The High-Pressure Response of 9,9′-Spirobifluorene Studied by Raman Spectroscopy

Abstract

The pressure response of crystalline 9,9′-spirobifluorene up to 8 GPa was studied by means of Raman spectroscopy using a diamond anvil cell as a pressure chamber. With increasing pressure, the observed Raman peaks shifted to higher frequencies, reflecting the bond hardening upon volume reduction, which was much more pronounced for the initially weaker intermolecular interactions than for the stronger intramolecular covalent bonds. The significant changes in the Raman spectrum and the pressure evolution of the frequencies at ~1.3 GPa for both the intermolecular and the intramolecular Raman peaks signaled a pressure-induced structural and molecular conformation transition with a little hysteretic behavior (~0.5 GPa) upon pressure release. For P > 4 GPa, the reversible decrease of the pressure coefficients of the majority of the intermolecular and some intramolecular peak frequencies indicated another structural modification of the studied molecular crystal. A value of ~9 GPa for the bulk modulus of the system at zero pressure was estimated from the logarithmic pressure coefficients of the frequencies of the intermolecular modes in the low-pressure phase. These coefficients were reduced by ~6 times at 4.2 GPa, indicating that the considerable stiffening of the material in the high-pressure phase emanated from the selective strengthening of the intermolecular interactions.

1. Introduction

Molecular crystals, which consist of molecules held together by weak intermolecular interactions—such as van der Waals and hydrogen bonding—are highly significant in the fields of material science, chemistry and life science. Among the organic molecules, polycyclic aromatic hydrocarbons (PAHs) are a diverse group of molecular structures made up of two or more interconnected aromatic rings. Apart from their intrinsic toxicity [1,2,3,4,5], these compounds have garnered considerable interest because of their unique physical and chemical properties due to the delocalization of π-electrons within their aromatic rings, strongly influenced by the exact molecular structure [1,4,6,7,8,9,10,11,12]. Therefore, PAHs and their derivatives have diverse applications in pharmaceuticals, plastics, dyes, liquid crystals and optoelectronics [7,8,9,10,11]. The formation of spiro compounds is a very promising way to improve the morphological stability—while retaining the functionality—of PAH molecules with a relatively low molecular weight, which is vital for optoelectronic applications [13,14]. Spiro compounds consist of two mutually perpendicular π-systems connected through a common tetracoordinated atom [15]. The perpendicular arrangement of the two parent molecular units leads to the high steric demand of the resulting rigid structure. This structural feature efficiently diminishes molecular interactions between the π-systems, which leads to the higher solubility of the spiro-linked compounds. The spiro-linking also considerably suppresses the excimer formation, frequently observed in the solid state, stabilizing the emission properties [16].

The 9,9′-spirobifluorene (SBF, C₂₅H₁₆) molecule consists of two fluorene-like subunits—each composed of two aromatic hexagonal rings connected by a non-aromatic pentagonal ring—in a nearly perpendicular configuration [17]. It exhibits the D_2d point group symmetry and thus presents an intriguing structural motif that has been applied in molecular recognition and catalysis [18,19] and integrated into materials with unusual optoelectronic properties [20]. SBF crystallizes in the monoclinic structure (space group: P2₁/c, lattice parameters: a = 10.173 Å, b = 10.241 Å, c = 16.722 Å, β = 96.52°, Z = 4), where the individual rings remain planar but the biphenyl groups of the molecule deviate significantly from planarity due to crystal packing [21]. Note that the formation of another, possibly metastable SBF polymorph has also been reported, having the same space group but larger lattice parameters: a = 18.2491 Å, b = 11.1522 Å, c = 18.6918 Å, β = 117.907° and Z = 8 [22]. This difference arises from the fact that the metastable polymorph contains two SBF molecules in the asymmetric unit instead of one in the case of its stable counterpart, owing to the formation of centrosymmetric SBF dimers through C-H···π(arene) hydrogen bonding [22].

Because of the coexistence of weak intermolecular interactions and strong intramolecular covalent bonds in molecular crystals, the application of pressure acts selectively, reducing much more pronouncedly the intermolecular distances. This is usually followed by changes in the relative orientations of the constituent molecules and/or structural transitions—even at moderate pressures—without altering their chemical structure. Raman spectroscopy, which probes in a facile way both intermolecular (external) and intramolecular (internal) vibrations, is usually the method of choice to investigate the response of molecular crystals and the different evolution of the inter- and the intramolecular modes with pressure [23,24,25]. The parent benzene and several PAHs that are comprised of 2–4 hexagonal aromatic rings (naphthalene, biphenyl, fluorene, anthracene, phenanthrene, tetracene, chrysene, pyrene and triphenylene) have been studied by means of Raman spectroscopy under high pressure, and subtle structural transitions at moderate pressures (below 8 GPa) were reported [26,27,28,29,30,31,32,33,34,35,36,37,38,39]. These transitions are usually sluggish and reversible and are associated with symmetry changes, molecular reorientations and stacking rearrangements, charge transfer processes, or changes in the π-electron density [34,37].

Here, we report, for the first time, the structural stability and pressure-induced phase transitions of SBF crystals, adding to the existing knowledge of the physical properties of PAH systems. Further, an estimation of the bulk modulus of SBF is provided, which is not available in the literature. Such information is valuable for the potential use of PAHs in the various applications aforementioned, as well as for understanding the pressure response of molecular crystals in general. More specifically, high-pressure Raman measurements of SBF crystals were conducted up to 8 GPa using a diamond anvil cell (DAC) for pressure application and Daphne 7474 as a pressure transmitting medium (PTM). Significant alterations in the Raman spectrum profile and the pressure evolution of the Raman peak frequencies at ~1.3 GPa were observed, which were attributed to a pressure-induced structural and molecular conformation transition of SBF with small hysteresis upon pressure release. Above 4 GPa, the reversible changes in the pressure coefficients of the majority of the intermolecular and some intramolecular peak frequencies in the low (<300 cm⁻¹) and 480–900 cm⁻¹ frequency regions indicated either a second structural transition of the system or the completion of the first one at ~1.3 GPa.

2. Results and Discussion

2.1. Raman Spectrum of SBF at Ambient Pressure

The Raman spectrum of the crystalline SBF at ambient conditions excited with λ_exc = 632.8 nm is illustrated in Figure 1. Because of the complexity of the studied system and the large number of atoms included in its molecule and monoclinic unit cell, the spectrum was very rich in terms of the number of observed peaks and could be divided into three main frequency regions. In the frequency region below 250 cm⁻¹, intermolecular (external) vibrational modes and intramolecular (internal) torsional and skeleton deformation modes are expected due to the smaller values of the corresponding force constants and the heavier masses involved. The majority of the intramolecular modes exhibited frequencies above 250 cm⁻¹ owing to the strong covalent bonding and the relative motion of lighter masses. In the frequency region 300–1600 cm⁻¹, the intramolecular vibrations included C–H and CCC bending and ring deformations in the two fluorene-like subunits, while C–H stretching vibrations are expected in the frequency region 2900–3250 cm⁻¹. In this high-frequency region, higher-order (overtones and combinational) vibrational modes with relatively low intensities are also expected, particularly in the range of 3100–3250 cm⁻¹. Contrary to the case of the fluorene molecule, the absence of the CH₂ methylene group in SBF—due to the formation of the spiro-linking—caused the disappearance of the corresponding stretching vibrations in the frequency region 2900–3000 cm⁻¹ of the Raman spectrum [37,40,41]. Thus, the C-H stretching vibrations in SBF were limited in the range of 3000–3100 cm⁻¹.

The frequency positions of the various Raman peaks appearing in Figure 1 and their relative intensities were in fair agreement with those reported earlier for SBF crystals by Boo et al. using λ_exc = 514.5 nm [43]. Boo et al. also assigned their experimentally observed Raman peaks to specific intramolecular vibrational modes by means of ab initio Hartree–Fock (HF) and Becke 3 Lee–Yang–Parr (B3LYP) density functional theory (DFT) calculations using the 4–31 G basis set. They further confirmed their assignment for some of the peaks by solution Raman measurements with the depolarization method of SBF in CCl₄ [43]. For the isolated SBF molecule with 41 atoms and the D_2d point group symmetry, 88 fundamentals are expected, with symmetries 20A₁ + 9A₂ + 10B₁ + 20B₂ + 29E. From these, the A₂ modes are inactive in both Raman and infrared (IR) spectroscopy, while the A₁ and B₁ modes are active only in Raman scattering. On the other hand, the B₂ and E modes—the latter were doubly degenerate modes because of the presence of two identical fluorene-like subunits in the SBF molecule—are active in both Raman and IR spectroscopy [43].

In this work, we adopted the same assignment for the various Raman peaks by comparing the extrapolated frequencies to zero pressure from the low-pressure ω vs. P data with those experimentally obtained at ambient conditions by Boo et al. (Appendix A,

Table A1. Mode symmetry in the D_2d point group, approximate mode description and calculated (ω_calc, cm⁻¹) and experimental (ω_exp, cm⁻¹) frequencies of the vibrational modes of 9,9′-spirobifluorene, as obtained from the literature [43]. The frequencies (ω_i0 at 0.0 GPa, ω_j0 at 1.1 GPa or at the lowest pressure they appear and ω_k0 at 4.4 GPa or at the lowest pressure they appear in cm⁻¹) extracted from the least-squares fits and the corresponding ω vs. P data of the present work, along with their linear (b₁, cm⁻¹GPa⁻¹) and quadratic pressure coefficients (b₂, cm⁻¹GPa⁻²), are also presented. The corresponding data for the well-resolved Raman peaks are shown in bold. The symmetry, approximate mode description and ω_calc from [43] for the Raman peaks tentatively assigned in this work are shown in italics.

Peak	Sym.*	Approx. Mode Description *	ωcalc *	ωexp **	P < 1.3 GPa			1.1 < P < 4.1 GPa			P > 4.2 GPa
					ωi0	b1	b2	ωj0	b1	b2	ωk0	b1
ω1								50	4.5		60	3.1
ω2											69	2.4
ω3								54	6.8		67	2.9
ω4	B1	ring tor	47	48	48	10.8		56	12.6	−0.95	81	4.0
ω5								75	4.9		85	3.6
ω6	E	ring tor and defm	46		51	14.4		63	12.6	−0.77	92	4.9
ω7		external			54	16.5
ω8		external			59	15.9		72	13.1	−1.03	97	5.1
ω9		external			73	21.1		95	23.3	−2.53	129	6.1
ω10								110	20.4	−1.64
ω11	E	ring tor and defm or external	92		95	20.9		106	22.1	−2.06	143	5.8
ω12											155	6.1
ω13								121	27.5	−2.38	169	4.2
ω14								139	25.5	−2.50
ω15		external			111	47.4	−11.60	163	8.6		179	6.5
ω16								155	13.9	−0.15	196	8.6
ω17		external			139	29.0
ω18	B1	ring tor	156	156	155	18.1		172	15.7	−0.59	207	7.7
ω19		external			163	22.5		177	24.8	−2.20	217	6.1
ω20		external			177	18.8
ω21		external			187	22.0		213	14.0		267	7.2
ω22	B2	ring defm	216		220	4.5		217	−7.8	1.95	229	9.7
ω23	E	ring tor, ip (CH + CCC) bend	240		245	4.6		246	4.3		260	4.1
ω24								259	11.3		274	5.4
ω25	E	ring tor, ip (CH + CCC) bend	240		248	7.0		260	9.8	−0.62	283	8.2
ω26								273	21.5	−2.17
ω27								285	6.2			6.2
ω28	B1	ring tor	319	320	321	6.5		323	3.6			3.6
ω29	A1	ip CH and CCC bend	331	336	337	5.2		335	3.0			3.0
ω30	B2	ring defm	407	412	412	3.5
ω31	E	ring tor	424	419	418	2.8		420	1.9			1.9
ω32	A1	ring defm	420	424	426	3.3		427	1.7			1.7
ω33								487	3.5	−0.25	494	0.6
ω34	B1	oop CH and CCC bend	501		498	3.2		502	6.3		515	1.9
ω35	E	ring tor, ip (CH + CCC) bend	513	514	515	1.5		516	2.9		526	2.0
ω36	E	ring tor, ip (CH + CCC) bend	513		519	5.5		520	3.7		532	2.8
ω37								569	2.3			2.3
ω38	E	ring tor, ip (CH + CCC) bend	644	636	635	1.3		633	0.8			0.8
ω39								640	1.9	−0.10	644	0.9
ω40	A1	ip CH and CCC bend	705	704	703	2.7		708	2.7	−0.12	715	1.9
ω41	E	oop CH and CCC bend	735	727	728	0.2		728	0.9			0.9
ω42	E	oop CH and CCC bend	735		738	0.8
ω43	E	oop CH bend	752	750	750	2.7		750	1.0			1.0
ω44	E	oop CH bend	752		756	2.6
ω45	A1	ip CH and CCC bend	766	760	763	2.4		764	1.9			1.9
ω46	B1	oop CH bend	762		765	3.6		769	7.8	−0.79	781	1.1
ω47	B1	oop CH and CCC bend	800	796	796	1.6		795	0.3	0.25	800	2.7
ω48	E	oop CH bend	864		861	1.0		854	1.1		858	0.2
ω49								869	5.3	−0.44	879	1.7
ω50								875	2.7			2.7
ω51								915	2.1			2.1
ω52	B1	oop CH and CCC bend	943		932	5.9
ω53	E	oop CH and CCC bend	947	946	948	2.8		951	1.8			1.8
ω54	B2	ip CH and CCC bend	948	948	951	3.4
ω55	E	oop CH and CCC bend	947		959	0.4
ω56								980	2.4			2.4
ω57	B1, E	oop CH and CCC bend, ip CH and CCC bend	984, 1005	1004	1005	2.4		1008	1.9			1.9
ω58											1020	3.2
ω59	A1, B2	ip CH and CCC bend	1020	1020, 1021	1020	2.5		1026	4.0			4.0
ω60	E	ip CH and CCC bend	1035	1031	1029	4.0		1041	3.7			3.7
ω61	A1	ip CH and CCC bend	1083	1088	1084	3.4
ω62	B2	ip CH and CCC bend	1105	1104	1105	0.7		1102	2.0			2.0
ω63	E	ip CH and CCC bend	1103		1110	4.0		1105	2.4			2.4
ω64								1111	2.4			2.4
ω65	A1	ip CH and CCC bend	1148	1152	1152	3.0		1155	1.7			1.7
ω66	E	ip CH and CCC bend	1155		1157	4.2		1158	2.3			2.3
ω67	E	ip CH and CCC bend	1155	1164	1160	4.0		1163	2.5			2.5
ω68								1170	3.2			3.2
ω69								1171	4.8			4.8
ω70	A1, E	ip CH and CCC bend	1178, 1179	1176	1175	6.2		1182	4.5			4.5
ω71	B2	C(sp3)—C str, ip CH and CCC bend	1206	1208	1210	2.6
ω72	B2	ip CH and CCC bend	1222		1214	3.8		1215	2.4			2.4
ω73	A1	ring str, ip CH bend	1221	1224	1226	4.6		1226	2.5			2.5
ω74					1232	4.7
ω75								1249	3.9			3.9
ω76	E	ip CH and CCC bend	1288	1282	1282	2.6		1286	2.5			2.5
ω77	B2	ip CH and CCC bend	1299	1295	1294	1.6		1294	1.9			1.9
ω78	A1	ip CH and CCC bend	1298	1297	1297	4.8
ω79	E	ip CH and CCC bend	1305		1301	3.9		1304	4.3			4.3
ω80	B2	ip CH and CCC bend	1352	1351	1352	5.5		1360	4.9			4.9
ω81	A1	ip CH and CCC bend	1354	1352	1359	4.0		1368	4.6			4.6
ω82								1477	2.3			2.3
ω83	E	ip CH and CCC bend	1473	1474	1476	6.4
ω84	A1	ip CH and CCC bend	1471	1480	1480	3.8		1485	3.1			3.1
ω85	B2	ip CH and CCC bend	1474	1480	1485	4.8		1488	3.2			3.2
ω86					1498	2.9
ω87	A1, B2	ip CH and CCC bend	1573, 1572	1580, 1581	1581	5.1		1587	4.5			4.5
ω88					1591	4.7
ω89								1609	3.3			3.3
ω90	E	ip CH and CCC bend	1594		1601	3.3
ω91	E	ip CH and CCC bend	1594	1604	1605	5.1		1609	4.6			4.6
ω92	A1	ip CH and CCC bend	1596	1608	1610	6.1		1614	4.7			4.7
ω93	B2	ip CH and CCC bend	1591	1628	1624	3.7		1630	3.1			3.1
ω94		2nd order			3004	6.7
ω95		2nd order			3012	6.6		3019	8.7			8.7
ω96	E	CH str	3047		3024	5.8
ω97	E	CH str	3047		3033	3.0		3036	7.8			7.8
ω98	B2	CH str	3048		3040	7.0
ω99	A1	CH str	3048		3047	8.4		3060	6.9			6.9
ω100	A1	CH str	3058		3060	9.6		3068	6.4			6.4
ω101											3100	8.0
ω102	A1	CH str	3068		3065	11.3		3081	8.7			8.7
ω103	E	CH str	3067		3075	11.1		3081	11.7			11.7
ω104	A1	totally sym CH str	3080		3081	13.3		3092	8.6			8.6
ω105								3105	12.8			12.8
ω106	B2	CH str	3080		3088	12.3		3099	13.7			13.7
ω107								3118	11.7			11.7
ω108		2nd order			3159	9.4		3171	6.4			6.4
ω109		2nd order			3196	10.1		3222	7.2			7.2
ω110		2nd order			3209	13.8		3218	9.9			9.9

* Symmetries, approximate mode descriptions and ω_calc values are those extracted from the DFT calculations (4–31 G basis set) by Boo et al. [43]. Approximate mode description: tor, torsion; defm, deformation; bend, bending; str, stretching; sci, scissoring; twi, twisting; roc, rocking; ip, in-plane; oop, out-of-plane. ** ω_exp values are taken from the Raman spectroscopic measurements of SBF crystals and saturated solution in CCl₄ by Boo et al. [43].

). Several additional peaks appearing in the Raman spectrum of crystalline SBF were also tentatively assigned to intramolecular or intermolecular (external) vibrations (low-frequency region). This was accomplished by taking into account their similar ambient pressure frequencies to those obtained from the DFT calculations by Boo et al. for the SBF molecule in the D_2d point group symmetry [43] and assuming a possible crystal field and molecular deformation [21] splitting of the degenerate E modes (shown in italics in Table A1). Some weak Raman peaks with relatively high frequencies could not be assigned to specific fundamentals of the SBF molecule and may have originated, as also mentioned above, from second-order Raman scattering. Noticeably, the similarity of some Raman peak frequencies of SBF with those of fluorene monomer and other fluorene-containing spiro compounds (dicarbomethoxy and dimethyl-substituted spiro-cyclopropane-1,9′-fluorene as well as bis(2,2′-biphenylene)silane and bis(2,2′-biphenylene)germane), was remarkable, being indicative of their common origin [40,41,43,44,45].

2.2. Pressure Dependence of the Raman Spectrum in the Low- and High-Frequency Regions

In this subsection, the results of the high-pressure Raman study of SBF in the low- (ω < 300 cm⁻¹) and high-frequency (3000–3250 cm⁻¹) regions of the spectrum are summarized and discussed. As mentioned in the previous subsection, the first region contained the intermolecular and intramolecular torsional and skeleton deformation modes, while the second included C–H stretching vibrations. The character of the former modes and the peripheral positions of the hydrogen atoms, associated with the latter vibrations, rendered these modes more sensitive—compared to the intermediate frequency intramolecular modes—to volume contraction and the possible occurrence of any pressure-induced structural and/or molecular conformation modifications [37].

Representative Raman spectra of SBF at various pressures in the low-frequency region are illustrated in Figure 2a. With increasing pressure, all Raman peaks shifted to higher frequencies, reflecting the bond hardening upon volume contraction. Moreover, considerable relative intensity changes of the various peaks were also observed in the whole pressure range investigated (0–8 GPa). Bond hardening with increasing pressure forced additional Raman peaks, initially located below 50 cm⁻¹, to gradually enter the spectral window depicted in the figure (peaks ω₁, ω₃, ω₄, ω₆ and ω₇ in Table A1). Note that the low-frequency edge of this spectral region was limited by the edge filter cut-off, which was used in the spectrometer to eliminate the strong, elastically scattered light (Section 3). Apart from these observations, abrupt changes regarding the appearance of the Raman peaks and their frequency shifts with pressure were apparent when going from 1.23 to 1.56 GPa, signaling the occurrence of a structural phase transition.

The pressure dependence of the frequencies of the various Raman peaks in the low-frequency region is illustrated in Figure 2b. For each peak, the ω vs. P data upon pressure increase (open circles) and decrease (solid circles) were fitted by linear (ω = ω₀ + b₁P) or parabolic (ω = ω₀ + b₁P + b₂P²) functions in three different pressure ranges (P < 1.3 GPa, 1.1 < P < 4.1 GPa and P > 4.2 GPa). The linear pressure coefficients (b₁) of the stronger peaks (thicker circles and lines) are also shown in the figure. The frequencies ω₀ obtained from these fits and the corresponding pressure coefficients b₁ and b₂ for all the observed Raman peaks are summarized in Table A1 (Appendix A). As can be inferred from Figure 2b, the pressure dependencies of the frequencies for both intramolecular and intermolecular modes were smooth and linear up to ~1.3 GPa, with the exception of the external ω₁₅ mode, which showed a strong parabolic dependence. The Raman peaks with frequencies below 200 cm⁻¹ exhibited relatively large pressure coefficients, compatible with the small force constants involved [24]. However, abrupt changes and discontinuities occurred above this pressure value. More specifically, the Raman peaks attributed to intermolecular vibrations ω₇, ω₁₇ and ω₂₀ disappeared, whereas new Raman peaks (ω₅, ω₁₀, ω₁₃, ω₁₄, ω₂₄ and ω₂₆) progressively appeared in the spectrum as pressure increased from 1.5 to 3 GPa. In addition, as the pressure increased from 1.23 to 1.56 GPa, the frequencies of several Raman peaks attributed to intermolecular (ω₈ and ω₁₉) and intramolecular (ω₄, ω₆, ω₁₁, ω₁₈, ω₁₉, ω₂₂ and ω₂₃) modes either remained constant or softened by 1–8 cm⁻¹. This behavior indicated a weakening of the corresponding intermolecular and intramolecular interactions within this pressure range, probably caused by the abrupt anisotropic shift of the molecular units from their initial positions in the unit cell. The pressure dependencies (pressure coefficients b₁ and b₂) of the various peak frequencies were also different below and above 1.3 GPa. All these pressure-induced peculiarities were compatible with the aforementioned evolution of the overall Raman spectrum profile when going from 1.23 to 1.56 GPa (Figure 2a) and supported the hypothesis of a pressure-induced structural and molecular conformation transition for P > 1.3 GPa. This transition showed some hysteresis (~0.5 GPa) upon pressure release (solid symbols in Figure 2b); the high-pressure phase survived down to ~1.1 GPa and the low-pressure phase was recovered at ~0.7 GPa, suggesting the first-order nature of the observed phase transition. Since there was no significant change in the number of the intermolecular modes appearing in the Raman spectrum in the low- and high-pressure phases, the crystal structure most probably remained monoclinic.

In the pressure region 1.3–4 GPa, the pressure dependencies of the frequencies of the low-frequency Raman peaks were mainly sublinear (negative quadratic pressure coefficients, b₂), reflecting the gradual strengthening of the initially weak intermolecular interactions upon volume contraction and the stiffening of SBF [25,37]. Noticeably, the Raman peak ω₂₂, possibly originating from an intramolecular mode associated with ring deformation, showed peculiar behavior upon compression. Its frequency remained nearly constant within the pressure range of 2–3 GPa, whereas it increased quasi-linearly for P > 3 GPa, resulting in an overall superlinear pressure response (positive pressure coefficient b₂) in the pressure range under consideration (1.3–4 GPa). For pressures higher than 4 GPa, all the observed Raman peaks exhibited linear ω vs. P dependencies with reduced pressure coefficients b₁ (Figure 2b). The decrease of the pressure coefficients indicated the considerable stiffening of the studied molecular crystals at elevated pressures. We recall that similar behavior has also been observed in the case of the parent orthorhombic fluorene for P > 6 GPa [37]. High-pressure X-ray diffraction (XRD) measurements have shown that fluorene undergoes a pressure-induced isostructural phase transition for P > 3 GPa, caused by the molecular rearrangement from the ambient and low-pressure herringbone pattern to π-stacking, resulting in abrupt changes in the lattice parameters (increase in a and b and decrease in the c lattice constant). The low- and high-pressure phases co-existed up to ~5 GPa, where crystalline fluorene was completely transformed to the stiffer high-pressure phase [46]. This structural transition was also apparent in the pressure evolution of the Raman peak frequencies since two distinct reversible changes in the linear pressure coefficients—mainly to lower values—were observed at 2.5 and 6 GPa, but without any spectroscopic evidence for phase coexistence in the intermediate pressure region [37]. In the case of SBF, the absence of high-pressure XRD data in the literature prevented us from conclusively judging whether two different structural phase transitions occurred at ~1.3 and ~4.2 GPa or whether a single one—sluggish, with phase coexistence—took place in the pressure range of 1.3–4.2 GPa, possibly characterized by the reorientation and different stacking of the SBF molecules.

Representative high-pressure Raman spectra of SBF in the high-frequency intramolecular mode region, including C–H stretching vibrations and higher-order modes as well as the pressure evolution of the corresponding Raman peak frequencies, are illustrated in Figure 3. Similarly to the low-frequency region, all Raman peaks shifted to higher frequencies with increasing pressure and the concomitant volume contraction. The pressure dependencies of the peak frequencies were quasi-linear up to ~1.3 GPa, where the pressure-induced phase transition took place and the overall spectrum profile changed significantly when going from 1.23 to 1.56 GPa. The ω vs. P dependencies for all the observed peaks were also quasi-linear up to the maximum pressure attained in our experiments. Nevertheless, the pressure coefficients of the peak frequencies above 1.3 GPa were different from those for lower pressures; the coefficients at high pressures increased for the first-order Raman peaks ω₉₇, ω₁₀₃ and ω₁₀₆, attributed to C–H stretching vibrations, and the second-order Raman peak ω₉₅, but decreased for the first-order Raman peaks ω₉₉, ω₁₀₀, ω₁₀₂ and ω₁₀₄, attributed to C–H stretching vibrations, and the high-frequency second-order Raman peaks ω₁₀₈–ω₁₁₀. The ω vs. P data for the high-frequency intramolecular modes exhibited the same hysteretic behavior upon pressure release to that of the intermolecular and low-frequency intramolecular modes, as the low-pressure phase of SBF was recovered at ~0.7 GPa.

2.3. Pressure Dependence of the Raman Spectrum in the Intermediate-Frequency Region

Raman spectra of SBF at selected pressures in the intermediate-frequency region of the intramolecular modes (300–1700 cm⁻¹) are illustrated in Figure 4a.

The Raman peaks in this frequency region also shifted to higher frequencies upon compression but with smaller rates compared to the Raman peaks in the low- and high-frequency regions. Moreover, the changes in the overall spectral profile were also much less pronounced and compatible with their internal character. The pressure dependencies of the frequencies of the observed Raman peaks in the intermediate-frequency region are shown in Figure 4b and Figure 5. Changes in the pressure evolution of the peak frequencies as well as the appearance or disappearance of some Raman peaks were also observed at ~1.3 GPa, with some hysteresis upon pressure release. The majority of the Raman peaks exhibited quasi-linear ω vs. P dependencies in the pressure ranges of 0–1.3 GPa and 1.1–7.8 GPa, with generally smaller pressure coefficients in the high-pressure phase. However, there were several Raman peaks in the frequency region of 480–900 cm⁻¹, attributed to in- and out-of-plane C–H and CCC bending (Table A1) that had parabolic ω-P dependencies for 1.1 < P < 4.1 GPa and/or underwent changes in the linear pressure coefficients of their frequencies above 4.2 GPa.

2.4. Force Hierarchy and Elastic Properties of SBF

The mode Grüneisen parameter γ_i = B₀·∂(lnω_i)/∂P (where B₀ is the bulk modulus and ω_i is the frequency of the ith mode) is related to the pressure-induced volume contraction and the concomitant modifications of the force constants of the bonds involved in the vibrational mode. In the Grüneisen approximation, all the γ_i parameters of the various modes of a given solid are equal to the same value γ, yielding the scaling law ω ~ V^−γ, in which the frequency spectrum uniformly expands as the crystal contracts upon compression [24,47]. In molecular crystals like SBF, the mode Grüneisen parameters, apart from the mode frequency anharmonicity, express the force (bond stiffness) hierarchy. Although the Grüneisen approximation still works well for the intermolecular modes, it fails completely for the intramolecular modes, and a rough γ_i ~ ${\mathsf{\omega }}_{\mathrm{i}0}^{-2}$ dependence is observed, where ω_i0 is the frequency of the ith mode at zero pressure [23,24]. As the bulk modulus for the SBF crystals is not available in the literature, and in order to compare its various binding forces, we can use the experimentally obtained logarithmic pressure coefficients of the frequencies (frequency normalized pressure slopes) Γ_i = ∂(lnω_i)/∂P= (1/ω_i0)·∂ω_i/∂P of all the observed Raman peaks, which are proportional to the γ_i parameters (γ_i = B₀Γ_i).

The normalized pressure slopes Γ_i of all the intermolecular and intramolecular Raman peaks of SBF below 1.3 GPa (low-pressure phase—open symbols) or above 4.2 GPa (high-pressure phase—solid symbols) as a function of their frequency ω_i0 at 0.0 or 4.2 GPa, respectively, are summarized in Figure 6. As can be inferred from this figure, the Γ_i parameters spanned more than two orders of magnitude in the low-pressure phase of SBF, reflecting the coexistence of the weak van der Waals intermolecular interactions and the strong covalent intramolecular bonding. The parameters were almost constant for the intermolecular modes (rhombi), with an averaged value of ~0.23 (blue horizontal line), whereas they roughly followed Γ_i ~ ${\mathsf{\omega }}_{\mathrm{i}0}^{-2}$ dependence (olive line) for the low- and intermediate-frequency intramolecular modes (circles). On the other hand, the Γ_i parameters were again nearly constant for the higher-frequency intramolecular modes (ω > 1000 cm⁻¹), including the C–H stretching vibrations (squares). This behavior reflected the similar force constants involved in these vibrations.

From the averaged value of the Γ_i parameters for the intermolecular modes, we could obtain a rough estimation for the bulk modulus of SBF in the low-pressure phase, assuming that the corresponding Grüneisen parameter equaled 2 [23,24,25]. This estimation yielded B₀ ~ 9 GPa (~2/0.23 GPa), which was of the same order of magnitude—though 50% larger—as that experimentally obtained for the parent fluorene crystals in the low-pressure phase, B₀ ~ 6 GPa, with a pressure derivative of $B_0^{\prime }=$ 7.5–8 [46,48]. In the high-pressure phase, the corresponding XRD data showed that fluorene crystals became stiffer with B₀ = 11.3 GPa and $B_0^{\prime }=$ 5.4 [46]. In the present case of SBF, its intense stiffening at 4.2 GPa, where the second phase transition or the completion of the structural phase transition at ~1.3 GPa occurred, was associated with the considerable strengthening of the intermolecular interactions. This was clearly revealed by the strong—almost by 6 times—decrease in the Γ_i parameters for the intermolecular and low-frequency intramolecular modes (solid symbols in Figure 6). These parameters at 4.2 GPa become more comparable to those of the intermediate- and high-frequency intramolecular modes, reflecting a more homogenized state of the studied system in terms of the strength of the binding forces.

3. Materials and Methods

The studied SBF (C₂₅H₁₆) crystals (purity > 98%) were purchased from Tokyo Chemical Industry Co., Ltd. (TCI, Tokyo, Japan). The as-received sample was structurally characterized by means of powder X-ray diffraction (XRD) using a Brücker D8 Advance diffractometer (Billerica, MA, USA) equipped with a Cu X-ray tube (Cu Kα radiation, λ = 1.5418 Å). The XRD profiles at ambient conditions were collected at 2θ from 3° to 40° with an increment of 0.01° and a 0.2 s scan time. The corresponding data are illustrated in Figure A1 (Appendix A) along with the reflection positions expected for the main monoclinic SBF polymorph reported by Schenk (CSD Entry: BIPHME, Deposition Number: 1111373) and those for the rather metastable one by Douthwaite et al. (CSD entry: BIPHME01, deposition number: 273066) [21,22,42]. From these, it was apparent that the studied material crystallized in the monoclinic structure reported by Schenk [21].

Raman spectra were acquired in the backscattering geometry using a micro-Raman LabRAM HR spectrometer (HORIBA, Kyoto, Japan) equipped with a Peltier-cooled charge-coupled detector (CCD). The 632.8 nm line of a He-Ne laser was used for excitation, focused on the sample by means of a 100× objective (focusing spot diameter: ~1 μm) for ambient pressure or a 50× objective (focusing spot diameter: ~2 μm) for the high-pressure measurements at a power lower than 0.4 or 2 mW, respectively, to avoid any laser heating-induced effects. The elastically scattered light from the sample was eliminated by means of an appropriate edge filter, while the inelastically scattered light was dispersed by a 600 g/mm grating. The combination of this grating with an entrance pinhole of 80 μm yielded a spectral width of ~3.5 cm⁻¹. Prior to each measurement, the frequency axis in the Raman spectrum was calibrated by the corresponding spectral lines of a Ne lamp.

The high-pressure Raman experiments were conducted in a Mao-Bell-type diamond anvil cell (DAC) using a ~80 μm thick drilled (~120 μm hole diameter) stainless steel gasket [49]. Daphne 7474 oil, which is a non-polar medium and, hence, was expected to affect to a lesser extent the molecular crystals of SBF and their pressure response compared to polar fluids, was used as a pressure transmitting medium (PTM) [50]. This PTM solidified at 3.7 GPa at room temperature and exhibited very good hydrostatic behavior up to ~4.1 GPa since the standard deviation of pressure remained lower than 0.013 GPa. At higher pressures, the standard deviation increased quasi-linearly, reaching the value of 0.25 GPa for P = 8 GPa [51]. The pressure in the sample chamber inside the DAC was calibrated by means of the ruby fluorescence (R₁ line) method [52,53].

4. Conclusions

In conclusion, we reported our detailed high-pressure Raman study of crystalline 9,9′-spirobifluorene (SBF) up to 8 GPa, using a diamond anvil cell for pressure generation and Daphne 7474 oil as a pressure transmitting medium. The abrupt changes observed for pressures higher than 1.3 GPa for the Raman peaks attributed to both external and internal vibrational modes and the pressure evolution of their frequencies signaled the occurrence of a pressure-induced phase transition of the studied material, characterized by structural and molecular conformation alterations. This phase transition, presumably of first-order in nature, showed a little hysteretic behavior and the initial phase was recovered at ~0.7 GPa upon pressure release. For pressures higher than 4.2 GPa, the linear pressure coefficients of the intermolecular and some low- and intermediate-frequency intramolecular modes reversibly decreased, indicating the occurrence of a second structural transition or the completion of the one initiated at ~1.3 GPa. From the low-pressure ω vs. P data for the intermolecular modes, we estimated a value of ~9 GPa for the bulk modulus of the low-pressure phase of crystalline SBF. In the high-pressure phase and at 4.2 GPa, the considerable strengthening of the intermolecular interactions significantly narrowed the initially broad range of the force constants in the SBF crystal.