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Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study
Spectrochimica Acta Part A 59 (2003) 75 /86 www.elsevier.com/locate/saa
Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study Ivan Baraldi a,*, Anna Spalletti b, Davide Vanossi a a
Dipartimento di Chimica, Universita` di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy b Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy Received 28 February 2002; accepted 25 March 2002
Abstract The experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans -stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO CI level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the 1H-NMR experiments, the ab initio geometry of ZE -2-pyridylphenylbutadiene is consistent with the presence of the N ×/H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (pH, pL) character in all compounds except for E -4,4?-dipyridylethene, where S1 has (n, p*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the 1(n, pL) and 1(pH, pL) states which occurs in solvents of high polarity. The inclusion of the most important doubly- and triply-excited configurations in the CI calculations shows that the 1 A g excited state is above the spectral region analyzed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: aza-Derivatives of stilbene and 1,4-diphenylbutadiene; Ab initio and CS INDO CI calculations; Electronic spectrum; 1HNMR; Rotamerism
1. Introduction The replacement of one or more CH groups with nitrogen atoms in the phenyl rings of a,v-
* Corresponding author. Tel.: /39-59-2055085; fax: /3959-373543 E-mail address: [email protected] (I. Baraldi).
diphenylpolyenes, including stilbene (S), generates aza-compounds, isoelectronics to the parent hydrocarbon, that display important differences in their physical-chemistry properties, namely the presence of 1,3(n, p*) states of low energy, a significant permanent electric dipole moment (d) in the compounds without inversion centre, the formation of new conformational equilibrium (rotational isomerism) [1,2], and the possibility to
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 1 7 - 8
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form intramolecular hydrogen-type bonds [3]. All such aspects can modify the spectroscopic, photophysical and photochemical properties of the azaderivatives with respect to those of the parent hydrocarbon. Several experimental techniques and theoretical models have been applied to study the rotational isomerism, not only in aza-derivatives, but also in their parent hydrocarbons (see Refs. [1,2] for recent reviews). Much work has been done based on spectroscopic and fluorimetric measurements, in particular through the analysis of the lexc dependence of fluorescence spectra as well as by the effect of temperature and solvent. In the case of n -styrylpyridines (n-StPs n /2, 3, 4) and n,n?dipyridylethenes (n,n ?-DPEs n,n ?/2, 3, 4), where the position of the nitrogen was found to play an important role on electronic spectrum and photobehaviour [4 /10], the characterization of rotamers was investigated but no satisfactory conclusions were obtained [1]. The molecules bearing the nitrogen in the meta position (with respect to the double bond) display a photobehaviour very similar to stilbene, while those bearing the heteroatom in ortho and para positions show reduced fluorescence quantum yields and lifetimes, their S1 state deactivating significantly via S1 0/S0 internal conversion (IC). More recently, the study of the excited state properties and rotamerism was extended to asymmetric aza-analogues of 1,4-diphenylbutadiene (DPhB), n-pyridylphenylbutadienes (n-PPhBs n /2, 3, 4) [11]. Also in this case, the EE isomers bearing the heteroatom in meta position behave similarly to the corresponding hydrocarbon. On the contrary, a marked decrease in the yield of the radiative decay and an increase of the IC yield are operative in the other two isomers with the nitrogen in ortho and para positions. Concerning the presence of rotamers in the ground electronic state, no clear indication has been reached. From a theoretical point of view, several papers have appeared on the rotational isomerism and electronic spectrum of aza-analogues of stilbene [12 /18]. The main aspects of these works were the interpretation of the electronic spectra in relation to the rotational isomerism, and the characterization of the first singlet excited state (S1). A brief
summary of the recent works may be useful. Marconi et al. [17] carried out a quantum chemistry study on 2-StP, 3-StP, 2,2?-DPE and 3,3?DPE, using the MNDO method for the groundstate properties, without geometry optimisation, and INDO/S method to evaluate the electronic spectrum. For 2-StP and 2,2?-DPE they found that the rotamer A (see Fig. 1 for our notation of rotamers) was the most stable one, although the energy differences between rotamers were probably overestimated. Moreover, as concerns the electronic spectrum, they confirm the (pH, pL) character of S1, as shown in a previous work [16]. More recently, Hamed et al. [18] carried out a theoretical (INDO/S) study on the spectra of StPs and other heterocompounds, but only one rotamer was considered and, in consequence, no discussion on rotamerism was presented. To our knowledge, no theoretical study has been made on azaanalogues of DPhB. A spectroscopic investigation on 2,2?-DPE in Spolskii matrices appeared some years ago [19]. The experimental study was combined with AM1 computation of the ground state potential energy curve for the symmetrical conrotatory twisting of the pyridyl moieties around the exocyclyclic Ca(a?) /pyridyl single bonds. It was suggested that the trapped rotamer in the n -alkane matrices is the rotamer A (according to our notation in Fig. 1). In conclusion, while the photophysical and photochemical properties of aza-derivatives of stilbene and DPhB have been widely investigated, the aspect of the rotameric equilibrium, even if always well present, did not reach a completely adequate solution. Also some questions concerning the electronic spectra of aza-derivatives are still open, especially that connected with the effect of rotational isomerism and solvent. For all these reasons, in the present paper we report the results of a combined experimental and theoretical study carried out with the aim of elucidating the rotameric behaviour and the electronic spectra of E -2-StP, E -2,2?-DPE, EE -2-PPhB and ZE -2PPhB. Fig. 1 shows the possible rotameric equilibrium for the molecules investigated. Even if the E -4-StP, E -4,4?-DPE, EE -4-PPhB and ZE -4PPhB have not rotational isomerism, they have been also included in this work because their
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excited state properties are very interesting for purposes of comparison. We studied the rotational isomerism by simple ab initio calculations of geometry and total molecular energy of rotamers; moreover, we compared the UV absorption spectra, particularly the splitting of the conjugation band, with CS INDO CI electronic spectra. The spectra calculations were first performed in gas phase approximation and were then corrected for the dielectric solvent effects, according to the simple virtual charge model [20], to describe the dependence of the spectral behaviour on the solvent polarity. The presence of interactions
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between N and ethenic H (hydrogen-type bond), suggested by calculations, was confirmed by 1HNMR experiments. In this paper we also pay attention to the relative position of the symmetry-allowed 11Bu state with respect to the symmetry-forbidden 1 A g state, in the C2h symmetry. The 11Bu /1 A g energy gap is a function of polyene length, and has an important role on the photophysics and photochemistry of (a,v-diphenyl)polyenes [21]. For this reason, we performed CI calculations adding doubly- and triply-excited configurations to the
Fig. 1. Rotamer equilibria of the aza-analogues of E -S and DPhBs.
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usual CI approach limited to singly excited configurations. The study allowed us to draw some important conclusions about the rotamerism and electronic spectra of aza-compounds investigated.
2. Experimental and computational details 2.1. Experimental All the investigated compounds were synthesized for previous works. The absorption spectra, already published [6,9,11], are now reported in the same solvent, a mixture of methylcyclohexane and 3-methylpentane (MCH/3MP, 9/1, v/v), for comparison purposes. They were recorded by a Lambda 800 double beam spectrophotometer. The experimental oscillator strengths were calculated by equation: fexp 4:39109 fo(n)d ˜ n˜ [22]. 1 H-NMR spectra were carried out in a nonpolar solvent (deuterated benzene) and in a protic solvent (deuterated methanol) on Bruker AC 200 and DMX 500 spectrometers, using TMS as reference. 2.2. Quantum chemistry calculations Ab initio calculations on the ortho aza-derivatives of trans stilbene (E -S) and DPhBs were executed using the HyperChem computational package [23]. For each molecule of Fig. 1, after the complete optimisation of geometry at 3-21G level, the total molecular energy was obtained using the basis set 6-31G*. In addition, for the azaanalogues of E -S, the correlation effects at MP2 level were also evaluated. The quantum chemical calculations of the electronic spectra of all the studied aza-compounds and their parent hydrocarbons (E -S, DPhBs) were performed by the CS INDO CI method [24,25] using geometries obtained after complete optimisation according to PM3 Hamiltonian model [26]. The characteristic of the CS INDO is the use of a hybrid atomic orbital basis set and of specific screening constants (kij i, j/s, p, n) for the various chemically distinct interactions (s /s, s /p, p /p, etc.). In this work the
screening constants, entering the calculation of the resonance integrals, were given the values kss /1, ksp /0.65, kpp /0.50, kns /0.72, knp /0.60, knn /0.68, chosen according to criteria previously discussed [24,25]. The hybrid atomic orbitals were determined by the maximum overlap criterion [27]. All other details of the CS INDO calculations were as in the original version [24]; in particular the gAB integrals were calculated by the Ohno /Klopman formula [28]. The so obtained sets of CS INDO MOs were the basis for variational configuration interaction (CI) calculations. Both a singly-excited CI (S-CI) and a more extended CI, involving double (D) and triple (T) excitations (S/D/T/CI), were carried out to calculate the electronic absorption spectra. The poly-excited configurations were included to study their effect on the electronic states of different symmetry, and were selected by a perturbative criterion with respect to a suitable set of monoexcited reference configurations. The parameter h , defining the threshold of the perturbative selection of D and T configurations, was set to a value near or equal to 0.05. In all CI calculations, the MO active space included all p, p* and n type orbitals. The solvation energy was evaluated within the virtual charge model, using a slightly simplified version of the formula proposed by Heidrich et al. [29] 1 1 Esol 1 2 o X 2 X X QA QA QB (1) Reff A A B("A) rAB Reff In Eq. (1) o is the static dielectric constant of the solvent, QA(B) is the atomic charge of atom A(B), Reff is an average effective atomic radius, here ˚ , and rAB is the interatomic taken equal to 4 A distance. For each electronic state considered, the total energy of the solvated molecule was taken equal to the sum of the energy of the isolated molecule and the solvation energy Esol. It is worth noting that the evaluation of Esol by formulas like that of Eq. (1) has been shown to be basically consistent with the ZDO approximation [30]. Adopting the above-described procedure, we have obtained reasonable descriptions of the di-
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electric solvent effects on the ground and excited states of cyanines and other dyes [20,31].
3. Results and discussion 3.1. Rotamerism and hydrogen bonding The results of NMR spectra of the investigated compounds are reported in Table 1. For all investigated hydrocarbons (E -S, EE -DPhB and ZE -DPhB) and their aza-derivatives (E -4-StP, E 4,4?-DPE, EE -4-PPhB and ZE -4-PPhB) with the chain linked in para position with respect to nitrogen, the effect of deuterated benzene is that of reducing the chemical shift (d ) of all the ethenic hydrogens because of the ring current. Only for the 2-derivatives (E -2-StP, E -2,2?-DPE, EE -2-PPhB and ZE -2-PPhB) does the change from a protic to a non-polar solvent have remarkable effects. Indeed, the ethenic proton near the nitrogen shows more or less strong de-shielding (underlined figures in Table 1) while the others are shifted towards smaller d . This behaviour is in agreement with the presence of an intramolecular hydrogenbond type interaction, in non-polar solvent, between the nitrogen and the nearest ethenic proton [32], that is reduced or broken in polar and protic solvent in favour of stronger intermolecular hydrogen bonds. In the case of ZE -2-PPhB, the huge effect (more than 1 ppm of shift) is connected with a stronger N ×/H bond due to a shorter distance between the interacting atoms (see Fig. 1 and following discussion). So, NMR experiments suggest the presence of a detectable N ×/H interaction in the 2-pyridyl-derivatives. Table 2 presents the results of ab initio calculations on the total molecular energy and permanent electric dipole moment of rotamers of Fig. 1, using the optimised geometry obtained at 3-21G level. All investigated rotamers are found to be planar. As regards the relative energy between rotamers, Table 2 shows that the correlation effects evaluated at MP2 level for the ethene derivatives are not very important, so they will not be discussed. We concentrate our attention on the results obtained using the basis set 6-31G*. The rotamer A is the most stable for all aza-compounds of Fig. 1. The
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energy difference between the rotamer A and B of E -2-StP and EE -2-PPhB is 1.35 kcal/mol, a value equal to the one calculated in Ref. [33] for the rotamers of E -1-(2-pyridyl)-2-(4-pyridyl)-ethene. It is to be noted that the intramolecular N×/H distance calculated for A and B rotamers of E -2˚ , respecStP, is very similar, 2.455 and 2.449 A tively, and it is shorter than the sum of the van der Waals radii of H and N atoms. The calculated C / H×/N angle in A and B rotamers is 998 and 738, respectively. While the N×/H distances are in agreement with the standard rules for the formation of hydrogen bond, the C /H ×/N angles are too small, in particular in the rotamer B. These results suggest that in both trans rotamers it is possible to consider the presence of intramolecular hydrogenbond type interaction(s) and, consequently, the relative stability of rotamers A and B is to be mainly attributed to the differences in geometry, as reported for E -1-(2-pyridyl)-2-(4-pyridyl)-ethene [33]. The E -2,2?-DPE(B) and E -2,2?-DPE(C) are less stable than E -2,2?-DPE(A) by 2.02 and 2.28 kcal/mol, respectively, and the energy difference between the ZE -2-PPhB(B) and ZE -2-PPhB(A), is even more pronounced because of the calculated value of 4.00 kcal/mol. In ZE -2-PPhB(A), the ˚, calculated intramolecular N ×/H distance is 2.266 A a shorter value in comparison with that found in trans conformations, and the calculated C /H ×/N angle is 122.758. This suggests the formation of a ‘real’ intramolecular hydrogen bond between N×/H in ZE -2-PPhB(A). With regard to the rotamers in trans geometry, in ZE -2-PPhB(A) there is a greater N ×/H interaction, as found from NMR data. Such theoretical results show that in the gas phase or in non-polar solvents the equilibrium between rotamers is markedly shifted towards the rotamer A and only in E -2-StP and EE -2-PPhB is a small presence of rotamer B possible, about 10% at room temperature. The polarity of the solvent may change the rotameric equilibrium, especially in E -2,2?-DPE where the rotamer B is polar while the others are non-polar because of the presence of an inversion centre. Also the proticity of the solvent may affect the conformational equilibrium, favouring the formation of intermolecular hydrogen bonds destabilizing the conform-
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Table 1 Chemical shifts (d ) and coupling constants (J ) for olefinic hydrogens of the investigated compounds in two solvents at room temperature
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Table 2 Ab initio results on molecular energy (Etot) and permanent electric dipole moment (d) of aza-compounds investigated Molecule
Etot(6-31G*) (kcal/mol)
Etot(6-31G*/MP2) (kcal/mol)
½d½ (D) (6-31G*)
E -2-StP (A) E -2-StP (B)
/347 097.42 /347 096.07
/348 231.65 /348 230.41
1.51 1.93
E -2,2?-DPE (A) E -2,2?-DPE (B) E -2,2?-DPE (C)
/357 135.71 /357 133.69 /357 133.43
/358 288.65 /358 286.53 /358 286.33
0.0 3.18 0.0
EE -2-PPhB (A) EE -2-PPhB (B)
/395 345.82 /395 344.47
1.41 1.99
ZE -2-PPhB (A) ZE -2-PPhB (B)
/395 342.89 /395 338.89
1.72 2.43
ations where the intra-molecular N×/H bonds are present. 3.2. Electronic spectrum
of the ortho derivatives 2-StP and 2,2?-DPE are markedly different from that of the parent hydrocarbon. Indeed, in these two compounds the stilbenic A band splits into two components, a main one (A? band) red-shifted and a secondary
The electronic spectra of aza-analogues of stilbene and DPhB have been reported and discussed in various papers [9,12 /18]. In this work, we re-analyzed the spectra by focusing our attention on the 26 000/40 000/cm spectral region, i.e. the region of the first UV absorption band (A band) of the parent hydrocarbons, E-S and DPhBs. First, we discuss the results on E -S and its azaderivatives, and then we pass to those of DPhBs and corresponding aza-analogues. For both sets of compounds, after a concise summary of the experimental observations, only the S/D/T/ CI calculations will be reported and discussed. The input geometry used for CI calculations was obtained after full optimization using the PM3 model. A planar geometry is found for almost all compounds, except for ZE -DPhB and ZE -4PPhB. 3.2.1. Stilbene and its aza-derivatives In Fig. 2, the UV absorption spectra in MCH/ 3MP at room temperature of the two StPs (part a) and the two DPEs (part b) in their trans configuration, are compared with the spectrum of E -S showing a strong A band (fexp /0.76) with maximum at 4.20 eV and a not much pronounced vibrational structure. It is evident that the spectra
Fig. 2. UV absorption spectra of: (a) 4-StP(2), 2-StP(3) and S(1); (b) 4,4?-DPE (2), 2,2?-DPE (3) and S(1) in trans configuration in MCH/3MP at room temperature.
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one (Aƒ band) blue-shifted relative to the A band of E -S. The intensity of the A? band is ‘similar’ to that of the A band of E -S, while that of the Aƒ band is smaller. More precisely, in 2,2?-DPE the oscillator strengths are fexp /0.48 and fexp /0.22 for A? and Aƒ bands, respectively. The Dn˜ between the two maxima increases on passing from 2-StP (Dn$4590=cm) ˜ to 2,2?-DPE (Dn$5940=cm) ˜ where two well resolved bands are observed. Such a splitting is not observed in the spectra of para derivatives (4-StP and 4,4?-DPE). Moreover, the A band of EE -4,4?-DPE is appreciably blueshifted with respect to that of E -S, while the A band of E -4-StP does not present significant changes. As is well known [24], in E -S the A band is due to the S0 0/S1 transition, 11Ag 0/11Bu in C2h symmetry classification or 1A 0/1B in Platt’s notation. The S1 (11Bu) excited state is well described by the 1[pH, pL] configuration involving the HOMO and LUMO delocalized MOs; for this reason, the A band is usually referred to as the conjugation band. Two quasi-degenerate transitions lie at higher energies in the same spectral region. The first one, S0 0/S2 (11Ag 0/21Bu), is symmetry-allowed yet very weak (ftheo /0.009), while the second, S0 0/S3 (11Ag 0/21Ag), is symmetry-forbidden. Owing to these characteristics these two transitions are hidden by the intense 11Ag 0/11Bu transition in the one-photon absorption spectrum of E -S. It is to be noted that the excited state 21Ag is not the excited state 1 A g ; which is found at higher energy. Fig. 3 shows the correlation diagram between the CS INDO S/D/T/CI energies of the Franck-Condon lowest excited electronic states of E -S and those of rotamers of trans StPs, Fig. 3(a), and DPEs, Fig. 3(b), for the isolated molecules. The values of the oscillator strengths of the more intense transitions are also reported near the horizontal lines representing the energy levels. From a general point of view, the theoretical spectrum obtained for the A rotamer of E -2-StP and E -2,2?-DPE is the one that best reproduces the UV absorption spectrum of these compounds (Fig. 2), confirming the results of ab initio calculations on total energy, i.e. a net prevalence of the rotamer A exists in the conformational equilibrium. More-
Fig. 3. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of E -S. The oscillator strengths of more intense transitions are indicated near to levels. (a) n -StPs; (b) n ,n ?-StPs. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited states.
over, these theoretical results predict the 1(n, p*) Franck-Condon excited states to lie above the 1 (pH, pL) state in the ortho derivatives and in the E -4-StP, whereas both 1(n, p*) levels are found below the 1(pH, pL) level in the E -4,4?-DPE. (Anyway, in view of their forbidden character, the 1(n, p*) states can be ignored as far as the interpretation of the UV absorption spectra is concerned.) That said, theory accounts quite well for the A-band splitting observed on passing from E -S to E -2-StP and E -2,2?-DPE. Briefly, the redshifted A? component appears to be due to the pH 0/pL transition (11A? 0/21A? in E -2-StP and 11Ag 0/11Bu in E -2,2?-DPE), while the blue-shifted Aƒ component involves a 1(p, p*) state deriving from the quasi-degenerate 21Bu-21Ag pair of E -S. All the observed spectral features, e.g. intensity
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ratios and energy splitting of the A? and Aƒ bands, are well reproduced by the calculations. In more detail, the observed splitting between A? and Aƒ band is 0.57 eV for E -2-StP, and 0.74 eV for E 2,2?-DPE. The calculated values for the A rotamers are 0.60 and 0.75 eV, respectively, in good agreement with the experimental finding. As for E -S, the state corresponding to 1 A having g doubly-excited character, is found above the states responsible for the Aƒ band. The different behaviour of the UV spectra of the para derivatives with respect to those of ortho derivatives is also fairly well described by the theoretical results of Fig. 3. In particular, the absence of splitting of the conjugation band is clearly explained by the fact that the energy difference between the intense pH 0/pL transition and the second allowed p 0/p* transition decreases on passing from E -S to E -4,4?-DPE, while it increases on going from E -S to ortho derivatives. The blue shift of the A band of E -4,4?-DPE with respect to that of E -S is underestimated by the calculation. Solvent effects observed in the absorption and emission spectra of E -2-StP, E -4-StP and E -2,2?DPE support a1(p, p*) nature of the lowest singlet excited state, while they suggest a1(n, p*) nature of S1 for E -4,4?-DPE. The CS INDO CI main results on energy levels of the aza-derivatives of stilbene plunged in a dielectric medium of high dielectric constant (o /40) indicate a significant blue shift on the 1(n, p*) states, about 0.2 eV, and a smaller red shift on the 1(pH,pL) state. Consequently our calculations indicate for E -4,4?-DPE, in solvent of high polarity, the presence of a crossing between the 1(n, p*) states and 1(pH, pL) state is that now the S1 state. Such theoretical results are in agreement with the experimental finding on the blue shift of the fluorescence maximum on going from solvent of low polarity to protic solvent (unpublished results). 3.2.2. Diphenylbutadiene and its aza-derivatives The absorption spectra of 2- and 4-PPhB together with those of DPhB in EE and in ZE configurations are reported in Fig. 4(a) and (b), respectively. In the ZE -DPhB, the A band is blueshifted with respect to that of EE -DPhB of about
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1570/cm, the intensity is lowered (fexp(EE DPhB) /1.11, fexp(ZE -DPhB) /0.78), and the vibrational structure disappears. This behaviour is typical of the cis forms of a,v-diphenylpolyenes having sterical interactions [34,35]. Analogously to the spectra of StPs, the spectra of the EE - and ZE 2-PPhBs show a splitting of the A band, if compared to the spectrum of the corresponding hydrocarbon. The first (A?) band is intense and red-shifted, while the second (Aƒ) band is blueshifted with respect to that of the parent hydrocarbon. In particular, the comparison between Fig. 4(a) and (b) shows that A? band is more redshifted, with respect to the A band, in ZE configuration. The Aƒ band, that is only a weak shoulder in EE -2-PPhB, is more evident in ZE -2PPhB. fexp(A?)/0.73 and fexp(Aƒ)/0.27 were found for ZE -2PPhB. The UV absorption spectrum of 4-PPhBs is similar to that of the parent hydrocarbon both in EE and ZE configurations.
Fig. 4. UV absorption spectra of 4-PPhb (2), 2-PPhB (3) and that of DPhB (1) in MCH/3MP at room temprature in (a) EE configuration and (b) ZE configuration.
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Fig. 5. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of EE -DPhB. The oscillator strengths of more intense transitions are indicated near to levels. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited state.
In the UV absorption spectrum of EE -DPhB, the strong A band, with resolved vibrational structure and maximum at 3.76 eV (Fig. 4(a)), is related to the S0 0/S1, (11Ag 0/11Bu) transition having (pH, pL*) character. HOMO and LUMO are delocalized, thus the A band is the conjugation band, like in stilbene. Because of the presence of four delocalized MOs (HOMO, HOMO/1, LUMO, LUMO/1), several p 0/p* transitions, symmetry-forbidden or very weak, some with charge transfer character, are found in the calculated electronic spectrum between the tail of the A band and the second absorption band starting at 250 nm. The p) character. HOMO and LUMO are delocalized, thus the A band is the conjugation band, like in stilbene. Because of the presence of four delocalized MOs (HOMO, HOMO/1, LUMO, LUMO/1), several p 0/p* transitions, symmetry-forbidden or very weak, some with charge transfer character, are found in the calculated electronic spectrum between the tail of the A band and the second absorption band starting at 250 nm. The pH 0/pL1 (11Ag 0/31Ag) transition is of particular interest for the discussion below on the spectra of aza-analogues of DPhB. This transition is symmetry-forbidden in the EE DPhB but allowed in aza-derivatives without inversion centre. Since the pH 0/pL1 transition involves delocalized MOs, when it is allowed, it can acquired moderate strength. The 31Ag state is not much influenced by doubly-excited configura-
tions, while the upper 1Ag excited states are more sensitive to poly-excited configurations. With the suitable variation for a non-planar molecule, a similar classification is found for the UV absorption spectrum of ZE -DPhB of Fig. 4(b), and hence it is not reported. Fig. 5 shows the correlation diagram of the Franck-Condon lowest CS INDO S/D/T/CI singlet energy levels of EE -DPhB with those of the two rotamers, the stable and unstable forms of EE -2-PPhB and that of EE -4-PPhB, obtained for the isolated molecules. The oscillator strengths are reported on levels of intense transitions. First of all, the theoretical results show that the S1 state is 1 (pH, pL) whether in ortho or para derivatives, i.e. the 1(n, p*) state is always above S1. Secondly, the calculated spectrum of the A rotamer of EE -2PPhB is the only one in agreement with the UV absorption spectrum of the ortho derivative. In particular, only the calculated spectra for A rotamer of the ortho derivative shows the presence of a second transition of moderate intensity in the region of the Aƒ band. It is possible to note that the calculated red shift of the A band of EE DPhB, with respect to that of E-S, is of 0.49 eV, in good agreement with the experimental value of 0.44 eV. Fig. 6 shows the correlation diagram of the Franck-Condon lowest CS INDO S/D/T/CI singlet energy levels of ZE -DPhB together with those of ZE -2-PPhB (A and B rotamers) and ZE -
Fig. 6. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of ZE -DPhB. The oscillator strengths of more intense transitions are indicated near to levels. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited state.
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4-PPhB, obtained by considering the isolated molecule model. The oscillator strengths are reported on levels giving intense transitions. Also for these compounds, the calculated first singlet excited state has (pH, pL) character both in ortho and para derivatives. Again, Fig. 6 clearly shows that the CS INDO CI electronic spectrum of the rotamer A of ZE -2-PPhB is the only one in agreement with the UV absorption spectrum of the ortho derivative. For A rotamer, the calculated red shift of the first absorption band is 0.27 eV, in very good agreement with the experimental value of about 0.26 eV. The calculated energy difference between S3 and S1 excited states is 0.58 eV, while the experimental value corresponding to energy difference between maximum of the Aƒ and A? band is :/0.64 eV. Considering the blue shift of the conjugation band on going from EE -DPhB to ZE -DPhB, the calculated value is about half of the experimental one. A significant decrease of intensity is also calculated (Dftheo /0.48), in agreement with experimental finding (Dfexp /0.33). It can be mentioned that the spectral behaviour of the ortho derivative(s) in ZE configuration is related to the planarity of the rotamers, while the parent hydrocarbon is not planar. The poly-excited configurations have not much importance on the excited states responsible for A? and Aƒ band of azaderivatives of EE - and ZE -configurations. Experimental studies of the solvent effect on the spectral, photophysical and photochemical behaviour of the EE and ZE isomers are not very numerous; in particular, they do not stem from methodical work. It is found that the UV absorption spectrum of EE -DPhB is red-shifted with increasing solvent polarizability [36,37] as well as the emission spectrum [36]. The absorption spectrum of ZE -2-PPhB shows a blue shift of the maximum of the first absorption band in protic solvents [11]. The substitution of an intramolecular hydrogen bond with an intermolecular one destroys the planarity of the ZE -2-PPhB, returning toward the spectrum of ZE -DPhB. Moreover, the calculated solvent effect on the aza-derivatives of DPhBs is similar to that found for the azaderivatives of E -S, i.e. the 1(pH, pL) excited state is red-shifted while the 1(n, p*) ones are more blueshifted. Because of this, no crossing between the S1
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and S2 states is possible. The lack of crossing between the first and second singlet excited state strengthens the hypothesis that the solvent effect on the absorption and emission spectra of the azaderivatives and the parent hydrocarbon should be similar.
4. Conclusions The simple ab initio quantum chemical model used in this work for molecular energies, i.e. geometry optimization at 3-21G level followed by the energy calculation with a more extended basis set (6-31G*), seems to be suitable for the description of rotational isomerism of the investigated compounds. Moreover, the combined theoretical and experimental study of rotamerism and electronic spectra of ortho and para aza-derivatives of E -S and DPhBs allowed us to reach some important conclusions. About rotamerism, our ab initio calculations on ground electronic state in conjugation with theoretical analysis of the conjugation band, in particular its splitting, clearly indicate that the stable forms of the ortho azaderivatives of E -S and DPhBs are the rotamers A of Fig. 1. The ab initio calculations and the 1HNMR spectra suggest the presence of N ×/H intramolecular hydrogen bond in ZE -2-PPhB. In trans derivatives, we prefer to speak of intramolecular hydrogen-bond type interactions. Considering the Franck-Condon excited electronic states of aza-derivatives of E -S and DPhBs, the CS INDO S/D/T/CI results indicate, apart from the E -4,4?-DPE, that the S1 state has (pH, pL) character also in non-polar solvents. As regards E 4,4?-DPE, and in agreement with the experimental results, the theoretical calculations show that the crossing between the excited states of different character is possible in a solvent of high o , and thus the solvent may have an important role on the spectroscopic and photophysics properties of this molecule. No involvement of the 1 A excited state is g found in the spectral region analyzed.
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Acknowledgements The authors are grateful to Prof. G. Bartocci, Prof. U. Mazzucato and Prof. F. Momicchioli for stimulating discussions and assistance in the course of this work. Thanks are also due to Prof. G. Ponterini for help concerning the solvent effect on E -4,4?-DPE. This research was funded by the Ministero per l’Universita` e la Ricerca Scientifica e Tecnologica (Rome) and the Universities of Modena and Perugia in the framework of the Programmi di Ricerca di Interesse Nazionale (project: Mechanisms of Photoinduced Processes in Organized Systems).
References [1] U. Mazzucato, F. Momicchioli, Chem. Rev. 91 (1991) 1679. [2] G. Bartocci, A. Spalletti, U. Mazzucato, in: J. Waluk (Ed.), Conformational Analysis of Molecules in Excited States (Chapter 5), Wiley-VCH, 2000. [3] G. Gennari, G. Galiazzo, P. Bortolus, J. Photochem. Photobiol. A 43 (1988) 293. [4] (a) G. Favaro, U. Mazzucato, F. Masetti, J. Phys. Chem. 77 (1973) 601; (b) G. Bartocci, P. Bortolus, U. Mazzucato, J. Phys. Chem. 77 (1973) 605. [5] G. Bartocci, U. Mazzucato, Chem. Phys. Lett. 47 (1977) 541. [6] G. Bartocci, U. Mazzucato, J. Luminesc. 27 (1982) 163. [7] G. Bartocci, F. Masetti, U. Mazzucato, S. Dellonte, G. Orlandi, Spectrochim. Acta 38A (1982) 729. [8] U. Mazzucato, Pure Appl. Chem. 54 (1982) 1705. [9] (a) G. Marconi, G. Bartocci, U. Mazzucato, A. Spalletti, F. Abbate, L. Angeloni, E. Castellucci, Chem. Phys. 196 (1995) 383; (b) G. Marconi, G. Bartocci, U. Mazzucato, A. Spalletti, F. Abbate, L. Angeloni, E. Castellucci, Am. Inst. Phys Conf. Proc. 364 (1996) 175. [10] L. Angeloni, E. Castellucci, G. Bartocci, U. Mazzucato, A. Spalletti, G. Marconi, Gazz. Chim. Ital. 126 (1996) 609. [11] G. Bartocci, G. Galiazzo, U. Mazzucato, A. Spalletti, Phys. Chem. Chem. Phys. 3 (2001) 379. [12] G. Favini, S. Fasone, M. Raimondi, Gazz. Chim. Ital. 97 (1967) 1434. [13] H.-H. Perkampus, J.V. Knop, A. Knop, Ber. Bunsenges. Phys. Chem. 72 (1968) 623.
[14] G. Distefano, U. Mazzucato, A. Modelli, S. Pignataro, G. Orlandi, J. Chem. Soc. Faraday Trans. II 71 (1975) 1583. [15] A.R. Gregory, W. Siebrand, D.F. Williams, J. Am. Chem. Soc. 101 (1976) 1903. [16] G. Orlandi, G. Poggi, G. Marconi, J. Chem. Soc. Faraday Trans. II 76 (1980) 598. [17] G. Marconi, G. Orlandi, G. Poggi, J. Photochem. 18 (1982) 329. [18] M.M. Hamed, R.H. Abu-Eittah, A.A. Mohamed, J. Chem. Soc. Faraday Trans. 88 (1992) 955. [19] M. Tachon, E. Davies, M. Lamotte, K.A. Muszkat, T. Wismonntski-Knittel, J. Phys. Chem. 98 (1994) 11870. [20] (a) S. Marguet, J.C. Mialocq, P. Millie, G. Berthier, F. Momicchioli, Chem. Phys. 160 (1992) 265; (b) F. Momicchioli, I. Baraldi, A. Carnevali, M. Caselli, G. Ponterini, Coord. Chem. Rev. 125 (1993) 301. [21] M.T. Allen, D.G. Whitten, Chem. Rev. 89 (1989) 1691. [22] J.B. Birks, Photophysics of Aromatic Molecules, Wiley, London, 1970. [23] HyperChem, Computational Chemistry Manual, Hypercube, Inc., 1996. [24] F. Momicchioli, I. Baraldi, M.C. Bruni, Chem. Phys. 82 (1983) 229. [25] F. Momicchioli, I. Baraldi, A. Carnevali, G. Ponterini, in: Y. Ellinger, M. Defranceschi (Eds.), Strategies and Applications in Quantum Chemistry, Kluwer, 1996, pp. 379 / 399. [26] J.J.P. Stewart, J. Comp. Chem. 10 (1989) 221. [27] G. Del Re, Theoret. Chim. Acta 1 (1963) 188. [28] (a) K. Ohno, Theoret. Chim. Acta 2 (1964) 219; (b) G. Klopman, J. Am. Chem. Soc. 86 (1964) 4550. [29] D. Heidrich, U. Goering, W. Forster, C. Weiss, Tetrahedron 35 (1979) 651. [30] I. Jano, C.R. Acad. Sci. Paris 261 (1965) 103. [31] I. Baraldi, A. Carnevali, M. Caselli, F. Momicchioli, G. Ponterini, G. Berthier, J. Mol. Structure (Theochem) 330 (1995) 403. [32] R.K. Harris (Ed.), Nuclear Magnetic Resonance Spectroscopy, Longman, Harlow, 1987, pp. 204 /205. [33] S. Melandri, G. Maccaferri, P.G. Favero, W. Caminati, G. Orlandi, F. Zerbetto, J. Chem. Phys. 107 (1997) 1073. [34] F. Momicchioli, I. Baraldi, M.C. Bruni, J. Chem. Soc. Faraday Trans. II 68 (1972) 1556. [35] I. Baraldi, F. Momicchioli, M.C. Bruni, J. Chem. Soc. Faraday Trans. II 68 (1972) 1571. [36] S.P. Velsko, G.R. Fleming, J. Chem. Phys. 76 (1982) 3553. [37] S.E. Wallace-Williams, S. Møller, R.A. Goldbeck, K.M. Hanson, J.W. Lewis, W. Atom Yee, D.S. Kliger, J. Phys. Chem. 97 (1993) 9587.
Rotamerism and electronic spectra of aza-derivatives of stilbene and diphenylbutadiene. A combined experimental and theoretical study Ivan Baraldi a,*, Anna Spalletti b, Davide Vanossi a a
Dipartimento di Chimica, Universita` di Modena e Reggio Emilia, Via Campi 183, 41100 Modena, Italy b Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy Received 28 February 2002; accepted 25 March 2002
Abstract The experimental results on the rotameric equilibrium and electronic spectra of aza-derivatives of trans -stilbene and 1,4-diphenylbutadiene, have been rationalized by a theoretical study which combines simple ab initio calculations of molecular energies for the ground state with a theoretical analysis of the splitting of the conjugation band developed at CS INDO CI level. All results indicate that the stable conformer of each ortho aza-derivative is that corresponding to A species. As suggested by the 1H-NMR experiments, the ab initio geometry of ZE -2-pyridylphenylbutadiene is consistent with the presence of the N ×/H intramolecular hydrogen bond. As regards the Franck-Condon excited states of aza-derivatives, our theoretical results show that the first singlet excited state has (pH, pL) character in all compounds except for E -4,4?-dipyridylethene, where S1 has (n, p*) character in non-polar solvent. In this last compound, the theoretical study of solvatochromism indicates a crossing between the 1(n, pL) and 1(pH, pL) states which occurs in solvents of high polarity. The inclusion of the most important doubly- and triply-excited configurations in the CI calculations shows that the 1 A g excited state is above the spectral region analyzed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: aza-Derivatives of stilbene and 1,4-diphenylbutadiene; Ab initio and CS INDO CI calculations; Electronic spectrum; 1HNMR; Rotamerism
1. Introduction The replacement of one or more CH groups with nitrogen atoms in the phenyl rings of a,v-
* Corresponding author. Tel.: /39-59-2055085; fax: /3959-373543 E-mail address: [email protected] (I. Baraldi).
diphenylpolyenes, including stilbene (S), generates aza-compounds, isoelectronics to the parent hydrocarbon, that display important differences in their physical-chemistry properties, namely the presence of 1,3(n, p*) states of low energy, a significant permanent electric dipole moment (d) in the compounds without inversion centre, the formation of new conformational equilibrium (rotational isomerism) [1,2], and the possibility to
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 1 7 - 8
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form intramolecular hydrogen-type bonds [3]. All such aspects can modify the spectroscopic, photophysical and photochemical properties of the azaderivatives with respect to those of the parent hydrocarbon. Several experimental techniques and theoretical models have been applied to study the rotational isomerism, not only in aza-derivatives, but also in their parent hydrocarbons (see Refs. [1,2] for recent reviews). Much work has been done based on spectroscopic and fluorimetric measurements, in particular through the analysis of the lexc dependence of fluorescence spectra as well as by the effect of temperature and solvent. In the case of n -styrylpyridines (n-StPs n /2, 3, 4) and n,n?dipyridylethenes (n,n ?-DPEs n,n ?/2, 3, 4), where the position of the nitrogen was found to play an important role on electronic spectrum and photobehaviour [4 /10], the characterization of rotamers was investigated but no satisfactory conclusions were obtained [1]. The molecules bearing the nitrogen in the meta position (with respect to the double bond) display a photobehaviour very similar to stilbene, while those bearing the heteroatom in ortho and para positions show reduced fluorescence quantum yields and lifetimes, their S1 state deactivating significantly via S1 0/S0 internal conversion (IC). More recently, the study of the excited state properties and rotamerism was extended to asymmetric aza-analogues of 1,4-diphenylbutadiene (DPhB), n-pyridylphenylbutadienes (n-PPhBs n /2, 3, 4) [11]. Also in this case, the EE isomers bearing the heteroatom in meta position behave similarly to the corresponding hydrocarbon. On the contrary, a marked decrease in the yield of the radiative decay and an increase of the IC yield are operative in the other two isomers with the nitrogen in ortho and para positions. Concerning the presence of rotamers in the ground electronic state, no clear indication has been reached. From a theoretical point of view, several papers have appeared on the rotational isomerism and electronic spectrum of aza-analogues of stilbene [12 /18]. The main aspects of these works were the interpretation of the electronic spectra in relation to the rotational isomerism, and the characterization of the first singlet excited state (S1). A brief
summary of the recent works may be useful. Marconi et al. [17] carried out a quantum chemistry study on 2-StP, 3-StP, 2,2?-DPE and 3,3?DPE, using the MNDO method for the groundstate properties, without geometry optimisation, and INDO/S method to evaluate the electronic spectrum. For 2-StP and 2,2?-DPE they found that the rotamer A (see Fig. 1 for our notation of rotamers) was the most stable one, although the energy differences between rotamers were probably overestimated. Moreover, as concerns the electronic spectrum, they confirm the (pH, pL) character of S1, as shown in a previous work [16]. More recently, Hamed et al. [18] carried out a theoretical (INDO/S) study on the spectra of StPs and other heterocompounds, but only one rotamer was considered and, in consequence, no discussion on rotamerism was presented. To our knowledge, no theoretical study has been made on azaanalogues of DPhB. A spectroscopic investigation on 2,2?-DPE in Spolskii matrices appeared some years ago [19]. The experimental study was combined with AM1 computation of the ground state potential energy curve for the symmetrical conrotatory twisting of the pyridyl moieties around the exocyclyclic Ca(a?) /pyridyl single bonds. It was suggested that the trapped rotamer in the n -alkane matrices is the rotamer A (according to our notation in Fig. 1). In conclusion, while the photophysical and photochemical properties of aza-derivatives of stilbene and DPhB have been widely investigated, the aspect of the rotameric equilibrium, even if always well present, did not reach a completely adequate solution. Also some questions concerning the electronic spectra of aza-derivatives are still open, especially that connected with the effect of rotational isomerism and solvent. For all these reasons, in the present paper we report the results of a combined experimental and theoretical study carried out with the aim of elucidating the rotameric behaviour and the electronic spectra of E -2-StP, E -2,2?-DPE, EE -2-PPhB and ZE -2PPhB. Fig. 1 shows the possible rotameric equilibrium for the molecules investigated. Even if the E -4-StP, E -4,4?-DPE, EE -4-PPhB and ZE -4PPhB have not rotational isomerism, they have been also included in this work because their
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excited state properties are very interesting for purposes of comparison. We studied the rotational isomerism by simple ab initio calculations of geometry and total molecular energy of rotamers; moreover, we compared the UV absorption spectra, particularly the splitting of the conjugation band, with CS INDO CI electronic spectra. The spectra calculations were first performed in gas phase approximation and were then corrected for the dielectric solvent effects, according to the simple virtual charge model [20], to describe the dependence of the spectral behaviour on the solvent polarity. The presence of interactions
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between N and ethenic H (hydrogen-type bond), suggested by calculations, was confirmed by 1HNMR experiments. In this paper we also pay attention to the relative position of the symmetry-allowed 11Bu state with respect to the symmetry-forbidden 1 A g state, in the C2h symmetry. The 11Bu /1 A g energy gap is a function of polyene length, and has an important role on the photophysics and photochemistry of (a,v-diphenyl)polyenes [21]. For this reason, we performed CI calculations adding doubly- and triply-excited configurations to the
Fig. 1. Rotamer equilibria of the aza-analogues of E -S and DPhBs.
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usual CI approach limited to singly excited configurations. The study allowed us to draw some important conclusions about the rotamerism and electronic spectra of aza-compounds investigated.
2. Experimental and computational details 2.1. Experimental All the investigated compounds were synthesized for previous works. The absorption spectra, already published [6,9,11], are now reported in the same solvent, a mixture of methylcyclohexane and 3-methylpentane (MCH/3MP, 9/1, v/v), for comparison purposes. They were recorded by a Lambda 800 double beam spectrophotometer. The experimental oscillator strengths were calculated by equation: fexp 4:39109 fo(n)d ˜ n˜ [22]. 1 H-NMR spectra were carried out in a nonpolar solvent (deuterated benzene) and in a protic solvent (deuterated methanol) on Bruker AC 200 and DMX 500 spectrometers, using TMS as reference. 2.2. Quantum chemistry calculations Ab initio calculations on the ortho aza-derivatives of trans stilbene (E -S) and DPhBs were executed using the HyperChem computational package [23]. For each molecule of Fig. 1, after the complete optimisation of geometry at 3-21G level, the total molecular energy was obtained using the basis set 6-31G*. In addition, for the azaanalogues of E -S, the correlation effects at MP2 level were also evaluated. The quantum chemical calculations of the electronic spectra of all the studied aza-compounds and their parent hydrocarbons (E -S, DPhBs) were performed by the CS INDO CI method [24,25] using geometries obtained after complete optimisation according to PM3 Hamiltonian model [26]. The characteristic of the CS INDO is the use of a hybrid atomic orbital basis set and of specific screening constants (kij i, j/s, p, n) for the various chemically distinct interactions (s /s, s /p, p /p, etc.). In this work the
screening constants, entering the calculation of the resonance integrals, were given the values kss /1, ksp /0.65, kpp /0.50, kns /0.72, knp /0.60, knn /0.68, chosen according to criteria previously discussed [24,25]. The hybrid atomic orbitals were determined by the maximum overlap criterion [27]. All other details of the CS INDO calculations were as in the original version [24]; in particular the gAB integrals were calculated by the Ohno /Klopman formula [28]. The so obtained sets of CS INDO MOs were the basis for variational configuration interaction (CI) calculations. Both a singly-excited CI (S-CI) and a more extended CI, involving double (D) and triple (T) excitations (S/D/T/CI), were carried out to calculate the electronic absorption spectra. The poly-excited configurations were included to study their effect on the electronic states of different symmetry, and were selected by a perturbative criterion with respect to a suitable set of monoexcited reference configurations. The parameter h , defining the threshold of the perturbative selection of D and T configurations, was set to a value near or equal to 0.05. In all CI calculations, the MO active space included all p, p* and n type orbitals. The solvation energy was evaluated within the virtual charge model, using a slightly simplified version of the formula proposed by Heidrich et al. [29] 1 1 Esol 1 2 o X 2 X X QA QA QB (1) Reff A A B("A) rAB Reff In Eq. (1) o is the static dielectric constant of the solvent, QA(B) is the atomic charge of atom A(B), Reff is an average effective atomic radius, here ˚ , and rAB is the interatomic taken equal to 4 A distance. For each electronic state considered, the total energy of the solvated molecule was taken equal to the sum of the energy of the isolated molecule and the solvation energy Esol. It is worth noting that the evaluation of Esol by formulas like that of Eq. (1) has been shown to be basically consistent with the ZDO approximation [30]. Adopting the above-described procedure, we have obtained reasonable descriptions of the di-
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electric solvent effects on the ground and excited states of cyanines and other dyes [20,31].
3. Results and discussion 3.1. Rotamerism and hydrogen bonding The results of NMR spectra of the investigated compounds are reported in Table 1. For all investigated hydrocarbons (E -S, EE -DPhB and ZE -DPhB) and their aza-derivatives (E -4-StP, E 4,4?-DPE, EE -4-PPhB and ZE -4-PPhB) with the chain linked in para position with respect to nitrogen, the effect of deuterated benzene is that of reducing the chemical shift (d ) of all the ethenic hydrogens because of the ring current. Only for the 2-derivatives (E -2-StP, E -2,2?-DPE, EE -2-PPhB and ZE -2-PPhB) does the change from a protic to a non-polar solvent have remarkable effects. Indeed, the ethenic proton near the nitrogen shows more or less strong de-shielding (underlined figures in Table 1) while the others are shifted towards smaller d . This behaviour is in agreement with the presence of an intramolecular hydrogenbond type interaction, in non-polar solvent, between the nitrogen and the nearest ethenic proton [32], that is reduced or broken in polar and protic solvent in favour of stronger intermolecular hydrogen bonds. In the case of ZE -2-PPhB, the huge effect (more than 1 ppm of shift) is connected with a stronger N ×/H bond due to a shorter distance between the interacting atoms (see Fig. 1 and following discussion). So, NMR experiments suggest the presence of a detectable N ×/H interaction in the 2-pyridyl-derivatives. Table 2 presents the results of ab initio calculations on the total molecular energy and permanent electric dipole moment of rotamers of Fig. 1, using the optimised geometry obtained at 3-21G level. All investigated rotamers are found to be planar. As regards the relative energy between rotamers, Table 2 shows that the correlation effects evaluated at MP2 level for the ethene derivatives are not very important, so they will not be discussed. We concentrate our attention on the results obtained using the basis set 6-31G*. The rotamer A is the most stable for all aza-compounds of Fig. 1. The
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energy difference between the rotamer A and B of E -2-StP and EE -2-PPhB is 1.35 kcal/mol, a value equal to the one calculated in Ref. [33] for the rotamers of E -1-(2-pyridyl)-2-(4-pyridyl)-ethene. It is to be noted that the intramolecular N×/H distance calculated for A and B rotamers of E -2˚ , respecStP, is very similar, 2.455 and 2.449 A tively, and it is shorter than the sum of the van der Waals radii of H and N atoms. The calculated C / H×/N angle in A and B rotamers is 998 and 738, respectively. While the N×/H distances are in agreement with the standard rules for the formation of hydrogen bond, the C /H ×/N angles are too small, in particular in the rotamer B. These results suggest that in both trans rotamers it is possible to consider the presence of intramolecular hydrogenbond type interaction(s) and, consequently, the relative stability of rotamers A and B is to be mainly attributed to the differences in geometry, as reported for E -1-(2-pyridyl)-2-(4-pyridyl)-ethene [33]. The E -2,2?-DPE(B) and E -2,2?-DPE(C) are less stable than E -2,2?-DPE(A) by 2.02 and 2.28 kcal/mol, respectively, and the energy difference between the ZE -2-PPhB(B) and ZE -2-PPhB(A), is even more pronounced because of the calculated value of 4.00 kcal/mol. In ZE -2-PPhB(A), the ˚, calculated intramolecular N ×/H distance is 2.266 A a shorter value in comparison with that found in trans conformations, and the calculated C /H ×/N angle is 122.758. This suggests the formation of a ‘real’ intramolecular hydrogen bond between N×/H in ZE -2-PPhB(A). With regard to the rotamers in trans geometry, in ZE -2-PPhB(A) there is a greater N ×/H interaction, as found from NMR data. Such theoretical results show that in the gas phase or in non-polar solvents the equilibrium between rotamers is markedly shifted towards the rotamer A and only in E -2-StP and EE -2-PPhB is a small presence of rotamer B possible, about 10% at room temperature. The polarity of the solvent may change the rotameric equilibrium, especially in E -2,2?-DPE where the rotamer B is polar while the others are non-polar because of the presence of an inversion centre. Also the proticity of the solvent may affect the conformational equilibrium, favouring the formation of intermolecular hydrogen bonds destabilizing the conform-
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Table 1 Chemical shifts (d ) and coupling constants (J ) for olefinic hydrogens of the investigated compounds in two solvents at room temperature
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Table 2 Ab initio results on molecular energy (Etot) and permanent electric dipole moment (d) of aza-compounds investigated Molecule
Etot(6-31G*) (kcal/mol)
Etot(6-31G*/MP2) (kcal/mol)
½d½ (D) (6-31G*)
E -2-StP (A) E -2-StP (B)
/347 097.42 /347 096.07
/348 231.65 /348 230.41
1.51 1.93
E -2,2?-DPE (A) E -2,2?-DPE (B) E -2,2?-DPE (C)
/357 135.71 /357 133.69 /357 133.43
/358 288.65 /358 286.53 /358 286.33
0.0 3.18 0.0
EE -2-PPhB (A) EE -2-PPhB (B)
/395 345.82 /395 344.47
1.41 1.99
ZE -2-PPhB (A) ZE -2-PPhB (B)
/395 342.89 /395 338.89
1.72 2.43
ations where the intra-molecular N×/H bonds are present. 3.2. Electronic spectrum
of the ortho derivatives 2-StP and 2,2?-DPE are markedly different from that of the parent hydrocarbon. Indeed, in these two compounds the stilbenic A band splits into two components, a main one (A? band) red-shifted and a secondary
The electronic spectra of aza-analogues of stilbene and DPhB have been reported and discussed in various papers [9,12 /18]. In this work, we re-analyzed the spectra by focusing our attention on the 26 000/40 000/cm spectral region, i.e. the region of the first UV absorption band (A band) of the parent hydrocarbons, E-S and DPhBs. First, we discuss the results on E -S and its azaderivatives, and then we pass to those of DPhBs and corresponding aza-analogues. For both sets of compounds, after a concise summary of the experimental observations, only the S/D/T/ CI calculations will be reported and discussed. The input geometry used for CI calculations was obtained after full optimization using the PM3 model. A planar geometry is found for almost all compounds, except for ZE -DPhB and ZE -4PPhB. 3.2.1. Stilbene and its aza-derivatives In Fig. 2, the UV absorption spectra in MCH/ 3MP at room temperature of the two StPs (part a) and the two DPEs (part b) in their trans configuration, are compared with the spectrum of E -S showing a strong A band (fexp /0.76) with maximum at 4.20 eV and a not much pronounced vibrational structure. It is evident that the spectra
Fig. 2. UV absorption spectra of: (a) 4-StP(2), 2-StP(3) and S(1); (b) 4,4?-DPE (2), 2,2?-DPE (3) and S(1) in trans configuration in MCH/3MP at room temperature.
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one (Aƒ band) blue-shifted relative to the A band of E -S. The intensity of the A? band is ‘similar’ to that of the A band of E -S, while that of the Aƒ band is smaller. More precisely, in 2,2?-DPE the oscillator strengths are fexp /0.48 and fexp /0.22 for A? and Aƒ bands, respectively. The Dn˜ between the two maxima increases on passing from 2-StP (Dn$4590=cm) ˜ to 2,2?-DPE (Dn$5940=cm) ˜ where two well resolved bands are observed. Such a splitting is not observed in the spectra of para derivatives (4-StP and 4,4?-DPE). Moreover, the A band of EE -4,4?-DPE is appreciably blueshifted with respect to that of E -S, while the A band of E -4-StP does not present significant changes. As is well known [24], in E -S the A band is due to the S0 0/S1 transition, 11Ag 0/11Bu in C2h symmetry classification or 1A 0/1B in Platt’s notation. The S1 (11Bu) excited state is well described by the 1[pH, pL] configuration involving the HOMO and LUMO delocalized MOs; for this reason, the A band is usually referred to as the conjugation band. Two quasi-degenerate transitions lie at higher energies in the same spectral region. The first one, S0 0/S2 (11Ag 0/21Bu), is symmetry-allowed yet very weak (ftheo /0.009), while the second, S0 0/S3 (11Ag 0/21Ag), is symmetry-forbidden. Owing to these characteristics these two transitions are hidden by the intense 11Ag 0/11Bu transition in the one-photon absorption spectrum of E -S. It is to be noted that the excited state 21Ag is not the excited state 1 A g ; which is found at higher energy. Fig. 3 shows the correlation diagram between the CS INDO S/D/T/CI energies of the Franck-Condon lowest excited electronic states of E -S and those of rotamers of trans StPs, Fig. 3(a), and DPEs, Fig. 3(b), for the isolated molecules. The values of the oscillator strengths of the more intense transitions are also reported near the horizontal lines representing the energy levels. From a general point of view, the theoretical spectrum obtained for the A rotamer of E -2-StP and E -2,2?-DPE is the one that best reproduces the UV absorption spectrum of these compounds (Fig. 2), confirming the results of ab initio calculations on total energy, i.e. a net prevalence of the rotamer A exists in the conformational equilibrium. More-
Fig. 3. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of E -S. The oscillator strengths of more intense transitions are indicated near to levels. (a) n -StPs; (b) n ,n ?-StPs. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited states.
over, these theoretical results predict the 1(n, p*) Franck-Condon excited states to lie above the 1 (pH, pL) state in the ortho derivatives and in the E -4-StP, whereas both 1(n, p*) levels are found below the 1(pH, pL) level in the E -4,4?-DPE. (Anyway, in view of their forbidden character, the 1(n, p*) states can be ignored as far as the interpretation of the UV absorption spectra is concerned.) That said, theory accounts quite well for the A-band splitting observed on passing from E -S to E -2-StP and E -2,2?-DPE. Briefly, the redshifted A? component appears to be due to the pH 0/pL transition (11A? 0/21A? in E -2-StP and 11Ag 0/11Bu in E -2,2?-DPE), while the blue-shifted Aƒ component involves a 1(p, p*) state deriving from the quasi-degenerate 21Bu-21Ag pair of E -S. All the observed spectral features, e.g. intensity
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ratios and energy splitting of the A? and Aƒ bands, are well reproduced by the calculations. In more detail, the observed splitting between A? and Aƒ band is 0.57 eV for E -2-StP, and 0.74 eV for E 2,2?-DPE. The calculated values for the A rotamers are 0.60 and 0.75 eV, respectively, in good agreement with the experimental finding. As for E -S, the state corresponding to 1 A having g doubly-excited character, is found above the states responsible for the Aƒ band. The different behaviour of the UV spectra of the para derivatives with respect to those of ortho derivatives is also fairly well described by the theoretical results of Fig. 3. In particular, the absence of splitting of the conjugation band is clearly explained by the fact that the energy difference between the intense pH 0/pL transition and the second allowed p 0/p* transition decreases on passing from E -S to E -4,4?-DPE, while it increases on going from E -S to ortho derivatives. The blue shift of the A band of E -4,4?-DPE with respect to that of E -S is underestimated by the calculation. Solvent effects observed in the absorption and emission spectra of E -2-StP, E -4-StP and E -2,2?DPE support a1(p, p*) nature of the lowest singlet excited state, while they suggest a1(n, p*) nature of S1 for E -4,4?-DPE. The CS INDO CI main results on energy levels of the aza-derivatives of stilbene plunged in a dielectric medium of high dielectric constant (o /40) indicate a significant blue shift on the 1(n, p*) states, about 0.2 eV, and a smaller red shift on the 1(pH,pL) state. Consequently our calculations indicate for E -4,4?-DPE, in solvent of high polarity, the presence of a crossing between the 1(n, p*) states and 1(pH, pL) state is that now the S1 state. Such theoretical results are in agreement with the experimental finding on the blue shift of the fluorescence maximum on going from solvent of low polarity to protic solvent (unpublished results). 3.2.2. Diphenylbutadiene and its aza-derivatives The absorption spectra of 2- and 4-PPhB together with those of DPhB in EE and in ZE configurations are reported in Fig. 4(a) and (b), respectively. In the ZE -DPhB, the A band is blueshifted with respect to that of EE -DPhB of about
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1570/cm, the intensity is lowered (fexp(EE DPhB) /1.11, fexp(ZE -DPhB) /0.78), and the vibrational structure disappears. This behaviour is typical of the cis forms of a,v-diphenylpolyenes having sterical interactions [34,35]. Analogously to the spectra of StPs, the spectra of the EE - and ZE 2-PPhBs show a splitting of the A band, if compared to the spectrum of the corresponding hydrocarbon. The first (A?) band is intense and red-shifted, while the second (Aƒ) band is blueshifted with respect to that of the parent hydrocarbon. In particular, the comparison between Fig. 4(a) and (b) shows that A? band is more redshifted, with respect to the A band, in ZE configuration. The Aƒ band, that is only a weak shoulder in EE -2-PPhB, is more evident in ZE -2PPhB. fexp(A?)/0.73 and fexp(Aƒ)/0.27 were found for ZE -2PPhB. The UV absorption spectrum of 4-PPhBs is similar to that of the parent hydrocarbon both in EE and ZE configurations.
Fig. 4. UV absorption spectra of 4-PPhb (2), 2-PPhB (3) and that of DPhB (1) in MCH/3MP at room temprature in (a) EE configuration and (b) ZE configuration.
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Fig. 5. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of EE -DPhB. The oscillator strengths of more intense transitions are indicated near to levels. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited state.
In the UV absorption spectrum of EE -DPhB, the strong A band, with resolved vibrational structure and maximum at 3.76 eV (Fig. 4(a)), is related to the S0 0/S1, (11Ag 0/11Bu) transition having (pH, pL*) character. HOMO and LUMO are delocalized, thus the A band is the conjugation band, like in stilbene. Because of the presence of four delocalized MOs (HOMO, HOMO/1, LUMO, LUMO/1), several p 0/p* transitions, symmetry-forbidden or very weak, some with charge transfer character, are found in the calculated electronic spectrum between the tail of the A band and the second absorption band starting at 250 nm. The p) character. HOMO and LUMO are delocalized, thus the A band is the conjugation band, like in stilbene. Because of the presence of four delocalized MOs (HOMO, HOMO/1, LUMO, LUMO/1), several p 0/p* transitions, symmetry-forbidden or very weak, some with charge transfer character, are found in the calculated electronic spectrum between the tail of the A band and the second absorption band starting at 250 nm. The pH 0/pL1 (11Ag 0/31Ag) transition is of particular interest for the discussion below on the spectra of aza-analogues of DPhB. This transition is symmetry-forbidden in the EE DPhB but allowed in aza-derivatives without inversion centre. Since the pH 0/pL1 transition involves delocalized MOs, when it is allowed, it can acquired moderate strength. The 31Ag state is not much influenced by doubly-excited configura-
tions, while the upper 1Ag excited states are more sensitive to poly-excited configurations. With the suitable variation for a non-planar molecule, a similar classification is found for the UV absorption spectrum of ZE -DPhB of Fig. 4(b), and hence it is not reported. Fig. 5 shows the correlation diagram of the Franck-Condon lowest CS INDO S/D/T/CI singlet energy levels of EE -DPhB with those of the two rotamers, the stable and unstable forms of EE -2-PPhB and that of EE -4-PPhB, obtained for the isolated molecules. The oscillator strengths are reported on levels of intense transitions. First of all, the theoretical results show that the S1 state is 1 (pH, pL) whether in ortho or para derivatives, i.e. the 1(n, p*) state is always above S1. Secondly, the calculated spectrum of the A rotamer of EE -2PPhB is the only one in agreement with the UV absorption spectrum of the ortho derivative. In particular, only the calculated spectra for A rotamer of the ortho derivative shows the presence of a second transition of moderate intensity in the region of the Aƒ band. It is possible to note that the calculated red shift of the A band of EE DPhB, with respect to that of E-S, is of 0.49 eV, in good agreement with the experimental value of 0.44 eV. Fig. 6 shows the correlation diagram of the Franck-Condon lowest CS INDO S/D/T/CI singlet energy levels of ZE -DPhB together with those of ZE -2-PPhB (A and B rotamers) and ZE -
Fig. 6. CS INDO S/D/T/CI energy levels of investigated rotamers of aza-derivatives of ZE -DPhB. The oscillator strengths of more intense transitions are indicated near to levels. ( */), 1(p, p*) excited states. ( / / /), 1(n, p*) excited state.
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4-PPhB, obtained by considering the isolated molecule model. The oscillator strengths are reported on levels giving intense transitions. Also for these compounds, the calculated first singlet excited state has (pH, pL) character both in ortho and para derivatives. Again, Fig. 6 clearly shows that the CS INDO CI electronic spectrum of the rotamer A of ZE -2-PPhB is the only one in agreement with the UV absorption spectrum of the ortho derivative. For A rotamer, the calculated red shift of the first absorption band is 0.27 eV, in very good agreement with the experimental value of about 0.26 eV. The calculated energy difference between S3 and S1 excited states is 0.58 eV, while the experimental value corresponding to energy difference between maximum of the Aƒ and A? band is :/0.64 eV. Considering the blue shift of the conjugation band on going from EE -DPhB to ZE -DPhB, the calculated value is about half of the experimental one. A significant decrease of intensity is also calculated (Dftheo /0.48), in agreement with experimental finding (Dfexp /0.33). It can be mentioned that the spectral behaviour of the ortho derivative(s) in ZE configuration is related to the planarity of the rotamers, while the parent hydrocarbon is not planar. The poly-excited configurations have not much importance on the excited states responsible for A? and Aƒ band of azaderivatives of EE - and ZE -configurations. Experimental studies of the solvent effect on the spectral, photophysical and photochemical behaviour of the EE and ZE isomers are not very numerous; in particular, they do not stem from methodical work. It is found that the UV absorption spectrum of EE -DPhB is red-shifted with increasing solvent polarizability [36,37] as well as the emission spectrum [36]. The absorption spectrum of ZE -2-PPhB shows a blue shift of the maximum of the first absorption band in protic solvents [11]. The substitution of an intramolecular hydrogen bond with an intermolecular one destroys the planarity of the ZE -2-PPhB, returning toward the spectrum of ZE -DPhB. Moreover, the calculated solvent effect on the aza-derivatives of DPhBs is similar to that found for the azaderivatives of E -S, i.e. the 1(pH, pL) excited state is red-shifted while the 1(n, p*) ones are more blueshifted. Because of this, no crossing between the S1
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and S2 states is possible. The lack of crossing between the first and second singlet excited state strengthens the hypothesis that the solvent effect on the absorption and emission spectra of the azaderivatives and the parent hydrocarbon should be similar.
4. Conclusions The simple ab initio quantum chemical model used in this work for molecular energies, i.e. geometry optimization at 3-21G level followed by the energy calculation with a more extended basis set (6-31G*), seems to be suitable for the description of rotational isomerism of the investigated compounds. Moreover, the combined theoretical and experimental study of rotamerism and electronic spectra of ortho and para aza-derivatives of E -S and DPhBs allowed us to reach some important conclusions. About rotamerism, our ab initio calculations on ground electronic state in conjugation with theoretical analysis of the conjugation band, in particular its splitting, clearly indicate that the stable forms of the ortho azaderivatives of E -S and DPhBs are the rotamers A of Fig. 1. The ab initio calculations and the 1HNMR spectra suggest the presence of N ×/H intramolecular hydrogen bond in ZE -2-PPhB. In trans derivatives, we prefer to speak of intramolecular hydrogen-bond type interactions. Considering the Franck-Condon excited electronic states of aza-derivatives of E -S and DPhBs, the CS INDO S/D/T/CI results indicate, apart from the E -4,4?-DPE, that the S1 state has (pH, pL) character also in non-polar solvents. As regards E 4,4?-DPE, and in agreement with the experimental results, the theoretical calculations show that the crossing between the excited states of different character is possible in a solvent of high o , and thus the solvent may have an important role on the spectroscopic and photophysics properties of this molecule. No involvement of the 1 A excited state is g found in the spectral region analyzed.
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Acknowledgements The authors are grateful to Prof. G. Bartocci, Prof. U. Mazzucato and Prof. F. Momicchioli for stimulating discussions and assistance in the course of this work. Thanks are also due to Prof. G. Ponterini for help concerning the solvent effect on E -4,4?-DPE. This research was funded by the Ministero per l’Universita` e la Ricerca Scientifica e Tecnologica (Rome) and the Universities of Modena and Perugia in the framework of the Programmi di Ricerca di Interesse Nazionale (project: Mechanisms of Photoinduced Processes in Organized Systems).
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