2.1. Effect of Pressure on the Crystal Structure
In phase II benzene, the aromatic rings are arranged in a slipped-parallel configuration, i.e., the molecules lie on parallel planes [
43]. The compression of benzene phase II induces a decrease in the intermolecular distances, resulting in covalent intermolecular bonds shorter than 3.0 Å at around 30 GPa [
43,
44]. Through X-ray diffraction (XRD) experiments, Heimel et al. [
17] found that LPPs’ compression decreases intermolecular distances from about 4.045 Å [
45] to about 3.5 Å at 6 GPa (see
Table S1). Therefore, it is expected that the compression of [6]LPP would lead to the formation of intermolecular covalent linkages.
[12]CPP, whose diameter is about 1.65 nm, has a crystal structure characterised by a T-shaped arrangement of the molecules, which strongly impedes π-π intermolecular contact formation (see
Table S2). [6]CPP, with a diameter of about 0.88 nm, is polymorphic and either forms tubular or herringbone arrangements [
46]. Pressure-dependent XRD studies of these systems are beyond the scope of this work.
In this work, we conduct a pressure-dependent X-ray diffraction study on a single crystal of H
4[6]CPP in order to relate structural changes with the spectroscopic changes observed upon compression.
Table S3 reports all the experimental details. The crystal structure of H
4[6]CPP was investigated using single-crystal X-ray diffraction between ambient pressure and 4.59 GPa. H
4[6]CPP crystallises in the monoclinic space group
P2
1/
c, with four molecules per unit cell and one molecule in the asymmetric unit. The effect of pressure on the unit cell parameters is shown in
Figure 2a. The cell axes and volume smoothly decrease with each step in the pressure series, showing the absence of any first-order phase transitions. The
a,
b and
c-axes compress by 3.1%, 13.4% and 8.6%, respectively, up to 4.59 GPa.
The molecular structure is shown in
Figure 2b. The closest intramolecular distance between phenyl units under ambient conditions is formed between the phenyl rings containing C13 and C25: the centroid–centroid distance is 3.79 Å and the angle between the planes is 10.59(9)°. At 4.59 GPa, these geometries change to 3.33 Å and 6.1(7)°, respectively. In the same pressure range, the length of the molecule as measured by the distance between the centroid of the hexadiene rings based on C1 and C19, undergoes a modest change from 10.65 to 10.69 Å such that the molecule becomes more oblate overall.
The molecules are packed in a distorted body-centred cubic topology in which a central reference molecule is surrounded in a plane by six others, with four additional molecules in the planes above and below, giving an overall molecular coordination number of 14 (
Figure 3a). This arrangement persists throughout the pressure range studied (
Table S4 lists extended structural information). An analysis of the intermolecular energies is shown in
Table 1, where colours shown for each contact are also indicated in the molecular centroids of
Figure 3a.
The pattern of intermolecular contacts is shown using energy frameworks in
Figure 3b. The lack of framework struts in the
b-direction is presumably the reason why the crystal experiences most compression along this direction (
Figure 2a). The strongest interaction is a non-specific dispersion contact formed by alignment of the long axes of two molecules; thus, at −50 kJ mol
−1, the energy of this interaction is like a moderately strong hydrogen bond. The weakest interactions (the pale blue contact with −10.7 kJ mol
−1 and the yellow contact with −4.8 kJ mol−
1) are built along the
a-axis, where the facing phenylene ring fragments from different molecules interact with each other through their respective double bonds. These interactions are also the shortest, with the ring centroids being separated by 4.24 Å at ambient pressure and 3.54 Å at 4.59 GPa. The variation of other intra- and intermolecular ring and centroid distances are shown in
Figure 4. The fact that at 4.59 GPa the shortest intramolecular distance is 3.33 Å while the shortest intermolecular distance is still 3.54 Å suggests that further compression could lead to the formation of covalent intramolecular links.
2.2. FTIR
It has been demonstrated that the most significant spectral changes occurring in paraphenylenes as a function of pressure are seen in the C-H stretching modes as they are sensitive to the sp
3 or sp
2 hybridisation of carbon [
47,
48]. The clearest example is provided by the pressure-induced polymerisation of benzene itself in which the C-H stretching from aromatic benzene at around 3000–3200 cm
−1 in the IR spectrum vanishes in decompression to 27 GPa and is replaced by a C-H stretching band characteristic of saturated C(sp
3) at around 2900 cm
−1 [
47,
48]. Therefore, this section will mainly discuss this high-frequency region, although data have been collected in the frequency range between 500 and 3200 cm
−1 and are available in the Supporting Information (
Figures S1–S4). In
Figure 5, the FTIR spectra in the 2800–3300 cm
−1 region at selected pressures are shown in order of increasing cyclic conjugation: [6]LPP, H
4[6]CPP, [12]CPP and [6]CPP.
FTIR experiments were performed to explore the possibility of pressure-induced reactivity. In addition, IR experiments were used to calibrate the frequencies of the vibrational modes as a function of pressure and to use them as an internal pressure gauge in the fluorescence measurements. For each of the phenylene systems, one or two polycrystalline samples were investigated using ruby fluorescence as a pressure marker [
49].
In
Figure 5a, we show the FTIR spectra at different pressures for [6]LPP in the region corresponding to the C-H stretching modes. With increasing pressure, there is a change in the C-H profile that can be related to the change from a twisted to a planar configuration (see
Figure S1). When the maximum pressure reached is below 15 GPa, decompression re-establishes the initial [6]LPP D
2 configuration. However, if [6]LPP is compressed above 25 GPa, then a saturated C-H stretching mode at around 2900 cm
−1 appears in the infrared spectrum. This new band rapidly grows to become the most intense feature of the spectrum after the pressure is released. These results indicate that compression at around 25 GPa can induce the loss of aromaticity within the phenyl units through the formation of intermolecular linkages.
Figure 5b shows the pressure-dependent FTIR of H
4[6]CPP. At the lowest pressure, the H
4[6]CPP high-frequency spectrum is characterised by two sets of bands: one at lower frequencies (around 2887 cm
−1 and 2900 cm
−1) that correspond to C-H stretching modes of saturated carbons and of those from the olefinic C-H bonds of the cyclohexadiene units, respectively. Bands at 3030 cm
−1 correspond to the C-H stretches of the phenylene units. As seen in
Figure 5b, compression of H
4[6]CPP leads to an intensity increase of the C-H stretching band derived from the phenylene moieties, which could be interpreted by the pressure-induced strengthening of the π-π interactions as we know that the intramolecular distances are decreasing in compression. Interestingly, decompression from 17 GPa shows the irreversibility of the process as there is a new contribution around 2930 cm
−1 that is consistent with an increase in C-H stretching involving C with higher sp
3 character. Such reactivity might be induced by intra- or intermolecular reactivity through the phenylene units, or through the cyclohexadiene units, respectively. The shorter intramolecular distances of 3.33 Å at 4.59 GPa and obtained in the XRD results indicate that intramolecular reactivity might be favoured in this case.
It has been reported that compression of [12]CPP induces the deformation of the cycle towards a more oval configuration, creating shorter and longer diameters [
7,
40]. The onset of ovalisation is below 1 GPa [
40]; however, the FTIR spectra presented in
Figure 5c show that compression within the oval regime does not seem to affect the sp
2 framework as the C-H stretching modes broaden and upshift while maintaining their band shapes. After pressure is released, the spectrum is identical to that of the pristine sample.
The compression of tubular [6]CPP above 6 GPa causes a permanent deformation of the cycle that could lead to the formation of intermolecular covalent linkages [
39].
Figure 5d shows that at 10 GPa, the C-H stretching modes of the aromatic phenyl units upshift and broaden but their overall profile persists. After decompression, a new contribution at lower frequencies appears at around 2990 cm
−1, indicating the irreversibility of the compression cycle. This band would correspond to a contribution from more saturated carbons and could be explained by a pressure-induced polymerisation.
The pressure shift of aromatic C-H stretching has been estimated for all the systems and compared with that of benzene in
Table 2. As seen in the table, benzene and [6]LPP show similar values, which in increasing order are as follows: benzene, [6]LPP, [6]CPP, H
4[6]CPP and [12]CPP. It is interesting that [12]CPP is the one with largest coefficient as it is the only one showing a reversible response to compression within the explored pressure range.
2.3. Raman Spectroscopy
Pressure-induced intramolecular interactions can be responsible for the general increase in the positions of the Raman bands. It has already been established that there is a common Raman response that reveals structural ovalisation of [n]CPP. The deformation of the cycle is revealed by a change of the rate of frequency shift as a function of pressure [
40]. The most intense Raman bands appear in the 1560–1600 cm
−1 range, having originated from symmetric vibrations arising from collective C-C stretching modes of the benzenoid rings along the transversal direction. Different sp
2 carbon systems as single-wall carbon nanotubes or graphite also present C-C stretching modes in the 1560–1600 cm
−1 region; therefore, we will refer to them as ‘G
A1g’ modes (G is aligned to graphite, which also shows this mode, and A
1g to the symmetry) [
7]. In this work, we measure the Raman spectra of selected powdery systems within an anvil cell at selected pressures. The Raman shift GA
1g [
7] is shown for all of the systems in
Figure 6 (see also
Figures S5–S8 for raw data and analysis of the decompressed sample). As in the compression of phase II of benzene [
45], and in line with former results, the GA
1g mode of [6]LPP linearly upshifts [
16,
50]. The pressure at which the [n]CPPs change the slope and, thus, the cycle, becomes oval is higher the more strained and rigid the cycle is, in agreement with the change in slope at around 5 GPa in [6]CPP and below 1 GPa in [12]CPP [
40]. In the case of the non-conjugated cycloparaphenylene H
4[6]CPP, the C-C stretching mode from the aromatic units of H
4[6]CPP linearly upshift to approximately 8 GPa, where there is a change in slope.
Figure 6b shows the described deformations for the conjugated and non-conjugated CPP.
Equation (1) describes approximate piecewise linear trends in the Raman frequencies:
A
,i and B
,i correspond to the pressure coefficients at pressures below and above
(ovalisation pressure), respectively, and ω
i(0) is the Raman shift at ambient pressure. Thus, for each band we have three parameters, (ω
i(0), A
,i and B
,i) and these are compared in
Table 3. The similar of A
.i for benzene [
45], [6]LPP and H
4[6]CPP demonstrate their similar physical, chemical, and structural response to compression. In previous work, we demonstrated that the similar large values of the
A coefficients for CPPs are related to the deformation of the cycle and, therefore, of the phenyl units [
40]. Moreover, the H
4[6]CPP C=C stretching mode corresponding to the cyclohexadiene units linearly upshift to 10 GPa while not showing a change in slope.
2.4. Absorption and Fluorescence at Low Pressures
In linear paraphenylenes, the absorption spectrum is a consequence of the allowed HOMO-LUMO absorption. The HOMO-LUMO band gap in [n]LPPs decreases with the lengthening of the conjugation that occurs as
n increases, but it can also be enhanced by the decrease of inter-phenyl torsional angles [
53,
54]. Thus, the absorption spectrum of [6]LPP can be fitted to three different vibronic contributions, the 0-0, 0-1 and 0-2 as shown in
Figure 7a. [6]LPP is a good light emitter, with a lifetime value of 0.78 ns [
55]. The PL spectrum of [6]LPP is characterised by a vibronic progression with three main contributions at 422, 447 and 467 nm. The highest energy band at 422 nm is the 0-0 transition, followed by 0-1 and 0-2.
In the case of the [n]CPPs, the complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls the photophysical properties. These molecules have high optical absorbance in the blue spectral region that increases with the ring size; however, the absorption maxima (at ~340 nm) are independent of the ring size as seen in the absorption spectra of [12]- and [6]CPP shown in
Figure 7b,c, respectively. Vertical absorption happens between the HOMO-1/HOMO-2 to LUMO, HOMO to LUMO+1/LUMO+2 and S
2 and S
3 states, respectively, which are degenerate (or almost-degenerate in [6]CPP) by symmetry [
22].
The absorption spectra of both CPPs contain shoulder-like bands at around 396 nm in [12]CPP and 475 nm in [6]CPP, which correspond to the HOMO-LUMO absorption S
0 to S
1 [
56]. By contrast to the LLPs, a higher value of
n implies a larger HOMO-LUMO band gap [
30,
31]. Although these HOMO-LUMO transitions are forbidden by symmetry, the imperfect geometry of CPPs leads to small perturbations in the electronic wave function that result in a small but non-zero oscillator strength and the presence of HOMO-LUMO absorption at low intensity [
55]. The inhomogeneous broadening of the [6]LPP and [n]CPP spectra is due to conformational structural variations related to irregular variations of the dihedral angles [
30,
31,
55].
A peculiar feature of cycloparaphenylenes is their unusual optoelectronic behaviour as a function of molecular size. Specifically, the emission and quantum efficiency diminishes, which lengthens fluorescence lifetimes for smaller
n [
35]. This diminution in quantum efficiency implies that the [6]CPP S
1 state is not fluorescent [
57]. We find that the emission of [12]CPP shows a maximum at 463 nm and lifetime of 2.6 ns, which is in agreement with previous results [
35]. The explanation for the observation of this emission from the forbidden S
1 state has been approached through a variety of hypotheses: phonon-assisted transitions [
36]; efficient fluorescence in large CPP hoops to a broken Condon approximation due to exciton self-trapping [
55]; vibrational intensity borrowing from the higher states [
58]; Jahn−Teller distortion effects due to coupling to circle-to-oval vibrational modes breaking the selection rules [
37]; and strong exciton−vibration couplings [
36,
55]. None of these occur in the small molecules, which remain inefficient emitters [
55].
While the S
0→S
1 transition is forbidden in the linear response, it can be probed by nonlinear spectroscopies such as two-photon absorption [
36]. In this work, one-photon (OP) and two-photon (TP) absorption were tested for both [12]- and [6]CPP. Emission is a one-photon transition and its activity is not affected by the excitation mechanism (OP or TP). Thus, in the case of [12]CPP, no significant change in the PL spectrum is observed, which is in agreement with previous results [
30,
36]. However, in the [6]CPP case, no fluorescence was detected.
The absorption and emission spectra of H
4[6]CPP are presented in
Figure 7d. The absorption UV-vis spectrum was measured in a KBr matrix. The band profile analysis of the absorption spectrum shows that it is formed from two contributions. To understand the origin of these two contributions, we conducted TD-DFT B3LYP/6-31G(d,p) calculations on the optimised B3LYP/6-31G(d,p) structure of a single molecule. These calculations show that there is only one absorption allowed that would correspond to the maximum of the absorption band at 252 nm corresponds mainly to the HOMO-1 to LUMO/HOMO to LUMO+1 transition with an oscillator strength of 0.91. On the other hand, the HOMO-LUMO absorption is formally symmetry-forbidden absorption (see supporting information for more information); however, we relate the experimentally observed band at 264 nm to this HOMO-LUMO transition. As in the case of [12]CPP, in symmetry distortion of H
4[6]CPP should be responsible for perturbing symmetry rules, which explains why the HOMO-LUMO absorption is observed. On the other hand, the fluorescence spectrum at low pressure was measured through one-photon excitation within the diamond anvil cell at 0 GPa. As seen in
Figure 7d, H
4[6]CPP is a strong fluorophore whose lifetime is 10 ns, which is lower than that found for diphenyl units at 16.0 ns [
55] as expected for systems with larger π-conjugation.
2.5. Optical Absorption in Compression
The UV-visible absorption spectra in the region of 370–500 nm were measured for all the samples at selected pressures. Powdered samples were diluted with NaBr and loaded into the gasket chamber together with a ruby chip, whose fluorescence shift was used as pressure marker [
48]. As reference in the absorption measurements, the sample chamber was loaded with NaBr and its UV-vis transmission spectrum was measured.
UV-vis measurements of [6]LPP up to 12.5 GPa (
Figure 8a) show a broadening of the absorption bands with pressure, accompanied by new contributions that appear at lower energy (marked in green). Computational work demonstrates that the band gap decreases with the decreasing angle between neighbouring phenyl units [
49]. With support from TD-DFT calculations (
Figure S10), additional absorption bands at lower energy can thus be assigned to the planar conformer. The absorption intensity rapidly decreases with compression although spectra could not be measured beyond 12.5 GPa as seen in
Figure 8a.
The H
4[6]CPP pressure-dependent absorption measurements were done using H-silicon carbide as anvils, which limits the spectral range to 300 nm and above, and only part of the S
0→S
1 absorption 252 nm could be detected. In
Figure 8b, the absorption edges at different pressures are presented.
Figure 8c shows the absorption spectra of [12]CPP at selected pressures. In addition to the redshift of the bands, the contribution from the S
0→S
1 transition increases in intensity on compression. This agrees with the already described pressure-induced ovalisation of the cycle, which would perturb the symmetry of the excited S
1 state. Indeed, in the case of [6]CPP, whose pressure-induced deformation occurs at around 5 GPa[
39], in the visible region at around 4.7 GPa, the S
0→S
1 contribution seems to grow in intensity (
Figure 8d). However, the S
0→S
1 intensity above
Poval is not as high as expected for the completely allowed transition. Moreover, pressure seems to change the relative intensities of the bands for the S
0→S
2 and S
0→S
3 transitions. While in [12]CPP pressure favours π-conjugation [
40], in [6]CPP pressure leads to a lowering of symmetry within the cycle leading to its polymerisation [
39,
40].
In
Figure 9, the shift of the absorption bands as a function of pressure is shown for the four systems discussed here. For all the phenylene systems, the absorption redshifts with pressure as a result of a decrease in the band gap. However, the rate at which this occurs is strongly dependent on the π configuration.
Table 4 reports the coefficients of the absorption shift estimated for all the systems studied here. In systems with large π conjugation, such as picene, the S
1 band has been observed with a redshift of ~8 nm/GPa. This energy gap closure is a result of the decrease in the intramolecular distances involving π-π interactions [
59]. The torsional freedom of [6]LPP compared with that of picene leads to subtle changes in which the absorption bands redshift by ~1 nm/GPa. In the case of H
4[6]CPP, there are slight changes in the slope of the absorption edge going from ~2 nm/GPa to ~−1 nm/GPa at around 8 GPa, which can be associated with the pressure-induced interactions between intramolecular biphenyl as inferred from the FTIR and Raman observations. As such, [6]LPP and H
4[6]CPP present analogous trends, whereas the pressure-induced interactions between cofacial phenyl units in the cycle deviates the slope (see FTIR section).
Interestingly, in the case of [12]CPP, both the S
0→S
2 and S
0→S
1 transitions redshift with compression, presenting a change in the slope at around 1 GPa,
Poval, going from ~15 nm/GPa and ~30 nm/GPa to ~1 nm/GPa and ~2 nm/GPa, respectively. These large slopes indicate important configurational rearrangements induced by pressure, as previous theoretical calculations estimated a band decrease of 10 nm/GPa in the order of magnitude of the results we obtain [
39,
40].
In the case of [6]CPP, the S
0→S
2 and the S
0→S
3-S
4 transitions at around 5 GPa show a variation in the trend from red-shifting to blue-shifting: the S
0→S
2 goes from ~4 nm/GPa to ~−5 nm/GPa and the S
0→S
3-S
4 from ~8 nm/GPa to ~−9 nm/GPa. At pressures above
Poval, [6]CPP is expected to become highly strained and deformed, with shorter intermolecular interactions [
39], which explains the sudden change in the absorption trends.
2.6. Fluorescence in Compression
Fluorescence measurements were performed on polycrystalline samples loaded with different PTM. To avoid sample irradiation, pressure was measured through the calibrated FTIR shifts measured in the previous section; thus, the cell had to be moved between pressure points to the different setups. The dependence of the fluorescence signal intensity upon the laser power is linear or quadratic depending on whether the fluorescence follows a one-photon (OP) or two-photon (TP) absorption process. Thus, at each pressure step, the fluorescence intensity dependence was measured to determine the process type. The fluorescence spectra and pressure-trends for the three systems are shown in
Figure 10 and
Figure 11.
Table 5 gathers the pressure coefficients of the bands estimated for all the systems studied here.
TP fluorescence spectra of [6]LPP were measured using 740 nm as an excitation line. The fluorescence spectra of [6]LPP are characterised by the S
1→S
0 emission, which shows a well-defined vibronic pattern. [6]LPP initially was loaded without a PTM; however, during the high-pressure fluorescence experiments of [6]LPP, the formation of an excimer at pressures below 2 GPa was characterised by a fluorescence band at around 550 nm. The excimer formation is a very interesting result because it has strong intensity at very low pressure. This behaviour is different to benzene, which showed a sudden intensity exchange between the monomer and excimer transitions at around 6.1 GPa [
61]. Here, formation of the excimer is likely due to the presence of defects among the crystal grains and not the reactivity trigger as IR showed much higher pressures are required. Consequently, it was necessary to work with an inert PTM to avoid the formation of such defects. In these conditions, the excimer was still present. On the other hand, as seen in
Figure 10a, the different vibronic transitions are observed as they all redshift with increasing pressure.
Figure 11a shows the shift of the three vibronic contributions as a function of pressure. Our results are in excellent agreement with those of Guha et al. [
59]. All the data reported show a significant band gap decrease with increasing pressure, with an average linear pressure coefficient 2 nm/GPa calculated above planarisation (ca. 1 GPa), which is in agreement with the absorption measurements.
In H
4[6]CPP, fluorescence spectra were measured using different excitation wavelengths between 540 and 660 nm. The spectra at selected pressures are presented in
Figure 10b. In contrast to [6]LPP, H
4[6]LPP fluorescence is a broad single band, which is assigned as a 0-0 vibronic transition. The TP excitation profile was also measured at selected pressures, detecting fluorescence around its maximum. As in the absorbance experiments, H
4[6]CPP shows a significant redshift in compression at about 150 nm in the 12 GPa range (
Figure 11b). Moreover, the lifetime of the S
1→S
0 emission was measured as a function of pressure and a striking increase in lifetime is observed, going from 9 ns at low pressure to a maximum of 38 ns at 9 GPa as shown in
Figure 11c. Measurements in compression and decompression were conducted to ensure reversibility of the process. Interestingly, unlike in [6]LPP, three regimes are observed: 0–4 GPa with a coefficient of ~5 nm/GPa and lifetime increase of ~0.8 ns/GPa; 4–9 GPa with a redshift coefficient of ~20 nm/GPa and lifetime increase of ~5 ns/GPa; and a third regime up to 12 GPa in which the coefficient is smaller with ~8 nm/GPa and lifetime decrease from a rate of −3 ns/GPa. This change in rate coincides approximately with the maximum pressure for which X-ray data could be collected. The diffraction data at 4.59 GPa are much weaker than at lower pressures, which may point to gradual transformations. These results in the change in the optical properties with pressure confirm that H
4[6]CPP undergoes intramolecular changes involving π-π interactions. This can be interpreted as the increase in the dipole moment of the excited state with stronger π-π intramolecular interactions causing a decrease in the transition energies [
62]. At pressures above 9 GPa, further compression leads to stronger intramolecular connections that do not seem to be so energetically favoured as the energy gap does not decrease so steeply, whereas compression up to 12 GPa seems to lead to a product without covalent bond formation as the process is fully reversible.
In
Figure 10c, the TP fluorescence spectra of [12]CPP at selected pressures are presented. These were measured with an excitation line of 760 nm. The asymmetry in the fluorescence profile indicates that the spectra are formed by at least two different contributions as previously described [
35,
63]. We have fitted the spectrum to two contributions where the energy spacing between these peaks is approximately 40 nm, corresponding to approximately 1450 cm
−1 and matching the phenylene stretching vibration [
64]. [12]CPP fluorescence shows a redshift of the same order of magnitude as in the absorption measurements (
Figure 11d). The average shift of the vibronic modes gives a redshift of 50 nm/GPa up to 1 GPa and of 10 nm/GPa above 1 GPa. These results are in line with the compression of a low-pressure rigid phase in which pressure would favour the cyclic conjugation [
40,
49,
65].
[6]CPP fluorescence was measured with OP and TP excitations, and pressures below and above ovalisation pressure. However, significant fluorescence could not be detected for any configuration. This lack of results indicates that the deformation of the cycle does not allow the S
0→S
1 transition and confirms the fact that the absorbance observed in the bigger cycles at room pressure cannot be related with a Jahn–Teller effect [
37].