 -0.35 (3.76) 3.35 2.92 3.66 -0.37 (3.73) 3.33 2.88 3.63 -0.40 (3.72) 3.33 2.92

(>+0.4)

3-CH3

3.46 3.45 3.43 3.45

(>+0.8)

F3

4.07 3.86 3.99 3.85 4.03 3.84 4.03 3.86

(>+0.8)

(>+0.4)

F6

2-NH2

-0.39

5-NH2

+0.85

+0.34

F10

6-NH2

3.75 3.78 3.09 3.32 3.52 3.37 3.36 3.38 3.34 3.69

+0.04

F9

3.41 2.91 2.78 2.44 3.09 2.49 2.85 2.33 3.04 2.98

(>+0.4)

4-NH2

Monoamino Substitutions -0.41 3.69 3.03 (3.49) 2.94 -0.34 3.13 2.63 -0.17 3.70 2.97 -0.57 3.29 3.34

(>+0.7)

F8

3.56 3.39 3.34 3.68 3.35 3.77 3.45 3.19 3.71

-0.06

3-NH2

3.55 3.28 3.47 3.67 3.44 3.57 3.64 3.84

+0.28

F7

3.97 3.97 3.53 3.88 4.02 3.86 3.94 4.08 3.76 3.94

+0.25

-0.05

F11

2,5-di-NH2

(>+0.4)

3,6-di-NH2 2,6-di-NH2

+0.43 -0.18

+0.18 -0.41

F15

4,6-di-NH2

-0.09

-0.40

F16

2,4,6-tri-NH2

-

-0.74

F17

2,4,6-tri-CH3

3.76 3.89 3.08 3.37 3.20 3.37 4.07 3.72 3.70 3.65 3.95 3.28 3.30

(>+0.8)

F13 F14

Polyamino and Polymethyl Substitutions 3.63 -0.47 3.62 3.36 3.63 3.14 2.93 (3.57) 2.71 3.45 2.58 2.37 3.17 -0.47 3.38 2.95 2.79 -0.62 2.96 3.14 3.58 3.25 3.27 3.24 -0.64 3.32 3.41 3.79 3.36 3.16 2.75 -0.66 2.91 3.63 3.25 3.26 3.56 -0.42 (3.70) 2.89 3.31 2.87 2.40

-0.14

3,5-di-NH2

3.73 3.73 3.30 3.33 3.60 4.14 3.63 3.44

+0.26

F12

4.10 4.20 3.55 4.09 3.64 3.41 4.01 3.88 3.92 3.41 3.99 3.98 3.83

(>+0.8)

(>+0.4)

a The lack of stable minimum in the excited state is indicated by a dash or by the estimated value of the adiabatic energy in the S1(ππ*) state (given in the parentheses) for a given form for the frozen OH/NH distance of 1.7 Å (arbitrarily chosen).

as well as the unsubstituted system F1. In the F1-F5 systems, the S1c(ππ*) form is not stable and its relaxed geometry optimization ends up in the S1d(ππ*) stable form. Methylation at any considered position does not alter the S1 energetical landscape either in ordering of the S1(ππ*) and S1(nπ*) states or in the exoergicity of the system defined by the enthalpy of the given ESIPT reaction. For any methylated-F1 system, each of the S1(nπ*) stable minima is lower in energy than the corresponding S1(ππ*) minimum. Inspecting the different monoamino-substituted systems (F6-F10 in Table 1) one notices that the effect is quite opposite to the monomethylated systems (F2-F5). The ∆Hrdc enthalpy for the PTdc reaction tends to be smaller for 2-, 4-, and 6-monoamino derivatives, while it increases for 3- and 5-aminosubstituted systems. For 3-NH2 (F7) substitution, not only PTdc but also PTab reaction is endoergic. The latter is, however, more exoergic for the 2-, 4-, and 6-monoamino derivatives and less for the 5-NH2 substitution (F9). The substituent effect discussed above reflects the fundamental principle of the effect of monosubstituents on the reactivity of benzene ring molecules.50 For moieties strongly activating the benzene ring for electrophilic substitution, such

as the amino or hydroxy groups, an increase in occupancy of the π orbitals at the ortho- and para- positions is observed, whereas for the meta-position the effect is weak and is opposite in sign; i.e., the π orbital occupancy decreases. Although, the effect was originally found for the ground state of a single aromatic ring molecule, in this study we found a similar effect for excited ππ* state of 7HQ double-ring system. The general conclusion resulting from the above discussion is the following one: two substitution positions, 6 and 2, tend to decrease the ∆Hr enthalpy of both PTab and PTdc processes in comparison with the nonsubstituted system; position 4 has a tendency to decrease ∆Hr enthalpy only of PTdc, while positions 3 and 5 tend to increase the ∆Hr of both PT reactions. This observation suggests that the choice of the position of the side-group substituted to the 7HQ frame is more important for the switching functionality than the kind of substituent used, as long as the electron-donating/withdrawing character of the given substituent is strong enough. In the case of amino groups, it might be both the strong π-electron-donating character (which may stabilize ππ* state by decreasing the energy of the occupied π orbital, which makes the π-electron less accessible for the detachment) and the strong σ-electron-withdrawing character

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SCHEME 1: The Considered Tautomeric Forms of the System (F1) with Numbering of Atoms Indicated

(which may destabilize nπ* state due to increase of the n-electron energy). Furthermore, the nπ/ππ* state energy ordering measured by the nπ/ππ* energy gap is almost the same for 2- and 4-aminated positions when compared to 2- and 4-methylated derivatives, with the nπ* state being always lower in energy. The only exception is the 6-amino substitution (F10), which changes the nπ/ππ* state energy ordering dramatically by stabilizing the ππ* state versus nπ*. This fact makes the 6-amino substitution the most valuable for improvement of the switching performance. It seems that it is the strong π-electron-donating character of the amino group that is crucial for stabilizing the ππ* versus nπ* state and, thus, for forcing the exoergicity of the PTdc reaction. b. Other Substitutions at Positions 2 and 6 of F1. In order to check if the conclusions drawn above are applicable for a broader range of substituents that are characterized by stronger (dimethylamino (DMA)) or weaker (F, OH) π-electron-donating or even by π-electron-withdrawing (CHO and CN) properties, positions 2 and 6 of 7HQ were chosen for monosubstitution with different side groups (see Table S1 in the Supporting Information) The results for 2-substututed-F1 systems F6, F18, F21, F24, F28 allow us to put the substituents in the following series in order of increasing ∆Hrdc enthalpy:

NH2 (+0.28) ∼ DMA (+0.25) < F (+0.47) < OH (+0.52) < OCH3 (+0.59) (3) This ordering shows that the NH2 (F6) and DMA (F18) groups in position 2 have a positive impact on lowering the PTdc enthalpy. A similar conclusion can also be derived for the series of 6-substituted-F1 systems (F10, F19, F22, F26, F29 in Table 1 and Table S1 in the Supporting Information) in order of increasing ∆Hrdc enthalpy, where:

NH2 (+0.25) ∼ DMA (+0.18) < OH, OCH3 (+0.47) < F (+0.78) (4) Here the result for the 6-DMA (F19) derivative (0.18 eV) is even more promising, since it is found to be the smallest among all 2- and 6-substituted systems (see also F32-F40 in Table S1 in the Supporting Information), although the smallest ∆Hrdc value of +0.04 is obtained for the 4-NH2 substitution (F8 in Table 1) among all the monosubstitutions to the 7HQ frame. Moreover, 6-substituted systems seem to have a general feature of destabilizing nπ* states in comparison to other positions, so that the local nπ*-state minima are close to or even higher than the respective ππ* stable forms. Contrary to that, in the case of 2-susbtituted derivatives, even if ∆Hr was decreased upon a given 2-substitution, still the nπ* state is adiabatically lower than the respective ππ* state. The series of ∆Hrdc enthalpies, tested here on 2- and 6-substitutions to the F1 system, is in a good accord with the series of pEDA (π-electron donor-acceptor parameter) for

different substituents presented by Ozimin´ski and Dobrowolski49 on the series of substituted benzenes in its ground state. The pEDA reflects the ability of a given substituent to shift the π-electron density toward the benzene ring in comparison to nonsubstituted benzene. In this series, NH2 and DMA groups have the largest π-electron-donating character among all the considered substituents (F6, F10 and F18, F19); F and OH are moderate π-electron donors (F21, F22 and F24, F26); CH3 and phenyl benzene derivatives have comparable electron impact character to nonsubstituted benzene (F2, F5, and F32, F33). In contrast, CN and CHO groups are moderate π-electron acceptors (F34-F37 and F40), while the BH2 group is the strongest π-electron-acceptor (F39). For such substituents as CN in any position (F34-F37), phenyl either in the 2- or 6-position (F32, F33), CHO (F40), Cl (F38), BH2 (F39), and as discussed above CH3 (in any position), the adiabatic ∆Hrdc(ππ*) enthalpy could not even be estimated due to lacking the S1c minimum, as it was also observed for specific positions of certain substituents, e.g. 3-NH2 (F7) and 4-OH (F25). While for three substituents in position 2, CN (F34), CHO (F40), and BH2 (F39), both PTab and PTdc reactions were found to be endoergic, and none of the proton-transferred excitedstate structures, in neither ππ* nor nπ* state, have stable minima. This observation is reflected in the pEDA,49 according to which the more π-electron-accepting character a substituent has, the higher the barrier for the PTdc process is expected to be. One can see by inspecting Tables 1 and S1 (Supporting Information) that for both nπ* and ππ* states the S1-energetical landscape is comparable for strong π-electron-donating NH2 and DMA substituents. This allows us to presume that the conclusions derived from amino substitutions may be extended to DMA derivatives. However, in the following we used NH2 instead of DMA group for further tests to keep computational time within a reasonable limit. On the other hand, this conclusion might be useful for future synthesis of functional materials, if the DMA derivative is easier to synthesize than the NH2 one. Besides, it is a known fact that amines easily undergo hydrolysis while DMA-substituted compounds are resistant to it. 3.2. Double and Triple Substitutions to the Molecular Frame. Having in mind the conclusions on monosubstitutions to the molecular frame obtained above, we can examine whether the effect of double or even triple substitutions with the π-electron-donating groups to the molecular frame would be qualitatively additive, so that eventually the barrier for the PTdc reaction could be eliminated. As a first step, different diamino-substituted systems were checked (F11-F15 in Table 1). It was found that in comparison to monoamino substitutions either ∆Hrab, ∆Hrdc, or even ∆Hrdc,VE decreases substantially, if both substituents are in the 2-, 4-, or 6-position (F14, F15). It is worth noticing that in the case of 4,6-diamino- (F15), and especially in the case of the 2,6-diamino derivative (F14), but not 2,5-diamino (F11) and 3,5-diamino- (F12), the nπ* excited state local minima were also found to be higher in energy than the respective

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SCHEME 2: Tautomeric Structures of 7HQ Considered in Modeling of the Proton Crane (C)

ππ* state minima. This is apparently an effect of using 6-monoamino substitution, because for the 3,5-diamino derivative (F12) the effect is opposite. In the latter case (F12), both PTab and PTdc reactions are endoergic, and yet protontransferred S1b and S1c forms could not be determined (due to the barrierless back-proton transfer to S1a and S1d forms, respectively). Eventually, all three amine groups were substituted simultaneously in 2,4,6-positions, and the resulting 2,4,6triamino-7-hydroxy-8-carbaldehydequinoline system (F16) was the first molecular system characterized by a barrierless ESIPT reaction starting from the FC region of either keto or enol form. Due to the fact that also for the 2,6-diamino derivative (F14) the nπ* state is adiabatically higher in energy than the respective ππ* state, both of these types of substitutions applied to the 7HQ frame are thus recommended to improve functionality of the ESIPT-type optical switch. What can be concluded from double- and triple-amino substitutions is their qualitatively additive effect on the S1-

energetical landscape. Subsequent substitutions of the amino group increases exoergicity of the PTdc reaction. This conjecture was tested with other substituents such as DMA (F20), OCH3 (F31), F (F23), and CH3 (F17), all 2,4,6-triplesubstituted on the 7HQ frame (Table S1, Supporting Information). The additivity of the effect was indeed observed, and a decrease of ∆Hrdc,VE in the triple-substituted systems could then be ordered in a series:

NH2 (-0.74) > DMA (-0.60) > OCH3 (-0.39) > F (-0.19) > CH3 (∼+0.4) (5) However, none of these triple substitutions resulted in a barrierless PTdc process as in the case of NH2. To emphasize the necessity of using π-electron-donating substituents with respect to forcing the exoergicity of the ESIPT reaction, the series 5 shows that their negative ∆Hrdc,VE enthalpies are in contrast to the positive value, of +0.4 eV, of

TABLE 2: Adiabatic (∆Hrdc) and Vertical (bold) (∆Hrab,VE and ∆Hrdc,VE) Enthalpies of the Two ESIPT Reactions, PTab and PTdc (characterized by adiabatic and vertical energy of the two respective excited-state forms), in the “Reactive” 1ππ* State (Upper Row) and the Respective Adiabatic Minima in the Intruding “Nonreactive” 1nπ* State (lower row, for selected systems) for Derivatives (C) of 7-Hydroxyquinoline Substituted with Different Open-Ring Crane Moieties at Position 8

* Indicates proximity of the given excited state with S0.

Effect of Chemical Substitutions on Energetics ∆Hrdc,VE for 2,4,6-trimethyl substitution (the F17 system). The latter value is comparable to monomethyl-substituted systems and may once again be explained by the negligible influence of methylation on electron distribution in aromatic species. 3.3. Modeling the Proton Crane. In this section we concentrate on a search for the most appropriate functional group for a proton-craning moiety (see Scheme 2), the unit which is supposed to transfer the proton between the two stable forms (S0a and S0d) of a molecular frame upon optical excitation. a. Open-Ring Molecular Systems. As a first step, open-ring moieties were considered as proton cranes (C1-C8 in Table 2). The first five proton cranes were the carbaldehyde moiety (C1), treated as a reference system, and its four derivatives (C2-C5). For all of them the PTdc reaction was found to be endoergic (Table 2). Among these, the C2 and C4 systems exhibited a lack of both S1b and S1c proton-transferred stable

J. Phys. Chem. A, Vol. 114, No. 44, 2010 11885 minima, thus suggesting profound endoergicity for the PT process from the FC region, while for the C1, C3, and C5 systems the PTab process was spontaneous (barrierless). Among these five proton-crane moieties, the C5 system exhibits the smallest ∆Hr enthalpies for both PT processes, suggesting that a good proton crane should not only be electronically conjugated with the molecular frame, but also electronically conjugated by itself. The last three examples of open-ring proton cranes are such conjugated systems in which the oxygen atom in C5 was exchanged with a nitrogen atom and additionally substituted by remaining hydrogen atoms or an ethylenic group. In this way, the C8 system was found to have the most exoergic ∆Hrab and ∆Hrdc values of -1.75 and -0.58 eV, respectively. The C8 system was found to most closely resemble the sixmembered ring heterocycles such as pyridine10 or oxazine28 in structure and electron conjugation. This is not surprising, since

TABLE 3: Adiabatic (∆Hrdc) and Vertical (bold) (∆Hrab,VE and ∆Hrdc,VE) Enthalpies of the Two ESIPT Reactions, PTab and PTdc (characterized by adiabatic and vertical energy of the two respective excited-state forms), in the “Reactive” 1ππ* State (upper row) and the Respective Adiabatic Minima in the Intruding “Nonreactive” 1nπ* State (lower row, for selected systems) for Derivatives (C) of 7-Hydroxyquinoline Substituted with Different Six-Membered-Heterocycle Crane Moieties at Position 8

* Indicates proximity of the given excited state with S0. a Indicates possible dissociation chanel.

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pyridine is known to possess strong hydrogen atom affinity, which allows for abstraction of a hydrogen atom in the excited state through an intramolecular hydrogen bond.51,52 For these reasons, both oxazine and pyridine moieties have already been positively tested as the promising proton cranes in the different optical molecular switches;28,10,53 The pyridine-crane switch has already been considered as a current flow switch when the conjugated polymer is inserted between two golden electrodes10 or is connected to nanotubes.53 For these reasons, several different six-membered-ring heterocycles (C10-C18, Table 3) were tested as potential proton cranes and compared to the pyridine-based system, C9. However, in the case of C9 the PTdc reaction still is endoergic (+0.33 eV), while the other modification of its aromatic ring considered by us gave a significant decrease of the PTdc enthalpy, as discussed below. b. Insertion of a Nitrogen Atom into the Pyridine Ring. The considered modification of a crane (C in Scheme 2) was by insertion of a single N atom into the pyridine ring, first into position 3′ (numbering of atoms according to C9 in Table 3) to get a pyrimidine system (C10) and next into 4′ to get a pyrazine moiety (C11). It turned out that the insertion of an additional N atom into the pyridine ring decreases ∆Hrdc in comparison to its value obtained for the pyridine (C1), but this also stabilizes the intruding nπ* states (versus ππ*). This undesirable effect results from addition of the n-orbital of the heteroatom (N3′ or N4′) into the electronic system. However, the effect is smaller in the C11 system, and that is why N4′ insertion was kept in further tests. Such substitution required additional group insertion at position 5′ of the system. The carbonyl group seemed to be the most likely candidate, since it is known to have a strong electron-withdrawing character. In this way, we came to a point when the oxazine moiety could eventually be introduced as a proton crane (see C1228 and C13 systems). The comparison of the energetics of the C12 and C13 systems points toward C12 as the best candidate (see its PE profiles in Figure 2) for the proton crane. This is reflected by the stronger exoergicity of both-side PT reactions for the C12 system versus C13 and with respect to all other systems considered here so far, as well. c. Addition to N4′ Atom with the Insertion of Carbonyl Group to the 5′ Position. Encouraged by the success of the C12 and C11 proton-crane systems we decided to gather two types of modification, the substition to the N4′ atom and the insertion of the carbonyl group into the pyridine ring, to get a 2(1H)-pyrazinone system (C14). The C14 proton-crane system as well as its N4′-CH3 (C15) and N4′-OH (C16) derivatives are characterized by quite comparable 1ππ*-energetical landscape, giving also reasonably promising exoergicity of both PT enthalpies with values situated between those of the C12 and C11 systems and good nπ*/ππ* state ordering. d. CH2 Group Insertions into Position 4′- or/and 5′-. The last type of the proton-crane modification applied by us to the pyridine ring was the insertion of the CH2 group into the aromatic ring. Here we show two examples: with a single CH2 group inserted into position 4′ (C17) and with simultaneous insertion of these groups into positions 4′ and 5′ (C18). Both systems show strong exoergic PT reactions; however, the PTdc reaction is much less exoergic for the latter system. Moreover, in the case of the C17 system, the PTdc reaction is not only the most exoergic among all the considered systems so far in the 1 ππ* excited state, but it is also barrierless from the FC area. For both C17 and C18 systems, the correct ππ*-nπ* state ordering was found. However, PE profiles for PT are more descent than in previously considered systems, and the 1ππ*

Rode and Sobolewski state approaching the ground state in the planar conformation may result in a nonadiabatic transition to the ground state without a significant twisting of the crane. This could make switching to another rotameric form more difficult. e. Effect of 2,4,6-Triamino Substitution to the 7HQ Molecular Frame with Different Proton Cranes. As discussed above, the 2,4,6-triamino substitution (F16 in Table 1) is the most promising from the point of view of this study, since it results in the barrierless ESIPT both from the S1d toward the S1c form and from the S1a toward the S1b. Although the latter is observed in many other systems considered so far, the former one is quite unique. This effect is expressed 2-fold, as (i) providing the exoergicity of the ESIPT reactions for which both ∆Hr enthalpies are negative and (ii) stabilizing the local minima of ππ* character with respect to nπ* for both S1b and S1c forms. This is based on comparison of the adiabatic energies for the respective S1 minima of the nonsubstituted system (F1 in Table 1) with the 2,4,6-triamino-substituted system (F16 in Table 1) with the carbaldehyde moiety acting as a proton crane. It is natural to ask whether the effect of 2,4,6-triamino substitution could even be more pronounced, if the proton crane were a stronger electron acceptor. To answer this question, a series of different proton-crane moieties, such as carbaldehyde (C1 in Table 4), vinyl carbaldehyde (C2), pyridine (C3), and 2(1OH)-pyrazinone (C4), when attached to 2,4,6-triamino-7hydroxyquinoline were considered (see also Table 3 for comparison with nonsubstituted systems). As one can see from the inspection of Table 4, the exchange of proton crane from the carbaldehyde (C1) to one of the three C2-C4 systems resulted in a decrease of both ∆Hab and ∆Hdc enthalpies, the largest being the case of C4. Also, destabilization of the local minima of the nπ* versus the ππ* singlet state was confirmed for all the considered 2,4,6-triamino-substituted 7HQ systems. The only undesirable effect for 2,4,6-triamino-substituted 7HQ switch systems is stronger overlapping of the excitation wavelengths for both tautomeric forms of the system, S0a and S0d, versus nonsubstituted quinoline (compare F1 and F16, Table 1). Thus, application of the appropriate six-membered ring heterocycle as a proton-crane moiety would be a compromise in this respect. For example, for the carbaldehyde moiety as a crane and with a nonsubstituted frame system (F1) yields ∆λ(abs) ) 0.90 eV; 2,4,6-triamino substitution reduces this gap to -0.29 eV (F16), while an application of six-membered-ring proton crane such as 2(1OH)-pyrazinone gives a relatively larger ∆λ(abs) of 0.52 eV for a substituted system (C4, Table 4) [∆λ(abs) ) 0.73 eV without substitution, C16-Table 3] (when comparing first ππ* excitations of S0a and S0d forms). f. Pyridine Substituent Effect. The last but not least idea of how to improve the functionality of the molecular switch based on ESIPT is the question of what is the influence of the substituent applied to the proton-crane moiety on modeling its S1-energetical landscape. To answer this question, pyridine was chosen as a reference model proton crane attached to the 7HQ frame with different monosubstituted pyridine 3′-, 4′-, 5′-, and 6′-positions (P1 in Table 5). It was found that for two substitutions, 4′-F (P3) and 6′-F (P5), the ∆Hrdc enthalpy increases, while for the remaining two, 3′-F (P2) and 5′-F (P4), the enthalpy is decreased in comparison to nonsubstituted pyridine (P1), though position 5′ was next chosen to be tested for substitution with different π-electrondonating side groups. The strong π-electron donor, NH2 (P6), increased both ∆Hrab and ∆Hrdc enthalpies while π-electron-acceptors CHO (P9), CN (P11), and BH2 (P13) decreased ∆Hrdc when being 5′-substituted

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TABLE 4: Adiabatic (∆Hrdc) and Vertical (bold) (∆Hrab,VE and ∆Hrdc,VE) Enthalpies of the Two ESIPT Reactions, PTab and PTdc (characterized by adiabatic and vertical energy of the two respective excited-state forms), in the “Reactive” 1ππ* State (upper row) and the Respective Adiabatic Minima in the Intruding “Nonreactive” 1nπ* State (lower row, for selected systems) for 2,4,6-Triamino and Diamino Derivatives of 7-Hydroxyquinoline Substituted with Different Open-Ring Crane Moieties (C) at Position 8

a

2,6-diamino-susbstitution to 7HQ. b 2,4-diamino-susbstitution to 7HQ.

TABLE 5: Adiabatic (∆Hrdc) and Vertical (bold) (∆Hrab,VE and ∆Hrdc,VE) Enthalpies of the Two ESIPT Reactions, PTab and PTdc (characterized by adiabatic and vertical energy of the two respective excited-state forms), in the “Reactive” 1ππ* State (upper row) and the Respective Adiabatic Minima in the Intruding “Nonreactive” 1nπ* State (lower row, for selected systems) for Derivatives (P) of 7-Hydroxy-8-pyridinequinoline Substituted at Different Positions of the Pyridine Moiety

to the proton crane. Once again the neutral CH3 (P7) substituents almost negligibly influenced the S1 energetics of the model system (P1). Having in mind the promising result for π-electron-withdrawing groups, also other positions were further tested for substitution to the pyridine-crane moiety (see P10, P12, P14). Among these the 4-BH2 and 6-BH2 substitution gave the largest decrease of ∆Hrdc enthalpy of -0.42 and -0.68 eV, respectively. This result obtained for the substituted proton crane is qualitatively opposite to the one obtained for the molecular

frame of the molecular switch. This may be simply explained by the function that the given moiety plays in the molecular switch after the UV light is absorbed. Thus, the function of the molecular frame is to donate a hydrogen atom (electron/proton pair) to the proton crane. After the UV excitation, which shifts electron density from molecular frame to the crane, the proton follows the electron density changes, which results in proton transfer between these moieties. That is why the role of the substituent is to push the electron density from the frame (in the S0) toward the proton crane in the S1 state as much as

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possible. This implies that the molecular frame should be substituted by the electron-donating (NH2, DMA moieties, for instance) and the proton crane by electron-withdrawing substituents (BH2, CN moieties, for instance). Moreover, since the role of the S1(ππ*) state is to drive the PT reaction, our results point to the conclusion that the π-donating character of the substituent has the most significant impact on S1-energetical landscape of the molecular switch. We can also conclude that the substituent proton-crane effect on the S1-energetical landscape is weaker than the substituent molecular-frame effect. Also, the choice of the proper proton crane is crucial for switching functioning. Nevertheless, its functionality may additionally be improved by the proper substitutions of the molecular frame by the π-electron-donating amino or dimethylamino groups. Worth noticing is the fact that in general six-membered heterocycles used as proton cranes seem to be much more promising proton-crane candidates than the carbaldehyde derivatives. 4. Conclusions The substituent effect on the S1-energetical landscape of the molecular optical switch applied to the molecular frame and to the proton crane was studied. The answer for the central question of this study, i.e., could the substituent effects be used to strengthen the driving force for the ESIPT reactions, is positive. To show explicitly the driving force for the ESIPT reaction the visualization of changes of the electron distribution in the molecular system upon different types of substitutions was presented and are discussed in Table S2 (Supporting Information) in terms of HOMO/LUMO orbitals. The main conclusions arising from this study may be summarized as follows: (1) π-electron-donating groups such as amino (NH2) and perhaps even better dimethylamino [N(CH3)2] should be substituted to the molecular frame of the switch to amplify the shift of the electron density from the molecular frame toward the proton crane of the switch. On the other hand, the π-electron withdrawing groups, such as BH2 or CN, should be substituted to the proton crane to pull the electron density from the molecular frame due to the UV-excitation. In this way, the cooperative effect of both types of side groups is expected to provide a better performance of the switch, which is in accord with the theoretical suggestions found for fyrylfulgides.27 (2) The 2,4,6-triamino and 2,6-diamino derivatives of quinoline should particularly result in the barrierless ESIPT from the keto-side of the switch. Substitutions by BH2, CHO, or CN groups at positions 4′, 5′ and 6′ of pyridine-like proton cranes should additionally improve the switching performance. (3) The S1-energetical landscape for NH2- and DMAsubstituted systems are similar to each other as well as to the OH- and OCH3-substituted models. An additional advantage of a DMA or OCH3-substituted system would be the lack of free and easily accessible protons, which may be important when studying such systems in bulk. (4) Although the effect of substitution to the proton-crane on the S1-energetical landscape is generally weaker than the effect of substitution to the molecular frame, the proper choice of crane is crucial for efficient operation of the switch. We hope that the results of our theoretical explorations can provide helpful hints for the future synthesis of such systems. Supporting Information Available: Additional material as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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