Article pubs.acs.org/IC
Group 14 Dithienometallole-Linked Ethynylene-Conjugated Porphyrin Dimers Mitsuhiko Morisue,*,† Yuki Hoshino,† Masashi Nakamura,‡ Takashi Yumura,§ Shinjiro Machida,∥ Yousuke Ooyama,‡ Masaki Shimizu,† and Joji Ohshita‡ †
Faculty of Molecular Chemistry and Engineering, §Faculty of Chemistry and Materials Technology, and ∥Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ‡ Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan S Supporting Information *
ABSTRACT: The considerably conjugated π systems of the group 14 dithienometallole-linked ethynylene-conjugated porphyrin dimers (1Ms) were described based on comprehensive experimental and theoretical studies. The electronic absorption spectra of 1M displayed a large splitting in the Soret band and a red-shifted Q-band, indicating that the dithienometallole spacer was effective in facilitating the porphyrin− porphyrin electronic coupling. Torsional planarization behaviors of 1M were observed in the time-resolved fluorescence spectra. Density functional theory (DFT) calculations revealed that the dithienometallole spacer is an ideal partner for the ethynylene-conjugated porphyrin to produce fully delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels due to their similar HOMO and LUMO levels. Finally, 1M exhibited a strong propensity for the quinoidal−cummulenic conjugation in the dithienometallole spacer when in a photoexcited state.
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INTRODUCTION Expanded π-conjugated networks of porphyrin dimers have provoked considerable attention in versatile studies of porphyrin-based materials.1 The ethynylene-conjugated porphyrin dimers are of particular interest because of their excellent porphyrin−porphyrin electronic coupling.2 The electronic delocalization processes of meso−meso butadiynelinked porphyrin dimers have been extensively studied.3 Porphyrin−porphyrin electronic coupling is maximized when two porphyrin rings adopt a coplanar conformation. With an energy barrier of only 0.7 kcal/mol for rotation around the ethynylene linkage, porphyrin dimers exhibit conformational disorder between staggered and planar conformations in the ground state; upon photoexcitation, however, the conformational disorder redistributes to repopulate the planar conformation. Therefore, the porphyrin framework conjugated to ethynylene linkages leads to the quinoidal−cummulenic conjugation,4 as demonstrated by the preference for the planar conformation in the excited state, a trait typically assigned to the participation in the quinoidal conjugation geometry.5 The proquinoidal spacer-linked ethynylene-conjugated porphyrin dimer is a preeminent π-conjugated network exhibiting considerable electronic delocalization. Other proquinoidal spacers, such as anthracene or benzobis(thiadiazole), sufficiently enhance the porphyrin−porphyrin electronic coupling, as reported by Anderson and co-workers6 and Therien and coworkers,7 respectively. The enlarged coherent domain led to © XXXX American Chemical Society
nonlinear optical properties and a narrowed HOMO−LUMO gap. Thiophene and its related derivatives have attracted a great deal of attention as key semiconducting components because of their organic electronic materials. Their strong propensity to impose a quinoidal geometry makes them well suited for mediating electronic communication through oligomeric and polymeric thiophene backbones.8,9 Among the thiophene homologues, the metallole-fused bithiophenes, which are dithienometalloles, such as dithienosilole10 and dithienogermole,11 have provided a recent surge of discovery of novel πconjugated materials using the thiophene-related compounds. The main motivation for development of these compounds originates from their solid-state performance as organic electronic devices that are superior to their carbon analogues.12 The primary advantage of dithienometallole stems from the planarity of bithiophene that is enforced by the silylene and germylene bridge. In this bridge, the orbital of the exocyclic σbonds on the silicon or germanium atom manipulates the parent π* conjugation through the σ*−π* conjugation,13 while the Si−C or Ge−C bonds are longer than the C−C bonds exhibiting reduced C(β)−C(β′) antibonding interactions with the metallole ring.10a The present study explored the group 14 dithienometallole as a longitudinally oriented proquinoidal Received: March 17, 2016
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DOI: 10.1021/acs.inorgchem.6b00667 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. (A) Molecular Structures of 1M and 2; (B) Possible Resonance Structure of 1M
Scheme 2. Synthetic Routes to 1M
Scheme 3. Synthetic Route to 2
Sonogashira−Hagihara coupling reaction from the corresponding precursors 3M.12a,c The 3,3′-di-n-hexyl 2,2′-bithiophenelinked ethynylene-conjugated porphyrin dimer 2 to be used as a reference compound was also synthesized using a similar synthetic strategy (Scheme 3 in the Experimental Section, vide infra). On the other hand, we altered the synthetic strategy for
spacer to enhance quinoidal−cummulenic conjugation in the ethynylene-conjugated porphyrin dimer 1M (Scheme 1).
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RESULTS AND DISCUSSION We synthesized 1M (M = Ge, Si, and C) (Scheme 2). The porphyrins 1Ge and 1C were synthesized via the repetitive B
DOI: 10.1021/acs.inorgchem.6b00667 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Absorption and normalized fluorescence spectra of 1Ge (red), 1Si (blue), 1C (light blue), and 2 (green) in cyclohexane (A). Fluorescence spectra were obtained upon excitation at 450 nm. Plot of the absorption maxima of the Q-band (B) and the Stokes shift (C), which was defined as the shift from the Q-band absorption maximum to the emission maximum. These values were determined in cyclohexane (ETN = 0.006, Δf = −0.001, red circle), toluene (ETN = 0.099, Δf = 0.014, green circle), and THF (ETN = 0.207, Δf = 0.210, blue circle). The orientation polarizability of the solvent (Δf) is defined as Δf = (εr − 1)/(2εr + 1) − (nD2 − 1)/(2nD2 + 1), wherein εr is the relative dielectric constant and nD is refractive index.
electronic absorption spectra moderately obeyed the ETN index (Figure S2−S4). The solvent effect on the Stokes shift for each 1M was obscured in the Lippert−Mataga plot (Figures S2− S4).16 These analyses unveiled that the solvent polarity stabilized the charge-transfer character of the parent π conjugation of 1M. The exceptional π conjugation facilitated by the dithienometallole spacer of 1M dominated over any effects caused by the central group 14 element. Still, the heavier central group 14 element slightly blue shifted the Q-band (Figure 1B), indicating that the heavier central group 14 elements reduce the polarization of 1M in the ground state. The trend observed in the Stoke’s shift indicated that the inherent polarization of photoexcited 1Si was smaller than 1Ge and 1C according to the Lippert−Mataga principle (Figure 1C). One interpretation of this result is the flexibility of the M−C bonds compensates for the polarization strength along the long axis in the excited state. This interpretation is in agreement with the larger red shift of 1Ge than that of 1Si in the time-resolved fluorescence spectra (vide infra). Nevertheless, the marginal effects of the central group 14 element on the electronic spectra of 1M indicated that the σ*−π* conjugation was overwhelmed by the strong parent π conjugation. Therefore, regardless of the central metal that is present, the dithienometallole spacer produces an excellent π-conjugation system to electronically couple two porphyrin rings.
1Si to avoid the desilylation step, although this strategy was not effective because of tedious elimination procedures for the homocoupled dimer derived from 7.14 All of the compounds were stable under ambient conditions and satisfactorily and fully spectroscopically identified as described in the Experimental Section. A comprehensive study of the electronic structures of 1M is shown in Figure 1A. The largely split Soret band (approximately 400−540 nm) and intense Q-band (approximately 610−730 nm) of 1M were indicative of the extensive porphyrin−porphyrin electronic coupling through the dithienometallole spacer. In contrast, the electronic coupling through the twisted bithiophene spacer, in which steric repulsion of two hexyl groups prohibited the planar conformation of two thiophene rings (Figure S1I), was not significant enough to largely split the degenerated Soret band of 2. This comparison unambiguously revealed that the enforced planarity of the dithienometallole spacer played a key role in the enhanced πconjugation. Expecting the quinoidal−cummulenic geometry is admixed with a charge-transfer characteristic, the degree of the π conjugation may have been enhanced as the solvent became more polar. The lowest singlet absorption maxima of the Qband of 1M was red shifted as the empirical solvent polarity parameter (ETN) increased from 0.006 (cyclohexane) to 0.207 (THF) (Figure 1B).15 The solvatochromic shifts of 1M in the C
DOI: 10.1021/acs.inorgchem.6b00667 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. Orbital energies in fragments of a proquinoidal spacer linking ethynylene-conjugated porphyrin dimers obtained from DFT B3LYP calculations using the absorption maxima of the Q-band of the proquinoidal spacer-linked ethynylene-conjugated porphyrin dimers, wherein Ar = in 2,3-di(3,3-dimethylbutoxy)phenyl and R = H in Therien’s porphyrin dimers with the blue proquinoidal spacer6a and Ar = 3,5-di(t-butyl)phenyl and R = tri(n-hexyl)silylethynyl in Anderson’s porphyrin dimers with green proquinoidal spacer,7a wherein orange and green arrows indicate transition dipole moment in the x and y directions, respectively. Detailed discussion for transition dipole moments is shown in Figure S6 and Table S1−S2.
The absorption maxima of the Q-band of 1Ge (686 nm), 1Si (693 nm), and 1C (701 nm) in THF were comparable to those of the Anderson’s porphyrin dimers with the p-phenylene (686 nm), 2,5-thiophene (704 nm), and 9,10-anthracene spacer (706 nm) in chloroform with 1% pyridine.6a However, the spectra presented in this work did not deeply penetrate into the nearinfrared (NIR) wavelength region, unlike Therien’s homologues with the 6,13-pentacene (823 nm) or 4,8-{benzo[1,2c:4,5-c′]bis([1,2,5]thiadiazole)} spacer (1006 nm) in THF (Figure 2).7a According to the point-dipole approximation for electronic coupling,17 the strength of the electronic coupling is steeply diminished as a function of the reciprocal of the cube of the interchromophore distance. Considering that the longer porphyrin−porphyrin distance in 1M (∼21 Å of the Zn···Zn distance) was elongated compared to the previous porphyrin dimers created by a single aromatic ring (∼18 Å of the Zn···Zn distance) based on the optimized structures (Figure S1), the dithienometallole spacer is ideally positioned to enhance longrange electronic coupling. Considering the tensor product of each transition dipole moment of the spacer and two porphyrins, the primary advantage of a longitudinally linked proquinoidal spacer is the parallel arrangement of each transition dipole moment to enhance the polarization along the long axis (x axis) of 1M
(given in orange arrows in Figure 2), unlike the laterally linked spacers (given in green arrows). Moreover, the lack of steric conflicts between the dithienometallole spacer and the porphyrin-β protons makes the fully planar conformation possible (Figure S1), which aids in delocalizing the π electrons over the proquinoidal spacer-linked ethynylene-conjugated porphyrin dimers. Further insights into the orbital energies of the HOMO and LUMO levels of the proquinoidal spacers relative to those of the porphyrin allow for interpretation of the effectiveness of the dithienogermole spacer (Figure 2). Density functional theory (DFT) B3LYP calculations revealed that the HOMO and LUMO levels of the dithieonometallole spacers are close to the HOMO and LUMO levels of the ethynyleneconjugated porphyrin. The magnitude of the orbital interactions between the two fragments is inversely proportional to the energy difference of their interacting orbitals according to second-order perturbation theory. Then the dithienogermole spacer engages, leading to fully delocalized HOMO and LUMO levels over the porphyrin dimers via strong orbital interactions. Thus, the dithienometallole spacer can surmount the relatively long porphyrin−porphyrin separation to achieve considerable π-electronic coupling in 1M. The low rotational barrier around the ethynylene linkage should statistically disorder the torsional conformations of 1Ge D
DOI: 10.1021/acs.inorgchem.6b00667 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 3. Time-resolved fluorescence spectra recorded every 0.1 ns (upper to lower) of 1Ge upon (A), streak scope image (B) and fluorescence decay profile at 655−665 nm (blue), 700−710 (green), and black (630−720 nm) together with the fitted curves (red), where the instrumental response function is shown in gray (C). Conditions: excitation by a 200 fs pulse at 452 nm at 298 K in methylcyclohexane/isopentane (4/1, v/v). Time window of the streak image (B) and the decay profile (C) (5 ns) was longer than that for the time-resolved spectra (A) (1 ns).
Table 1. Fluorescence Decay Profiles of 1M and 2 in Methylcyclohexane/Isopentane (4/1, v/v) upon Excitation at 452 nm at 298 K shorter wavelengthsa 1Ge 1Si 1C 2
τ1/ns (α1)
τ2/ns (α2)
τ3/ns (α3)
0.12 0.16 0.14 0.23
0.38 0.34 0.60 1.18
0.91 (0.15) 0.86 (0.21) 1.25 (0.05)
(0.58) (0.55) (0.73) (0.31)
(0.27) (0.24) (0.22) (0.69)
λem/nm (Φ)c
longer wavelengthsa τ1/ns (α1) 0.13 0.15 0.09 0.09
(−0.30) (−0.77) (−1.08) (−0.36)
τ2/ns (α2)
τ3/ns (α3)
kemb/s−1
0.57 0.66 0.78 1.13
0.89 (0.74) 0.95 (0.88) 0.99 (0.40)
1.8 1.4 2.3 1.1
(0.56) (0.89) (1.68) (1.36)
× × × ×
108 108 108 108
77 K 711 710 722 669
(0.14) (0.17) (0.21) (0.14)
298 K 691 688 710 649
(0.13) (0.14) (0.17) (0.12)
a
Shorter and longer emission wavelengths for 1Ge = 655−665 and 700−710 nm, 1Si = 655−665 and 700−710 nm; 1C = 670−680 and 715−725 nm; 2 = 640−650 and 675−685 nm, respectively. Fluorescence lifetime (τ1, τ2, and τ3) was assigned to torsional planarization, locally excited state of the twisted conformer, and fully π-conjugated state of the planar conformer, respectively (see the text). bRadiative rate constant at 298 K defined as kem = Φ/τ wherein τ is the overall value based on the entire emission at all wavelengths. cFluorescence maxima (λem) and absolute fluorescence quantum yield (Φ) at 77 (glassy media) and 298 K (fluidic media) upon excitation at 440 nm for 1M and 450 nm for 2.
Time-resolved fluorescence spectroscopy provided direct insight into the photodynamics of 1Ge (Figure 3 and Table 1). The incident laser at 452 nm excited mostly the porphyrinlocalized Soret band to produce the locally excited Frank− Condon state of 1Ge, followed by the fast S2 → S1 internal conversion (
