Intersystem crossing (ISC) in DMABN and DMABA : Laser induced optoacoustic spectroscopy and semi-empirical (AM1) calculations Pradipta Purkayastha, Pranab Kumar Bhattacharyya, Subhash Chandra Bera* and Nitin Chattopadhyay* Department of Chemistry, Jadavpur University, Calcutta-700 032, India Received 16th March 1999, Accepted 12th May 1999
The individual contribution of the intersystem crossing (ISC) process in the total non-radiative deactivation of the excited Ñuorophores, p-N,N-dimethylaminobenzonitrile (DMABN) and p-N,N-dimethylaminobenzaldehyde (DMABA) was determined by laser-induced optoacoustic spectroscopy (LIOAS). It was noted that, for DMABN, the ISC yield is almost insensitive to the solvent polarity, while, it is highly solvent dependent for DMABA. Semi-empirical (AM1) calculations have been performed to optimize the ground state geometry of both the Ñuorophores. The AM1-SCI (singly excited conÐguration interaction) method has been adopted to get the energies and the dipole moments of their excited states. From the calculated oscillator strengths for the optical transitions and also from absorption studies in solvents of di†erent polarity it is revealed that DMABN is excited to the S state and DMABA to the S state selectively with a laser beam of 320 nm. The experimental 2 3 results for the large variation of the ISC yield (/ ) in DMABA has been ascribed to the reordering of the ISC np* and pp* states with a change in the solvent polarity.
Introduction
Experimental and calculations
The dual luminescence of DMABN and some related compounds have drawn the attention of photophysicists and photochemists since its discovery by Lippert.1h9 The importance of the phenomenon lies in its potential application in laser dyes, photochemical energy conversion, etc.2 The studies of such phenomena have also been extended to di†erent microheterogeneous environments.8h12 According to the concept introduced by Grabowski et al. the appearance of the lower energy emission in DMABN is due phenomenologically to a twisting of the donor dimethylamino group about the single bond connecting the phenyl ring.3 Zachariasse interpreted this dual emission as a solvent-induced pseudo JahnÈ Teller e†ect4 which is, however, contradicted by many groups.5,8 Several groups have tried to explain the experimental observations using theoretical calculations.13h16 In general, the total Ñuorescence quantum yield of the Ñuorophores like DMABN decreases along with the appearance of a red-shifted emission in the polar solvents. Recently the non-radiative deactivation pathways of the photoexcited DMABN and DMABA has been studied through the LIOAS technique.17h18 It was revealed that the intersystem crossing yield (/ ) is almost insensitive to the solvent polarity for ISC DMABN while the same is highly sensitive to the factor for DMABA. It is interesting to note that for the latter Ñuorophore, / is +1.0 in apolar solvents indicating that the deacISC tivation process is fully through the triplet state. The present project is one of the rare endeavors to see how / in nitrile and aldehyde can be rationalized theoretically. ISC In this paper, we have ascribed the dependence of the / of ISC the two Ñuorophores on the solvent polarity, principally to the relative positioning of the np* and pp* states. We have performed the semi-empirical (AM1-SCI) calculations to Ðnd out the geometries in the ground state and transition energies, oscillator strengths and dipole moments of the excited singlet and triplet states.
The procurement and puriÐcation of DMABN and DMABA and the solvents were as mentioned in an earlier publication.17 The absorption and Ñuorescence spectra were recorded in a Shimadzu MPS 2000 spectrophotometer and a Spex Ñuorolog spectroÑuorimeter. Although ab initio calculations involving extended basis sets with extensive conÐguration interaction (CI) have been successful in explaining structures, energetics and reactivities of small molecules in di†erent electronic states, such reports are still limited in number for large molecular systems. However, semi-empirical molecular orbital methods have established their wide utilities in this respect. The methods provide acceptable approximations to give results which are quite close to the experimental Ðndings.19h22 Here, we have used the Hyperchem package (Hypercube Inc., Canada) for our theoretical calculations. The ground state (S ) geometry of 0 the compounds were optimized by both AM1 and PM3 methods.23,24 AM1-SCI (singly excited conÐguration interaction) and PM3-SCI were performed to get the energy (E ) g and dipole moment in the ground state and the transition energies (*E ), dipole moments and oscillator strengths of i?j di†erent excited states. Since the results of AM1 and PM3 calculations were very close, we have tabulated only the AM1 results. We have taken care of all the singly excited conÐgurations (67 conÐgurations) within an energy window of 13 eV from the ground state. *E corresponds to the excitation of i?j an electron from the orbital / (occupied in the ground state) i to the orbital / (unoccupied in the ground state). The total j energy of the excited state (E ) was then calculated as E \ E j j g ] *E . The CI wavefunction has been used to generate i?j orbitals and one-electron density matrices, which were used to calculate the dipole moments of the excited states of the compounds. The nature of the electronic transitions have been determined from the individual eigenvectors. The stabilization of the di†erent states due to solvation has been calculated from the solvation energies based on Phys. Chem. Chem. Phys., 1999, 1, 3253È3258
3253
OnsagerÏs theory.25 Assuming that the solute molecule having a dipole k in the ith electronic state is fully solvated, the soli vation energy is given by 2k2(e [ 1) *E \ i solv a3(2e ] 1)
(1)
where e is the relative permittivity of the solvent and a is the cavity radius. We have taken the maximum molecular length as the cavity diameter for both the molecular systems (8.68 A for DMABN and 8.46 A for DMABA). Fig. 1 AM1 optimized geometries of DMABN and DMABA. Hydrogens have been omitted for clarity of the Ðgure.
Results and discussion The optimized (AM1) geometrical parameters, energies and dipole moments of DMABN and DMABA have been tabulated in Table 1. The results show that in the isolated condition both the molecules have a potential minimum around a torsional (3È2È1È10) angle of 16¡. Fig. 1 shows the ground state optimised geometries for the molecular systems. Our results di†er appreciably from the report of Majumdar et al. for DMABN where the most stable conformer was shown to have a torsional angle of around 50¡.15 They, however, adopted the MNDO method for the geometry optimization. Furthermore, we have noted that the optimized geometry for both the molecular systems show that the N,C,C centres of the dimethylamino group are not in the molecular plane. The ÈN\ centre is above and the two C atoms of the methyl groups are below the plane of the phenyl ring (Fig. 1). It is pertinent here to establish the reliability of our calculations. We have applied both AM1 and MNDO methods to calculate the ground state optimised geometric parameters of a third and somewhat similar system, namely, p-N,N@dimethylaminobenzoic acid (DMABA) for which the crystallographic data are readily available to us.26 While the AM1 method gives benzeneÈdimethylamino and benzeneÈcarboxilic acid torsion angles to be 14.5 and 0¡, respectively, the MNDO method gives the values 72 and 90¡ for the same torsional angles. The crystal structure analysis,26 however, refers to the
Table 1 Calculated ground state geometrical parameters, energies (kcal mol~1) and dipole moments (D) for the most stable forms of DMABN and DMABA in isolated condition Geometrical parameters and propertiesa r r(1h2) r(2h3) r(3h4) r(4h5) r(5h6) r(6h7) r(5h8) r(8h9) r(1h10) h(1h11) h(1h2h3) h(2h3h4) h(3h4h5) h(4h5h6) h(5h6h7) h(8h5h4) h(9h8h5) h(10h1h2) (11h1h2) E/kcal mol~1 k/D
Calculated valuesb for DMABN
Calculated valuesb for DMABA
1.40 1.42 1.39 1.40 1.40 1.39 1.42 1.16 1.44 1.44 121.14 120.91 120.68 119.15 120.68 120.42 180.00 118.20 118.19
1.40 1.42 1.39 1.40 1.40 1.39 1.47 1.24 1.44 1.44 121.18 120.71 121.07 118.74 121.04 120.95 123.99 118.34 118.33
[2224.0964 5.35
[2285.2858 4.75
a Numbering is as per the structures given in Fig. 1. b Bond lengths (r) and bond angles (h) are in A ngstroms and degrees respectively.
3254
Phys. Chem. Chem. Phys., 1999, 1, 3253È3258
values 3.7 and 2.2¡, respectively, for the said angles. The individual structural parameters (bond lengths and angles) also agree well with the calculated values through the AM1 method. Hence, at least in the present case, the AM1 method is superior to the MNDO method. Our AM1-SCI data for DMABN get further support from the experimental dipole moments (k) of the probe in di†erent electronic states. The k values (D)¤ coming out from our calculation are 5.35, 7.41 and 6.69 for ground, lowest excited singlet and triplet states, respectively, (refer to Tables 1 and 2) which agree reasonably well with the experimental values for the corresponding states, viz., 6.6, 9.1 and 8.8 as reported by Weisenborn et al.27 and the k values of the former two states 6.05 and 8.36 reported by Baumann.28 Moreover, our calculations reveal that for DMABN, the S and S states are 1 2 nearly degenerate in apolar solvents which is consistent with the Ðndings of Baumann.28 That the oscillator strength for the S ] S transition is an order of magnitude greater than the 0 2 S ] S transition is also supported by the work of Visser et 0 1 al.29 Table 2 shows the calculated transition energies and dipole moments of the excited states of the two Ñuorophores. The nature of the electronic transitions have also been mentioned in Table 2 for the states relevant to our purpose. We used a laser radiation of 320 nm in our LIOAS experiments, and hence we have tabulated transitions up to 280 nm. For both the molecules there are several triplet states below the Ðrst excited singlet (S ) state. The solvation energies of the mol1 ecules in di†erent states in solvents of di†erent polarity (nheptane (HEP), e \ 1.9 ; tetrahydrofuran (THF), e \ 8.2 and acetonitrile (ACN), e \ 38) have been calculated. Although the reliability of our method of calculation has already been discussed, we have calculated the Ñuorescence energy for DMABN in HEP (where the emission originates only from the so-called non-polar form resulting in a pure single emission). Since the solvation dynamics is faster than the Ñuorescence lifetime (ns) of the Ñuorophore, the molecule gets solvated before it Ñuoresces. Hence, the energy di†erence between the solvated S state and the unsolvated S state 1 0 should correspond to the Ñuorescence peak (28930 cm~1) which actually matches with the calculated value (28948 cm~1). For DMABN, out of the three lowest singlet states, S has 2 an oscillator strength much higher than the other two (Table 2). For DMABA, out of the four lowest singlet states, S pos3 sesses higher oscillator strength than the others. So, preferably S for DMABN and S for DMABA would be the excited 2 3 states where the electronic transitions, using the 320 nm laser beam, take place. As mentioned above, these results are in agreement with those reported by Visser et al.29 Assignment of these states to the excitation process gets further support from the shift of the absorption spectra of the compounds in ¤ D B 3.335 64 ] 10~30 C m.
Table 2 Calculated transition energies, dipole moments and oscillator strengths of some low lying excited states of DMABN and DMABA in isolated conditions DMABN
DMABA
Transition energy/nm
Nature of the state
Dipole moment/D
Oscillator strength
Transition energy/nm
Nature of the state
Dipole moment/D
Oscillator strength
280.29 297.84 317.32 330.74 334.96 368.67 375.72 396.56 523.08
T 6 S T3 5 S (pp*) 2 S (pp*) 1 T (pr*) T4 (pp*) 3 T (pp*) 2 T (pp*) 1
8.11 3.48 5.29 7.41 6.24 3.69 6.21 6.19 6.69
0.000 0.019 0.000 0.270 0.013 0.000 0.000 0.000 0.000
283.52 297.77 330.30 332.19 336.33 361.89 370.31 376.25 391.29 399.47 517.24
T 7 S S4 (pp*) 3 T (pp*) 6 S (pp*) 2 S (np*) 1 T (pr*) 5 T (pp*) 4 T (pp*) T3 (np*) 2 T (pp*) 1
9.18 3.14 6.87 6.21 5.93 2.40 3.31 5.71 5.67 2.45 5.80
0.000 0.021 0.266 0.000 0.009 0.000 0.000 0.000 0.000 0.000 0.000
Table 3 /
ISC
values for DMABN and DMABA in di†erent solventsa
Solvent / for DMABN ISC / for DMABA ISC a From ref. 17.
n-Heptane
p-Dioxane
Dibutyl ether
Tetrahydrofuran
Acetonitrile
0.73 1.05
0.63 1.07
0.74 0.84
0.69 0.50
0.60 0.24
solvents of di†erent polarity. The excited state dipole moments can be estimated from the absorption shifts in solvents of di†erent polarity using the OoshikaÈLippertÈMataga equation.30 In solvents having relative permittivities e and e@ and refractive indices n and n@ the relevant Ðnal equation is given by :
C GA
k [ k 2k g g l6 [ l6 @ \ e a a a3 Åc [
A
B
n2 [ 1 n@2 [ 1 [ 2n2 ] 1 2n@2 ] 1
BHD
e[1 e@ [ 1 [ 2e ] 1 2e@ ] 1
(2)
where, l6 and l6 @ are the wavenumbers of the 0È0 absorption a a bands in the two solvents and a is the cavity radius approximated to half of the maximum end-to-end distance of the molecules. We have, however, taken the absorption band maxima for the determination of the k values. For both the e compounds we see a small bathochromic shift in the absorption spectra with an increase in the solvent polarity reÑecting that the dipole moment of the excited singlet state (k ) is more e than that of the ground state (k ). This rules out the imporg tance of the S state (k \ 3.48 D) of DMABN and S and S 3 1 4 states (k \ 2.40 and 3.14 D, respectively) of DMABA for the absorption phenomena as these states have lower k values than the respective k values (5.35 and 4.75 D, respectively). g The dipole moments of the excited singlet states, estimated from the solvent shift of the absorption spectra, have been found to be 7.5 ^ 0.5 D for DMABN and 7.0 ^ 0.5 D for DMABA. These values agree well with the calculated dipole moments of the S (7.41 D) and S (6.87 D) states of the two 2 3 Ñuorophores, respectively, conÐrming that the excitation of the probes takes place to these states selectively. The quantum yields of the non-radiative intersystem crossing process (/ ) for DMABN and DMABA in di†erent solISC vents, as obtained from the LIOAS experiment, are presented in Table 3. The deactivation of an excited singlet state depends on three competing processes ; Ñuorescence, internal conversion (IC) to the ground state and intersystem crossing (ISC) to lower states. The triplet quantum yield depends on the efficient ISC process. The triplet state further decays through two competitive processes, phosphorescence and another ISC to
the ground singlet state. The important ISC process is the excited singlet to lower triplet state crossing because unless this is efficient, the overall ISC yield will be low. Hence, in the following discussion we shall analyse the excited singlet to lower triplet state ISC process critically. The ISC is strongly dependent on the connecting states of the spinÈorbit coupling (L Æ S) operator. The major contributors to the L Æ S matrix elements are one centre components having large atomic coefficients in the connecting states. Table 4 presents the low lying singlet and triplet states of DMABN and DMABA in di†erent environments. For DMABN, two close singlet states S and S are low lying and 1 2 both of them are pp* in nature. They are most likely to take part in the ISC process. For DMABN, although S is above the S state in isolated 2 1 condition, in HEP medium they are nearly degenerate. In the other two solvents (THF and ACN) S gets more stabilization 2 and thus becomes the lowest excited singlet state. So, in the Table 4 Energies (kcal mol~1) of the electronic states, relevant to the ISC process, of the solvated Ñuorophores DMABN State
Isolated
HEP
THF
ACN
S (pp*) S2 (pp*) T1 (pr*) T4 (pp*) T3 (pp*) T2 (pp*) 1
[2137.68 [2138.77 [2146.57 [2148.03 [2152.03 [2169.46
[2141.31 [2141.34 [2147.47 [2150.58 [2154.56 [2172.42
[2145.70 [2144.44 [2148.56 [2153.66 [2157.62 [2175.98
[2146.99 [2145.36 [2148.88 [2154.57 [2158.52 [2177.04
State
Isolated
HEP
THF
ACN
S (pp*) S3 (pp*) S2 (np*) T1 (pr*) T5 (pp*) T4 (pp*) T3 (np*) T2 (pp*) 1
[2198.76 [2200.31 [2206.31 [2208.11 [2209.33 [2212.25 [2213.74 [2230.03
[2202.14 [2202.83 [2206.72 [2208.89 [2211.66 [2215.15 [2214.17 [2232.43
[2206.21 [2205.86 [2207.22 [2209.83 [2214.48 [2217.32 [2214.69 [2235.33
[2207.42 [2206.76 [2207.36 [2210.11 [2215.31 [2218.13 [2214.84 [2241.49
DMABA
Phys. Chem. Chem. Phys., 1999, 1, 3253È3258
3255
solution phase, the equilibrated lowest excited singlet state for DMABN is always the state that is assigned as S in the iso2 lated condition. Below this state, there are four low lying triplet states T , T , T and T . Of these four states, the lower 1 2 3 4 three triplets are of 3pp* in nature while the T state is 3pr* 4 in nature. So it is likely that the T state contributes most to 4 the singletÈtriplet intersystem crossing. From Table 5, representing the atomic coefficients of molecular orbitals for DMABN, it can be noted that the ring carbon atom [C(2)] attached to the donor dimethylamino group contributes to the ISC from both SS o L Æ S oT T and SS o L Æ S oT T matrix ele1 4 2 4 ments. The electronic states involved in the ISC process are, thus, principally S (pp*) and T (pr*). Hence, / is quite 2 4 ISC large, as found from LIOAS experiment,17 since the states involved are pp* and pr* in nature. The contribution of the one centre components of the matrix elements also corroborates efficient ISC process in all the solvents studied. Again, because both the states (pp* and pr*) are known not to be perturbed very much by external perturbation, / is ISC expected to be less sensitive to solvent which is consistent with our experimental results where we have observed that the / ISC of DMABN hardly varies with solvent polarity. For DMABA, the situation is di†erent. Although there are three low lying excited singlet states, viz., S , S and S , and 1 2 3 the molecule is excited to the S state, the very fast internal 3 conversion takes it to the lowest excited singlet state prior to the occurrence of the slow ISC process. The S state is 1np* in 1
nature and the other two are 1pp* in nature. With an increase in the solvent polarity, however, there is a reversal of stability of the S (np*) and S (pp*). The low lying triplet states are 1 3 T , T , T , T and T . The T state is 3pr* and T is 3np* in 1 2 3 4 5 5 2 nature while the others are of 3pp* type. Table 6 shows that the important atomic centres for the intersystem crossing are C(2), C(5) and C(8), when S and T states are connected by 1 5 the L Æ S operator, while only the atomic centre C(2) is important in the SS o L Æ S oT T matrix element. Among the three 3 5 carbon centres, C(2) and C(5) are on the ring and directly attached to the dimethylamino and aldehyde groups respectively and the C(8) centre is the aldehydic carbon itself. The S 1 state can be deactivated via all the three centres to the T 5 state, while the S state can cross over to the T state through 3 5 the C(2) centre only. The intersystem crossing of both the singlets directly to further down triplets will, in general, be much less efficient due to the large energy gap. However, the relative magnitude of S deactivation appears to be more 1 because a signiÐcant cross over through carbon and oxygen centres of the aldehyde group and the ring carbon centre anchored by the aldehyde group will occur through the matrix element SS o L Æ S o T T whereas only the carbon centre 1 1 anchored by the aldehyde will be involved in the SS o L Æ S o T T element. Other triplet states do not have a sig3 2 niÐcant contribution to one centre components of the matrix elements. All these facts taken together, indicate quite strongly that when the S state is lower in energy than the S state (in 3 1
Table 5 Atomic coefficients (relevant for the S , S , T , T , T and T states)a to the molecular orbitals of DMABN 1 2 1 2 3 4 Molecular orbital
27
28
29
s [0 0.108 [0.071 p 0 0.009 0.018 x p 0.001 0.073 [0.037 y p [0 0.549 [0.187 z C(2) s 0 [0.021 0.005 p 0 0.009 0.038 px 0 [0.045 0.054 y p [0 [0.287 0.544 z C(3) s [0.001 0.006 [0.002 p 0.038 [0.032 [0.013 x p 0.041 [0.021 [0.017 y p 0.506 [0.344 [0.177 z C(4) s 0 [0.003 0.001 p 0.038 0.017 [0.028 x p 0.045 0.005 [0.029 py 0.486 0.120 [0.334 z C(5) s 0 0 [0 p [0.001 0.034 0.038 px [0.002 0.042 0.044 y p 0 0.442 0.473 z C(6) s [0 [0.003 0.001 p [0.038 0.019 [0.028 x p [0.045 0.007 [0.031 py [0.486 0.120 [0.334 z C(7) s 0.001 0.006 [0.002 p [0.044 [0.038 [0.013 px [0.051 [0.033 [0.017 y p [0.505 [0.342 [0.177 z C(8) s [0 [0.002 0.001 p 0 [0.001 0.015 x p 0 [0.005 0.018 y p [0 [0.042 0.095 z N(9) s [0 [0 [0 p 0 [0.017 [0.022 px 0.001 [0.018 [0.026 py [0 [0.198 [0.284 z a S : 28 ] 30 (0.563) and 27 ] 29 (0.404) ; S : 28 ] 29 ; T : 28 ] 29 ; T : 28 ] 30 ; 1 2 1 2 N(1)
3256
Phys. Chem. Chem. Phys., 1999, 1, 3253È3258
30
31
32
33
[0 [0 [0 0
0.001 [0 [0 0
[0.385 [0.008 0.017 0.087
0.291 0.052 [0.061 [0.218
0 0.002 0.004 [0.001
[0 [0.004 [0.007 0.001
0.214 [0.190 0.146 0.251
[0.219 0.224 [0.085 0.327
0.003 [0.037 [0.041 [0.487
0.042 0.028 [0.038 0.002
[0.067 [0.055 [0.086 [0.218
0.071 0.021 0.047 [0.259
[0 0.040 0.045 0.506
[0.110 [0.100 [0.077 0.014
0.051 [0.049 0.034 0.170
[0.052 0.074 [0.003 0.140
[0 0 0 [0
[0 [0.067 [0.114 0.016
[0.003 [0.023 0.008 [0.042
0.001 0.021 [0.011 0.012
0.001 [0.042 [0.048 [0.506
0.110 [0.018 [0.125 0.013
0.051 [0.033 0.061 0.167
[0.052 0.056 [0.033 0.145
[0.003 [0.042 0.048 0.486
[0.042 [0.047 0.005 0.003
[0.067 0.020 0.043 [0.236
0.071 [0.061 [0.093 [0.240
0 [0 [0 0
[0 [0.341 [0.585 0.082
0.021 [0.039 [0.022 [0.314
[0.021 [0.017 [0.043 [0.380
[0 0 0 [0
0 0.334 0.573 [0.080
[0.001 0.032 0.024 0.303
0.001 0.021 0.037 0.355
T : 27 ] 30 ; T : 28 ] 32 (0.485) and 28 ] 33 (0.390). 3 4
Table 6 Atomic coefficients (relevant for the S , S , S , T , T , T , T and T states)a to the molecular orbitals of DMABA 1 2 3 1 2 3 4 5 Molecular orbital
27
28
29
30
31
32
33
s [0.007 0.002 0.106 [0.065 [0.005 [0.108 0.473 p [0.024 [0 0.009 0.015 0.002 0.019 0.038 x p 0.016 0.001 0.074 [0.035 [0.002 [0.024 [0.053 y p 0.031 0.014 0.557 [0.180 [0.012 [0.083 [0.214 z C(2) s 0 [0.021 0.004 0 0 0.031 [0.299 p 0 0.009 0.038 0.002 [0 [0.007 0.290 x p 0 [0.045 0.054 0.004 [0 0.056 [0.165 y p [0 [0.287 0.544 [0 0.038 0.394 0.072 z C(3) s [0.001 0.006 [0.002 0.003 [0.003 [0.010 0.092 p 0.038 [0.032 [0.013 [0.037 0.033 [0.024 0.048 x p 0.020 [0.039 [0.018 [0.016 0.036 [0.033 0.092 y p 0.013 [0.514 [0.349 [0.174 0.465 [0.278 [0.032 z C(4) s 0.027 [0 [0.003 0.001 0 0.008 [0.068 p 0.137 [0.036 0.016 [0.025 [0.039 [0.004 0.080 x p 0.041 [0.043 0.003 [0.026 [0.044 0.010 [0.029 y p [0.029 [0.490 0.104 [0.316 [0.521 0.069 [0.040 z C(5) s [0.124 0 [0 [0 0 [0.001 [0 p [0.328 [0 0.031 0.030 0.001 0.014 0.029 x p 0.157 [0 0.040 0.035 0.001 0.020 [0.007 y p [0.018 [0.021 0.444 0.405 0.011 0.211 0.083 z C(6) s 0.016 0 [0.003 [0.001 [0 0.007 [0.072 p 0.042 0.035 0.017 [0.028 0.038 [0.004 0.057 x p [0.085 0.042 0.005 [0.031 0.044 0.012 [0.069 y p [0.012 0.475 0.106 [0.357 0.499 0.052 [0.033 z C(7) s [0.021 [0.001 0.006 [0.002 0.003 [0.009 0.094 p [0.075 0.041 [0.037 [0.008 [0.040 [0.014 [0.059 x p 0.062 0.049 [0.031 [0.011 [0.047 [0.016 [0.094 py 0.007 0.506 [0.351 [0.120 [0.498 [0.264 [0.008 z C(8) s 0.021 [0 [0.002 0 [0 0.003 [0.026 p 0.249 [0 0.005 0.025 0.001 [0.046 0.011 x p 0.035 [0 0.001 0.030 0 [0.049 [0.022 y p [0.019 [0 0.033 0.367 0.011 [0.630 [0.109 z O(9) s [0.001 [0 [0 0 0 0 [0 p [0.753 0.002 [0.015 [0.023 [0.001 0.030 0.001 x p [0.073 0.001 [0.013 [0.026 [0.001 0.034 0.012 y p 0.073 0.008 [0.175 [0.324 [0.011 0.429 0.069 z a S : 27 ] 30 (0.533) and 27 ] 32 (0.455) ; S : 29 ] 31 (0.536) and 28 ] 30 (0.415) ; S : 29 ] 30 ; T : 29 ] 30 ; T : 27 ] 30 (0.528) and 27 ] 32 1 2 3 1 2 (0.462) ; T : 29 ] 31 ; T : 28 ] 31 and T : 29 ] 33. 3 4 5 N(1)
high polar solvents), the intersystem crossing will be much less. Thus, the experimental observation of a sharp decrease of the ISC yield in the polar solvents are corroborated by our semi-empirical calculations.
2
Conclusion
5
The efficiency of the non-radiative intersystem crossing process depends on the relative positioning of the solvent stabilized excited singlet and the nearest triplet states. The electronic transition associated with the states relevant for the ISC process of DMABN are of pp* and pr* nature in all the solvents making the ISC quantum yield poorly solvent sensitive. For DMABA, however, a variation of the solvent polarity changes the ordering of the solvent stabilized singlet states resulting in a drastic change in the ISC yield.
Acknowledgements Financial help from the C.S.I.R., Govt. of India, is gratefully acknowledged. NC sincerely thanks Professors M. Van der Auweraer and F. C. De Schryver for introducing him to the Ðeld of LIOAS.
References 1
E. Lippert, W. Luder and H. Boos, Advances in Molecular Spectroscopy, ed. A. Mangini, Pergamon Press, Oxford, 1962, p. 443.
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