Intramolecular Charge Transfer with Fluorazene ... - ACS Publications

J. Phys. Chem. A 2010, 114, 1621–1632

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Intramolecular Charge Transfer with Fluorazene and N-Phenylpyrrole Sergey I. Druzhinin,*,†,‡ Sergey A. Kovalenko,*,§ Tamara A. Senyushkina,† Attila Demeter,| and Klaas A. Zachariasse*,† Max-Planck-Institut fu¨r biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Go¨ttingen, Germany, Institut fu¨r Chemie, Humboldt UniVersita¨t zu Berlin, Brook-Taylor Strasse 2, 12489 Berlin, Germany, and Institute of Materials and EnVironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences. P. O. Box 17, 1525 Budapest, Hungary ReceiVed: October 9, 2009; ReVised Manuscript ReceiVed: December 3, 2009

The reaction from the initially prepared locally excited (LE) precursor to the intramolecular charge transfer (ICT) state of the planarized fluorazene (FPP) is investigated and compared with its flexible counterpart N-phenylpyrrole (PP). The fluorescence spectra of FPP and PP at 25 °C in solvents of different polarity reveal that the onset of a LE f ICT reaction occurs at lower polarity (tetrahydrofuran, ε ) 7.39) for FPP than for PP (1,2-dichloroethane, ε ) 10.4). In accordance with this observation, the ICT reaction enthalpy -∆H is larger for FPP than for PP, 16.7 versus 6.7 kJ/mol in ethyl cyanide (EtCN). The larger ICT efficiency of FPP is related to the smaller energy gap between the two lowest excited singlet states ∆E(S1,S2): 3680 cm-1 for FPP and 4070 cm-1 for PP in n-hexane, as would be expected in the context of the PICT model. From picosecond fluorescence decays in EtCN at -45 °C it is found that the LE f ICT reaction rate constant ka of FPP is with 9.8 × 1010 s-1 considerably larger than that of PP with 3.9 × 1010 s-1. From femtosecond transient absorption spectra in acetonitrile (MeCN) at 22 °C, an ICT reaction time of 1.6 ps is obtained for FPP, shorter than the 4.0 ps determined for PP. The results show that a perpendicular twist of the pyrrole and phenyl subgroups is not required for an efficient ICT reaction with PP, the planarization of FPP even making this reaction faster. The similarity of the ESA spectra of FPP with those of PP in MeCN, with ICT absorption maxima at 365 nm (FPP) and 370 nm (PP), leads to the conclusion that both ICT states have a planar structure. CHART 1

Introduction As part of the ongoing investigations on the molecular structure of the intramolecular charge transfer (ICT) state formed from a locally excited (LE)1,2 precursor in electron donor/ acceptor (D/A) molecules, the rigidified fluorazene (FPP; Chart 1) was synthesized and studied in comparison with its flexible parent molecule N-phenylpyrrole (PP).3-6 FPP and PP do not undergo ICT in the unpolar solvent n-hexane, a fast and efficient ICT reaction only taking place in solvents more polar than diethyl ether (DEE).3 In the strongly polar alkyl cyanides acetonitrile (MeCN, ε25 ) 36.7) and ethyl cyanide (EtCN, ε25 ) 29.2), a red-shifted additional fluorescence band appears besides the LE emission of FPP. The new fluorescence originates from an ICT state, having a dipole moment µe(ICT) ) 13 D, much larger than µe(LE) ∼ 1 D. The ICT reaction of FPP is more efficient than that of PP. In the dual fluorescence spectrum of FPP in MeCN at -45 °C, for example, the ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) has increased to 5.48 as compared to 1.25 for PP under these conditions.3,4 Calculations of the LE and ICT states of FPP and PP lead to results that are different from our interpretation. Whereas the ICT state of FPP is, not surprisingly, computed to be planar, a structure with strongly twisted D and A subgroups is calculated * To whom correspondence should be addressed. E-mail: [email protected] (S.I.D.); [email protected] (S.A.K.); [email protected] (K.A.Z.). Fax: +49-551-201-1501 (K.A.Z.). † Max-Planck-Institut fu¨r biophysikalische Chemie, Spektroskopie und Photochemische Kinetik. ‡ Affiliation: Chemistry Department, Moscow State University, 119991 Moscow, Russia. § Humboldt Universita¨t zu Berlin. | Hungarian Academy of Sciences.

for the ICT state of PP.7,8 These results are in contrast to our conclusion that the ICT state is planar, which was based on the photophysical similarity of FPP and PP.3 The possibility may exist that calculations presently tend to preferentially stabilize twisted structures, for which the electronic coupling between the D and A subgroups is strongly reduced. This stabilizing effect is caused by dynamic electron correlation, which is minimized by the electronic decoupling.9 It should be noted that dynamic electron correlation generally requires a substantial correction to the energies calculated for the ICT states of, e.g., DMABN, NTC6, and PP.2,10-12 To increase the ICT efficiency of FPP, 4-cyanofluorazene (FPP4C) was made. With FPP4C, ultrafast ICT with a reaction time below 100 fs was observed, even in the nonpolar n-hexane.13 In this solvent at 25 °C, Φ′(ICT)/Φ(LE) ) 22, whereas a LE emission could not be detected in more polar media. Similar results were obtained with 4-cyano-N-phenylpyrrole (PP4C),13-16 again3,13,17,18 establishing that the formation of a perpendicular twist of the donor moiety relative to the benzonitrile acceptor group is not a requirement for fast and efficient ICT. In the case of a related rigidified D/A molecule, 4-fluorofluorazene (FPP4F), the ICT reaction is faster than with FPP, but clearly slower than for FPP4C. This is caused by the electron acceptor strength of the fluorophenyl group being larger than that of phenyl but substantially smaller relative to ben-

10.1021/jp909682p  2010 American Chemical Society Published on Web 12/31/2009

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Figure 1. Absorption (Abs) and fluorescence spectra of FPP and PP in n-hexane, diethyl ether (DEE), tetrahydrofuran (THF), and 1,2-dichloroethane (12DCE) at 25 °C. The overall fluorescence spectra of FPP in THF and 12DCE and of PP in 12DCE are separated into the contributions from the locally excited (LE) and intramolecular charge transfer (ICT) states. This separation is carried out by employing the spectrally shifted LE band of PP4M (no ICT) as a model for the LE emission. Excitation wavelength λ(exc): around 270 nm.

zonitrile.19 In the present article, photostationary and picosecond time-resolved experiments with FPP, as well as femtosecond transient absorption measurements with FPP and PP, are presented. These transient spectra support our previous analysis of the spectra reported for FPP4F and FPP4C.13,19 The solvent polarity dependence of the fluorescence spectra and the ICT thermodynamics of FPP are also investigated. Experimental Section The synthesis of 9H-pyrrolo-[1,2-a]-indole (fluorazene, FPP) was described previously.3 With FPP and 4-methyl-N-phenylpyrrole (PP4M, Aldrich), HPLC was the last purification step. All solvents were chromatographed over Al2O3 just prior to use. The solutions, with an optical density between 0.4 and 0.6 at the maximum of the first band in the absorption spectrum, were deaerated by bubbling with nitrogen for 15 min. The measurement and treatment of the fluorescence spectra, quantum yields, and single photon counting (SPC) decays and also of the femtosecond transient absorption spectra have been described elsewhere.2,13,14,18,20 Broadband transient absorption measurements were carried out by using a Ti:Sa amplifier CPA-2001 (Clark).

Results and Discussion Absorption and Fluorescence Spectra of FPP and PP at 25 °C. The absorption and fluorescence spectra of FPP and PP at 25 °C in a series of solvents from the nonpolar n-hexane to the medium polar 1,2-dichloroethane (12DCE) are shown in Figure 1. The spectra of FPP in the more polar (Table 1) alkyl cyanides n-butyl cyanide (BuCN), n-propyl cyanide (PrCN), EtCN, and MeCN are depicted in Figure 2. The corresponding spectra for PP can be found in refs 4 and 21. In the absorption spectra of FPP, the minor S1 band appears at the low-energy side of the main S2 band. The energy gap between the two lowest excited singlet states ∆E(S1,S2) ) ν˜ max(S2,abs) - ν˜ max(S1,abs) is approximated by ν˜ max(S2,abs) E(S1), where ν˜ max(Sn,abs) is the maximum of the Sn absorption band and E(S1) is the crossing point of the absorption and fluorescence spectra.13,19,22,23 In each solvent, ∆E(S1,S2) is smaller for FPP than for PP, 3680 vs 4070 cm-1 in n-hexane, as an example. This explains, within the context of the PICT model,3-5,13-18,20 the larger ICT reaction efficiency of the former

38030 (263.0) 1c

34040 4070

37900 (263.9) 14860

34220 3680

DEE

4.24 32090 0.50 0 0 0.26 34080 (293.4) 0.150c

1.88 32110 0.40 0 0 0.38 33880 (295.2) 2640

n-hexane

38050 (262.8) 1c 33990 4060

34220 (292.2) 0.150c 38020 (263.0) 1c 33940 4080

34220 (292.2) 0.161c

34170 (292.6) 0.149c

33920 4020

33850 4030

37880 (264.0) 1c

34330 (291.3) 0.151c

0.16

0.14

37940 (263.6) 1c

PrCN 24.2 31900 27310 0.183 0.037 0.200

BuCN 19.8 31900 27470

12DCE 10.4 31730 27920

THF 7.39 31930 29480 0.42 0.012 0.03

MeCN 36.7 31740 26540 0.082 0.069 0.841 0.65 34440 (290.4) 2250 38190 (261.8) 14730 34010 4180

EtCN 29.2 31900 27110 0.136 0.042 0.308 34390 (290.8) 0.159c 38110 (262.4) 1c 34010 4120

35240 4070

39310 (254.4) 13910

-

1.88 32970 0.194 0 0 0.43 -

n-hexane

DEE

35190 4290

39480 (253.3)

-

4.24 32720 0.213 0 0 0.33 -

35190 3950

39130 (255.5)

-

35140 4200

39340 (254.2)

-

-

0.10

0 -

10.4 32540 29870

12DCE

7.39 32660

THF

35150

-

-

0.087

19.8 32760 29140

BuCN

PP for given solventa PrCN

35190 4040

39320 (254.3) 13320

-

-

24.2 32610 28850 0.146 0.017 0.118

EtCN

35060 4340

39400 (253.8) 13490

-

-

29.2 32410 28590 0.120 0.021 0.174

MeCN

35310 4270

39580 (252.7) 13210

-

36.7 32670 28040 0.074 0.031 0.418 0.74 -

b

a Solvents: diethyl ether (DEE), tetrahydrofuran (THF), 1,2-dichloroethane (12DCE), n-butyl cyanide (BuCN), n-propyl cyanide (PrCN), ethyl cyanide (EtCN), and acetonitrile (MeCN). Measurements as in ref 4; Φ(ISC) for PP4M: 0.54 (n-hexane); 0.41 (DEE); 0.60 (MeCN). c Relative to the absorbance of ν˜ max(S2,abs). d Crossing point of the fluorescence and absorption spectra (Figure 2). e The energy difference ν˜ max(abs) - E(S1) is taken as an approximation for the energy gap ∆E(S1,S2) between the two lowest excited singlet states.2,14

ε25 ν˜ max(LE) (cm-1) ν˜ max(ICT) (cm-1) Φ(LE) Φ′(ICT) Φ′(ICT)/Φ(LE) Φ(ISC)b ν˜ max(S1,abs) (cm-1) (λmax (nm)) εmax (S1,abs) (M-1 cm-1) ν˜ max(S2, abs) (cm-1) (λmax (nm)) εmax (S2,abs) (M-1cm-1) E(S1)d (cm-1) ∆E(S1,S2)e (cm-1)

FPP for given solventa

TABLE 1: Data Obtained from the Fluorescence and Absorption Spectra of FPP and PP in a Series of Solvents at 25°C

Intramolecular Charge Transfer with FPP and PP J. Phys. Chem. A, Vol. 114, No. 4, 2010 1623

Figure 2. Absorption and fluorescence spectra of FPP in (a) acetonitrile (MeCN), (b) ethyl cyanide (EtCN), (c) n-propyl cyanide (PrCN), and (d) n-butyl cyanide (BuCN) at 25 °C. Excitation wavelength λ(exc): around 270 nm. See the caption of Figure 1.

molecule in solvents more polar than DEE, as deduced from the ICT/LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) (Table 1). From Figure 1, it can be seen when ICT fluorescence starts to appear for FPP as compared with PP. As mentioned in the Introduction, FPP (as well as PP) does not undergo an LE f ICT reaction in n-hexane. The limiting solvent, in which dual fluorescence just becomes visible for FPP is tetrahydrofuran (THF, ε25 ) 7.39), with Φ′(ICT)/Φ(LE) ) 0.03 (Figure 1c, Table 1), whereas for PP in this solvent the presence of an additional ICT fluorescence band on the red edge of the LE emission cannot be detected (Figure 1g, Table 1).24 The spectra presented in Figures 1 and 2 (Table 1) show that the planarization brought about by the presence of the methylene bridge in FPP enhances the ICT efficiency with respect to PP, as also observed for FPP4F and FPP4C as compared with PP4F and PP4C.3,13,19 Fluorescence Spectra of FPP and PP in Alkyl Cyanides at -40 °C. The fluorescence spectra of FPP and PP in the alkyl cyanides PrCN, EtCN, and MeCN at about -40 °C are presented in Figure 3. These spectra show that for FPP Φ′(ICT)/Φ(LE) becomes larger with increasing solvent polarity (ε data at -40 °C): 0.79 (PrCN, ε ) 34.0), 1.31 (EtCN, ε ) 38.2), and 4.44

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Figure 4. Plots of ln(Φ′(ICT)/Φ(LE)) vs the reciprocal absolute temperature 1000/T for FPP in MeCN, EtCN, PrCN, and BuCN. The LE f ICT reaction enthalpies ∆H(SB) obtained by employing eq 1, are indicated in the figure (Table 2).

When kd . 1/τ′0(ICT), the high-temperature limit (HTL)4,18,19,27,28 (see Figure 4), eq 1 simplifies to

Φ′(ICT)/Φ(LE) ) k'f(ICT)/kf(LE)(ka /kd) Figure 3. Fluorescence spectra of FPP and PP in PrCN, EtCN, and MeCN at about -40 °C. Excitation wavelength λ(exc): around 270 nm. See the caption of Figure 1.

(MeCN, ε ) 48.9). A similar behavior is found with PP at this temperature: 0.21 (PrCN), 0.34 (EtCN), and 1.11 (MeCN). The larger Φ′(ICT)/Φ(LE) ratios observed with FPP than for PP again demonstrate that the ICT reaction is more efficient for the former molecule (Table 1). Stevens-Ban Plots for FPP: ICT Reaction Enthalpy ∆H(SB). From experiments in sufficiently polar solvents (the alkyl cyanides MeCN, EtCN, PrCN, and BuCN) in which FPP shows dual (LE + ICT) fluorescence, the ICT reaction enthalpy ∆H is determined from plots of the logarithm of Φ′(ICT)/Φ(LE) (eq 1) versus the reciprocal absolute temperature. The so-called Stevens-Ban (SB)25,26 plots of ln(Φ′(ICT)/Φ(LE)) vs 1000/T of FPP in MeCN, EtCN, PrCN, and BuCN are presented in Figure 4. By fitting the plots, the enthalpy difference ∆H(SB) for the LE f ICT reaction is determined (eq 1a, Table 2). In MeCN, a larger -∆H(SB) is obtained with FPP (14.5 kJ/mol) than for PP (8.8 kJ/mol, Table 2), which explains the larger Φ′(ICT)/Φ(LE) ratio of the former system (Figure 4 and Table 1): 0.84 (FPP), 0.42 (PP) at 25 °C. A similar situation is found with FPP and PP in EtCN, PrCN, and BuCN (Table 1). In the last two solvents, Φ′(ICT)/Φ(LE) of FPP passes through a maximum: at -92 and at -105 °C (Figure 4). At these temperatures, kd ) 1/τ′0(ICT).20 This condition is not reached for FPP in MeCN and EtCN; see Figure 4. In Scheme 1 and eq 1, ka and kd are the rate constants of the forward and backward ICT reaction, τ0(LE) and τ′0(ICT) are the fluorescence lifetimes, and kf(LE) and k′f(ICT) are the radiative rate constants.

Φ′(ICT)/Φ(LE) ) k'f(ICT)/kf(LE){ka /(kd + 1/τ′0(ICT))} (1)

(1a)

Franck-Condon Energy E(FC,ICT). The energy E(FC,ICT), relative to the equilibrated S0 state, of the Franck-Condon ground state populated by the ICT fluorescence (Figure 5) can be determined via eq 2, employing the data for E(S1), ∆H(SB), and ν˜ max(ICT) from Table 2.

E(FC,ICT) ) E(S1) + ∆H - ν˜ max(ICT)

(2)

For FPP the results are 75 (MeCN), 70 (EtCN), 68 (PrCN), and 66 kJ/mol (BuCN). With PP, somewhat higher energies are obtained: 78 (MeCN), 72 (EtCN), 71 (PrCN), and 68 kJ/ mol (BuCN); see Table 2. The large magnitude of E(FC,ICT) means that these FC states are not thermally accessible from S0, ruling out a competitive direct ICT excitation bypassing LE; see Figure 5.22,29,30 The lower E(FC,ICT) values found for FPP as compared with PP can indicate that the structural changes involved in the LE f ICT reaction are somewhat smaller for the rigidified molecule. A similar situation has been encountered with the pair FPP4F/PP4F.19 Picosecond LE and ICT Fluorescence Decays of FPP in Acetonitrile at -45 °C. The LE and ICT fluorescence decays of FPP in MeCN at -45 °C (Figure 6) are double-exponential (eqs 3-9), having decay times τ2 ) 3.6 ps and τ1 ) 15.36 ns with an amplitude ratio A ) 24.4 (eq 5). In the global analysis shown, the ICT amplitude ratio A22/A21 (eq 4) is close to -1 (-0.94), indicating that LE is the precursor for ICT.4,20,26,27,29 From these data, together with τ0 ) 11.66 ns (PP4M in MeCN at -45 °C),4 the rate constants ka and kd, as well as the ICT lifetime τ′0(ICT), are determined (eqs 10-12). It is seen (Table 3) that ka of FPP (27 × 1010 s-1) is considerably larger than that of PP (9.9 × 1010 s-1). A similar finding was reported for the pairs FPP4F/PP4F and FPP4C/PP4C,13,19 showing that planarization leads to an increase of the ICT rate constant ka

Intramolecular Charge Transfer with FPP and PP

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TABLE 2: Data for FPP and PP in Four Alkyl Cyanides FPP for given solventa 25

ε -∆H(SB)c (kJ/mol) E(S1)d (cm-1) ν˜ max(ICT) (cm-1) E(FC,ICT)e (kJ/mol)

PPb for given solventa

BuCN

PrCN

EtCN

MeCN

BuCN

PrCN

EtCN

MeCN

19.8 11.2 33.94 27.47 66.2

24.2 12.3 33.99 27.31 67.6

29.2 12.8 (16.7) 34.01 27.11 69.7 (65.8)

36.7 14.5 34.01 26.54 74.9

19.8 3.9 35.15 29.14 68.0

24.2 5.1 (4.6) 35.19 28.85 70.7 (71.2)

29.2 5.9 (6.7) 35.06 28.59 71.5 (70.7)

36.7 8.8 (10.0) 35.31 28.04 78.1 (77.0)

Solvents: n-butyl cyanide (BuCN), n-propyl cyanide (PrCN), ethyl cyanide (EtCN), and acetonitrile (MeCN). Data are at 25 °C, except for ∆H. b References 4 and. 5. c Enthalpy difference between LE and ICT, determined from a plot of ln(Φ′(ICT)/Φ(LE)) vs 1000/T (eq 1). The values in parentheses come from ∆H ) Ed - Ea, determined from the temperature dependence of the ICT rate constants ka and kd (Scheme 1); see ref 4 and Table 3. d Crossing point of the fluorescence and absorption spectra (Figure 2). e Energy of the Franck-Condon (FC) state reached upon ICT emission employing ∆H(SB), see eq 2 and Figure 5. The values in parentheses are calculated by employing ∆H (Table 4). a

SCHEME 1

for the fluorazenes as compared with the corresponding flexible N-phenylpyrroles.

if(LE) ) A11 exp(-t/τ1) + A12 exp(-t/τ2)

(3)

if(ICT) ) A21 exp(-t/τ1) + A22 exp(-t/τ2) with A22 ) -A21 (4) A ) A12 /A11

(5)

The expressions for τ1, τ2, and A appearing in eqs 3-5 are20,26,29

1 1/τ1,2 ) {(X + Y) - √(X - Y)2 + 4kakd} 2

(6)

X - 1/τ1 kakdτ02 A) ) 1/τ2 - X (1 + kaτ0 - τ0 /τ1)2

(7)

where τ0 is the fluorescence lifetime of the model compound (no ICT) and X and Y are given by eqs 8 and 9.

X ) ka + 1/τ0(LE)

(8)

Y ) kd + 1/τ0(ICT)

(9)

ka ) (1/τ1 + A/τ2)/(1 + A) - 1/τ0(LE)

(10)

kd ) {(1/τ2 - 1/τ1)2 - (2ka + 2/τ0(LE) - 1/τ1 1/τ2)2}/4ka (11) 1/τ0(ICT) ) 1/τ1 + 1/τ2 - ka - kd - 1/τ0(LE)

(12) Temperature Dependence of the Fluorescence Decay Parameters of FPP in EtCN and PrCN. A global analysis of the LE and ICT fluorescence response functions of FPP in EtCN

at -65 °C is depicted in Figure 7. Similar to FPP in MeCN at -45 °C (Figure 6), the ratio A22/A21 of the ICT amplitudes (eq 4) is close to -1 (-1.03), again indicating that the ICT state is not formed directly upon excitation but originates from the LE precursor. The fluorescence decay times τ1 and τ2 with the amplitude ratio A (eqs 3-9) of FPP in EtCN, measured as a function of temperature, are presented in Figure 8. The lifetimes τ0 of the model compound PP4M (no ICT) are also included (Table 3). A global analysis of the LE and ICT decays of FPP in PrCN at -60 and -100 °C (Table 3) is shown in Figure S1 of the Supporting Information. Thermodynamic ICT Parameters of FPP in EtCN. From the data for τ1, τ2, A, and τ0 of FPP in EtCN in Figure 8, the rate constants ka and kd, as well as the ICT lifetime τ′0(ICT), are calculated (Scheme 1) by employing eqs 10-12. The Arrhenius plots for ka, kd, and τ′0(ICT) are presented in Figure 9. The activation energies Ei and preexponential factors ki° determined from these plots are collected in Table 4. From these data, ∆H ()Ea - Ed) and ∆S ) R ln(ka°/kd°) are determined. The enthalpy difference ∆H ) -16.7 kJ/mol is somewhat larger than that obtained from the Stevens-Ban plot (-12.8 kJ/mol; Figure 4, Table 4). A similar difference has been found for PP.4 LE/ICT Fluorescence Decay Analysis for FPP in MeCN, EtCN, and PrCN. The LE and ICT fluorescence decays of FPP in MeCN and EtCN are double-exponential (Figures 6 and 7, Table 3). As further proof that only the two states LE and ICT are involved in the excited-state reaction (Scheme 1), the ICT decay if(ICT) has been deconvoluted with the LE decay if(LE) as a formal excitation pulse profile.4,31,32 A single-exponential LE/ICT decay is obtained, with a decay time τ(LC); see eq 13 and Table 3. Good agreement between the experimental τ(LC) and the calculated 1/(kd + 1/τ′0(ICT)) is found. These results show that the two excited states LE and ICT are sufficient for an analysis of the ICT reaction with FPP in the alkyl cyanides. The same conclusion was reached for PP.4

τ(LC) ) 1/(kd + 1/τ0(ICT))

(13)

Comparison of ICT Reaction Data for FPP and PP: Influence of Planarization. Rate Constants. The planarization of FPP brings about an enhancement of the ICT reaction efficiency as compared with its flexible counterpart PP.3,4 This becomes evident from the LE f ICT rate constant ka, which is considerably larger for FPP (27 × 1010 s-1) than for PP (9.9 × 1010 s-1) in MeCN at -45 °C (Table 3) and likewise in EtCN at this temperature: 9.8 × 1010 s-1 (FPP) and 3.9 × 1010 s-1 (PP). For the ICT f LE back-reaction kd, the opposite influence is observed: 1.1 × 1010 s-1 (FPP) vs 1.6 × 1010 s-1 (PP) in

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TABLE 3: Fluorescence Decay Parameters (τ2, τ1, A, τ0; Equations 3-9 and Scheme 1), ICT Reaction Rate Constants ka and kd, ICT Lifetime τ′0(ICT), LE/ICT Decay Time τ(LC) (Equation 13), and Free Enthalpy Difference ∆G for FPP, PP, FPP4F, PP4F, and DMABN in Acetonitrile (MeCN), Ethyl Cyanide (EtCN), and n-Propyl Cyanide (PrCN) FPP MeCN EtCN EtCN EtCN EtCN PrCN PrCN PPe MeCN EtCN PrCN FPP4F MeCN EtCN EtCN PP4F MeCN EtCN PrCN DMABNf MeCN

T (°C)

τ2 (ps)

τ1 (ns)

A

τ0(PP4M)a (ns)

ka (1010 s-1)

kd (1010 s-1)

τ′0(ICT) (ns)

-45 -45 -55 -65 -75 -60 -100

3.6 9.0 10.0 11.1 13.2 26 108

15.36 12.10 13.15 13.61 14.44 13.24 14.78

24.4 7.54 11.2 17.7 28.5 8.55 49.17

11.66 11.73 11.89 12.06 12.23 12.1 12.7

27 9.8 9.2 8.5 7.3 3.4 0.90

1.09 1.30 0.82 0.48 0.26 0.40 0.018

15.57 12.15 13.28 13.71 14.54 13.39 14.83

-45 -45 -45

8.7 15.4 31.3

5.88 6.41 7.38

6.39 1.42 0.71

11.66 11.73 11.77

9.92 3.88 1.33

1.55 2.66 1.85

-45 -45 -85

2.0 2.8 6.4

7.42 6.79 7.99

75.8 40.6 108

5.64 5.64 5.96

48.8 34.8 15.5

-45 -45 -45

5.10 14.0 35.0

4.83 4.08 4.60

31.4 5.76 2.60

5.75 5.75 5.75

-45

7.63

3.53

14472g

3.72h

τ(LC)b (ps)

∆Gc (kJ/mol)

97 (91) 76 (90) 122 (121) 206 (205) 363 (375) 260 (244) (3986)d

-6.07 -3.83 -4.38 -4.97 -5.94 -3.79 -5.63

5.46 4.87 4.86

67 (64) 39 (37) 56 (53)

-3.5 -0.81 +0.63

0.64 0.86 0.14

7.45 6.81 8.02

(152) 150 (115) 707 (642)

-8.2 -7.0 -7.4

19.0 6.1 2.1

0.60 1.05 0.79

4.81 3.88 4.27

155 (160) 96 (93) 124 (124)

-6.6 -3.3 -1.8

13.08

0.0009

3.53

(3421)i

-18.2

a Decay times measured in isooctane (no ICT reaction). b Value in parentheses is the calculated decay time τ(LC) ) 1/(kd + 1/τ′0(ICT)) (eq 13). c ∆G ) -RT ln(ka°/kd°). d Time determined by solvent relaxation; cf. PP in PrCN.5 e References 4 and 5. f Reference 20. g Estimated from data in ref 20. h Decay time of 4-(methylamino)benzonitrile.20 i Experimental τ(LC) not accessible because of too high LE amplitude ratio A (eq 7); see ref 20.

Figure 5. Potential energy surfaces for the ground state S0 and the excited states S1, S2, LE, and ICT. When excited to the S2 state (with an energy gap of ∆E(S1,S2) above S1), the system relaxes by internal conversion to the equilibrated LE state, having an energy E(LE) above S0. The ICT reaction proceeds from the LE to the ICT state, with a reaction barrier Ea and an enthalpy difference ∆H. Fluorescence from the LE and ICT states, with emission maxima ν˜ max(LE) and ν˜ max(ICT), reaches the corresponding Franck-Condon states E(FC,LE) and E(FC,ICT), respectively.

MeCN at -45 °C and 1.3 × 1010 s-1 (FPP) vs 2.7 × 1010 s-1 (PP) in EtCN at -45 °C. These differences can be understood by looking at the activation energies Ea and Ed, the enthalpy difference ∆H, and the preexponential factors ka° and kd°. Arhenius Parameters. The Arrhenius parameters Ea, Ed, ka°, kd°, ∆H, and ∆S for FPP (Figure 9) and PP in EtCN are listed in Table 4. By introducing a methylene bridge in PP, giving FPP, -∆H increases from 6.7 to 16.7 kJ/mol. The planarization

Figure 6. Global analysis for double-exponential LE and ICT fluorescence decays of FPP in acetonitrile (MeCN) at -45 °C. The emission wavelengths are 305 (LE) and 425 nm (ICT). The decay times τ2 and τ1 with the corresponding amplitudes A1i(LE) and A2i(ICT), see eqs 3-5, are given in the figure. The shortest decay time τ2 is listed first. The weighted deviations σ, the autocorrelation functions A-C, and the values for χ2 are also indicated: excitation wavelength, 272 nm; time resolution, 0.496 ps/channel with a time window of 1200 effective channels.

not only leads to an increase of -∆H but also results in a decrease of the activation energy Ea from 9.0 to 3.7 kJ/mol. This lowering of the ICT reaction barrier induces a smaller preexponential factor ka°: 0.69 × 1012 s-1 (FPP) and 4.2 × 1012 s-1 (PP); see Figure 10.4 As kd° similarly correlates with Ed, 610 × 1012 s-1 and 20.4 kJ/mol for FPP versus 110 × 1012 s-1 and 15.7 kJ/mol for PP (Figure 10),33 a relationship between


Figure 7. Global analysis for double-exponential LE and ICT fluorescence decays of FPP in ethyl cyanide (EtCN) at -65 °C. The emission wavelengths are 305 (LE) and 425 nm (ICT). The decay times τ2 and τ1 with the corresponding amplitudes A1i(LE) and A2i(ICT), see eqs 3-5, are given in the figure. The shortest decay time τ2 is listed first. The weighted deviations σ, the autocorrelation functions A-C, and the values for χ2 are also indicated: excitation wavelength, 272 nm; time resolution, 0.496 ps/channel with a time window of 1200 effective channels. See the caption of Figure 6.


Figure 9. FPP in EtCN. Arrhenius plots of the forward (ka) and backward (kd) ICT rate constants (eqs 10 and 11) vs the reciprocal absolute temperature. The reciprocal ICT lifetime 1/τ′0(ICT) is also shown (eq 12). See Scheme 1 and Table 3.

TABLE 4: Activation Energies Ea and Ed, Preexponential Factors ka° and kd°, Enthalpy Difference ∆H, and Entropy Difference ∆S for the ICT Reaction of FPP, PP, PP4F, and DMABN in n-Propyl Cyanide (PrCN), Ethyl Cyanide (EtCN), and Acetonitrile (MeCN) ∆Sc Ed k a° k d° -∆H -∆H(SB) (J K-1 Ea (kJ/mol) (kJ/mol) (1012 s-1) (1012 s-1) (kJ/mol) (kJ/mol) mol-1) a

FPP EtCN PPd MeCN EtCN PrCN PP4Fe EtCN DMABNf MeCN

b

-56.5

3.7

20.4

0.7

610

16.7

12.8

5.7 9.0 12.4

15.7 15.7 17.0

2.1 4.2 9.2

65 109 150

10.0 6.7 4.6

8.8 5.9 5.1

5.9

20.0

1.5

300

14.0

14.0

-44.2

5.0

32.0

1.83

186.8

27.0

23.2

-38

-28 -27 -23

∆H ) Ea - Ed. From plots of Φ′(ICT)/Φ(LE) vs 1000/T (eq 1), Figure 4, and Table 2. c ∆S ) R ln(ka°/kd°). d References 4 and 21. e Reference 19. f Reference 20. a

Figure 8. Fluorescence decay parameters of FPP in EtCN as a function of temperature: decay times τ1 and τ2, amplitude ratio A (eqs 3-9). The lifetime τ0 of the model compound PP4M is also indicated; see Table 3.

∆H and ∆S ()R ln(ka°/kd°)) is obtained for FPP relative to PP: -16.7 and -6.7 kJ/mol versus -56.5 and -27 J K-1 mol-1 (Table 4). Such a correlation also was found for PP in the solvent series MeCN, EtCN, and PrCN (Table 4).19 A similar ∆H/∆S relationship has been observed for intramolecular excimer formation with 1,3-dipyrenylpropanes.34,35 The increase in ka observed for FPP relative to PP discussed above (Table 3) is caused by the smaller Ea of the former molecule. The lower values for kd in the case of FPP are due to the larger -∆H, in EtCN 16.7 kJ/mol (FPP) and 6.7 kJ/mol (PP), and for MeCN -∆H(SB), 14.5 kJ/mol (FPP) and 8.8 kJ/mol (PP). Comparison of ICT Reaction Data of FPP and FPP4F: Influence of F-Substitution. Upon F-substitution of FPP, the LE f ICT rate constant ka in MeCN at -45 °C increases, from

b

27 × 1010 to 49 × 1010 s-1 (FFP4F), with a corresponding decrease of the back-reaction kd from 1.1 × 1010 to 0.64 × 1010 s-1 (Table 3). A similar observation is made for the two molecules in EtCN at -45 °C. These effects are both caused by the increase of the enthalpy difference for FPP4F with respect to FPP: -19.2 and -14.5 kJ/mol (Table 2) in MeCN.19 For the ultrafast ICT reaction of FPP4C, SPC data cannot be obtained.13 A comparison of FPP4C with FPP and FPP4F will hence be made in a later section by employing the femtosecond transient absorption experiments. Transient Absorption Spectra of FPP in n-Hexane. The transient absorption spectra of FPP in n-hexane at 290 nm excitation are presented in Figure 11a. After correcting for the stimulated emission (SE), the excited-state absorption (ESA) spectra are obtained (Figure 11b), with a main band maximum at 825 nm and a smaller maximum at 425 nm (Table 5). Because only a LE emission is observed in the fluorescence spectrum of FPP in n-hexane (Figure 1a),3 the ESA band is attributed to the LE state. In support of this conclusion, the band integral BI(780,860), between 780 and 860 nm in the ESA spectrum, does not show any time development (Figure 11c). With an

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Figure 10. Plot of ln(ka°), full circles, and ln(kd°), closed circles, versus the corresponding activation energies Ea and Ed, for FPP, PP, and PP4F. See Table 4.

excitation wavelength of 266 nm, practically the same ESA spectrum is found (Figure S2 in the Supporting Information).

BI ) A2 exp(-t/τ2) + A1 exp(-t/τ1) + A0

(14)

Transient Absorption Spectra of PP in n-Hexane. The transient absorption spectra of PP in n-hexane at 266 nm excitation, with pump-probe delay times between 0.2 and 110 ps, are shown in Figure 12a. The ESA spectra (Figure 12b), obtained by correcting for the stimulated emission, again consist of a main band, with a maximum at 840 nm and a smaller peak at 410 nm (Table 5). The spectra closely resemble those of FPP (Figure 11), indicating that both molecules have a planar LE structure. Because an ICT reaction does not take place with PP in n-hexane (Figure 1e), the ESA spectra are therefore likewise attributed to the LE state. The band integral BI(780,880) of PP shows a decay of 4.5 ps, caused by vibrational cooling, as observed with the ICT reaction of FPP4F/PP4F,19 FPP4C/ PP4C,13 DMABN,20 and NTC6.18 LE ESA Spectrum of PP4M. The transient absorption and ESA spectra of PP4M in MeCN at pump-probe delay times between 0.4 and 120 ps are shown in Figure S3 (Supporting Information). From the fact that PP4M does not undergo an ICT reaction, irrespective of solvent polarity,4,19,36 it follows that the ESA spectra of PP4M in Figure S3, having a main maximum at 770 nm (Table 5), are those of the LE state of PP4M in MeCN. Their similarity with the ESA spectra of FPP and PP in n-hexane (Figures 11b and 12b) fully supports the identification of these spectra as those of a LE state. Transient Absorption Spectra of FPP in MeCN. The transient absorption spectra of FPP in MeCN with an excitation wavelength of 290 nm are shown in Figure 13a. After correction for stimulated emission SE(LE) and SE(ICT), the ESA spectra are obtained at pump-probe delay times between 0.2 and 120 ps (Figure 13b). The ESA absorption peak at 800 nm is attributed to the LE state (Table 5), based on the similarity with the ESA spectra of FPP in n-hexane (Figure 11b). The peak at 365 nm shows a growing-in and is hence part of the ICT ESA spectrum. The residual absorption around 800 nm at a delay of

Figure 11. FPP in n-hexane at 290 nm excitation. (a) Transient absorption spectra and (b) excited-state absorption (ESA) spectra after correction for the LE stimulated emission (SE(LE)) at pump-probe delay times between 0.2 and 100 ps. The SE spectrum (LE, cf. Figure 1a) is also depicted. The spectral range extends from 340 to 1040 nm. (c) The band integral BI(780,860), between 780 and 860 in the ESA spectrum, does not show any time development. The offset A0 (eq 14) is also indicated. m∆OD is the optical density/1000.

120 ps is due to LE, in equilibrium with ICT. The BI(760,840) decays with a time τ2 ) 1.6 ps (eq 14). This time τ2 is the LE f ICT reaction time of FPP in MeCN (Table 6). A rise time of the ICT absorption band with a maximum at 365 nm is found from BI(350,395), likewise with a time of 1.6 ps obtained from a global analysis of the two BI’s in Figure 13c. From the transient absorption and ESA spectra of FPP in MeCN at 266 nm excitation (Figure S4, Supporting Information), at pumpprobe delay times between 0.2 and 120 ps, a similar decay time τ2 ) 1.55 ps is determined from BI(760,840). The longer decay time τ1 ) 13.5 ps (Figure 13c) is again due to vibrational cooling.13,18,20 Transient Absorption Spectra of PP in MeCN. The transient absorption spectra of PP in MeCN at an excitation wavelength of 290 nm are presented in Figure 14a. After correction for stimulated emission SE(LE) and SE(ICT), the ESA spectra are obtained (Figure 14b). The absorption peaks at 495 and 840 nm are part of the LE ESA spectrum. BI(780,880) shows a decay with a time τ2 of 4.0 ps (Figure



TABLE 5: Excited-State Absorption (ESA) Maxima for FPP, PP, and PP4M in n-Hexane and Acetonitrile (MeCN) at 290 and 266 nm Excitationa ESA maximab (nm) n-hexane FPP

825, 425 (LE)

PP

840, 410 (LE)

PP4M FPP4Fc

320, 410, 730 (LE)

PP4Fc FPP4Cd PP4Cd DMABNe

315, 415, 495, 740 (LE) 444, 700 (LE + ICT) 378, 444, 729 (LE + ICT) 300-320, 445, 745 (LE)

MeCN 800 (LE) 365, (605) (ICT) 840 (LE) 370, 495 (ICT) 400, 770 (LE) 295, 325, (405), 570, 735 (ICT + LE) 320, 405, 740 (ICT + LE) 377, 500 (ICT) below 340, 453 (ICT) 320, 355, 440, 710 (LE) 315, 425 (ICT)

a See Figures 11-17 and Figures S2-S6. Data for FPP4F, PP4F, FPP4C, PP4C, and DMABN are included for comparison. Main maxima are in bold. b ICT maxima in italics. c Reference 19. d Reference 13. e Reference 20.

Figure 13. FPP in acetonitrile (MeCN) at 290 nm excitation. (a) Transient absorption spectra and (b) ESA spectra after correction for the stimulated emission SE(LE) and SE(ICT), with pump-probe delay times between 0.2 and 120 ps. The SE(LE) and SE(ICT) spectra and their time development are also shown; cf. Figure 2a. The downward arrow (800 nm) represents the decay of the LE state. The spectral range extends from 340 to 1040 nm. (c) Analysis (eq 14) of the band integrals BI(350,395) and BI(760,840): a global analysis of the two BI’s results in a time τ2 ) 1.6 ps. Note the growing-in of BI(350,395) in the spectral range of the ICT absorption. See the caption of Figure 11.

TABLE 6: Decay Times and LE f ICT Reaction Times Derived from Femtosecond ESA Spectra of FPP, PP, FPP4F, PP4F, FPP4C, PP4C, and DMABN in Acetonitrile (MeCN) at 22 °C

Figure 12. PP in n-hexane at 266 nm excitation. (a) Transient absorption spectra and (b) ESA spectra, attributed to LE, after correction for the stimulated emission (SE(LE); cf. ref 4), at pump-probe delay times between 0.2 and 110 ps. The spectral range extends from 340 to 1040 nm. (c) The band integral BI(780,880) shows a decay of 4.5 ps; see text. See the caption of Figure 11.

14c). A growing-in is found for BI(350,410), covering part of the ICT ESA spectrum, with the same time τ2 ) 4.0 ps (global

FPP PP FPP4Fc PP4Fc FPP4Cd PP4Cd DMABNe

τ2a (ps)

1/τ2b (1010 s-1)

1.6 4.0 1.2 3.3 0.087f 0.067f 4.07f

63 25 94 30 1200 1500 24.6

a See eq 14. b ICT reaction time 1/τ2 ) ka; see Scheme 1. This value is an upper limit; see footnote 53 in ref 19. c Reference 19. d Reference 13. e Reference 20. f Shortest decay time in femtosecond excited-state absorption spectrum.13

analysis). This time corresponds to the LE f ICT reaction time (Table 6). At 266 nm excitation (Figure S5c, Supporting

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Figure 15. ESA spectra of (a) FPP, (b) FPP4F, and (c) FPP4C in n-hexane at a pump-probe delay time of 0.2 ps, with 290 nm excitation. The maxima at 825 and 425 nm (FPP), 730 and 410 nm (FPP4F), and around 740 nm (FPP4C) are attributed to LE. In the ESA spectrum of FPP4C an ICT band appears with a maximum at 440 nm. The spectral range extends from 340 to 1040 nm. See the caption of Figure 11. Figure 14. PP in acetonitrile (MeCN) at 290 nm excitation. (a) Transient absorption spectra and (b) ESA spectra after correction for the stimulated emission (SE), at pump-probe delay times between 0.2 and 120 ps. The time development of the SE(ICT) fluorescence spectrum is also shown; cf. ref 4. In b, the downward arrow (840 nm) indicates the decay of the LE absorption, whereas the upward arrow (370 nm) represents the growing-in of the ICT absorption. The spectral range extends from 340 to 1040 nm. (c) Decay curves of the band integrals BI(350,410) and BI(780,880), eq 14. See the caption of Figure 11.

Information) τ2 ) 4.0 ps, from BI(350,420) and BI(780,880). The observation that the ICT ESA spectra of FPP as well as those of PP show a growing-in over the same spectral range could be an indication that the ICT structure of PP is planar, contrary to the results of calculations.7,8 LE f ICT Reaction Times. The LE f ICT reaction times obtained from the ESA spectra of FPP and PP at 22 °C (Figures 13 and 14, and Figures S4 and S5 of the Supporting Information) are collected in Table 6. The reaction time τ2 in MeCN is smaller for FPP than for PP, as also found for the rate constant ka at -45 °C in this solvent (Table 3). The introduction of a F-substituent (FPP4F and PP4F) makes the ICT reaction faster, although to a much smaller extent than for CN substitution in the case of FPP4C and PP4C. LE and ICT ESA Band Maxima of FPP and PP (Table 5). LE Maxima in n-Hexane and MeCN. The main maximum in the LE ESA spectrum of FPP occurs at 825 nm in n-hexane and 800 nm in MeCN, whereas with PP an LE maximum is found at 840 nm in both solvents (Table 5). For PP4M in MeCN an LE spectrum with a peak at 770 nm is determined. These

maxima occur in the same spectral range as those observed around 730-740 nm for the LE state of the fluorazene and N-phenylpyrrole derivatives FPP4F, FPP4C, PP4F, and PP4C, as well as for DMABN in n-hexane and MeCN (Table 5). ICT Maxima in MeCN. The main ICT ESA maxima in MeCN are located at 365 nm for FPP and at 370 nm for PP. With FPP4F, these maxima occur at 295 nm and at 405 nm for PP4F. In the case of FPP4C ESA maxima are found at 377 nm and below 340 for PP4C. For comparison, the ICT ESA maximum of DMABN in MeCN is observed at 315 nm. The similarity of the ESA spectra of FPP with those of PP in MeCN leads to the conclusion that both ICT states have a planar structure. This conclusion differs from the results of calculations that find a perpendicularly twisted configuration for the lowestenergy ICT structure of PP,7,8 as mentioned in the Introduction. ESA Spectra of FPP, FPP4F, and FPP4C in n-Hexane and MeCN at 0.2 and 120 ps Pump-Probe Delay Times. The ESA spectra of FPP in n-hexane are compared with those of FPP4F and FPP4C at two different pump-probe delay times ∆t in Figure 15 (∆t ) 0.2 ps) and Figure S6 in the Supporting Information (∆t ) 120 ps). From this comparison, support is obtained for the identification of the LE and ICT bands in the ESA spectra of fluorazenes FPPX and the N-phenylpyrroles PPX. As for FPP3 and FPP4F19 an ICT reaction does not occur in n-hexane; the ESA spectra at both delay times with main maxima at 825 (FPP) and 730 nm (FPP4F) can therefore be attributed to LE. FPP4C undergoes an ultrafast ICT reaction, a rapid equilibrium being established between the LE and ICT states.13 The LE (around 740 nm; see Figure 15) as well as the ICT (440 nm) absorption maxima are already present at ∆t )


Figure 16. ESA spectra of (a) FPP, (b) FPP4F, and (c) FPP4C in acetonitrile (MeCN) at a pump-probe delay time of 0.2 ps, with 290 nm excitation. The maxima at 800 and 405 nm (FPP), 730 and 405 nm (FPP4F), and around 700 nm (FPP4C) are attributed to LE. In the ESA spectrum of FPP4C an ICT band appears with a maximum at ∼400 nm. The spectral range extends from 340 to 1040 nm. See the caption of Figure 11.

0.2 ps, the contribution of the ICT band to the overall ESA spectrum becoming larger at ∆t ) 120 ps. In MeCN, an LE f ICT reaction has been observed for all three fluorazenes FPP, FPP4F, and FPP4C, although with different ICT reaction times (Table 6).3,13,19 Because of the relatively slow ICT reactions of FPP (1.6 ps) and FPP4F (1.2 ps), the ESA spectra of FPP and FPP4F at the short time delay ∆t ) 0.2 ps (Figure 16a,b) with a just-developing ICT contribution are similar to the LE spectra in n-hexane (Figure 15a,b). In the ESA spectrum of FPP in MeCN at ∆t ) 120 ps (Figure 17a), an ICT maximum has appeared at 365 nm and also at 605 nm, alongside the LE maximum at around 790 nm. The maximum at ∼605 nm is attributed to the ICT state. With FPP4F at a longer delay time of 120 ps (Figure 17b) a new weak maximum is present at 570 nm. For FPP4C in MeCN, with an ICT reaction time below 100 fs (Table 6),13 the LE band (around 740 nm) is still present in the ESA spectrum at ∆t ) 0.2 ps (Figure 16c) but has practically disappeared at ∆t ) 120 ps (Figure 17c). The ESA spectrum in Figure 17c, having maxima at 380 and 500 nm, therefore is that of the ICT state of FPP4C in MeCN. The development of the weaker maxima 605, 570, and 500 nm in the series FPP, FPP4F, and FPP4C is taken as a support for their identification as ICT bands. πσ* State Unlikely as an ICT Intermediate. For all three fluorazenes FPPX (FPP, FPP4F, and FPP4C) as well as for the N-phenylpyrroles PPX (PP, PP4F, PP4C, and PP4M), the main LE absorption band in n-hexane and MeCN occurs between 700 and 825 nm (fluorazenes) and between 730 and 840 nm


Figure 17. ESA spectra of (a) FPP, (b) FPP4F, and (c) FPP4C in acetonitrile (MeCN) at a pump-probe delay time of 120 ps, with 290 nm excitation. In the ESA spectrum of FPP4C an ICT band appears with a maximum at 380 nm. The spectral range extends from 340 to 1040 nm. See the caption of Figure 11.

for the N-phenylpyrroles (Table 5). These LE spectra are similar to that of DMABN, which has LE maxima at 745 nm in n-hexane and 710 nm in MeCN.20 The LE ESA band of DMABN was recently ascribed to a πσ* state with a nonlinear rehydbridized (bent) cyano group, on the basis of computations.37-39 Such a state is, however, not likely for FPPX and PPX molecules without a substituent that can be bent such as C-C≡N or C-C≡C.40 Also the fact that several DMABN derivatives with a para-subtituent different from C≡N, such as C(dO)OR, C(dO)R, C(dO)NR2, and CF3, undergo an ICT reaction,22,41 speaks against the intermediacy of a πσ* state in these reactions. Conclusions The planarization and rigidization introduced into FPP by its methylene linker group lead to an enhancement of the ICT reaction as compared with PP. This is seen from the fluorescence spectra, in which an ICT emission appears in solvents of lower polarity than for PP. With FPP, dual ICT + LE fluorescence is beginning to be observed in the medium polar THF at 25 °C, whereas only LE emission is found for PP in this solvent. In the polar solvent MeCN, the ICT rate constant ka of FPP is with 27 × 1010 s-1 at -45 °C considerably larger than that of PP (9.9 × 1010 s-1) under these conditions. A similar result is obtained in the less polar solvent EtCN at -45 °C, with 9.8 × 1010 s-1 for FPP and 3.9 × 1010 s-1 for PP. This larger efficiency of the ICT reaction of FPP with respect to PP is reflected in the larger enthalpy difference ∆H: 16.7 kJ/mol for FPP as compared with 6.7 kJ/mol for PP in EtCN. Related ∆H(SB) data, determined from Stevens-Ban plots of ln(Φ′(ICT)/Φ(LE)) vs 1000/T in MeCN, EtCN, PrCN, and BuCN, allow a similar conclusion.

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The femtosecond ESA spectra of FPP and PP in n-hexane show that an ICT reaction does not occur in this solvent. Both LE spectra are similar to that of PP4M, for which such a reaction likewise is not found. In the polar solvent MeCN at 22 °C, an ICT reaction time of 1.6 ps is determined, shorter than the 4.0 ps measured for PP in this solvent. The similarity of the ESA spectra of FPP with those of PP in MeCN, with ICT absorption maxima at 365 nm (FPP) and 370 nm (PP), leads to the conclusion that both ICT states have a planar structure. This conclusion differs from the results of calculations that find a perpendicularly twisted configuration for the lowest-energy ICT structure of PP. This could be due to an overemphasis caused by dynamic electron correlation of twisted and hence electronically decoupled phenyl and pyrrole subgroups in this molecule. A comparison of the ESA spectra of FPP with those of its derivatives FPP4F and FPP4C shows that the introduction of a F-substitution in FPP leads to a somewhat faster ICT reaction, in accordance with the relatively weak electron acceptor properties of F. A much faster ICT reaction takes place for FPP4C in n-hexane as well as in MeCN as a result of the incorporation of the CN substituent, a strong electron acceptor. The LE ESA spectrum of FPP is similar to that of PP, with a main band at 800 and 840 nm, respectively. Comparable LE ESA spectra have also been measured previously for FPP4F (730 nm) and PP4F (740 nm) as well as for FPP4C (700 nm) and PP4C (730 nm). Perhaps somewhat unexpectedly, the LE ESA spectrum of DMABN has similar features, also with a strong absorption band at 745 nm in n-hexane and 710 nm in MeCN. From these observations it is concluded that these LE bands of FPPX and PPX cannot be attributed to a πσ* state. Acknowledgment. Many thanks are due to Prof. N. P. Ernsting (Humboldt University Berlin) for the use of the femtosecond absorption equipment in the investigations reported here. We thank Mr. J. Bienert for carrying out HPLC purifications and Mr. H. Lesche for technical support. A.D. gratefully acknowledges the support of the Hungarian Science Foundation (OTKA ID No. 76278). Supporting Information Available: LE and ICT fluorescence decays and transient absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) The definition of the term “LE state”. When an electron donor/ acceptor (D/A) molecule only emits a single fluorescence band, this is called a LE fluorescence, irrespective of the dipole moment µe(LE) of this state. In such a case, we say that an ICT reaction does not take place. We do speak of an ICT reaction only when a new excited ICT state is produced from the relaxed initially excited S1(LE) state as the precursor. For this ICT state the condition µe(ICT) > µe(LE) generally holds. A detailed discussion of the limits of the LE nomenclature is presented in ref 2. (2) Zachariasse, K. A.; Druzhinin, S. I.; Galievsky, V. A.; Kovalenko, S.; Senyushkina, T. A.; Mayer, P.; Noltemeyer, M.; Boggio-Pasqua, M.; Robb, M. A. J. Phys. Chem. A 2009, 113, 2693. (3) Yoshihara, T.; Druzhinin, S. I.; Zachariasse, K. A. J. Am. Chem. Soc. 2004, 126, 8535. (4) Yoshihara, T.; Druzhinin, S. I.; Demeter, A.; Kocher, N.; Stalke, D.; Zachariasse, K. A. J. Phys. Chem. A 2005, 109, 1497. (5) Druzhinin, S. I.; Galievsky, V. A.; Yoshihara, T.; Zachariasse, K. A. J. Phys. Chem. A 2006, 110, 12760.

Druzhinin et al. (6) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: Berlin, 2006; p 220. (7) Xu, X.; Cao, Z.; Zhang, Q. J. Phys. Chem. A 2006, 110, 1740. (8) He, R.-X.; Li, X.-Y. Chem. Phys. 2007, 332, 325. (9) Borden, W. T.; Davidson, E. R. Acc. Chem. Res. 1996, 29, 67. (10) (a) Amatatsu, Y. J. Phys. Chem. A 2005, 109, 7225. (b) Go´mez, I.; Mercier, Y.; Reguero, M. J. Phys. Chem. A 2006, 110, 11455. (11) Ha¨ttig, C.; Hellweg, A.; Ko¨hn, A. J. Am. Chem. Soc. 2006, 128, 15672. (12) Neubauer, A.; Bendig, J.; Rettig, W. Chem. Phys. 2009, 358, 235. (13) Druzhinin, S. I.; Kovalenko, S. A.; Senyushkina, T. A.; Demeter, A.; Machinek, R.; Noltemeyer, M.; Zachariasse, K. A. J. Phys. Chem. A 2008, 112, 8238. Erratum: J. Phys. Chem. A 2009, 113, 520. (14) Yoshihara, T.; Galievsky, V. A.; Druzhinin, S. I.; Saha, S.; Zachariasse, K. A. Photochem. Photobiol. Sci. 2003, 2, 342. (15) Cornelissen-Gude, C.; Rettig, W. J. Phys. Chem. A 1998, 102, 7754. (16) Murali, S.; Rettig, W. J. Phys. Chem. A 2006, 110, 28. (17) Zachariasse, K. A.; Druzhinin, S. I.; Bosch, W.; Machinek, R. J. Am. Chem. Soc. 2004, 126, 1705. (18) Druzhinin, S. I.; Kovalenko, S. A.; Senyushkina, T.; Zachariasse, K. A. J. Phys. Chem. A 2007, 111, 12878. (19) Druzhinin, S. I.; Kovalenko, S. A.; Senyushkina, T. A.; Demeter, A.; Januskevicius, R.; Stalke, D.; Machinek, R.; Zachariasse, K. A. J. Phys. Chem. A 2009, 113, 9304. (20) Druzhinin, S. I.; Ernsting, N. P.; Kovalenko, S. A.; Pe´rez Lustres, L.; Senyushkina, T.; Zachariasse, K. A. J. Phys. Chem. A 2006, 110, 2955. (21) Druzhinin, S. I.; Galievsky, V. A.; Yoshihara, T.; Zachariasse, K. A. J. Phys. Chem. A 2006, 110, 12760. (22) Galievsky, V. A.; Zachariasse, K. A. Acta Phys. Pol., A 2007, 112, S-39. (23) Zachariasse, K. A.; Grobys, M.; Tauer, E. Chem. Phys. Lett. 1997, 274, 372. (24) In the subtraction procedure to separate a weak ICT emission from the main LE fluorescence band, a problem arises. The fluorescence spectrum adopted for LE is not exactly equal to the LE emission, but rather is the spectrum of a model compound, such as PP4M employed here, which is at best similar but certainly not identical to the real LE fluorescence band. Therefore, Φ′(ICT)/Φ(LE) ratios of 0.02 such as that reported by us4 for PP in THF at 25 °C are at the limit of the possible experimental accuracy, although the value of 0.02 was obtained by analyzing a series of measurements as a function of temperature in THF, with Φ′(ICT)/Φ(LE) ) 0.05 at -104 °C, at which temperature the ICT fluorescence band is clearly visible; see ref 4. (25) Stevens, B.; Ban, M. I. Trans. Faraday Soc. 1964, 60, 1515. (26) Leinhos, U.; Ku¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. 1991, 95, 2013. (27) Il’ichev, Yu, V.; Ku¨hnle, W.; Zachariasse, K. A. J. Phys. Chem. A 1998, 102, 5670. (28) Druzhinin, S. I.; Galievsky, V. A.; Zachariasse, K. A. J. Phys. Chem. A 2005, 109, 11213. (29) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (30) Go´mez, I.; Reguero, M.; Boggio-Pasqua, M.; Robb, M. A. J. Am. Chem. Soc. 2005, 127, 7119. (31) Conte, J. C.; Martinho, J. M. G. Chem. Phys. Lett. 1987, 134, 350. (32) Zachariasse, K. A.; Yoshihara, T.; Druzhinin, S. I. J. Phys. Chem. A 2002, 106, 6325. Erratum: J. Phys. Chem. A 2002, 106, 8978. (33) Leffler, J. E.; Grunwald, E. Rates and Equilibria of Organic Reactions; Dover: New York, 1989. (34) Duveneck, G.; Zachariasse, K. A. J. Am. Chem. Soc. 1987, 109, 3790. (35) Williams, A. Free Energy Relationships in Organic and BioOrganic Chemistry; The Royal Society of Chemistry: Cambridge, U.K., 2003. (36) Galievsky, V. A.; Druzhinin, S. I.; Saha, S.; Zachariasse, K. A. Photochem. Photobiol. Sci. 2003, 2, 342. (37) Lee, J.-K.; Fujiwara, T.; Kofron, W. G.; Zgierski, M. Z.; Lim, E. C. J. Chem. Phys. 2008, 128, 164512. (38) Zgierski, M. Z.; Lim, E. C. J. Chem. Phys. 2004, 121, 2462. (39) Zgierski, M. Z.; Lim, E. C. J. Chem. Phys. 2005, 122, 111103. (40) Zachariasse, K. A.; Druzhinin, S. I.; Kovalenko, S. A.; Senyushkina, T. J. Chem. Phys. 2009, 131, 224313. (41) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. ReV. 2003, 103, 3899.

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