Mechanistic Investigations on the Photoisomerization Reactions of Five-Membered Ring Heterocyclic Molecules Containing Sulfur and Selenium Atoms.

The restricted active space self-consistent field method in the 26-electron/27-orbital active space and the 6-311(d) basis set has been used to investigate the mechanisms of the photochemical isomerization reactions concerning the model systems of 1,2,3-thiadiazole and 1,2,3-selenadiazole. The computational works suggest that the preferred reaction paths for both 1,2,3-thiadiazole and 1,2,3-selenadiazole are as follows: reactant → Franck-Condon region → conical intersection → intermediate → transition states → photoproducts. As a result, the structures of the conical intersections, which play a decisive role in these photoisomerization reactions, are obtained. In particular, the present theoretical evidences demonstrate that the potential energy surfaces for the formation of 1,3-diradicals are quite flat. This may explain why their experimental detections are so difficult.

Introduction

1,2,3-Thiadiazoles and their derivatives have attracted intense interest because of their use in the study of fundamental chemistry problems and potential applications.[1] It is well-accepted that various 1,2,3-thiadiazoles can exhibit biological activity such as antibacterial,[2] antiviral,[3,4] antitumor,[5] antiallergic,[6] systemic acquired resistance,[7,8] fungicidal,[9−12] and insecticidal[13] activity, as well as being DNA photocleavers[14] and reagents in photolitography.[15−21] Also, 1,2,3-thiadiazoles have been introduced into other active compounds as an active moiety for lead discovery.[22]

The photochemistry of 1,2,3-thiadiazole (1) has been the subject of many investigations over the past 50 years.[23−42] The experimental observations by Krantz, Laureni, and many chemists indicate that direct irradiation of 1 can produce thiirene (2), thioketene (3), and ethynethiol (4).[23−42] See Scheme . These experimental results strongly imply that the product distribution detected so far from direct photolysis is basically a characteristic of reactivity associated with the 1,3-diradical (5).[23−42] Its experimental detection, however, was anticipated could be very difficult because the early studies revealed that the energy of the intermediate (5) was computed to be much higher than those of the final photoproduct species (2, 3, and 4).[43,44] Moreover, these photoproducts were proposed to be yielded directly in the thiadiazole-excited state, in concert with nitrogen extrusion.[45]

Scheme 1

It is well-known that selenium compounds show broad similarities with the corresponding sulfur analogues, that is, they offer utilizations in the syntheses of dyes, pharmaceuticals, and various fine chemicals.[46] Because the selenium element has a larger atomic size in comparison with oxygen and sulfur, molecules including the selenium atom always give an increased polarizability, which, in turn, make them generally less stable than the sulfur analogues. In addition, its physical properties make selenium-containing heterocycles desirable materials in the growth of organooptic and organoelectronic materials.[47−51] As a result, because 1,2,3-selenadiazoles undergo a wide variety of reactions as 1,3-dipoles and develop the synthesis of different organoselenium compounds, 1,2,3-selenadiazoles have also attracted much attention in several laboratories.[52,53] In fact, because of the reason that 1,2,3-selenadiazoles are easily separated with the loss of a nitrogen molecule and a selenium atom under both light irradiation and thermal conditions, 1,2,3-selenadiazoles have drawn attention as versatile intermediates for the preparation of alkynes.[54−67]

In contrast to the photochemistry of 1,2,3-thiadiazole (1) mentioned above, however, the photochemistry of 1,2,3-selenadiazole (6) has been little explored.[24,25,68,69] For instance, until now, Krantz and co-workers reported that after irradiating samples of matrix-isolated 1,2,3-selenadiazole at 8 K, the reactions were monitored by IR spectroscopy.[24−32] On the basis of comparisons with sulfur analogues, new absorption bands for selenirene (7), selenoketene (8), ethynylselenol (9), and acetylene (10) were obtained (Scheme ). It has to be noted here that the photoproduct 10 is not observed in the photolysis of 1. Presumably, once a 1,3-diradical (11) is formed, it can easily rearrange to selenoketene (8).[24−32] Therefore, no acetylene (10) can be found in the photochemical isomerization reactions of 6. Nevertheless, an intermediate with a five-membered ring was proposed to lead to a reaction between selenoketene (8) and the diradical (11).[56,68,69]

Scheme 2

The above experimental results and discussion inspire this study. The computation of the structures of the key points and the mechanisms for the photochemical isomerization reactions of 1 and 6 are thus of great academic interest. To the best of our knowledge, so far, there has been neither experimental nor theoretical study for the photochemical mechanisms of both 1 and 6. It is astonishing how little is known about the mechanisms of the photochemical reactions of 1 and 6 because these molecules are so important in bioorganic and organic chemistry.[1−69] Accordingly, using a more sophisticated quantum chemical theory, the study of the potential energy surfaces of 1,2,3-thiadiazole (1) and 1,2,3-thiadiazole (6) systems is undertaken. The goal of this study is to combine both observed experimental works and theoretical examinations to provide a comprehensive understanding of the excited-state behaviors of 1,2,3-thiadiazoles and 1,2,3-selenadiazoles. From this, one may obtain the design of some useful related systems and allow some practical applications.

However, one reviewer indicated that the selenium derivative likely seems to populate a triplet state because of the heavy-atom effect. This assumption is based on a recent paper.[70] Nevertheless, according to the available experimental reports,[24−32,68,69] no species on the triplet state have been observed so far. Therefore, the photochemical reactions studied in this work are all focused on the singlet surfaces.

Results and Discussion

1,2,3-Thiadiazole

To obtain the comprehension of the photoisomerization mechanisms of 1,2,3-thiadiazole (1), its reaction profiles at the restricted active space self-consistent field (RASSCF)(26,27)/6-311G(d) are summarized in Figure . Figure also contains the relative energies of the critical points with respect to the ground-state minimum 1. A general outline of the five p−π orbitals in unsubstituted 1,2,3-thiadiazole, which forms the basis for this study, is shown in Figure A (Supporting Information). Some important geometrical parameters for the key points are collected in Figure .

Figure 1

Energy profiles for the photoisomerization modes of 1,2,3-thiadiazoles (1). The abbreviations FC and CI stand for Frank–Condon and conical intersection, respectively. The relative energies were obtained at the RASSCF(26,27)/6-311G(d) level of theory. For RASSCF-optimized structures of the critical points see Figure . For more information see the text.

Figure 2

Selected RASSCF(26,27)/6-311G(d) geometries (in Å and deg) for the photoisomerization of 1,2,3-thiadiazoles (1). Their relative energies are given in Figure . The corresponding RASSCF vectors are shown in the inset. The heavy arrows indicate the main atomic motions in the transition-state eigenvector. For more information see the Supporting Information.

In the first step, 1 is promoted to its excited singlet state by a vertical excitation, FC-1, as outlined on the left-hand side of Figure . After this vertical excitation process, 1 is situated on the singlet surface but still has the ground-state (S0) geometry. The RASSCF(26,27)/6-311G(d) result indicates that this vertical excitation energy (S0 → S1(S0 geom)) is 113 kcal/mol. The experimental excitation energy of 1 is reported to be 133–95.2 kcal/mol (=215–300 nm),[25,39] which is in agreement with this computational value. Because of this agreement, it is therefore expected that the same computational accuracy should be suitable to discuss the mechanisms of photochemical isomerization reactions of 1 (vide infra).

From the FC-1 point, 1 relaxes to reach an S/S CI, from which the photoexcited 1 can easily decay to S0 in a nonradiative way. The RASSCF computations given in Figure suggest that the energy of CI-1 lies 63 kcal/mol above 1 and 50 kcal/mol lower in energy than FC-1. Funneling through the CI-1 point, different reaction pathways on the S0 surface can be anticipated by following either the gradient difference vector or the derivative coupling vector directions.[71−75] As seen in Figure , the large contribution of the gradient difference vector is regarded as S–N1 bond-breaking motion, whereas the derivative coupling vector is in accord with stretching motions about both S–C2 and N1–N2 bonds, which results in a vibrationally hot 1-S0 molecule. Accordingly, following the gradient difference vector from CI-1 (Figure ) leads to the production of 1,3-diradical (5). As far as the author is aware, this is the first theoretical verification that 5 can be produced through the extrusion of nitrogen from the reactant molecule, 1.

From the 5 + N2 point, both carbon and sulfur atoms in the 5 species can automatically combine together to form thiirene (2) possessing a three-membered ring. It has to be mentioned here that attempts to find the transition state between 5 and 2 using the RASSCF(26,27)/6-311G(d) level of theory are always unsuccessful. It is thus concluded that this transition state would not exist on the RASSCF(26,27)/6-311G(d) potential energy surface. Moreover, a [1,2]-hydrogen migration can occur via the transition state TS-2, which is computed to be about 10 kcal/mol above 5 and nitrogen, from which results in the final photoproduct 3 (thioketene). Also, Figure shows that the other 1,2-hydrogen shift process can take place through the transition state TS-3, which has a higher activation barrier (15 kcal/mol) than the energies of 5 and nitrogen. This pathway yields the other photoproduct 4 (ethynethiol).

It has to be emphasized that the activation barriers at TS-2 and TS-3 are computed, respectively, to be about 10 and 15 kcal/mol, which are much less than the energy difference of 67 kcal/mol between CI-1 and Int-1. In other words, once reactant 1 is photoexcited to jump to the FC-1 point, then this molecule can easily relax to the singlet ground state through the CI (CI-1). From CI-1 to Int-1, 1 can have an excess energy of around 67 kcal/mol, which is greater than the energy difference between 5 and TS-2 (10 kcal/mol) and 5 and TS-3 (15 kcal/mol). Accordingly, 1 can easily cross over the TS-2 and TS-3 barriers to yield the final products, 3 and 4, respectively. However, the available experimental observations indicate that the existence of the intermediate 1,3-diradical (5) is still uncertain.[23−42] According to the computational results represented in Figure , the barrier height from Int-1 to TS-1 is quite small (∼9 kcal/mol) and the activation barriers from 5 to the final photoproducts (2, 3, and 4) are small as well (at most 15 kcal/mol). In other words, the theoretical evidence strongly suggests that the potential energy surface for the formation of the 1,3-diradical (5) is quite flat. As a result, once 1,3-diradical (5) is formed, the final photoproducts (2, 3, and 4) can be easily obtained. The above theoretical result may explain why the experimental detection of 5 is so difficult.[23−42]

Besides these, the RASSCF results schematically shown in Figure reveal that finding a transition state for producing acetylene from the 1,3-diradical (5) is unsuccessful. The present theoretical examinations therefore indicate that on the RASSCF(26,27)/6-311G(d) surface, no transition states exist for the loss of a sulfur atom in the 1,3-diradical (5) to yield acetylene. Indeed, until now, no experimental observations about acetylene under the light irradiations of 1 have been reported yet.[23−42]

In consequence, the theoretical computations demonstrate that the mechanisms for the above pathways proceed as follows:

Path 1: 1(S0) + hν → FC-1 → CI-1(S/S) → Int-1 → TS-1 → 5 + N2 → 2 + N2

Path 2: 1(S0) + hν → FC-1 → CI-1(S/S) → Int-1 → TS-1 → 5 + N2 → TS-2 → 3 + N2

Path 3: 1(S0) + hν → FC-1 → CI-1(S/S) → Int-1 → TS-1 → 5 + N2 → TS-3 → 4 + N2

Furthermore, as seen in Figure , the RASSCF computations demonstrate that the barrier height increases in the order: TS-1 (5.8 kcal/mol) < TS-2 (10 kcal/mol) < TS-3 (15 kcal/mol), which strongly implies that paths 1 and 2 are preferred over path 3. On the basis of these theoretical evidences, it is thus anticipated that thiirene (2) and thioketene (3) are the main photoproducts in the singlet photoisomerization of 1,2,3-thiadiazole (1). The quantum yields of the photoproducts should decrease in the order: 2 > 3 > 4, which agrees well with the available experimental observations.[23−42]

1,2,3-Selenadiazole

We next consider the photorearrangements of 1,2,3-selenadiazole (6), as indicated in Scheme . Similar to the case of 1, the entire potential energy surface based on the RASSCF calculations is collected in Figure . The optimized structures of the key points on the mechanistic pathways of Figure are summarized in Figure . Again, the five p−π orbitals in unsubstituted 1,2,3-selenadiazole, which is the basis of the present study, are given in Figure B (Supporting Information).

Figure 3

Energy profiles for the photoisomerization modes of 1,2,3-selenadiazole (6). The relative energies were obtained at the RASSCF(26,27)/6-311G(d) level of theory. All energies (in kcal/mol) are given with respect to the reactant (6). The abbreviations FC and CI stand for Frank–Condon and conical intersection, respectively. For RASSCF-optimized structures of the key points see Figure . For more information see the text.

Figure 4

RASSCF(26,27)/6-311G(d) geometries (in Å and deg) for the photoisomerization of 1,2,3-selenadiazole (6), transition state (TS), and isomer products. The corresponding RASSCF vectors are shown in the inset. The heavy arrows indicate the main atomic motions in the transition-state eigenvector. For more information see the Supporting Information.

The RASSCF(26,27)/6-311G(d) method has been used to compute the first vertical excitation energy of 1,2,3-selenadiazole (6) at the FC structure, which is from the singlet ground state (S0) to the lowest singlet excited state (S1). As one can see FC-2 in Figure , this vertical excitation energy is computed to be 104 kcal/mol, which is in good accordance with the experimental findings (235–280 nm = 122–102 kcal/mol).[24−32]

Then, from the FC-2 point, 6 can return to the ground state via one radiationless path. As shown in Figure , the first step of this route includes a CI S/SCI-2 and eventually leads to the formation of several photoproducts. It is apparent that the computed structure of S/SCI-2 given in Figure is quite similar to the geometry found for S/SCI-1 shown in Figure . As seen in Figure , the derivative coupling vector[71−75] for CI-2 agrees well with the Se–C2 and C1–N2 stretching motions, which finally results in vibrationally hot species at the singlet ground (S0) state. The gradient difference vector,[71−75] however, fits in with the intramolecular formation of a ring-opening species (Int-2). In other words, increasing the Se–N1 distance and following the gradient difference vector from S/SCI-2 can lead to the formation of the cis-butadiene-like intermediate (Int-2). From the Int-2 point, this molecule can generate the singlet 1,3-diradical (11) with concomitant extrusion of nitrogen via the TS-4, whose activation barrier was computed to be 15 kcal/mol. Subsequently, four rearrangement pathways can take place from the 1,3-diradical (11): (i) both selenium and carbon atoms in the 1,3-diradical (11) can automatically combine together to produce selenirene (7) product. However, repeated attempts using the RASSCF(26,27)/6-311G(d) level of theory to find the transition state between 11 and 7 always failed. It is thus concluded that the transition state would not exist on the path (i) surface, (ii) one H atom of the CH group transfers to the other CH group leading to the formation of selenoketene (8) product via the TS-5, (iii) one H atom of the CH group undergoes the 1,2-shift to attach to selenium to result in ethynylselenone (9) product via the TS-6, and (iv) the 1,3-diradical (11) can extrude one selenium atom to yield acetylene (10) product via the TS-7.

As a consequence, the theoretical examinations reveal that the mechanisms for the photochemical isomerization reaction of 6 should be represented as follows:

Path (i): 6(S0) + hν → FC-2 → CI-2(S/S) → Int-2 → TS-4 → 11 + N2 → 7 + N2

Path (ii): 6(S0) + hν → FC-2 → CI-2(S/S) → Int-2 → TS-4 → 11 + N2 → TS-5 → 8 + N2

Path (iii): 6(S0) + hν → FC-2 → CI-2(S/S) → Int-2 → TS-4 → 11 + N2 → TS-6 → 9 + N2

Path (iv): 6(S0) + hν → FC-2 → CI-2(S/S) → Int-2 → TS-4 → 11 + N2 → TS-7 → 10 + Se + N2

It has to be noted that the geometrical structure of 11 shown in Figure is quite analogous to the previously studied intermediate 5 given in Figure . Both 1,3-diradicals (5 and 11) have an allyl-like structure with one unpaired electron lying on the carbon and the other odd-electron on the group 16 atom (i.e., S or Se, respectively). Also, as mentioned earlier, it was claimed that the existence of the 1,3-diradical (11) is still not experimentally confirmed.[24,25,68,69] Our computational results based on the RASSCF study suggest that the energy difference among Int-2, TS-4, and 11 is not much (∼15 kcal/mol), and the activation barriers from 11 to the final photoproducts (8, 9, and 10) are not much as well (at most 23 kcal/mol). These facts strongly imply that the 1,3-diradical 11 is kinetically unstable and may rearrange spontaneously to the stable minima if 11 is produced. This could be the reason why the spectroscopic signals of 11 have still not been experimentally detected yet.[24,25,68,69]

Additionally, the computational results represented in Figure show that on one hand, 1,2,3-selenadiazole (6) possesses an energy of about 73 kcal/mol because of its relaxation from CI-2 to the local minimum Int-2. On the other hand, the theoretical examinations also demonstrate that the barrier heights from Int-2 to TS-4 and from 11 to TS-5, TS-6, and TS-7 are estimated to be 15, 11, 13, and 23 kcal/mol, respectively. Because of a large excess energy (73 kcal/mol) acquired from the decay of CI-2 to Int-2, the above barriers can readily be surmounted. As a consequence, 6 can easily cross over the TS-5, TS-6, and TS-7 to form the final photoproducts, 8, 9, and 10, respectively. Besides these, on the basis of the computational results given in Figure , it is shown that paths (i) and (ii) are more favorable than paths (iii) and (iv) from a kinetic viewpoint. Accordingly, these theoretical findings anticipate that the quantum yields of 7 and 8 should be much larger than those for isomeric photoproducts 9 and 10. This prediction is verified by some experimental reports.[24,25,68,69]

Conclusions

In this work, the photoisomerization reactions of 1,2,3-thiadiazole (1) and 1,2,3-selenadiazole (6) have been theoretically examined by using the RASSCF(26,27) method to obtain a better understanding of their mechanisms. Their ground-state and excited-state potential energy surfaces are schematically illustrated in Figures and 3, respectively.

The present theoretical investigations demonstrate that upon photoirradiation, both 1 and 6 are vertically promoted to the S1 excited state. Subsequently, radiationless decay from S1 to S0 occurs via a CI, which leads to a rapid ring-opening process with concomitant extrusion of nitrogen. Starting from these CIs, various kinds of photoproducts can be reached on barrier-less ground-state relaxation paths. These theoretical findings, based on the CI viewpoint, have helped us to better understand their photochemical behaviors, which can support the available experimental observations.[23−42,56,68,69] Moreover, the theoretical examinations suggest that both 1,3-radicals (5 and 11) play a dominant role in the photoisomerization reactions of 1 and 6, respectively, whose existences are first verified by the theoretical work. However, because the computed potential energy surfaces around these 1,3-radicals are quite flat, it would be very difficult to detect them using the experimental equipment.

Methodology

RASSCF[76,77] calculations were performed using the multiconfigurational self-consistent-field program in GAUSSIAN 09.[78] It is already reported that RASSCF has some advantages over correlated methods, such as CASPT2 or multireference configuration interaction.[76,77] As a result, the key point structures on the S0 and S1 surfaces were optimized at the RASSCF level of computations using the standard 6-311G(d) basis set,[79] that is, RASSCF(26,27)/6-311G(d). In other words, 26 valence electrons in 27 valence orbitals are chosen as the active space for both 1,2,3-thiadiazole (1) and 1,2,3-selenadiazole (6). RAS1 contains all of the seven doubly-occupied σ orbitals, RAS2 is made up of three doubly-occupied nonbonding orbitals and five π orbitals (with six π electrons), and RAS3 is made up of seven σ* and five π* orbitals. When the maximum force and its root mean square were less than 0.00045 and 0.00005 hartree/bohr, respectively, the optimization was determined. Localization of crossing minima, transition states, and the minima is performed using Cartesian coordinates. The computational results are thus independent of any specific choice of internal variables. The Cartesian coordinates and energetics calculated for the various points are available as Supporting Information.