Photophysical studies on meta- and para-carboranes containing 1-naphthyl groups

Abstract

Four new carboranes, C-(1-naphthyl)-meta-, C,C‘-bis(1-naphthyl)-meta-, C-(1-naphthyl)-para- and C,C‘-bis(1-naphthyl)-para-carborane, reveal high-energy emissions at 332–343 nm with quantum yields of 21–38% and long lifetimes of 26–31 ns in degassed cyclohexane solutions. The experimental absorption and emission spectra of the mononaphthyl carboranes are successfully simulated by hybrid DFT vibronic calculations. The computed lowest energy excited state (S₁) geometries contain non-planar naphthyl groups due to shorter carborane C–aromatic C bonds and steric effects between the bulky naphthyl and carboranyl units. The high energy emissions are also reproduced in polymethylmethacrylate (PMMA) films and as solids. The lower energy emissions in 99 : 1 water : THF suspensions for these four compounds at 385–421 nm show quantum yields of 50–64% and long lifetimes of 28–40 ns. The findings indicate that meta- and para-carborane derivatives can generate different solid-state emissions as reported elsewhere for the intensively researched ortho-carborane analogues.

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Introduction

Organic luminophores containing icosahedral ortho-carborane (1,2-C₂B₁₀H₁₂) have been intensively studied in the last decade partly due to the ability of the cluster to take part in the charge transfer (CT) emissions as the acceptor in some dyads.^1–3 Dual emissions are observed in some derivatives where local emission (LE) at the organic fluorophore and CT emission involving both the organic moiety and the carborane cluster are simultaneously observed.^1–7

Solid-state emissions of these carborane fluorophores have been demonstrated to vary considerably depending on the solid morphologies.^8–11 While some emissions observed can be attributed to charge transfer involving the carborane cluster,¹² local, solid state intermolecular interactions and specific conformers were also inferred to be responsible for emissions present.¹³ Aggregation-induced emissions (AIEs) in these ortho-carborane fluorophores are usually reported from solid suspensions in 99 : 1 water : tetrahydrofuran solutions with high quantum yields.^14,15

Organic fluorophores involving meta- and para-carboranes (1,7- and 1,12-C₂B₁₀H₁₂ respectively) are less studied as these clusters do not generally participate in emissions with no flexible cluster C–C bond present to readily accept an electron.⁵ They are usually considered as spectators with little/no active role in the photophysics where the organic luminophore unit is present. Nevertheless, these clusters were recently described as active conduits between donor and acceptor units.¹⁶

Organic luminophores with fused aromatic rings (naphthalene,^17–23 anthracene,^19,24–33 phenanthracene,^34–38 pyrene,^39–43 fluoranthene⁴⁴ and chrysene⁴⁵) at the cage carbons of ortho-carborane clusters have been researched intensively and revealed many intriguing emissions. For example, C,C′-bis(1-naphthyl)-ortho-carborane gave three distinct emissions (LE, CT and intramolecular excimer).¹⁷ By contrast, the field of meta- and para-carborane analogues containing fused organic rings attached to the cluster carbons are under-developed with only three examples known.^26,28 Here, the photophysics of four meta- and para-carboranes with 1-naphthyl groups attached at one or both carborane carbon(s) are described with hybrid density functional theory (DFT) data successfully simulating their absorption and emission spectra (Fig. 1).

Fig. 1 Compounds investigated in this study. Each naked vertex represents BH.

Results and discussion

C-Naphthylcarboranes (m1, m2, p1, p2, Fig. 1) were synthesised in moderate yields (16–59%) from the corresponding parent carborane and iodonaphthalene using the well-established coupling method for C-aryl-meta- and para-carboranes (Fig. 2).^46,47 Replacing iodonaphthalene with bromonaphthalene, using this method also gave similar yields of the desired products, m1 and m2, from meta-carborane. Unlike the other three soluble carboranes, dinaphthyl-para-carborane is partly soluble or insoluble in many organic solvents.

Fig. 2 Scheme summarising the synthetic routes to C-naphthylcarboranes.

The compounds were characterised by multinuclear NMR, infrared (IR) and mass spectra (Fig. S1–S28). Sextet peaks corresponding to the carborane C–H proton were observed in the ¹H{¹¹B} NMR spectra of the mononaphthyl products arising from ³J_HH couplings with the five neighbouring B–H protons.⁴⁸ The ¹³C NMR peaks of the naphthyl group in all compounds were assigned with the aid of 2D COSY and correlation spectra.⁴⁹

Single crystal X-ray diffraction studies on these naphthalenes confirmed their identities with the expected planar naphthyl groups occupying orientations with negligible steric repulsions between the naphthyl group and the cluster.¹⁷ These observations differ from the reported²⁸C-anthracenyl meta- and para-carboranes where the anthracenyl groups are non-planar to accommodate the steric repulsions between the fused rings and the cluster. The carborane cluster can be disordered in crystals as both BH and CH vertices can interchange due to similar sizes.⁵⁰ The crystal structures for the mononaphthyl carboranes contain ordered clusters due to the carborane C–H⋯π(aryl) interactions present (Fig. 3 and Table 1).⁵¹

Fig. 3 Carboranyl C–H⋯.π(ring) intermolecular interactions in crystals of C-mononaphthyl-carboranes (m1 top and p1 bottom).

Table 1 Intermolecular interaction parameters, values in bold are interactions with the C5–10 ring. Hydrogens are normalised (C–H bond length = 1.089 Å). Cent = centroid of aromatic ring

	H⋯Cent (Å)	C ⋯Cent (Å)	C–H⋯Cent angle (°)
m1	2.687	3.562	137.0
m2	2.465, 2.538	3.552, 3.520	175.9, 149.5
p1	2.668, 2.620	3.625, 3.591	148.1, 146.3
p2	2.535	3.478	144.3

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The intermolecular interactions in the crystal structures of the dinaphthyl carboranes are aryl C–H⋯π(ring) interactions where one unique intermolecular interaction in m2 is substantially stronger than the other interactions in the crystal structures.(Fig. 4 and Table 1) This strong interaction is reflected by the shorter H⋯ring distance of 2.465 Å and the favourable directing C–H⋯ring angle of 175.9°.

Fig. 4 Aromatic C–H⋯.π(ring) intermolecular interactions in crystals of C,C′-dinaphthyl-carboranes (m2 left and p2 right).

Photophysics

Solution-state absorption spectra of the carboranes showed absorption maxima at 290 nm with vibronic structure indicating local absorption at the naphthyl groups⁵² (Fig. 5a, S29–S32 and Table S1). A strong higher energy broad band at 230 nm is also observed in cyclohexane for each compound and is likely to also arise from naphthyl group absorption since carborane absorption bands are extremely weak in this region.⁵³

Fig. 5 Photophysical data for C-naphthyl-para-carborane p1. (a) Absorption and emission spectra in cyclohexane (CHX). (b) Solid state emission spectra. (c) Solution state lifetimes. (d) Solid state lifetimes.

Solution state emissions for all compounds had high energy maxima at 332–341 nm with vibronic structures observed in cyclohexane (CHX) which become less well defined in the more polar solvent dichloromethane (DCM, Fig. 5a, S29–S32 and Table 2). These ultraviolet emissions arise from local excitation of the naphthyl groups. Naphthyl derivatives have long been known to be sensitive to oxygen quenching⁵² and are observed here for cyclohexane solutions where quantum yields and lifetimes in deaerated : aerated ratios of roughly 2 : 1 with a strong correlation between quantum yields and lifetimes.⁵⁴ Emissions of the carboranes in dichloromethane solutions remain unchanged when aerated or deaerated (Fig. S33). Oxygen-quenching also takes place in the emissions of toluene, tetrahydrofuran (THF), acetonitrile (MeCN) and dimethylformamide (DMF) solutions for dinaphthyl-meta-carborane (Fig. S36 and Table S3) with lower deaerated : aerated ratios.

Table 2 Solution state emission data

	Emission maxima^a (nm)	Quantum yield^b (QY, %)	Lifetime^c (τ, ns)	Radiative rate^d (kr, 10^7 s^−1)	Non-radiative rate^e (knr, 10^7 s^−1)
a Excited at 300 nm. b Measured with integrating sphere, Φ^F = QY/100. c Excited at 317 nm. Exponential decay fitting. d k r = Φ^F/τ in s^−1. e k nr = 1 − Φ^F/τ in s^−1. f Ratio of deaerated : aerated values; Φ^F(N2)/Φ^F(O2) and τ(N2)/τ(O2).
m1
Cyclohexane N2	332, 340	35.5	31.8	1.12	2.03
Cyclohexane O2	332, 340	14.2	13.8	1.03	6.22
N 2  : O2ratio^f		2.5	2.3
Dichloromethane	338	9.6	7.4	1.30	12.2
m2
Cyclohexane N2	333, 340	38.3	30.4	1.26	2.03
Cyclohexane O2	333, 340	15.2	14.3	1.06	5.93
Ratio		2.5	2.1
Dichloromethane	339	11.5	7.0	1.64	12.7
p1
Cyclohexane N2	333, 341	21.8	29.1	0.749	2.69
Cyclohexane O2	333, 341	14.4	15.0	0.960	5.71
Ratio		1.5	1.9
Dichloromethane	340	8.0	9.1	0.879	10.1
p2
Cyclohexane N2	332, 342	32.0	26.9	1.19	2.53
Cyclohexane O2	332, 342	16.2	10.5	1.54	7.98
Ratio		2.0	2.6
Dichloromethane	333, 340	10.7	7.1	1.51	12.6
Naphthalene
Cyclohexane N2	322, 334	29.4	105.3	0.28	0.67
Cyclohexane O2	322, 334	3.9	18.2	0.21	5.28
Ratio		3.6	5.8
Dichloromethane	322, 334	1.9	8.9	0.21	11.0

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Quantum yields and lifetimes (τ) of these four carboranes measured in deaerated cyclohexane solutions are in the ranges of 22–38% (Φ^F = 0.22–0.38) and 27–32 ns respectively (Table 2 and Fig. S35). These lifetimes are longer than the long lifetime of 22.5 ns reported for C-(1-pyrenyl)-ortho-carborane.⁷ When these cyclohexane solutions are aerated, the yields drop to 14–16% and lifetimes shortened to 11–15 ns. In dichloromethane solutions, the QY range drops further to 8–12% and the lifetimes are shorter at 7–9 ns. The radiative rates k_r for the cyclohexane and dichloromethane solutions of the carboranes are in the narrow region of 0.75–1.64 s⁻¹. This narrow range reflects virtually identical high-energy emissions from the naphthyl groups (LE) in these carboranes.

Solid state photophysical data on the carboranes were also explored in the form of polymethylmethacrylate (PMMA) films, suspensions from rapid precipitation of tetrahydrofuran (THF) solution with water (1 : 99 THF : water ratio) and as evaporated solid films (Fig. 5b, S34 and Table 3). The PMMA films would contain isolated solid molecules – likely in different conformers – ruling out intermolecular interactions responsible for these solid-state emissions. The solid suspensions would logically form aggregates as amorphous solids where intermolecular interactions and multiple conformers are expected to be present and may contribute to the emissions observed. The slow evaporation of dichloromethane solutions onto quartz plates would be expected to result in microcrystalline solids and the emissions shown may arise from the specific monomer conformer or from excimers due to intermolecular interactions present in the crystal form.

Table 3 Solid state emission data

	Emission maxima^a (nm)	Quantum yield^b (QY, %)	Average lifetime^c (τ, ns)	Radiative rate^d (kr, 10^7 s^−1)	Non-radiative rate^e (knr, 10^7 s^−1)	CIE 1931 coordinates
a Excited at 300 nm. b Measured with integrating sphere, Φ^F = QY/100. c Excited at 317 nm. Non-exponential decay fitting. d k r = Φ^F/τ in s^−1. e k nr = 1 − Φ^F/τ in s^−1.
m1
PMMA film	343	26.0	13.8	1.88	5.36	0.172, 0.005
99 : 1 H2O : THF	407	56.7	31.1	1.82	1.39	0.157, 0.028
Solid film	333, 343	1.0	0.6	1.67	165	0.174, 0.005
m2
PMMA film	345	29.6	17.6	1.68	4.00	0.170, 0.006
99 : 1 H2O : THF	374	64.2	34.0	1.89	1.05	0.163, 0.094
Solid film	385	22.8	4.7	4.85	16.4	0.173, 0.042
p1
PMMA film	343	28.3	17.2	1.65	4.17	0.171, 0.006
99 : 1 H2O : THF	402	49.5	39.9	1.24	1.27	0.159, 0.021
Solid film	330, 339	13.4	2.2	6.09	39.4	0.173, 0.006
p2
PMMA film	344	18.6	7.0	2.66	11.6	0.172, 0.005
99 : 1 H2O : THF	421	52.9	28.7	1.84	1.64	0.154, 0.068
Solid film	337, 343	20.5	7.5	2.73	10.6	0.173, 0.006
Naphthalene
PMMA film	322, 335	46.2	67.6	0.68	0.80	0.175, 0.005
99 : 1 H2O : THF	334	29.7	35.5	0.84	1.98	0.174, 0.005
Solid film	333, 336	70.3	71.9	0.98	0.41	0.174, 0.005

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The high energy emissions from PMMA films with minima 343–345 nm for all four compounds indicate similar photophysical processes to those found in solutions. The broad and lower energy emissions from the solid carborane suspensions of 1 : 99 THF : water mixtures in all compounds with maxima at 374–421 nm suggest that several conformers combined with intermolecular interactions in the form of either naphthyl–naphthyl or naphthyl–cluster or both result in excimers to give these ill-defined emissions. The high energy shoulders at 320–350 nm in the emission spectra of these 1 : 99 THF : water mixtures indicate that monomers – and crystalline forms in the case of m1, p1 and p2 – are also present in the solid state particles. Unlike the pure crystalline forms, the solid particle morphologies in these THF : water mixtures are not determined but expected to contain irregular powder and crystalline forms.

More structured high energy emissions at 330–343 nm were observed for the solid films except for the dinaphthyl-meta-carborane m2 at 385 nm. Based on the crystal structures determined here, it is concluded that the stronger intermolecular interactions present in m2 with short H⋯ring distance of 2.465 Å and the favourable directing C–H⋯ring angle of 175.9° cause the red shifting of the emission maximum with respect to its solution state and PMMA emissions. This emission is likely to arise from intermolecular excimers with these short interactions between naphthyl groups in the crystalline state. Such excimers are not present in the crystalline forms of m1, p1 and p2 presumably as the intermolecular interactions in the crystal forms are too weak to give excimers thus only LE (monomer) emissions are present for m1, p1 and p2. The crystalline form of m2 may also exist in the 1 : 99 THF : water mixture of m2 along with excimers of different conformations.

The quantum yields and lifetimes in these three different solid forms were measured. The solid suspensions contain the highest quantum yields and average lifetimes, the solid films the lowest while the PMMA films are in between (Fig. 5d, S37, Tables 3 and S2). Unlike the lifetime measurements in solutions with exponential decay profiles, non-exponential decays are present in these solids.

The emissions are enhanced in the solid suspensions with quantum yields of 50–64% and attributed to aggregation induced emissions (AIE) reported in many studies elsewhere for ortho-carborane derivatives.^8–11 It is noted here that the desired carborane AIE properties are not exclusive to ortho-carboranes. The observed fluorescence lifetimes are also remarkably long with values in the region of 29–40 ns. These lifetimes are longer than the solid suspension (1 : 99 THF : water) of the ortho-carborane chrysene reported at 23 ns.⁴⁵

The quantum yields 18.6–29.6% and lifetimes 7.0–17.6 ns for the PMMA films in all four carboranes are slightly different to the corresponding values found in solutions. These results suggest that free naphthyl-carborane rotations taking place in solutions and varied but static conformers present in PMMA films do not change the observed emission processes. This contrasts with many reports of ortho-carborane derivatives where non-radiative processes dominate in solutions but not in solid states.

A monoaryl-ortho-carborane can have three distinct excited states where the cluster does not take part in the organic local emission (LE), the elastic cluster C–C bond lengthens to act as an acceptor in a charge transfer (CT) emission and the cluster C–C bond lengthens further to a third excited state where a non-radiative relaxed process is expected.⁷ The non-radiative processes for ortho-carborane derivatives in solutions may in part be from the third carborane excited states accessed in solutions. These non-radiative pathways are hampered in the solid states due to the rigid geometry constraints of the solid states. The reported CT and non-radiative excited states with the ortho-carborane cluster do not apply to meta- and para-carborane clusters as there are no intracluster carborane C–carborane C bonds in the latter clusters.

The crystalline solid films of the naphthyl carboranes generally have shorter lifetimes and lower quantum yields than the corresponding PMMA films and 1 : 99 THF : water solid suspensions. Clearly, the specific conformers in the crystals are not ideal for high emission efficiencies especially for m1 where a QY of only 1% was observed. The similar QY and lifetime values found for the PMMA film and crystalline solid of p2 indicate that mixed conformers in PMMA have little effect on the emissions for this compound.

The radiative rates k_r for the PMMA films, solid suspensions and solid films of the carboranes are 1.7–2.7, 1.2–1.9 and 1.7–6.1 × 10⁻⁷ s⁻¹ respectively. Despite the different emission types and maxima, the PMMA films and solid suspensions have similar radiative rates. However, the radiative rates for the solid films are varied and appear to subtly depend on the specific conformer in each crystal structure.

The CIE 1931 coordinates of 0.17, 0.01 reflect the UV-violet high energy emissions in solutions and in PMMA films when excited at 300 nm. The solid suspensions contain visible deep blue emissions with coordinates at 0.15–0.16, 0.03–0.09. The solid films show visible violet emissions as the bands trail to 460 nm (Fig. S38).

Low temperature emission spectra were also measured for all carboranes in cyclohexane and dichloromethane at 77 K. Highly structured, intense emissions with emission maxima in the region of 322–342 nm at 2 μM concentrations were observed and assigned as vibronic LE (monomer) emissions (Fig. S39 and Table S4). Broad low energy emissions were present along with the high energy vibronic bands at higher concentrations of 20 μM. These broad low energy emissions are from excimers. The low temperature broad emissions in the region between 350 and 500 nm in cyclohexane resemble the emissions from 1 : 99 THF : water mixtures for the carboranes. Naphthalene and its derivatives have long been known to form excimers at higher concentrations and at lower temperatures.⁵⁵

It is instructive here to compare the photophysics of naphthylcarboranes with naphthalene itself to assess the roles of the carboranyl groups on the photophysical aspects. Data measured here for naphthalene are listed in Tables 2 and 3 for direct comparison with the naphthylcarboranes (Fig. S40–S42 and Table S5). The carboranyl units have little effect on the emission maxima with red shifts in the region of 940 cm⁻¹ (0.11 eV) in energies and similar quantum yields with respect to the parent naphthalene in solutions. The lifetimes are decreased and radiative rates increased when the carborane is present compared to the parent naphthalene.

The differences between naphthalene and the naphthylcarboranes are more marked in the solid-state emission data as excimer emissions are not present for the parent naphthalene under these conditions. With respect to high energy emissions only, the carborane units significantly reduce the quantum yields and emission lifetimes and increase the radiation rates compared to pure naphthalene. When considering 1 : 99 THF : water mixtures, the high energy LE emission of naphthalene has a quantum yield of 30% which is lower than the 50–64% yields for the low energy excimer emissions of the naphthylcarboranes.

Computations

Optimised geometries for all four at S₀ ground states were explored with the range-corrected DFT hybrid CAM-B3LYP functional here. Mononaphthyl-para-carborane has only one minimum located due to its high symmetry and a planar naphthyl group thus the model of choice for detailed computations here. There are 3 minima located for mononaphthyl-meta-carborane and for dinaphthyl-para-carborane whereas 7 minima were identified for dinaphthyl-meta-carborane. Simulated and observed infrared spectra were in excellent agreement when the energy scaling factor⁵⁶ of 0.945 was applied (Fig. S21–S24). Optimised and X-ray determined geometries were also in very good agreement with all cluster C–naphthyl C bond lengths in the 1.522–1.542 Å range (Fig. S44 and Table 4). While the S₀ optimised geometries contain planar naphthyl groups, the observed non-planar naphthyl groups are present due to crystal packing forces (Fig. S43).

Table 4 Aromatic deformation parameters for 1-naphthylcarboranes

	Cage C–C bond length (Å)	Inter-ring plane angle (°)	α (°)	β (°)
X-ray
m1	1.524	5.03	4.63	2.18
m2	1.522	3.46, 5.08	2.99, 4.08	2.48, 1.67
p1	1.530	1.52, 0.70	1.34, 1.38	2.22, 1.78
p2	1.542	1.76	0.73	1.85
Computed S0
m1	1.526	0.76	0.87	1.53
m2	1.526	1.86	1.99	1.54
p1	1.529	0.00	0.00	1.67
p2	1.527	0.00	0.00	1.56
Computed S1
m1	1.494	7.81	12.98	4.91
m2	1.493, 1.526	8.68, 1.76	14.08, 1.87	4.93, 1.46
p1	1.494	8.63	14.00	5.19
p2	1.491, 1.526	8.57, 0.00	13.83, 0.00	4.12, 1.70

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The rotation barrier between a phenyl group and the carbon atom of a carborane cluster is negligible (0.2 kcal mol⁻¹)⁵⁷ whereas the rotation barrier has been estimated to be 10–15 kcal mol⁻¹ for a 9-anthacenyl group instead of phenyl.^25,28 Here, the transition state of the mononaphthyl-para-carborane located in the C–C rotation is higher than the minimum by 3.3 kcal mol⁻¹ (Gibbs free energy 4.0 kcal mol⁻¹) where the hydrogen at C8 is close to the neighbouring hydrogen at boron (Fig. 6). Unlike the 9-anthracenyl group, the 1-naphthyl group is freely rotating in solutions of 1-naphthyl carboranes at ambient temperatures as observed in the sharp solution state NMR peaks here (Fig. S1–S20).

Fig. 6 Definition of the angles, α and β, as a measure of non-planarity in carboranes with the numbering scheme for the naphthyl group. Each naked vertex represents BH.

Emissions of interest here involve S₁ ← S₀ transitions thus S₁ excited state geometries for all four compounds were optimised. Interestingly, the naphthyl groups in these geometries are non-planar due to shorter cluster C–naphthyl C bond lengths (Table 4) increasing the steric repulsions between these units. The distortion angles, α and β, used to measure the relative distortions in X-ray structures of anthracenyl carboranes (α = 13.3–14.9°, β = 5.6)²⁸ reveal essentially similar distortion values (Table 4 and Fig. 6). However, the transition state geometry for the rotation barrier between the naphthyl and carborane units in the excited state geometry of mononaphthyl-para-carborane has a planar naphthyl group and a slightly longer cluster C–naphthyl C bond length of 1.507 Å. This geometry is only 3.8 kcal mol⁻¹ higher in energy than the excited state minimum.

Absorption spectra of carborane compounds have been predicted by using TD-DFT methods on optimised S₀ geometries for decades. These calculations assume that the geometries of the excited states, S_n (n = 1 or higher), are identical to the optimised S₀ ground state geometry (Vertical Hessian = VH, e.g. S₁′ ← S₀, Fig. 7). This calculated VH absorption spectrum for mononaphthyl-para-carborane (Fig. S45, S46 and Table S7) gives a reasonable overall picture when compared with the observed spectrum but only contains broad bands. It does not show the observed vibronic structure of the low energy bands.

Fig. 7 Vertical (VH) and adiabatic Hessian (AH) calculations used in predicting absorption and emission energies.

Vibronic calculations provide more realistic absorption spectra as bands with vibronic structure can be modelled. Here, by optimising the S_n (n = 1–6) excited state geometries, each individual vibronic spectrum (Adiabatic Hessian = AH, e.g. S₁ ← S₀, Fig. 6) from every S_n ← S₀ transition is combined to generate the simulated absorption spectrum for mononaphthyl-para-carborane (Fig. 8).

Fig. 8 (a) Calculated absorption spectrum based on combination of vibronic spectra (Adiabatic Hessian) from optimised S_n geometries with cyclohexane as solvent. (b) Simulated absorption and emission spectra for mononaphthyl-para-carborane p1 from vibronic computations.

Emission data have also been predicted by calculating the S₁′ ← S₀ energy using the optimised S₀ geometries (identical to S₁′ ← S₀), which can be a good approximation assuming S₀ and S₁ optimised geometries are similar. A more recent and common procedure is determining the S₁ ← S₀′ energy using the optimised S₁ geometry. Again, these TD-DFT methods do not model emission bands with vibronic structures.

The vibronic calculation on the S₁ → S₀ transition for mononaphthyl-para-carborane reveals an emission band with vibronic structure (Fig. 8b and S47) which is in excellent agreement with the observed emission band in cyclohexane (Fig. 5a). Using the energy scaling factor of 0.945 to the calculated energies (as applied for simulated infrared spectra earlier) resulted in excellent agreement of the emission maxima between computed and experimental data. The calculated vibronic bands are predicted to arise from the vibrations of the naphthyl group at 1342 and 1581 cm⁻¹ (Fig. 9).

Fig. 9 Vibrations at 1342 cm⁻¹ (left) and 1581 cm⁻¹ (right) likely to be responsible for the vibronic structure present at 345 nm in the simulated emission of p1.

Simulated vibronic emissions were also generated for all four carboranes and compared with observed emissions in dichloromethane solutions (Fig. 10). While the computed and observed mononaphthylcarborane spectra match perfectly, the vibronic bands for the dinaphthylcarborane spectra differ with much lower intensities in the simulated spectra. The nearly identical experimental emission spectra for all compounds infer that the natural transition orbitals (NTOs) involved are located at one naphthyl group.

Fig. 10 Comparison between simulated and observed emission spectra with dichloromethane as solvent.

The orbitals for the optimised S₀ and S₁ states of mononaphthyl-para-carborane (Fig. 11) are indeed present at the sole naphthyl group. However, the degenerate NTOs in the S₀ optimised geometry of dinaphthyl-para-carborane are located at both naphthyl groups due to symmetry while the NTOs are located at the non-planar naphthyl group in the S₁ geometry. Similar NTOs are present in the meta-analogues (Fig. S48). The modelling of the NTOs at the ground state in the dinaphthyl carboranes thus does not model the vibronic calculation correctly. A better model for such disubstituted meta- and -para-carboranes would be the monosubstituted analogues when predicting their absorption and emission spectra given that the emissions observed are nearly identical.

Fig. 11 Natural transition orbitals (NTOs) based on the optimised S₀ and S₁ geometries for p1 (top) and p2 (bottom). Purple = particle. Yellow = hole.

Conclusions

Naphthalenes with meta- and para-carboranyl groups are emissive in both solution and solid states with photoluminescence quantum yields of up to 38% and 64% respectively. These emissions have relatively long radiative lifetimes with up to 34 ns in solutions and up to 40 ns in solids. Such yields and lifetimes are unusual in organic luminophores containing carboranes. It is hoped that the field of luminescent meta- and para-carboranes will be intensively explored in future like the area of so many reported luminescent ortho-carboranes in the past decade. Vibronic calculations are demonstrated here to correctly simulate the observed absorption and emission spectra where vibronic structure is observed in some bands. These calculations are expected to support many future photophysical studies of carborane luminophores.

Author contributions

RAH, TDM, JAHM and MAF were responsible for syntheses of the naphthyl carboranes. RAH, LB, AB and MAF carried out the photophysical measurements. MAF was also responsible for the methodology, conceptualisation, characterisation, data analyses, computations, visualisation, writing the original draft, review and editing of the manuscript. DSY determined three crystal structures. MAF supervised RAH, TDM and JAHM and AB supervised LB during their research periods. MAF and AB were in charge of the funding acquisition.

Data availability

Data are provided within the manuscript or supplementary information (SI). Supplementary information containing synthetic details, NMR, MS and IR spectra, further photophysical and computational details and cartesian coordinates of ten optimised geometries is available. See DOI: https://doi.org/10.1039/d5dt02179k.

CCDC 2485369–2485371, 2486934 (m1, m2, p1 and p2) contain the supplementary crystallographic data for this paper.^58a–d

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Engineering and Physical Sciences Research Council (EPSRC; EP/P505186/1) and the Deutsche Forschungsgemeinschaft (DFG) for financial support. We also thank Dr Toby J. Blundell and Dr Claire Wilson for the X-ray crystallography analysis of compound p2.

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