2.1. The Effect of Methoxy Group Presence on the Ground-State Energy Landscape and the UV Absorption Spectrum
As far as the UV absorption spectrum is concerned, the calculations for CF-OCH
3 molecule predict a very slight red-shift of the absorption maximum, in comparison to its position for the model chromene (CF-H), which in energy scale is not higher than 0.1 eV. This is in excellent agreement with experiment which gives the maximum of absorption at 361 nm in cyclohexane (3.44 eV) for CF-H [
28] and 370 nm (3.35 eV) for CF-OCH
3 (
Figure S1). The observed red-shift is the effect of H substitution with a π-electron donating group, O−CH
3, that decreases the excitation energy value of all the S
0→S
n(ππ*) electron transitions in CF. Such behavior has been observed earlier for methoxy substituted 3
H-naphthopyrans (6-, 7- and 8-methoxy derivatives) [
5,
29].
3
H-Naphthopyrans typically produce both TC and TT isomers (see
Scheme 1) under conditions of continuous UV irradiation tuned to the CF absorption band. These TC and TT isomers are structurally related through the rotation about the C
13−C
14=C
1−C
2 dihedral angle. The UV absorption spectra of TC and TT are then quite similar (see
Tables S1 and S2) since the π-electron conjugation in those two forms is alike. Even though, in both forms the strongest S
0→S
2 transition is slightly red shifted for TC relative to its position for TT (
Figure 1b). The methoxy group substitution does not change much the chromene’s ground-state energy landscape (calculated at the MP2/cc-pVDZ theory level) leaving the S
0-state energy barriers almost unchanged [
28]. Thus, similarly as in the H counterpart, TC-OCH
3 and TT-OCH
3 forms do not differ much energetically. The difference between the relative energies of the respective H and OCH
3 derivatives is marginal, less than 0.02 eV, and both forms lie much above the closed-pyran form CF, app. by + 0.6 eV.
2.2. TT Formation, Single-vs. Two-Photon Mechanism
To unmix TT and TC contribution at the photo-stationary state under UV irradiation, the usual method is to follow thermal recovery to CF. Indeed after the decay of TC, only TT form remains.
Figure 1a shows then time evolution of transient absorption spectra recorded for CF-OCH
3 in cyclohexane after switching off UV irradiation (experimental set-up in
Scheme S1). The initial absorption band corresponds to the mixture of TC (major) and TT (minor) species.
Figure 1b shows that the absorption maximum of TC-OCH
3 occurs at 437 nm (2.84 eV), while that of TT-OCH
3 isomer at 412 nm (3.01 eV), in cyclohexane (compare expt. vs theory in
Table S1), corresponding to the spectral locations for H-derivative (427 nm, for TC-H, and 412 nm, for TT-H,
Table S2). The TC lifetime is slightly longer for the methoxy form (11.7 s vs. 9.3 s in cyclohexane, see
Table 1). A polar solvent as acetonitrile stabilizes the TC-OCH
3 species (lifetime 17.0 s), indeed calculations show a substantial dipole moment of 4.2 D (
Table S1).
Figure 1c shows that TT isomer in cyclohexane is formed in smaller amount from CF-OCH
3 than from CF-H precursor, in agreement with the data reported by Inagaki et al. [
21]. On the basis of DAS amplitudes selected at the TT and TC maxima we estimate that CF-OCH
3 in relation to CF-H described recently [
25], produces four times less TT form under the same experimental conditions. Two parallel reaction mechanisms leading to TT isomer have been considered in literature [
12,
17]. One is a two-photon absorption process:
while the second one is a single photon excitation path:
Experiments performed for CF-H to characterize the influence of the UV irradiation power on the TT absorbance signal (
Figure 2a) show the slope of this relation to be 1.5 in agreement with reported data [
28]. The slope value close to 2 is expected for a purely biphotonic process. The discrepancy can be explained by a small contribution of the single-photon reaction, additionally to the main consecutive two-photon absorption reaction path. We performed numerical simulations (see SI for details) for CF-H in cyclohexane using photophysical properties determined recently [
28] that quantify for the first time the yields of both processes. A quantum yield of 0.003 for the single-photon channel explains the experimental data (
Figure 2b, the slope of 1.5). The single photon process leading to TT has intriguing nature, since it requires the initial ring-opening followed by significant changes in geometry, which contradicts the intuition. However, one can expect that upon ring-opening process TC form is initially generated in the vibrationally excited state. Such excess of vibrational energy can lead the hot TC molecule to overcome relatively high (~1.2 eV) S
0-state energy barrier to form TT isomer.
The experimental slope determined for CF-OCH
3 is closer to unity (1.2,
Figure 2a), which indicates a greater contribution of the single photon excitation path in the TT-OCH
3 formation (CF→TT) that correlates to a lower TT-OCH
3 signal level. The reason for the decrease in the share of biphotonic channel (CF→TC→TT) is linked to the photo-dynamics of TC-OCH
3 and needs thus to be determined precisely.
2.3. Photophysical Properties of TC-OCH3 in the Singlet Excited State
Ultrafast transient UV-vis absorption experiments were performed for TC-OCH
3 in cyclohexane with photoexcitation at 475 nm (2.61 eV). In these experiments, a solution of CF-OCH
3 was under a continuous LED UV irradiation at 365 nm (3.40 eV) to ensure a constant TC-OCH
3 concentration. Although TT is also produced, its concentration is over 20 times lower than that of TC. Moreover the selected pump excitation wavelength at 475 nm (2.61 eV) favors TC excitation over TT on the basis of the respective absorption band locations (
Figure 1b). Thus, the measured transient absorption spectra can be securely assigned to the sole excitation of TC-OCH
3.
Figure 3a shows the evolution of the transient absorption bands, which resemble the data reported for TC-H [
25]. The initial positive transient absorption band peaking at 525 nm (2.36 eV) corresponds to TC-OCH
3 in the singlet excited state (S
1→S
n), which is in agreement with theoretical calculations (
Table S3). The band undergoes a substantial decay (88%) in the time window 0.3–2 ps which is concomitant with the recovery of the negative band peaking at 435 nm. The negative band corresponds to the depopulation band, i.e., depletion of the TC-OCH
3 in the S
0 state caused by laser pulse excitation at 475 nm (2.61 eV). The global analysis indicates two characteristic time-constants (see
Figure 3b): 0.45 ps - related to the lifetime τ
S1 of TC-OCH
3 in the S
1 state, and 5.1 ps corresponding to the vibrationally hot S
0 species produced by S
1→S
0 internal conversion. The lifetime τ
S1 can be also obtained from analysis of the band integral kinetics (0.47 ± 0.05 ps,
Figure S2). The offset shows a weak positive band at 535 nm (2.32 eV) (see the offset × 15 in
Figure 3b), which is assigned to the triplet excited state T
1 produced by intersystem crossing S
1→T
1 as in the parent TC-H compound [
25].
A significant shortening of singlet excited state lifetime for the methoxy TC derivative in comparison to the H-parent compound (0.47 vs. 0.87 ps) is observed in cyclohexane (
Table 1).
Moreover, data comparison at 50 ps delay (
Figure 3a) shows less pronounced S
0 depopulation. Both observations can be explained by a more effective S
1→S
0 internal conversion channel in TC-OCH
3 compound that rationalizes the decrease in TT formation in two photon process and then lower TT concentration in the photostationary state in comparison to those for the unsubstituted parent compound. The next step is to understand the reason for this increase in internal conversion rate and this requires the help of quantum chemical calculations.
2.4. Theoretical Modelling of TC→TT Photoisomerization Mechanism
In order to study the mechanism of the TC→TT photoisomerization reaction, it should be noted first that the molecular mechanism of rotation of the whole rotor unit vs. naphthalenone skeleton may be realized along the two different pathways (variations). They can be classified as
single-twist or
bicycle-pedal motion [
30,
31,
32,
33,
34].
To visualize these two variations of the TC→TT photoisomerization process, the excited state (S
1) and ground-electronic state (S
0) two-dimensional minimum potential energy surfaces (S
1-PES, and S
0-PES, respectively) were constructed to compare the photoisomerization mechanism between the OCH
3 and H derivatives (see
Figure 4). The molecule first evolves along the S
1-PES toward the region where the S
1→S
0 internal conversion process takes place (
Figure 4a,b, for OCH
3 and H derivatives). Further evolution of the molecule follows the S
0-state gradient toward the stable minimum (
Figure 4c,d). The PES minima of the ground state, S
0, and of the lowest excited state, S
1, were calculated using the MP2/cc-pVDZ and ADC(2)/cc-pVDZ methods, respectively. Each energy point at PES was obtained by optimization of the geometry of a given molecule imposing two constraints for driving coordinates: θ
1(C
14 = C
1) and θ
2(C
2 = C
3), separately, in a given electronic state. These two driving coordinates were frozen while all the remaining 3N-8 coordinates were optimized for each point in given electronic state. Thus, each PES is spread over the two driving coordinates: θ
1(C
14 = C
1) and θ
2(C
2 = C
3) (see
Scheme 1, for definition) defined as the dihedral angles describing rotation about the respective double bond. Note also that the two phenyl rings in the rotor unit were distinguished and marked with letters: “a” and “b” in the molecular structure (see
Scheme 1 and
Figure 4) to discriminate between the
single-twist and
bicycle-pedal variant motions. In the excited state, the photoisomerization can be realized as the
single-twist described as the rotation of the rotor unit vs. naphthalenone skeleton about the sole C
14 = C
1 bond and could be observed along the direction parallel to the θ
1(C
14 = C
1)-axes in
Figure 4a,b, for OCH
3 and H derivatives, respectively. The isomerization process, however, can be realized alternatively – as a
bicycle-pedal motion – in which the concerted rotation about the two double bonds: θ
1(C
14 = C
1) and θ
2(C
2 = C
3) takes place simultaneously. This movement can be seen as the two benzene rings: a and b moving in direction parallel to the naphthalenone moiety plane. The
bicycle-pedal motion can be observed along the direction close to the diagonal of
Figure 4a,b. It links upper-left and bottom-right corners of the respective S
1-PES so that the condition Δθ
1 = −Δθ
2 is approximately fulfilled.
Photoexcitation of TC isomer in its Franck-Condon region populates the ππ* excited state. Next, the excited-state relaxation proceeds in a barrierless fashion from the TC Franck-Condon region toward the relaxed excited-state minimum, S
1(TC) form. As shown for the unsubstituted molecule [
25], this relaxation process is accompanied by elongation of the C
13 = O
4 carbonyl double bond which eventually becomes single and the excited state gains the nπ* character. This mechanism resembles that discovered for DNA bases in which the nπ* state is responsible for driving the system in the region of the conical intersection with the ground electronic state CI(nπ*/S
0) [
35,
36]. Thus, the populated excited-state minimum S
1(TC) is observed in the ultrafast experiment with transient absorption detection (
Figure 3). This molecule evolves further along a few deactivation channels [
25], including the one that drives the molecule toward the TT photoproduct. A detail analysis of the mechanism of this process was possible thanks to the use of the calculated PES. The relevant species obtained as a result of the excited-state geometry evolution are shown in
Figure 5 and
Table 2.
We start our analysis of the process with the ground-state S
0TC geometry whose position is marked with the green dot in the upper-left corner of
Figure 4a,b, respectively, for methoxy and H derivatives. The single-photon excitation of S
0TC form of each molecule in a barrierless manner populates the excited-state S
1TC minimum. The first difference between the molecules occurs in the position of the S
1TC minimum on the corresponding S
1-PES (blue dot in the upper left corner of
Figure 4a,b).
During the initial S
1-state relaxation process of TC, both double bonds (θ
1, θ
2) become twisted from their initial Franck-Condon region values (1.8°, −11.7°), for S
0TC(OCH
3), down to (25.5°, −27°), for S
1TC(OCH
3) minimum; and from the initial values (2.5°, −9.5°), for S
0TC(H), down to (22.5°, −18.7°), for S
1TC(H) minimum (see
Table 2). In case of the H derivative, the S
1TC geometry points toward the
single-twist motion path along the direction parallel to the θ
1(C
14 = C
1)-axis (θ
1 = 22.5°, θ
2 = −18.7°).
However, the S
1TC(OCH
3) geometry (θ
1 = 25.5°, θ
2 = −27.0°) lies much closer to the diagonal of the S
1-PES, where the θ
1 = −θ
2 bicycle-pedal condition is fulfilled. The S
1TC form becomes an intermediate, whose geometry may determine further evolution of the molecule in the electronic excited state toward TT along the
single-twist or
bicycle-pedal pathway. These two mechanism variations populate a given type of the excited-state minimum: S
1BP, for
bicycle-pedal, or S
1TW, for
single-twist motion, respectively, shown in
Figure 5.
For OCH
3 derivative (
Figure 4a), the violet region forms a double minimum valley linking the two almost isoenergetic excited-state minimum geometries (blue dots): S
1BP and S
1TW separated by a very low energy barrier. S
1TW [θ
1 = 82.6°, θ
2 = −13.7°] is located in the midpoint between the S
0TC and S
0TT geometries (green and red circles) illustrating t
wisting motion of the molecule. The second minimum, S
1BP [θ
1 = 58.3°, θ
2 = −69.1°], is located in the region close to the diagonal of
Figure 4a suggesting that the
bicycle-pedal mechanism should prevail over the
twisting motion in the OCH
3 molecule, for the energy reasons.
The S
1BP and S
1TW minima were also determined for the unsubstituted parent compound. In this case, the S
1TW excited-state global minimum [θ
1 = 84.7°, θ
2 = −14.9°] is more energetically favored. It is by 0.05 eV more stable than that of the corresponding S
1BP form [θ
1 = 59.2°, θ
2 = −68.5°]. As a consequence, the position of the deeper twist-type S
1TW global minimum on S
1-PES suggests that the
twisting motion prevails over the
bicycle-pedal, for the model of unsubstituted compound. Furthermore in agreement with recently reported experiments [
25] we can also rationalize that the use of polar solvent is another way to control the equilibrium between
bicycle-pedal and
single-twist motion leading to the suppression of the TT formation in the H derivative. Indeed, in a polar solvent, the
bicycle-pedal pathway will be favored over
single-twist motion since the former path leads to a more polar intermediate S
1BP (
Figure 5), which likely produces back TC form after S
1→S
0 internal conversion. Moreover, in a polar solvent such as acetonitrile, the lifetime τ
S1 of TC is shorter in comparison to that in a non-polar cyclohexane (0.27 ps vs. 0.47 ps for OCH
3 derivative and 0.31 ps vs. 0.87 ps for H derivative, see
Table 1) [
25].
Even though after photoexcitation of TC-OCH
3 the excited-state gradient pulls the molecule toward the S
1BP region, the S
1-state PES still indicates the existence of a low-lying flat energy valley reaching toward the region located around θ
2 = −90° (blue region on the lower-left part of
Figure 4c,d). This is the region illustrating sole
single-twist motion around the C
2 = C
3 double bond of the H and OCH
3 derivatives. During this alternative
twist motion, the proton attached to the C
2 carbon atom in allene chain is found in vicinity of the carbonyl oxygen atom, O
4, that pulls the proton to form a stable AP minimum. The ground-state energy surface S
0-PES of both molecules indicates the existence of a high-energy ground-state equilibrium naphthalenol AP form (see the structure in
Table S2). Depopulation of the excited state in this region may generate the AP form in the electronic ground state. The formation of the AP form has been detected in NMR studies for the TC-H molecule [
8].
2.5. Role of the Methoxy Group
The π-electron-donating character of the methoxy group has been studied recently in both, ground- [
37] and excited state [
38] of monosubstituted benzenes. The electron donating effect of the methoxy group is even greater in the excited S
1 state than in the S
0 state. The presence of a methoxy group in the species shown in
Figure 5 results in a greater dipole moment of either the ground- or excited state due to a shift of electron-density from the methoxy group toward the carbonyl oxygen atom, O
4. The O
4 oxygen atom attracts phenyl b ring more in OCH
3 than in H derivative. It results in energetic stabilization of the corresponding excited-state species, S
1BP and S
1TW. In OCH
3 derivative, shortening of the C=O
4···H distance stabilizes the
bicycle-pedal type excited-state species S
1BP vs. the
single-twist one. Thus, we claim here that the role of the methoxy group is mostly to shift electron density toward the carbonyl oxygen atom which attracts the whole rotor unit more strongly than in unsubstituted derivative. This strengthen attraction results in a slight tilt of the rotor toward the carbonyl group in all the structures in the singlet excited state (C
10-C
1 and C
10-C
2 distances increase by 0.1 Å). The electronic effect is affecting the photophysics of the process and is dominating over the steric effect caused by the incorporation of the relatively small methoxy group into the molecule. This thesis is supported by experimental finding that 10-bromo substituted compound photoproduces a similar TT yield as the H derivative [
21]. However the steric effect with more bulky substituents needs further investigations.
The molecular system in the excited S
1 state can deactivate to the electronic ground state, S
0, through S
1→S
0 internal conversion. Thus, the ground state S
0-PES was constructed for both molecules (see
Figure 4a,b, for H and OCH
3, respectively). The height of the S
0-state energy barrier precludes the TC→TT photoisomerization process to be thermally activated.
As one can see, the blue dot representing the geometry of twisting type excited-state S1TW minimum is located close to the top of the green region representing the S0-state energy barrier for both molecules. The system after S1-state deactivation follows the S0-state energy gradient and eventually gains toward the TT photoproduct (S0TT), or goes back to TC form (S0TC).
In contrast to the twisting motion, the blue dot representing the bicycle-pedal type S1BP minimum is positioned on the left side of the S0-state energy barrier. The deactivation of S1BP favors return of the system toward the ground-state TC form, since once being in the ground state, the molecule meets a high barrier for the TC→TT isomerization and the ground-state energy gradient pulls it back to TC isomer. Thus, the channel path populating the S1BP minimum may result in stopping the photoisomerization process. The bicycle-pedal motion in the S1-state prevails over the single-twist for OCH3 derivative, thus a lower TC→TT isomerization quantum yield for this molecule can be expected, which is in agreement with experimental finding.