Influence of steric factors on exciplex energy and magnetic field effect

Spectrochimica Acta Part A 60 (2004) 97–102

Influence of steric factors on exciplex energy and magnetic field effect Suchandra Chatterjee a , Sharmistha Dutta Choudhury b , Samita Basu b,∗ , Nandita Ghosh c , Manas Chakrabarty c a

b

Serampore College, West Bengal, India Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Calcutta 700064, India c Department of Chemistry, Bose Institute, 93/1, A.P.C. Road, Calcutta 700009, India Received 31 October 2002; received in revised form 2 April 2003; accepted 4 April 2003

Abstract With the advent of spin chemistry, magnetic field effect (MFE) on exciplex luminescence has emerged as an important domain of research. MFE is a diffusion controlled phenomenon and hence is solvent dielectric (ε) dependent. It maximizes at a particular ε (εmax ) for a specific exciplex system. Various attempts have been made to explain the variation of this εmax from one exciplex to another. In our present work we have succeeded for the first time to enmark the energy of exciplex (Eex ) as the prime factor in determining the εmax . We have indicated a definite inverse correlation (1:1) between εmax and Eex . We have also tried to correlate some parameters that are important in exciplex formation, e.g. Charton’s steric constant (νc ), repulsive energy (Re ) and Eex . © 2003 Elsevier B.V. All rights reserved. Keywords: Exciplex energy; Steric effect; Magnetic field effect; Dielectric maximum; Carbazole derivatives; 1,4-Dicyanobenzene

1. Introduction From 1960 onwards, the ubiquitous nature of the photoinduced electron-transfer processes has made it a subject of extensive theoretical and experimental research [1]. Research activity continues on wide variety of fronts including the interesting area of spin chemistry. In this realm, study of magnetic field effect (MFE) on exciplex luminescence has been able to capture considerable attention and still remains a subject of intellectual challenge [2]. The physical requirement for an exciplex to show MFE is inherent in the competition between spin evolution and diffusion dynamics of its spin-correlated radical ion pairs (RIPs). By diffusion, the RIPs may attain a particular inter-radical distance where electron-electron exchange interaction becomes negligible making its singlet (S) and triplet (T0 , T± ) spin states degenerate. In this situation, inter-system crossing occurs from S↔T0 , T± through isotropic hyperfine interaction present in the system. An external low magnetic ∗ Corresponding author. Tel.: +91-33-2337-5354; fax: +91-33-2337-4637. E-mail address: [email protected] (S. Basu).

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1386-1425(03)00184-7

field of the order of 0.01 Tesla can affect this inter-system crossing rate by removing the degeneracy among the T0 , T± states due to Zeeman splitting and hence can alter the relative population of S and T states of RIPs. Any variation in the singlet RIP yield can be detected conveniently by noting the change in exciplex luminescence as the immediate outcome of the geminate recombination of the singlet RIPs. There are a number of examples where it has been cited that exciplex luminescence increases in the presence of an external magnetic field when RIPs are initially formed in singlet state [3–15]. Since MFE reflects the unique combination of spin evolution, diffusion dynamics and geminate recombination of the RIPs, solvent dielectric constant (ε) should have a major role in optimising this effect. Normally response to an external magnetic field is not observed in an extremely non-polar medium where most of the RIPs recombine at once as well as in a highly polar medium where free ion formation predominates. Only in a moderately polar medium, where a significant fraction of the RIPs can be separated, spin flipped and geminately recombined, the MFE on exciplex luminescence is detectable. Early researches on different unlinked donor–acceptor exciplex systems, viz. pyrene (PY)–N,N-dimethylaniline (DMA)

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[13,14], 9-cyano-phenanthrene-trans-anethole [15], etc. confirmed the need of moderate ε of the order of 14<ε<18 to maximize this MFE. Nevertheless, recently Aich and coworkers [11,12] showed that solvent ε is not the sole controlling factor to optimize this effect. They explored a new exciplex between N-ethyl carbazole (ECZ) and 1,4-dicyanobenzene (DCB) for which εmax comes at a much lower value of 9.0 than the usual limit 14<ε<18. Similar observations were also there with other exciplex systems, e.g. 1,4,5,8,9-pentamethyl-carbazole (PMC)–DCB [11,12] and all-s-trans-1,6-diphenylhexa-1,3,5-triene–DCB [10] having εmax around 12.0 and 10.0, respectively. This variation in εmax values among various exciplex systems was explained on the basis of the extent of charge transfer, δ, in the respective exciplex systems. For example, a lesser δ (0.15) in ECZ–DCB exciplex was pointed out as the principal cause for its lower εmax (9.0) compared to PY–DMA with δ (1.00) and εmax (14.0) [11,12]. Unfortunately, when we extended our work to other types of exciplex systems, the former hypothesis regarding dependence of εmax on δ did not hold well. We found that in the carbazole (CZ)–DCB exciplex system, εmax appears at a quite low value of 9.0 though δ is high enough, i.e. 0.90. This compelled us to investigate the overall thermodynamics of exciplex formation and assess all the energy parameters in detail in order to explain the variation of εmax from system to system.

2. Experimental The compounds ECZ and PMC were purchased from Aldrich whereas DCB and N-vinylcarbazole (VCZ) were obtained from Fluka. CZ was bought from BDH and N-methylcarbazole (MCZ), N-iso-propylcarbazole (ICZ), N-propylcarbazole (PCZ), N-butylcarbazole (BCZ) and N-secondary-butylcarbazole (S-BCZ) were all synthesised in laboratory [16]. CZ, DCB and ECZ were crystallised from 75% ethanol. Spectrograde tetrahydrofuran, N,N-dimethylformamide and benzene were used as solvents without further purification. All the solutions were deoxygenated by passing pure dry Ar gas through them before experiments. The absorption and fluorescence spectra were recorded by Shimadzu UV-2101 PC spectrophotometer and Hitachi spectrophotometer (model 4010), respectively. Increase in exciplex luminescence in the presence of an external magnetic field was studied using a full-wave phase-sensitive detection system described elsewhere [11,12]. The signal to noise ratio of this system is 1000:1. The dielectric constant, ε, of the medium was estimated using the relation ε = ε1 v1 +ε2 v2 where ε1 and ε2 are the dielectric constants of tetrahydrofuran and N,N-dimethylformamide respectively and v1 and v2 represent their volume fraction in the mixture [17].

Fig. 1. Potential energy diagram of the exciplex formation process in general, through interaction between the excited-state fluorophore (F*) and ground state quencher (Q).

3. Results and discussion 3.1. Determination of different energy parameters of exciplex Fig. 1 shows the energy profile of a photoexcited fluorophore (F*) interacting with a ground state quencher (Q) to form a stabilised exciplex releasing the binding energy −Hex . Upon radiative decay (hνmax ) the exciplex at first reaches the Franck–Condon ground state (FQ)FC , which is shown to have a repulsive energy (Re ) [18] and then relaxes non-radiatively along the original ground state energy surface (F,Q). We found both experimentally and analytically all the relevant energy parameters in benzene solvent. The molecular zero–zero excitation energy, hν00 , was obtained from the overlapping of excitation and emission spectra of fluorophore, the maximum energy of exciplex fluorescence, hνmax , was determined from the simple emission spectrum and −Hex, was estimated by temperature variation of respective monomer (Im ) and exciplex (Iex ) peak intensities using a modified form of van’t Hoff equation, i.e. the Stevens–Ban equation [19] ln(Iex /Im [Q]) = −Hex /RT + constant

(1)

instead of the simplified form (ln Iex = −Hex /RT+constant). The nature of plots (Fig. 2) obtained for all the exciplex systems are not linear. For the linear and near linear cases, −Hex values were estimated safely from the slopes of the plots themselves and then Eex were calculated using the expression, Eex = hν00 −Hex

(2)

This enabled us to calculate the values of Re using the expression, Eex = hνmax + Re

(3)

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Fig. 2. Representative Stevens–Ban plots for different exciplex systems with 2×10−4 M fluorophore concentration and 2×10−2 M quencher concentration. (a) ICZ–DCB, PCZ–DCB, BCZ–DCB, S-BCZ–DCB; (b) CZ–DCB; (c) VCZ–DCB, ECZ–DCB, MCZ–DCB; (d) PMC–DCB.

For other related exciplexes having non-linear Stevens–Ban plots, −Hex was determined indirectly [20] as described below. The Re values depend on the intrinsic size of the substituents on the fluorophores and hence can be correlated with the Charton’s steric parameter (νc ) [21]. The Charton’s steric parameter was preferred over other steric constants available due to its simplicity. Re versus νc curves were plotted for PCZ–DCB, BCZ–DCB, ICZ–DCB and S-BCZ–DCB all having linear Stevens–Ban plots (Fig. 3). This correlation was then utilised to read out the Re values for the non-linear cases. Finally, backtracking on the basis of the known values of hν00 and hνmax , the Eex and hence −Hex could be estimated easily. All the energy parameters are summarized in Table 1 and the indirectly obtained values are marked with parenthesis.

Due to the lack of availability of other carbazole derivatives, Fig. 3 is based on four experimental points only. So to check the reliability of the Re versus νc correlation line for carbazole exciplex systems, we have studied two other exciplex systems, PMC–DCB and VCZ–DCB, which are discussed later. 3.2. Correlation of energy parameters with MFE data The εmax values for all the exciplex systems were determined in tetrahydrofuran–N,N-dimethylformamide solvent mixtures using a saturated magnetic field of 0.014 ± 0.0005 T and are reported in Table 2. When Eex was plotted against εmax for all exciplex systems, we got a straight line with almost unit slope (Fig. 4), which indicates the presence of a direct (1:1) inverse correlation between Eex and εmax . This

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Table 1 Energy parameters of exciplex and Charton’s steric parameters, νc of the carbazole derivatives [21] System

hν00 (kcal/mol)

hνmax (kcal/mol)

CZ–DCB MCZ–DCB ECZ–DCB PCZ–DCB BCZ–DCB ICZ–DCB S-BCZ–DCB

85.11 82.29 81.65 81.96 81.61 82.19 81.98

72.11 64.86 64.52 62.55 62.59 62.19 62.06

−Hex (kcal/mol)

Re (kcal/mol)

Eex (kcal/mol)

(5.82) (4.58) (3.80) 4.79 ± 0.2 4.40 ± 0.2 4.00 ± 0.2 4.80 ± 0.2

(7.18) (12.85) (13.33) 14.62 ± 0.2 14.62 ± 0.2 16.00 ± 0.2 15.12 ± 0.2

(79.29) (77.71) (77.85) 77.17 ± 0.2 77.21 ± 0.2 78.19 ± 0.2 77.18 ± 0.2

νc 0.00 0.52 0.56 0.68 0.68 0.76 0.76

Values in parenthesis were found out indirectly using the correlation line (Fig. 3).

This explanation is similar to the previous argument which stresses the role of δ (the extent of charge transfer) in controlling the energy difference between contact ion pair and solvent separated ion pair and hence in shifting of εmax from system to system [10–12]. However in our case we have highlighted the role of Eex in determining εmax . Eex incorporates thermodynamic and steric parameters and correlates more accurately with the εmax variation. 3.3. Reliability of the νc versus Re correlation line for carbazole exciplex systems Now using the Eex versus εmax plot we have checked the reliability of the Re versus νc correlation line using VCZ–DCB (εmax =9.50) and PMC–DCB (εmax = 13.25) which have unknown Eex values. The Eex values for these systems were unavailable since their Stevens–Ban plots (Fig. 2) were non-linear and hence −Hex (binding energy of the exciplex) could not be obtained directly. Moreover, no νc value was available for the vinyl (–CH=CH2 ) group of VCZ and the methyl groups of PMC from literature [21]. Fig. 3. Correlation of the Franck–Condon ground state repulsive energy (Re ) with Charton’s steric parameter (νc ) for PCZ–DCB (1), BCZ–DCB (2), ICZ–DCB (3) and S-BCZ–DCB (4).

is due to the fact that with the decrease in the Eex value, the energy difference between contact ion pair and solvent separated ion pair increases [11]. As a result, a higher ε is required to attain that equilibrium between contact ion pair and solvent separated ion pair so that both separation and geminate recombination of RIPs is optimum to show maximum MFE. Table 2 % MFE data at εmax System

εmax

% MFE

CZ–DCB MCZ–DCB ECZ–DCB PCZ–DCB BCZ–DCB ICZ–DCB S-BCZ–DCB

9.00 10.50 10.25 11.00 11.00 10.25 11.00

1.1 4.3 4.4 4.8 5.2 5.6 5.4

± ± ± ± ± ± ±

0.02 0.01 0.01 0.01 0.01 0.01 0.01

Fig. 4. Correlation of energy of exciplex (Eex ) with εmax for CZ–DCB (1), MCZ–DCB (2), ECZ–DCB (3), PCZ–DCB (4), BCZ–DCB (5), ICZ–DCB (6), S-BCZ–DCB (7).

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Table 3 Some important energy parameters for VCZ–DCB and PMC–DCB System

hν00 (kcal/mol)

hνmax (kcal/mol)

εmax

Eex a (kcal/mol)

−Hex b (kcal/mol)

Re c (kcal/mol)

νc d

VCZ–DCB PMC–DCB

82.480 79.783

65.639 56.605

9.50 13.25

78.71 75.09

2.80 2.50

13.07 18.48

0.538 1.039

a b c d

Obtained using the correlation curve (Fig. 4) with known εmax values. Measured using Eq. (2) from known Eex and hν00 values. Calculated from Eq. (3) using known values of Eex and hνmax . Estimated from the correlation line (Fig. 3) with known Re values.

Fig. 5. 1,4,5,8,9-Pentamethylcarbazole (PMC).

So for these systems, using the Eex versus εmax correlation line (Fig. 4), we estimated their Eex values indirectly. All the other consequent parameters (i.e. Hex and Re ) were determined by analytical treatments using Eqs. (2) and (3). hν00 and hνmax were determined experimentally. Now using this Re value, νc was read out from the νc versus Re correlation line (Fig. 2). The results are summarised in Table 3. The νc value for VCZ was found to be ∼0.54. This value is midway between 0.52 for –CH3 and 0.56 for –CH2 CH3

group which is as expected. For PMC, νc was found to be ∼1.04 [ = 0.52+(0.52×4×1/4)] which means, the –CH3 group directly attached to the ‘N’ donor site has major contribution towards νc and the remaining four –CH3 groups [shown in parenthesis in Fig. 5] have 1/4th contribution each. This is again quite expected, as the exciplex luminescence following n␲* transition of the carbazole moiety, must be affected by the ‘N’-methyl group predominantly with minor contributions from the residual distant ones. Hence we have established that Eex is an appropriate parameter in explaining the variation of εmax for different systems. Another evidence that Eex is a good carrier of the exciplex characteristics can again be demonstrated by drawing the well-known Rehm Weller plots [22] for all the carbazole exciplex systems (Fig. 6). From this plot it is again clear that Eex has a better correlation with Eox D −Ered A (Table 4) than hνmax for different CZ–DCB systems. We have seen that Eex = hνmax +Re . Hence Re which describes the overall steric contribution of the substituents is the

Fig. 6. hνmax vs. (Eox D −Ered A ) (䊊) and Eex vs. (Eox D −Ered A ) (䊏) for different carbazole exciplex systems: CZ–DCB (1), MCZ–DCB (2), ECZ–DCB (3), ICZ–DCB (4), VCZ–DCB (5), PMC–DCB (6).

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Table 4 Eox D −Ered A values of some carbazole exciplex systems System

Eox D (V) [23]

Ered A (V) [23]

Eox D −Ered A (V)

CZ–DCB MCZ–DCB ECZ–DCB PCZ–DCB BCZ–DCB ICZ–DCB S-BCZ–DCB VCZ–DCB PMC–DCB

1.16 1.14 1.12 N.A. N.A. 1.14 N.A. 1.08 1.00

−1.65 −1.65 −1.65 −1.65 −1.65 −1.65 −1.65 −1.65 −1.65

2.81 2.79 2.77 – – 2.79 – 2.73 2.65

N.A., not available.

most important factor in determining the stability of an exciplex.

4. Conclusion In the present work the variation of εmax , i.e. the dielectric at which maximum MFE is observed, for different exciplex systems has evolved as a tool to establish the importance of steric effects in determination of exciplex parameters. The stability of an exciplex actually depends on Eex which in turn is guided by Re , the repulsive energy, and hence νc , the steric constants of the substituents on the fluorophore.

Acknowledgements We are indebted to Mrs. Chitra Raha (SINP) for fabrication of the phase sensitive detection system. Sincere thanks are due to Mr. Chandan Saha (Calcutta School of Tropical Medicine) and Mr. Pratip Bhattacharya (California Institute of Technology) for their valuable suggestions and as-

sistance. S. Chatterjee thanks CSIR, India for the fellowship (9/489(31)/98-EMR-I).

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