Conjugation in and Optical Properties of 1-R-1,2-Diphospholes and 1

Article pubs.acs.org/JPCA

Conjugation in and Optical Properties of 1‑R‑1,2-Diphospholes and 1‑R‑Phospholes Sergey A. Katsyuba,*,† Timur I. Burganov,† Elena E. Zvereva,† Almaz A. Zagidullin,† Vasily A. Miluykov,† Peter Lönnecke,‡ Evamarie Hey-Hawkins,‡ and Oleg G. Sinyashin† †

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov str. 8, 420088 Kazan, Russia ‡ Institut für Anorganische Chemie, Universität Leipzig, Johannisallee 29, 04103 Leipzig, Germany S Supporting Information *

ABSTRACT: The strength of conjugation between the diene moieties of 1-R-1,2-diphospholes and 1-R-phospholes and exocyclic phenyl groups of these P-heteroles has been quantitatively characterized by the use of Raman activities of the bands of the phenyl substituents. It is shown that conjugation in both types of phospholes is very similar to the conjugation of phenyl groups with the diene system of cyclopentadiene. Introduction of substituents (−OMe, −C(O)H, −NO2, −NMe2, and −CHCH2) in the para-position of the phenyl groups of 1-R-1,2-diphospholes extends π-delocalization of exocyclic groups into the electronic system of the 1,2-diphosphole ring, producing bathochromic shifts of the absorption bands up to 63 nm. In contrast, hypsochromic shifts up to 40 nm can be achieved by introduction of SnMe3 or SiMe3 groups at the phosphorus(III) atom of the 1,2-diphosphole and concomitant increase of aromaticity of the P-heterole. Conjugation shifts the “centre of gravity” of the whole electronic absorption spectrum, whereas positions of separate absorption bands are not simply dependent on conjugation lengths.

■

INTRODUCTION Since the 1990s, π-conjugated materials based on organophosphorus compounds represent a valuable addition to the pool of building blocks for molecular electronics.1−3 Phospholes (five-membered 6π-systems with one or more phosphorus atoms) as subunits for the π-conjugated systems are the most widely investigated P-heterocycles, because they display interesting electronic properties with both high electronaccepting and electron-transporting abilities.4,5 The key property of these P-heterocycles for molecular engineering of π-conjugated systems is based on their electronic structure6 with a reactive phosphorus center. Low aromatic character of these P-heteroles with a low-lying LUMO, σ*(P−R)−π*(1,3diene) hyperconjugation7,8 and high electronic affinity allow tuning the photophysical properties of phosphole-based πconjugated systems by chemical modifications of the reactive P atoms and influencing the extent of conjugation over the diene moiety.2,8,9 Richness and diversity of phosphorus chemistry together with the mentioned specific properties of organophosphorus derivatives should be fully exploited for the engineering of a wide range of novel conjugated materials.10,11 In comparison to the phospholes, which are the most studied P-heteroles described in the literature, 3,4,5-triaryl-1-R-1,2© 2014 American Chemical Society

diphospholes (some examples are presented in Figure 1) are much less explored. They combine the structural elements of both 1H- and 2H-phospholes in one molecule, which increases its aromaticity12−14 and, hence, is expected to change essentially their optical properties. Recently, we studied the UV/vis absorption spectra of these compounds and used quantum chemical calculations to better understand the origin of the absorption bands and their relationship with the electronic and geometrical structure of the molecules.15 We have demonstrated that the light absorption properties of the 1,2diphospholes can be easily tuned by variation of substituents introduced in the para-positions of the aryl moieties, whereas the influence of the R group at the phosphorus atom seemed to be rather moderate. The above-mentioned ways of possible modification of optical characteristics of the diphospholes are, most probably, related to the conjugational effects in these compounds, as light absorption by a molecule strongly depends on delocalization of electrons. An extent of delocalization usually results in a Received: October 25, 2014 Revised: December 4, 2014 Published: December 4, 2014 12168

dx.doi.org/10.1021/jp510712s | J. Phys. Chem. A 2014, 118, 12168−12177

The Journal of Physical Chemistry A

Article

Figure 1. 1-R-1,2-Diphospholes and 1-R-phospholes studied in the present paper and computed wavelengths of their longest-wavelength absorption bands (λmax, nm). The experimental wavelengths are given in parentheses. Computed Raman activities of the ν8a bands of the phenyl groups (in Å4· amu−1) are given in bold.

taken as a measure of conjugation between the phosphole ring and exocyclic substituents, the structure of these substituents and optical absorption of the whole system. Single-crystal X-ray structure analysis as well as UV/vis absorption spectroscopy data will be used as auxiliary tools. Molecules studied in this paper are shown in Figure 1.

bathochromic shift of the absorption bands in the UV/vis spectra, which applies also to phospholes.9 The main objective of the present work is to study the conjugation of the phosphole ring with exocyclic substituents and rationalize the most effective approaches to influence the optical properties of these promising compounds through their structural modification. Conjugational properties of the 1,2-diphospholes and phospholes will be compared by employing Raman spectroscopy combined with quantum chemistry, because the intensity of Raman lines is closely connected with the conjugational properties of molecules. Moreover, it is well established that conjugation leads to a many-fold increase in the intensity polarized lines, corresponding to the symmetric stretching vibrations of the bonds participating in conjugation.16,17 Thus, the intensity of these enhanced Raman lines can be viewed as observables of conjugation that can provide an estimation of its strength. We focus on interconnections between the Raman intensity,

■

EXPERIMENTAL SECTION Synthesis. 1,2-Diphospholes 1a, 1b, 1c, 1i, 2a, and 4a, were synthesized according to the earlier described protocol.18−20 X-ray Crystallography. The data were collected on a Gemini diffractometer (Agilent Technologies) using Mo Kα radiation (λ = 0.710 73 Å), ω-scan rotation. Data reduction was performed with CrysAlis Pro21 including the program SCALE3 ABSPACK22 for empirical absorption correction. The structures were solved by direct methods23 and the refinement of all non-hydrogen atoms was performed with SHELX-97.24 All non-hydrogen atoms were refined with anisotropic thermal 12169



Article

corresponding to the global minimum on potential energy surface of the ground state. Theoretical Background and Notations Used. The theory of Raman intensity enhancement on conjugation was elaborated by Shorygin and co-workers.16 In the simplest version of the semiclassical approximation related to the conditions for excitation of the spectrum in the transparency region of the substance, far from absorption bands, the derivative of electronic polarizability of a molecule (α) with respect to the normal vibrational coordinate of atoms (Q)

parameters. For 2a a difference-density Fourier map was used to locate all hydrogen atoms, whereas for 1a all hydrogen atoms were calculated on idealized positions using the riding model. Structure figures were generated with DIAMOND-3.25 CCDC 995511 (1a) and 995512 (2a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: (+44)1223-336-033; or [email protected]). More details can be found in Table 2S (Supporting Information). Spectroscopic Methods. Raman spectra were registered at room temperature on a BRUKER RAM II module (using a Ge detector operating at liquid nitrogen temperature) attached to a BRUKER VERTEX 70 FTIR spectrometer in the range 10− 4000 cm−1 with an optical resolution of 4 cm−1. Raman scattering radiation was collected in a backscattering configuration. A total of 1024 scans was averaged for each spectrum. A Nd:YAG laser with a wavelength of 1064 nm and power of 150−250 mW was used as the excitation source. The samples were inserted in a standard glass cell. UV/vis spectra were recorded at room temperature on a PerkinElmer Lambda 35 spectrometer, using 10 mm quartz cells. Spectra were registered with a scan speed of 480 nm·min−1, using a spectral width of 1 nm. All samples were prepared as solutions in n-hexane with concentrations of ca. ∼10−4 mol·l−1. Quantum-Chemical Calculations. All calculations were carried out using the Gaussian-03 suite of programs.26 Following full geometry optimizations, harmonic vibrational frequencies and Raman activities were calculated by the DFT method employed in this study, corresponding to the widely used Becke’s three-parameter exchange functional27 in combination with the Lee−Yang−Parr’s correlation functional28 (B3LYP). As demonstrated elsewhere,29 B3LYP offers the cost-effective choice for the computation of Raman spectra of organic molecules. In particular, the relative Raman activities for the bands in the frequency range ν ≤ 2300 cm−1 are better predicted by this hybrid functional than by second order Møller−Plesset perturbation theory (MP2).29 As inclusion of diffuse orbitals on heavy atoms is very important for simulations of Raman spectra,29,30 calculations were carried out with the 6-31+G* basis set.31−34 Test calculations with the much larger Dunning’s “correlation consistent” aug-cc-pVTZ basis set35 took considerably more CPU-time but did not lead to noticeable improvement in the simulated spectra (Figure 1S, Supporting Information), thus only B3LYP/6-31+G* computational results are discussed vide inf ra. Time-dependent density functional response theory (TDDFT)36−39 has been employed to compute the vertical excitation energy (i.e., absorption wavelength) and oscillator strength for the ground-state optimized geometries in the gas phase. Twenty lowest singlet excited states were taken into account. The procedure was analogous to the one described elsewhere,15 except that for the reason for consistency with present Raman spectra simulations the B3LYP/6-31+G* level of approximation was used instead of PBE0/def-TZVP. This induced only a small (1−5 nm) shift of the simulated absorption bands and almost negligible changes of oscillator strengths (see the Supporting Information for details). As shown elsewhere,15 possible conformational changes influence neither absorption wavelengths nor oscillator strengths for 1,2diphospholes, and for this reason all the calculations of the vertical excitations were obtained on a single structure

(α′)0 = const ·∑ [fe ′/(νe 2 − ν 2) − 2fe νeνe′/(νe 2 − ν 2)2 ] (1)

where fe is the oscillator strength calculated from the absorption band area, νe is the absorption band maximum in cm−1, ν is the wavenumber of the exciting line, derivatives with respect to Q are designated by primes. The summation is made over all the electronic excitation levels e. Conjugation typically leads to a decrease of νe values compared with that of a related nonconjugated molecule and thus to its approaching ν. The intensity of the Raman line, which is proportional to (α′)02, dramatically grows according to eq 2 if the Raman effect takes place in the so-called preresonance region. Preresonant enhancement of Raman lines were shown to come into play when the difference νe − ν reaches ∼5000 to ∼40 000 cm−1, which comprises the region of our Raman experiment ν of laser excitation = 9398 cm−1 and νe ≈ 25 000 cm−1). Thus, Raman bands of the moieties participating in conjugation should grow relative to similar bands of the same moieties incorporated into a related nonconjugated molecule. In the present work we demonstrate that intensity of the Raman band of the phenyl ν8a mode40 at ca. 1600 cm−1 (I8a) relative to the corresponding band of toluene (IT8a), where the conjugational effects are negligible, provides an empirical measure of the extent of conjugation of the phenyl group with the remainder of the molecule. It was already shown earlier17 that I8a can be viewed as observable of conjugation that can provide an estimation of its strength. For estimation of conjugational effects we use ratio of I8a in a compound under study and IT8a in the spectrum of toluene. As experimental Raman intensities are either not easily assessable or unavailable, for example, in the case of not yet synthesized molecules, I8a values were calculated by the following relationship derived from the intensity theory of Raman scattering:41 In =

f (ν0 − νn)4 Sn

⎡ νn⎢⎣1 − exp

( )⎤⎥⎦ −hcνn kT

(2)

where f is a proportionality constant, ν0 is the exciting laser wavenumber, νn is the wavenumber and Sn is Raman activity of the nth vibrational mode, c, h, and k are fundamental constants, and T is the temperature. The frequency of the ν8a vibration remains practically constant for all the studied compounds. Thus, it follows from eq 2 that relative intensities approximately equal to relative Raman activities: I8a T I8a

≈

S8a T S8a

(3)

This validates the use of the relative Raman activities (RRAs) in the subsequent discussion instead of the corresponding relative Raman intensities I8a/IT8a. 12170



Article

diffraction studies (Figure 2, Table 1). Bond angles and bond lengths of these molecules compare well with those observed

Static Raman activities were computed in the present study quantum-chemically in the double harmonic approximation, ignoring cubic and higher force constants and omitting secondand higher-order polarizability derivatives. In a few cases, where computed and experimental spectral curves were compared, the following procedure was applied: (i) the calculated Raman activities were converted, prior to the comparison with the experiment, to the corresponding Raman intensities with the use of eq 2, where f was a suitably chosen common normalization factor for all peak intensities; (ii) the calculated force constants were transformed to a redundant set of individual internal coordinates,42 and the scaling procedure was applied with the use of the program described in refs 43 and 44: Fij(scaled) = (si·sj)1/2Fij, where si and sj are scaling factors for internal coordinates i and j, respectively (transferable scaling factors, employed for this purpose,30,42,45 are summarized in Table 1S, Supporting Information); (iii) the computed frequencies and relative Raman intensities were plotted with a Lorentzian broadening (fwhm = 15 cm−1 was taken from the experimental Raman spectra). Shorygin’s theory is also applied in present study to establish interconnection between conjugational effects in Raman and absorption spectra. The first term of eq 1 reflects the dependence of the oscillator strength fe of electronic excitation on the vibrational coordinate. For some molecules experimental Raman excitation profiles have demonstrated the dominance of this term.17,46,47 Neglecting the second term of eq 1 and assuming that νe′ remains approximately constant for all the electronic excitations, one obtains (α′)0 = const ·∑ [fe ′/(νe2 − ν 2)]

Figure 2. ORTEP view of 1-propyl-3,4,5-triphenyl-1,2-diphosphacyclopenta-2,4-diene (1a, top) and 1-isopropyl-3,4,5-triphenyl-1,2diphosphacyclopenta-2,4-diene (2a, bottom). Hydrogen atoms are omitted for clarity.

Table 1. X-ray and DFT Calculated Structure Parameters of 1a and 2a 1a X-ray

(4) P1−P2 P1−C1 C1−C2 C2−C3 C3−P2 C1−C7 C2−C13 C3−C19 P1−C4

2.14(3) 2.17 1.79(6) 1.80 1.35(8) 1.39 1.44(8) 1.46 1.72(7) 1.74 1.49(9) 1.48 1.49(8) 1.50 1.49(8) 1.49 1.83(6) 1.87 Bond Angles, deg C1−C2−C3 117.4(6) 117.1 C2−C3−P2 119.8(5) 120.3 C3−P2−P1 91.0(2) 90.8 P2−P1−C1 96.5(2) 96.6 P1−C1−C2 113.6(5) 113.5 C1−P1−C4 110.8(3) 107.7 C4−P1−P2 111.6(2) 112.0 sum of bond angles at P1 318.9 316.3 Dihedral Angles, deg C12−C7−C1−C2 38.4(9) 47.9 C18−C13−C2−C3 65.4(8) 58.0 C20−C19−C3−C2 49.9(9) 54.5 Angle between Planes, deg C1−C2−C3−P2 and P2−P1− 12.6 (calc.) 12.7 C1

(5)

If a major role in the intensity enhancement due to conjugation is played by the second term of eq 1, a necessary condition for such an enhancement is a nonzero value of the derivative νe′, which is related to a change in molecular geometry upon electron excitation.16 Neglecting the first term of eq 1 and assuming that νe′ remains approximately constant for all the electronic excitations, one obtains (α′)0 ≈ const 2·∑ [fe νe/(νe 2 − ν 2)2 ]

(6)

Applying the same transformations as in the case of eq 5, one obtains RA1/2 ≈ const 2·∑ [fe /νe 3]

calc

X-ray

calc

2.14(5) 1.78(1) 1.38(2) 1.45(2) 1.72(1) 1.48(2) 1.49(2) 1.49(2) 1.86(1)

2.17 1.80 1.39 1.46 1.74 1.48 1.50 1.49 1.89

117.0(1) 120.2(9) 90.9(4) 97.0(4) 113.3(9) 108.4(6) 109.9(4) 315.3

117.0 120.3 90.9 96.5 113.7 107.4 112.3 316.2

40.4(8) 57.0(9) 54.3(8)

48.9 57.9 53.2

12.6 (calc)

12.3

Bond Lengths, Å

Raman activity (RA) is proportional to (α′)02; thus, (α′)0 in eq 4 can be substituted by RA1/2. As present quantum-chemical DFT calculations provide static RA, one should put ν = 0 and obtain RA1/2 = const1·∑ [fe ′/νe 2]

2a

(7)

RA of ν8a for a molecule under study can be substituted by RRA(ν8a)·RAT, where RAT is RA of ν8a for toluene. Thus, eqs 5 and 7 hold also for RRAs1/2. RRAs, fe and νe values for the systems under study were computed quantum-chemically in present study, and applicability of the simplified eqs 5 and 7 to the title compounds was tested (vide inf ra).

for similar phosphole derivatives. As expected from higher aromaticity of 1,2-diphospholes relative to phospholes (vide supra), the pyramidal environment of the phosphorus atom of the former is less pronounced than of the latter: the sum of bond angles at the tricoordinated phosphorus atom (319° for 1a and 315° for 2a) is larger than in the case of phospholes

■

RESULTS AND DISCUSSION Molecular Structure and Electronic Absorption Spectra. 1,2-Diphospholes 1a and 2a were characterized by X-ray 12171



Article

Table 2. Relative Raman intensities (RRI) of the Phenyl ν8 Banda (I/I0) in Raman Spectra of Some Monosubstituted Benzenes Ph−X and the Corresponding Computed Relative Raman Activities (RRAs)b,c

(∼300°). Nevertheless, 1,2-diphospholes absorb light at longer wavelengths than the less aromatic phospholes (Figure 1). Both the structural characteristics (Table 1) and spectroscopic features (Figure 1) of the phospholes are reproduced by DFT/TD-DFT computations quantitatively, which demonstrates the adequacy of the chosen level of the theoretical approximation for the description of both the ground and electronically excited states of the P-heteroles. This is important because the Raman effect in general, and the above-mentioned relationship between conjugation and intensity of Raman lines in particular, are closely connected to electronic excitations (vide inf ra). Raman Spectra and Conjugational Effect. In full accord with the reasons described in Theoretical Background, the experimental Raman spectra of the studied 1,2-diphospholes are dominated by very strong bands of the phenyl ν8a and ν1240 stretching modes at ca. 1600 and ca. 1000 cm−1, respectively (Figure 3). Other strong features in the spectra result from

X

computed RRA

experimental RRI

−H −Me −MeO −SH −NO2 −NMe2 −C(O)H −CHCH2 −Ph −NHPh

1.0 1.2 1.3 2.0 2.8 2.9 3.9 9.0 10.1 16.6

1.0 1.1 1.4 2.6 5.7 4.9 5.7 7.1 10.6 10.0

The RI sum of the ν8a and ν8b bands was measured experimentally because of their overlap in the experimental Raman spectra. Thus, the corresponding sums of computed RI of ν8a and ν8b are used for comparison. bSee Theoretical Background and Notations Used. cI0, intensity of ν8 band of benzene taken as unity. a

computed RRAs in the subsequent discussion instead of the corresponding experimental RRI values, which are either not easily assessable or unavailable, for example, in the case of not yet synthesized molecules. The RRAs of model phospholes 11−13 are compared in Table 3 with the corresponding values computed for toluene, where the conjugational effects are negligible, and styrene and cyclopenta-1,3-dienylbenzene, where conjugation of the phenyl group with the vinyl and the diene moieties, respectively, results in huge intensification of the phenyl ν8a band. It is clearly seen that these effects for phospholes 11−13 are much closer to the case of cyclopenta-1,3-dienylbenzene than to styrene, which suggests that the phenyl ring conjugates not only with the closest double bond but also with the whole diene moiety of the heterole ring. A replacement of the C5H− group of phosphole 13 by the dicoordinated phosphorus atom P− in the 1,2-diphosphole 11 does not change dramatically the conjugational abilities of the ring π-system but seems to moderately enhance them. The similarity of the conjugational properties of phospholes and 1,2-diphospholes is further illustrated by very similar RRA values for 5a and 6 (Table 3). An average contribution to RRA from one phenyl group, participating in conjugation with a diene moiety, amounts to ∼50/3 ≈ 17 and ∼59/4 ≈ 15 for 5a and 6, respectively. These slightly smaller values of RRA in comparison with model phospholes 11−13 can be traced back to mutual steric repulsion between the phenyl groups of 5a and 6, preventing them from adopting conformations optimal for conjugation with the diene moiety. Indeed, dihedral angles between the planes of the phenyl groups and the diene moiety increase from 35−38° in 11−13 to 48−58° in 5a and 6. Distal positions of the phenyl groups in 7 allow them to freely adopt orientations almost coplanar with the diene fragment. This results in very pronounced conjugational effects: immense growth of RRA (Table 3) and a concomitant bathochromic shift of λmax (Figure 1) relative to the spectra of the closely related phosphole 6. An introduction of the fused carbocycle to the heterole ring of 7 produces a sterically crowded phosphole 8, where phenyl groups adopt conformations similar to the case of 6. The resulting weakening of the conjugation in 8 comparative to 7 is reflected in a much smaller RRA value

Figure 3. Registered and simulated Raman spectra of 1,2-diphospholes 1a and 2a. Wilson’s notation is used for vibrations of phenyl rings (Figure 2S, Supporting Information).40

complex mixed vibrations of CC or PC bonds of the heterole and the phenyl rings, indicating a participation of these moieties in conjugation. Similar vibrations produce the strongest bands in Raman spectra of phospholes.48 Computations simulate quite accurately all the abovementioned features, reproducing not only relative intensities of the Raman bands within the spectrum of one compound (Figures 3 and 3S, Supporting Information)49 but also the differences between the Raman intenisties of ν8a bands found experimentally in the spectra of different representatives of the studied diphospholes (Figure 4S, Supporting Information). The same is true for the spectra of various monosubstituted benzenes used for assessment of an ability of simulations to reproduce relative Raman intensities (RRIs) of ν8a bands determined experimentally (Table 2). RRI and RRA values in Table 2 vary essentially because of quite different conjugation between the phenyl moiety and various substituents. For example, strong conjugation between phenyl and vinyl groups results in an almost 8-fold increase of RRI relative to toluene, where the conjugational effects are negligible. Taking into account that experimental RRIs are concerned with the condensed state of the compounds and besides are determined with at least 20% error, the computational “gas-phase” estimates of the RRA values should be regarded as quite satisfactory. This validates the use of the 12172



Article

Table 3. Relative Raman Activities (RRAs) of the Phenyl ν8a Band (S/S0) in the Raman Spectra of Some Monosubstituted Benzenes, 1,2-Diphospholes, and Phospholesa

properties of 1,2-diphospholes, as shown elsewhere,15 can be effectively tuned by variation of substituents introduced in the para-positions of the phenyl moieties. Table 4 clearly demonstrates the conjugational origin of these effects. Table 4. Relative Raman Activities (RRAs) of the Phenyl ν8a Band (S/S0) in the Raman Spectra of 1,2-Diphospholes 1a− 1i and the Related Monosubstituted Benzenes and the Longest-Wavelength Absorption Maxima (λmax, nm) in the UV/Vis Spectra of the Phospholesa

a

a

S0, Raman activity of the ν8a band of toluene taken as unity.

For example, a 33-fold increase of RRA for 1e relative to an already conjugated styrene molecule indicates that conjugation in the diphosphole not only involves the H2CCH− moiety and the adjacent phenyl ring but also spreads over the π-system of the diene moiety of the heterole. A similar effect was reported earlier: an increase of the length of the conjugated system from 5.6 Å for styrene to 8.4 Å for p-divinylbenzene resulted in an 8-fold increase of RRA.17 A similar comparison of RRA values collected in Table 4 demonstrates that the introduction of −OMe, −C(O)H, −NO2, −NMe2, and −CHCH2 groups in the para-positions of the phenyl moieties of the parent diphosphole 1a produces a two- to 8-fold increase of RRA for 1e−i relative to the parent 1a and a 33- to 83-fold increase of RRA relative to the corresponding monosubstituted benzenes. Thus, these groups bring essentially extended π-delocalization into the electronic system of the 1,2-diphospholes, leading to a narrower gap between frontier molecular orbitals (FMO) and a bathochromic effect,15 as a result. Nevertheless, it seems surprising that a vinyl group, producing (according to RRA values) by far the strongest conjugational effect in the diphospholes, induces a smaller batochromic shift of the longest-wavelength absorption band relative to the parent 1a than NMe2 groups (molecules 1e and 1h respectively). At a first glance it would also seem unclear why λmax for monophospholes are typically smaller than for analogous diphospholes (Figure 1) in spite of very close values of RRA for both types of phospholes (Table 3). These points are addressed vide inf ra. Interconnection between Conjugational Effects in Raman and Absorption Spectra. To rationalize influence of conjugation on light absorption properties of the compounds under study we apply simplified variants of Shorygin’s formula 1, viz. eqs 5 and 7. It should be noted that in these equations

S0, Raman activity of the ν8a band of toluene taken as unity.

(Table 3) and a concomitant hypsochromic shift of λmax (Figure 1). A similar effect is produced by a replacement of the phenyl group in 9 by more bulky isopropyl moiety in 10 (Figure 1): steric crowding forces the pyridyl groups of the latter to twist from the plane of the diene moiety, and the resulting weakening of conjugation causes a quite pronounced hypsochromic shift of λmax. In contrast to phospholes, the impact of the bulk of the substituent R at the tricoordinated phosphorus atom P1 on conjugation of the 1,2-diphospholes is negligible (compare 1a and 2a in Figure 1 and Table 3). The reasons are that (i) the R group is situated too far from the aryl moiety at the atom C3 of the diphosphole ring to influence its conformation; (ii) “free” space at the phosphorus atom P2 allows R to adopt a conformation minimizing steric repulsion from the aryl moiety at C5 (Figure 2). Thus, even moderate steric crowding essentially influences conjugational effects in phospholes, which can be used for tuning of their optical properties. The light absorption 12173



Article

the sum runs over all electronically excited states, which suggests that more than one electronic state should be taken into account. A truncation of the RA1/2 expression in eqs 5 and/or 7 after the lowest energy electronic excited state leads to a considerable error in the description of conjugational effects in Raman spectra of phospholes. The latter is evident from the fact that the vide supra discussed pronounced intensification of the Raman bands of phospholes and 1,2-diphospholes does not necessarily parallel the corresponding bathochromic shifts of the longest-wavelength absorption band caused by conjugation. Some striking examples of such nonparallelism are given at the end of the preceding subsection. Thus, to find interrelations between conjugational effects in Raman and absorption spectra of the molecules under study, we have tried to correlate the square roots of relative Raman activities of the phenyl ν8a band, (RRA)1/2, in the Raman spectra of phospholes with the sums ∑fe′/νe2 or ∑fe/νe3 computed for two, three, five, 10, 15, or 20 first electronic excitations, according to eqs 5 or 7, respectively. To apply eq 5, we assumed that fe′ derivatives are roughly proportional to the corresponding fe values, thus RA1/2 ≈ const3·∑ [fe /νe 2]

(5a)

The resulting correlations for the cases of eqs 5a and 7, truncated after two, three, or even five lowest energy electronic excited states, are rather poor (Figures 5S and 6S, and Table 2S Supporting Information), whereas the sum over the first 10−20 excitations reproduces the general trend quite well (Figure 4 and Table 2S, Supporting Information). This means that conjugation in phospholes results in both a dramatic growth of the Raman activity and a simultaneous shift of the electronic absorption bands in the quite broad UV/visible region. The shift of the “center of gravity” of the whole absorption spectrum parallels the Raman effect, whereas positions of separate absorption bands are not simply dependent on Raman activities (or conjugation lengths). In particular, insertion of parasubstituents −CHCH2 (1e), able to exhibit a strong conjugation with the aromatic rings, leads to the appearance of additional strong absorptions near 330−340 nm due to a substantial shift of bands belonging to the aromatic moieties (Figure 7S, Supporting Information) and to only a moderate bathochromic shift of the lowest transitions attributed to the 1,2-diphosphole unit.15 In contrast, the latter transitions are strongly and uniformly influenced by conjugational effects resulting from introduction of phenyl substituents to the phosphole ring (Figure 5). These effects are very similar for both the phospholes and 1,2-diphospholes, being more pronounced for the former just because of the larger number of phenyl groups, participating in conjugation with the phosphole ring (four for 6 vs three for 1a). The bathochromic shifts shown in Figure 5 parallel the Raman effects (Table 4). In spite of the larger conjugational effects in 6, it absorbs at shorter wavelengths than 1a, because absorption bands of the parent phosphole are essentially shifted to higher energies in comparison to the related 1,2-diphosphole (Figure 5). Influence of Substituents at the Tricoordinate Phosphorus Atom on Conjugational Effects. Pyramidal geometry of the tricoordinate phosphorus atom and a pronounced s character of its lone pair of electrons prevent efficient interaction between the lone pair and the endocyclic diene system. As a result, phospholes possess only a weak

Figure 4. Correlation of square roots from Raman activities (RA) of bands and parameters of absorption spectra of the studied compounds according to eqs 5a (top) and 7 (bottom) truncated after 20 lowestenergy electronic excited states. Here, W1 = Σ( feλe2) and W2 = Σ( feλe3), where R is the correlation coefficient, SD is the standard deviation, and N is the total number of data included in the analysis. The numbering of compounds is given in Figure 1.

aromatic character, which favors delocalization of the π-system to exocyclic unsaturated moieties. Effects of σ*(P−R)−π*(1,3diene) hyperconjugation found for phospholes7 are rather weak, and this suggests that variation of P−R groups should have only little influence on optical properties of P-heteroles, as actually was shown for 1-alkyl-1,2-diphospholes15 (see also Figure 1). Present computations demonstrate that Raman activities of the phenyl ν8a bands of 1a and 2a practically coincide, thus confirming that delocalization of the dienic π-system to the exocyclic aromatic moieties is not influenced by the P−R groups, provided that R = alkyl. Slight weakening of the πconjugational effects is found only in case of R = SiMe3 (3a, Figure 1) and SnMe3 (4a, Figure 1). These quite moderate changes are accompanied by hypsochromic shifts of the longest-wavelength absorption bands in the spectra of 3a and 4a amounting up to 36 nm. Obviously, these shifts, being among the largest shifts found for 1,2-diphospholes (Figure 1), are caused by conjugation different from the kind discussed vide supra. P−Sn and P−Si bonds, exhibiting high polarizability and a low σ−σ* gap, provide the characteristics favorable for σ−π conjugation50,51 with the dienic moiety of the diphosphole ring. 12174



■

CONCLUSIONS

■

ASSOCIATED CONTENT

Article

Quantum-chemical modeling of 1-R-1,2-diphospholes and phospholes in combination with experimental structural and spectroscopic methods has delivered the first quantitative description of the strength of conjugation between diene moieties of these P-heteroles and exocyclic phenyl groups. Raman activities (RA) of the ν8a bands of the exocyclic phenyl substituents were used as observables of the conjugation that provide an estimation of its length. It is shown that the conjugation in both types of phospholes is very similar to the conjugation of phenyl groups with the diene system of cyclopentadiene. The conjugation produces a bathochromic shift of the whole electronic absorption spectra of both types of phospholes, whereas positions of separate absorption bands are not simply dependent on conjugation lengths. Nevertheless, 1R-phospholes absorb at shorter wavelengths than 1-R-1,2diphospholes because absorption bands of the parent (unsubstituted) phosphole are essentially blue-shifted in comparison to the closely related 1,2-diphosphole. Thus, the two families of the P-heteroles complement each other as candidates for building blocks for optical materials because they absorb light in different, though partially overlapping, spectral regions. Their photophysical characteristics can be further tuned by influencing the conjugation in the phosphole ring and in the exocyclic moieties. In particular, introduction of such substituents as −OMe, −C(O)H, −NO2, −NMe2, and −CHCH2 in the para-position of the phenyl groups in 3,4,5triaryl-1-R-1,2-diphospholes leads to essentially extended πdelocalization into the electronic system of the heterocycle, which not only involves the exocyclic groups but also spreads over the π-system of the 1,2-diphosphole ring. This produces batochromic shifts of the absorption bands up to 63 nm. In contrast, hypsochromic shifts of ca. 30−40 nm can be achieved by introduction of SnMe3 or SiMe3 groups at the phosphorus(III) atom of the 1,2-diphosphole and concomitant increase of aromaticity of the P-heterole. The quantitative analysis described herein should facilitate further studies on the role of conjugation in phospholes and ultimately could aid the rational design of P-heterocycles with specific photophysical properties.

Figure 5. Comparison of the simulated absorption spectra of some 1,2-diphospholes (top) and phospholes (bottom).

The conjugation depends on the geometry of the molecule, being the most efficient if the σ-orbital is orthogonal to the plane of the dienic moiety. In full agreement with the expectations, a substitution of the exocyclic P−Pr moiety by P−SnMe 3 or by P−SiMe 3 groups results in essential planarization of the diphosphole ring (Figure 6). The latter,

S Supporting Information *

Figure 6. Comparison of some computed structural parameters of 1a and 4a.

Figure 1S. Comparison of simulated Raman spectra of a model 1,2-diphosphole at different levels of theory. Table 1S. Scaling factors for the force constants of the compounds studied. Figure 2S. The calculated modes for the normal vibrations of benzene and their Wilson’s notation. Figure 3S. Simulated and registered Raman spectra of compound 1c. Figure 4S. Comparison of fragments of simulated and registered Raman spectra of compounds 1a, 1b, and 1c. Figure 5S. Correlation of Raman activities of bands and parameters of absorption spectra of the studied compounds according to eq 5a. Figure 6S. Correlation of Raman activities of bands and parameters of absorption spectra of the studied compounds according to eq 7. Table 2S. Dependence of parameters of correlations obtained for eqs 5a and 7 on the number (n) of lowest-energy electronic excited states taken into account. Figure 7S. Simulated absorption spectra of compounds 1a and 1e computed at the B3LYP/6-31+G* level of theory for 20 singlet states. Figure 8S. Kohn−Sham orbital levels of highest occupied and lowest unoccupied molecular orbitals of molecules 1a, 3a, and 4a.

as well as a concomitant elongation of double bonds and shortening of the C−C single bond of the ring (Figure 6 and Table 3S, Supporting Information), suggests that the σ−π conjugation leads to higher aromaticity of 3a and 4a relative to diphosphole 1a. In accordance with general expectations, the aromatization of diphospholes is accompanied by a destabilization of lowest unoccupied molecular orbital (LUMO) (Figure 8S, Supporting Information), which results in a larger HOMO−LUMO gap and, thus, in shorter absorption wavelengths (Figure 1). Furthermore, stronger aromatic character of 3a and 4a disfavors delocalization of the ring π-system to exocyclic phenyl moieties, which explains the minor decrease of RRA of the phenyl ν8a band compared to 1a (Figure 1). 12175



Article

Containing Rings and Their Si-, N-, and P-Substituted Aromatic Heterocyclics. Struct. Chem. 2007, 18, 25−31. (13) Cyranski, M. K.; Krygowski, T. M.; Katritzky, A. R.; Schleyer, P. To What Extent Can Aromaticity Be Defined Uniquely? J. Org. Chem. 2002, 67, 1333−1338. (14) Josa, D.; Pena-Gallego, A.; Rodriguez-Otero, J.; Cabaleiro-Lago, E. M. A MP2 and DFT Study of the Aromatic Character of Polyphosphaphospholes. Is the Pyramidality the Only Factor to Take into Consideration? J. Mol. Model. 2011, 17, 1267−1272. (15) Zvereva, E. E.; Grimme, S.; Katsyuba, S. A.; Burganov, T. I.; Zagidullin, A. A.; Milyukov, V. A.; Sinyashin, O. G. Application of Time-Dependent Density Functional Theory and Optical Spectroscopy toward the Rational Design of Novel 3,4,5-Triaryl-1-R-1,2diphospholes. J. Phys. Chem. A 2013, 117, 6827−6834. (16) This was first demonstrated in the papers, published in Russian: Vol’kenstein, M. V. Zh. Fiz. Khim. 1943, 17, 62. Shorygin, P. P. Zh. Fiz. Khim. 1947, 21, 1125. The effect is discussed in depth in several reviews, published in English, e.g.: Shorygin, P. P. Raman Scattering and Conjugation. Russ. Chem. Rev. 1971, 40, 367−392. Leites, L. A.; Bukalov, S. S. Raman intensity and conjugation with participation of ordinary σ-bonds. J. Raman Spectrosc. 2001, 32, 413−424 See also ref 17 and references cited therein. (17) Schmid, E. D.; Topsom, R. D. Raman Intensity and Conjugation, 5. A Quantitative Relationship between Raman Intensity and the Length of Conjugation and an Analysis of the Raman Intensities of Some Substituted Benzenes and Biphenyls. J. Am. Chem. Soc. 1981, 103, 1628−1632. (18) Miluykov, V. A.; Bezkishko, I. A.; Zagidullin, A. A.; Sinyashin, O. G.; Hey-Hawkins, E. Reactions of Sodium 3,4,5-Triphenyl-1,2Diphosphacyclopentadienide With Alkyl Halides and Silicon and Tin Chlorides. Russ. Chem. Bull. 2010, 59, 1232−1236. (19) Miluykov, V. A.; Bezkishko, I. A.; Zagidullin, A. A.; Sinyashin, O. G.; Lonnecke, P.; Hey-Hawkins, E. Cycloaddition reactions of 1-alkyl3,4,5-triphenyl-1,2-diphosphacyclopenta- 2,4-dienes. Eur. J. Org. Chem. 2009, 1269−1274. (20) Bezkishko, I. A.; Miluykov, V. A.; Sinyashin, O. G.; HeyHawkins, E. The reaction of cyclopropenylphosphonium bromides with sodium polyphosphides as an advanced method of synthesis of sodium 1,2- diphosphacyclopentadienides: Scope and limitations. Phosphorus, Sulfur Silicon Relat. Elem. 2011, 186, 657−659. (21) CrysAlisPRO: Data collection and data reduction software package; Oxford Diffraction/Agilent Technologies UK Ltd.: Yarnton, England. (22) SCALE3 ABSPACK: Empirical absorption correction using spherical harmonics, CrysAlis - software package; Diffraction Ltd.: Abingdon, England. (23) SIR-92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (24) SHELX includes SHELXS97, SHELXL97. Programs for crystal structure determination; University of Göttingen, Germany, 1997. Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112−122. (25) DIAMOND 3 - Crystal and Molecular Structure Visualization. Crystal Impact - Dr. H. Putz & Dr. K. Brandenburg GbR, Kreuzherrenstr. 102, 53227 Bonn, Germany; http://www. crystalimpact.com/diamond. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (27) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into A Functional of the Electron Density. Phys. Rev. 1988, B37, 785−789. (29) Zvereva, E. E.; Shagidullin, A. R.; Katsyuba, S. A. Ab initio and DFT Predictions of Infrared Intensities and Raman Activities. J. Phys. Chem. A 2011, 115, 63−69. (30) Katsyuba, S. A.; Vandyukova, E. E. Scaled Quantum Mechanical Computations of Vibrational Spectra of Organoelement Molecules,

Table 3S. DFT calculated structural parameters of molecules 1a, 3a, and 4a. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Sergey A. Katsyuba. E-mail: [email protected], skatsyuba@ yahoo.com. Fax: (+7) 843-273-18-72 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are indebted to all staff members of the Supercomputer Centre of the Kazan Scientific Centre of the Russian Academy of Sciences and especially to Dr. D. Chachkov for technical assistance in these computations. A joint scholarship program of the German Academic Exchange Service (DAAD) and the government of the Republic of Tatarstan “Yevgeny Zavoisky” is gratefully acknowledged (EEZ). This work was supported by the Russian Foundation for Basic Research (grants 14-03-00920 A and 14-03-31796 mol_a).

■

REFERENCES

(1) Baumgartner, T.; Reau, R. Organophosphorus π-Conjugated Materials. Chem. Rev. 2006, 106, 4681−4727. (2) Hissler, M.; Lescop, C.; Reau, R. Organophosphorus πConjugated Materials: the Rise of a New Field. J. Organomet. Chem. 2005, 690, 2482−2487. (3) Hissler, M.; Dyer, P. W.; Reau, R. The Rise of Organophosphorus Derivatives in π-Conjugated Materials Chemistry. Top. Curr. Chem. 2005, 250, 127−163. (4) Saito, A.; Miyajima, T.; Nakashima, M.; Fukushima, T.; Kaji, H.; Matano, Y.; Imahori, H. Acenaphtho[1, 2-c]Phosphole P-Oxide: a Phosphole−Naphthalene π-Conjugated System with High Electron Mobility. Chem.−Eur. J. 2009, 15, 10000−10004. (5) Joly, D.; Tondelier, D.; Deborde, V.; Geffroy, B.; Hissler, M.; Reau, R. Phosphole-Based π-Conjugated Electroluminescent Materials for OLEDs. New. J. Chem. 2010, 34, 1603−1611. (6) Mathey, F. The Organic Chemistry of Phospholes. Chem. Rev. 1988, 88, 429−453. (7) Schäfer, W.; Schweig, A.; Mathey, F. Theory and Application of Photoelectron Spectroscopy. 60. Phospholes. Electronic Structure. J. Am. Chem. Soc. 1976, 98, 407−414. (8) Fadhel, O.; Szieberth, D.; Deborde, V.; Lescop, C.; Nyulaszi, L.; Hissler, M.; Reau, R. Synthesis, Electronic Properties, and Reactivity of Phospholes and 1,1′-Biphospholes Bearing 2- or 3-Thienyl CSubstituents. Chem.Eur. J. 2009, 15, 4914−4924. (9) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-Berthelot, J.; Toupet, L.; Nyulaszi, L.; Reau, R. Phosphole-Containing π-Conjugated Systems: from Model Molecules to Polymer Films on Electrodes. Chem.Eur. J. 2001, 7, 4222−4236. (10) Crassous, J.; Reau, R. π-Conjugated Phosphole Derivatives: Synthesis, Optoelectronic Functions and Coordination Chemistry. Dalton Trans. 2008, 48, 6865−6876. (11) Zagidullin, A. A.; Bezkishko, I. A.; Miluykov, V. A.; Sinyashin, O. G. Phospholes − development and recent advances. Mendeleev Commun. 2013, 23, 117−130. (12) Wang, L.; Wang, H. J.; Dong, W. B.; Ge, Q.Yu.; Lin, L. Using Three Major Criteria to Evaluate Aromaticity of Five-Member C12176



Article

Containing the Atoms P, S, and Cl. Chem. Phys. Lett. 2003, 377, 658− 662. (31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J. Chem. Phys. 1972, 56, 2257−2261. (32) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28, 213−222. (33) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (34) Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-consistent Molecular Orbital Methods 25. Supplementary Functions for Gaussian Basis Sets. J. Chem. Phys. 1984, 80, 3265. (35) Woon, D. E.; Dunning, T. H. Calculation of the Electron Affinities of the Second Row Atoms: Al-Cl. J. Chem. Phys. 1993, 99, 3730−3737. (36) Gross, E. U. K.; Dobson, J. F.; Petersilka, M. In Density Functional Theory II; Nalewajski, R. F., Ed.; Springer Series in Topics in Current Chemistry; Springer: Heidelberg, Germany, 1996. (37) Casida, M. E. In Recent Advances in Density Functional Methods; Chong, D. P., Ed.; World Scientific: Singapore, 1995; Vol. 1. (38) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454−464. (39) Furche, F. On the Density Matrix Based Approach to TimeDependent Density Functional Response Theory. J. Chem. Phys. 2001, 114, 5982−5992. (40) Wilson’s notation is used to describe the normal modes of phenyl ring. Graphical representation of the modes is given in Figure 2S, Supporting Information: Wilson, E. B., Jr. The Normal Modes and Frequencies of Vibration of the Regular Plane Hexagon. Phys. Rev. 1934, 45, 706−714. (41) Porezag, D.; Pederson, M. R. Infrared Intensities and Ramanscattering Activities within Density-functional Theory. Phys. Rev. B 1996, 54, 7830−7836. Keresztury, G. Raman Spectroscopy: Theory; Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley and Sons: Chichester, U.K., 2002; , Vol. 1. (42) Baker, J.; Jarzecki, A.; Pulay, P. Direct Scaling of Primitive Valence Force Constants: An Alternative Approach to Scaled Quantum Mechanical Force Fields. J. Phys. Chem. A 1998, 102, 1412−1424. (43) Sipachev, V. A. Local Centrifugal Distortions Caused by Internal Motions of Molecules. J. Mol. Struct. 2001, 567−568, 67−72. (44) Sipachev, V. A. Anharmonic Corrections to Structural Experiment Data. Struct. Chem. 2000, 2/3, 167−172. (45) Katsyuba, S. A.; Grunenberg, J.; Schmutzler, R. Vibrational Spectra and Conformational Isomerism of Calixarene Building Blocks. Part I. Diphenylmethane, (C6H5)2CH2. J. Mol. Struct. 2001, 559, 315− 320. (46) Ohtsuka, H.; Furukawa, Y.; Tasumi, M. Dependencies of the Raman Spectra of p-oligophenyls on the Chain Length and the Excitation Wavelength. Spectrochim. Acta, Part A 1993, 49, 731−737. (47) Bree, A.; Zwarich, R.; Taliani, C. Raman excitation profiles for some Ag modes of biphenyl in the pre-resonance region. Chem. Phys. 1982, 70, 257−267. (48) Casado, J.; Réau, R.; López Navarrete, J. T. Tuning of Electronic Properties in Thienyl-phosphole π-conjugated Systems Through Pfunctionalization Monitored by Raman Spectroscopy. Chem.Eur. J. 2006, 12, 3759−3767. (49) A discrepancy in the range 1250−1350 cm−1 is caused by intermolecular interactions in the condensed-phase experiment, which are not taken into account in the present gas-phase computations but specifically influence the experimental Raman intensities in this case. Experimental spectra of other 1,2-diphospholes (e.g., Figure 3S, Supporting Information) in this range practically coincide with the theoretical spectrum shown in Figure 3.

(50) Hoffmann, R. Interaction of orbitals through space and through bonds. Acc. Chem. Res. 1971, 4, 1−9. (51) Hoffmann, R.; Imamura, A.; Hehre, W. J. Benzynes, dehydroconjugated molecules, and the interaction of orbitals separated by a number of intervening σ bonds. J. Am. Chem. Soc. 1968, 90, 1499−1509.

12177


Conjugation in and Optical Properties of 1-R-1,2-Diphospholes and 1

Recommend Documents