Ultrafast Intramolecular Charge Transfer with N-(4

Nov 11, 2010 - (A) molecule N-(4-cyanophenyl)carbazole (NP4CN) in the polar ... n-hexane.1 This is different from most other D/A molecules .... S1, si...
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J. Phys. Chem. A 2010, 114, 12622–12638

Ultrafast Intramolecular Charge Transfer with N-(4-Cyanophenyl)carbazole. Evidence for a LE Precursor and Dual LE + ICT Fluorescence Victor A. Galievsky,*,†,‡ Sergey I. Druzhinin,*,†,4 Attila Demeter,§ Peter Mayer,| Sergey A. Kovalenko,*,⊥ Tamara A. Senyushkina,† and Klaas A. Zachariasse*,† Spektroskopie und Photochemische Kinetik, Max-Planck-Institut fu¨r biophysikalische Chemie, 37070 Go¨ttingen, Germany; B.I. StepanoV Institute of Physics, National Academy of Sciences of Belarus, pr. NezaVisimosti 68, 22072 Minsk, Belarus; Institute of Materials and EnVironmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary; Department Chemie und Biochemie, Ludwig-Maximilians-UniVersita¨t, Butenandtstrasse 5-13, Haus F, 81377 Mu¨nchen, Germany; and Institut fu¨r Chemie, Humboldt UniVersita¨t zu Berlin, Brook-Taylor Strasse 2, 12489 Berlin, Germany ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: October 15, 2010

The photophysics of N-(4-cyanophenyl)carbazole (NP4CN) was investigated by using absorption and fluorescence spectra, picosecond fluorescence decays, and femtosecond transient absorption. In the nonpolar n-hexane as well as in the polar solvent acetonitrile (MeCN), a locally excited (LE) state is detected, as a precursor for the intramolecular charge transfer (ICT) state. A LE f ICT reaction time τ2 at 22 °C of 0.95 ps in ethyl cyanide (EtCN) and 0.32 ps in MeCN is determined from the decay of the LE excited state absorption (ESA) maximum around 620 nm. In the ESA spectrum of NP4CN in n-hexane at a pump-probe delay time of 100 ps, an important contribution of the LE band remains alongside the ICT band, in contrast to what is observed in EtCN and MeCN. This shows that a LE a ICT equilibrium is established in this solvent and the ICT reaction time of 0.5 ps is equal to the reciprocal of the sum of the forward and backward ICT rate constants 1/(ka + kd). In the photostationary S0 f Sn absorption spectrum of NP4CN in n-hexane and MeCN, an additional CT absorption band appears, absent in the sum of the spectra of its electron donor (D) and acceptor (A) subgroups carbazole and benzonitrile. This CT band is located at an energy of ∼4000 cm-1 lower than for N-phenylcarbazole (NPC), due to the larger electron affinity of the benzonitrile moiety of NP4CN than the phenyl subunit of NPC. The fluorescence spectrum of NP4CN in n-hexane at 25 °C mainly consists of a structured LE emission, with a small ICT admixture, indicating that a LE f ICT reaction just starts to occur under these conditions. In di-n-pentyl ether (DPeE) and di-n-butyl ether (DBE), a LE emission is found upon cooling at the high-energy edge of the ICT fluorescence band, caused by the onset of dielectric solvent relaxation. This is not the case in more polar solvents, such as diethyl ether (DEE) and MeCN, in which a structureless ICT emission band fully overlaps the strongly quenched LE fluorescence. For the series of D/A molecules NPC, N-(4-fluorophenyl)carbazole (NP4F), N-[4-(trifluoromethyl)phenyl]carbazole (NP4CF), and NP4CN, with increasing electron affinity of their phenyl subgroup, an ICT emission in n-hexane 25 °C only is present for NP4CN, whereas in MeCN an ICT fluorescence is observed with NP4CF and NP4CN. The ICT fluorescence appears when for the energies E(ICT) of the ICT state and E(S1) of the lowest excited singlet state the condition E(ICT) e E(S1) holds. E(ICT) is calculated from the difference E(D/D+) - E(A-/A) of the redox potentials of the D and A subgroups of the N-phenylcarbazoles. From solvatochromic measurements with NP4CN an ICT dipole moment µe(ICT) ) 19 D is obtained, somewhat larger than the literature values of 10-16 D, because of a different Onsager radius F. The carbazole/phenyl twist angle θ ) 45° of NP4CN in the S0 ground state, determined from X-ray crystal analysis, has become smaller for its ICT state, in analogy with similar conclusions for related N-phenylcarbazoles and other D/A molecules in the literature. Introduction The fluorescence spectrum of the electron donor (D)/acceptor (A) molecule N-(4-cyanophenyl)carbazole (NP4CN) in the polar * 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. † Max-Planck-Institut fu¨r biophysikalische Chemie. ‡ National Academy of Sciences of Belarus. § Hungarian Academy of Sciences. | Ludwig-Maximilians-Universita¨t. ⊥ Humboldt Universita¨t zu Berlin. 4 Present address: Department of Physical Chemistry, University of Siegen, 57068 Siegen, Germany.

solvent acetonitrile (MeCN) at room temperature was found to consist of a single broad structureless emission band, whereas a structured fluorescence spectrum was observed in the nonpolar n-hexane.1 This is different from most other D/A molecules undergoing a reaction from an initially prepared locally excited (LE) to an intramolecular charge transfer (ICT) state, which generally show a dual LE + ICT emission.2 On the basis of its similarity with the broad ICT emission band of DMABN,2,3 the MeCN fluorescence of NP4CN was likewise attributed to an ICT state.1 By implication, the structured emission spectrum in n-hexane was assigned to the LE state, which means that a LE f ICT reaction was thought not to occur with NP4CN in this

10.1021/jp1070506  2010 American Chemical Society Published on Web 11/11/2010

Photophysics of N-(4-Cyanophenyl)carbazole solvent. Two possible reasons for not being able to detect a LE emission in the fluorescence spectrum of NP4CN in MeCN, (a) a very fast ICT reaction, with a rate constant much larger than the radiative rate constant kf(LE), or (b) direct excitation of the ICT state from the S0 ground state, bypassing LE, were not discussed. For N-phenylcarbazole (NPC), in contrast to NP4CN, the fluorescence spectra are structured in alkanes as well in more polar solvents, indicating that a LE f ICT reaction accompanied by an ICT emission does not take place.1,4,5 In view of the unclear situation outlined in this and the previous paragraph, an investigation of NP4CN in the excited state was undertaken and is described in the present article. Following ref 1, NP4CN appears among the compounds for which a twisted ICT (TICT) state has been postulated in the literature.1,2,6-10 This is the case for its single unstructured emission band in solvents more polar than alkanes. In alkanes, a LE state was assumed to emit, as mentioned above.1,6-10 In another interpretation, based on microwave dielectric loss measurements in benzene solution, the singlet excited state of NP4CN in benzene, toluene, p-dioxane, tetrahydrofuran (THF), and MeCN was attributed to LE.11 The TICT structure was adopted for NP4CN without experimental support, only based on the expected analogy with other D/A molecules, such as DMABN.2 Later, evidence has been presented that the ICT state of these substances has in fact a nonorthogonal conformation.12-16 D/A molecules for which planar ICT (PICT) states have been shown to exist are 4-(diisopropylamino)benzonitrile12 (DIABN) and the rigidified compounds 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline13 (NTC6), fluorazene14 (FPP), 4-fluorofluorazene15 (FPP4F), and 4-cyanofluorazene16 (FPP4C). The ICT state of the corresponding flexible analogues DMABN, N-phenylpyrrole (PP), N-(4fluorophenyl)pyrrole (PP4F), and N-(4-cyanophenyl)pyrrole (PP4C) was likewise assumed to have a planar structure (PICT).12-18 For 4-(3,6-di-tert-butylcarbazol-9-yl)benzonitrile (NP4CNdtBu), with a fluorescence spectrum consisting of a single ICT emission band at room temperature, comparable to that of NP4CN, a structure different from the 90° TICT conformation was reported.19 It was concluded that in the ICT state of NP4CNdtBu the carbazole and benzonitrile groups are twisted over an angle of around 40°, similar to that in the ground state S0. In the electrogenerated chemiluminescence spectrum of NP4CNdtBu in MeCN, a single intramolecular CT band appears, identical with the photoexcited fluorescence band.20 The ICT nature (A--D+) of this emission band was supported by its generation from the radical ions A--D and A-D+ (A, benzonitrile; D, 3,6-di-tert-butylcarbazole). This mechanism is the same as that producing the intermolecular exciplex (A-D+) directly from the radical anion A- (A, 1,4-dicyanobenzene) and the radical cation D+ (D, tri-p-tolylamine) in THF.21-23 It was already noted in ref 1 that the ICT fluorescence quantum yield Φ′(ICT) ) 0.38 of NP4CN in MeCN at room temperature (and also the ICT radiative rate constant k′f(ICT) ) 5 × 107 s-1) was much larger than for other D/A molecules such as DMABN (Φ′(ICT) ) 0.030 and k′f(ICT) ) 7.9 × 106 s-1 in MeCN at 25 °C).3 This observation, in conflict with the TICT ideas,2 was attributed to intensity borrowing by the ICT state from a zero-order LE state, which is closer in energy for NP4CN.1 The appearance of a single emission band for NP4CN in MeCN was considered to be similar to the observation of a single fluorescence band with MMD.2,24,25 However, when the LE f ICT reaction rate constant ka is much larger than the

J. Phys. Chem. A, Vol. 114, No. 48, 2010 12623 reaction rate constant kd for the ICT f LE back reaction, then it will become very difficult to detect the LE fluorescence next to the predominant ICT emission.26 This will be shown here to be the case for NP4CN. It also applies to MMD, for which a short-lived (230 fs in n-hexane at 22 °C) LE precursor has recently been detected via femtosecond transient absorption measurements.25 From experiments with NPC and NP4CN in the supersonic jet, it was concluded that the carbazole and phenyl subunits of NP4CN and NPC have a mutual perpendicular configuration in the electronic ground state S0, which would mean that they are electronically decoupled. This 90° twist angle θ(S0) was considered to be preserved on electronic excitation from S0 to S1, since the 0-0 transition is of maximum intensity, and there is no evidence of a torsional progression.27-31 Note, however, that for crystalline NP4CN and NPC, substantially smaller twist angles are obtained: θ(S0) ) 46° for NP4CN and θ(S0) ) 54 and 78° for NPC.32,33 An ICT emission could not be detected with NP4CN in the jet, not even for 1:1 NP4CN/MeCN complexes. This observation is similar to that with DMABN under supersonic jet conditions: no ICT emission for 1:1 complexes between DMABN and MeCN.27-31 N-phenylcarbazole derivatives have been employed as building blocks for organic light-emitting diodes (OLEDs).34-37 An investigation of the photophysics of two of these molecules, NP4CN and NPC, is presented in this article, with the goal to see whether the two excited states LE and ICT are involved in the excited-state reaction of NP4CN. Experimental Section NP4CN (mp 184.0-184.4 °C) was synthesized from carbazole and 4-fluorobenzonitrile.11 N-[4-(Trifluoromethyl)phenyl]carbazole (NP4CF, mp 166.0-166.8 °C) and N-(4-cyanophenyl)-3,6-dicyanocarbazole (NPTCN, mp >300 °C) were made following a similar procedure. N-(4-Fluorophenyl)carbazole (NP4F, mp 117.0-118.0 °C) was prepared in a reaction between 4-fluoroaniline and 2,5-dimethoxytetrahydrofuran.38 NPC was purchased from Aldrich. HPLC was the last purification step for all molecules presented here. The fluorescence decay times were obtained by employing the method of time-correlated single photon counting (SPC). The picosecond laser system consists of a mode-locked titanium-sapphire laser (Coherent, MIRA 900F) pumped by an argon ion laser (Coherent, Innova 415). The fluorescence records were detected at magic angle with a Hamamatsu MCP R3809U (-3100 V). The instrument response function of the laser SPC system has a fwhm of 18 ps. The analysis of the fluorescence decays was carried out by using modulating functions, extended by global analysis.39a In the femtosecond transient absorption experiments, the pumpinduced transient absorption signal was monitored with a supercontinuum probe in the range 280-690 nm. The experiments, with a time resolution of 30 fs, were carried out at magic angle. Further details of the X-ray crystal analysis and the measurement and treatment of the absorption and fluorescence spectra, quantum yields, SPC decays and femtosecond transient absorption spectra can be found elsewhere.3,13b,d,16,17a,18,39b Results and Discussion Crystal Structure of NP4CN and NPC from X-ray Analysis. The crystal structures of NP4CN and NPC are depicted in Figure 1. The results of a X-ray crystal analysis, bond lengths, bond angles, carbazole/phenyl twist angles θ, and nitrogen pyramidal angles φ are collected in Table 1. Literature data for NP4CN32 and NPC33 are also listed.

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Galievsky et al. CHART 1

Figure 1. Crystal structures of N-(4-cyanophenyl)carbazole (NP4CN) (left) and N-phenylcarbazole (NPC) (right). For both molecules, a view from above (a, c) and one along the axis from the phenyl substituent to the carbazole nitrogen (b, d) are presented. The amino twist angle θ is defined as (C(10)N(9)C(14)C(15) + C(13)N(9)C(14)C(19))/2. The pyramidal angle φ is the angle between the vector N(9)C(14) and the plane C(10)N(9)C(13). See Table 1.

Carbazole/Phenyl Twist Angle. With crystalline NP4CN, a carbazole/benzonitrile twist angle θ ) 45.2° is obtained, the mean value of the two dihedral angles 42.8°/47.5° and 42.9°/ 47.3° for the two molecules in the asymmetric unit (Figure 1b and Table 1). A twist angle of 46.1° can be found in the literature (Table 1).32,40 For NPC, the two molecules in the asymmetric unit have different twist angles θ, 53.7 and 76.5°, similar to the angles 54.4 and 77.8° of ref 33 (Table 1). The appearance of two different θ values for the same molecule may point to a relatively low rotational barrier for the N-phenyl bond of NPC; see Figure 2.13d Carbazole Nitrogen Pyramidal Angle. For the N atom in NP4CN, a pyramidal angle φ of around 4° is obtained (4.2 and 3.8° for the two molecules in the asymmetric unit, Table 1). With NPC, the angle φ of the carbazole nitrogen also has two values: 7.9 and 0.4° (Table 1). Carbazole/Phenyl Bond N(9)-C(14). The bond N(9)-C(14) between the carbazole and phenyl subgroups in NP4CN is with

TABLE 1: Data for the Ground-State Structure of NP4CN and NPC from X-ray Crystal Analysisa N(9)-C(14) N(9)-C(10) N(9)-C(13) C(1)-C(2) C(1)-C(10) C(2)-C(3) C(3)-C(4) C(4)-C(11) C(5)-C(6) C(5)-C(12) C(6)-C(7) C(7)-C(8) C(8)-C(13) C(10)-C(11) C(11)-C(12) C(12)-C(13) C(14)-C(15) C(14)-C(19) C(15)-C(16) C(16)-C(17) C(17)-C(18) C(18)-C(19) C(17)-C(20) C(20)-N(21) C(10)-N(9)-C(14) C(10)-N(9)-C(13) C(13)-N(9)-C(14) ∑Ng twist angle θh C(10)N(9)C(14)C(15) C(13)N(9)C(14)C(19) pyramidal angle φi quinoidalityj

NP4CNbAc

NP4CNbBc

NP4CNd

NPCeAc

NPCeBc

NPCfAc

NPCfBc

141.7 139.9 140.9 138.1 138.4 140.5 137.6 139.4 137.7 140.1 138.9 138.6 138.7 141.7 143.8 141.1 139.0 139.2 138.0 138.7 139.1 138.0 143.9 114.0 126.2 108.4 125.2 359.8 45.2 42.8 47.5 4.2 0.9950

141.8 140.2 140.5 138.2 139.1 139.0 137.9 138.5 137.3 140.3 140.0 139.1 137.9 141.3 144.5 140.5 139.2 138.5 138.5 138.2 139.1 138.8 144.2 113.7 126.3 108.4 125.2 359.8 45.1 42.9 47.3 3.8 1.0022

141.6 139.7 140.2 137.4 138.1 139.0 136.6 139.2 136.8 139.8 138.6 138.2 138.0 140.6 143.6 140.5 138.8 138.4 137.3 138.1 138.6 137.9 143.5 113.4 126.1 108.6 125.1 359.8 46.1 43.8 48.4 3.9 0.9949

142.7 138.9 139.8 138.3 138.9 140.0 137.4 139.0 137.4 139.9 139.6 138.6 138.4 141.2 144.6 141.0 137.4 138.0 139.1 137.0 137.2 138.1 125.0 108.5 125.7 359.2 76.5 81.2 71.8 7.9 1.0109

142.7 139.1 139.4 138.2 138.7 138.8 137.8 139.6 138.2 139.5 139.1 137.9 139.5 140.7 144.8 140.5 138.6 137.7 137.6 137.1 138.2 138.3 125.5d 108.3 126.2 360.0 53.7 53.2 54.2 0.4 1.0022

142.0 139.8 140.5 137.9 139.4 139.0 138.3 139.4 137.3 140.1 138.4 138.3 139.0 141.0 144.8 139.6 137.0 138.3 138.0 136.9 136.5 138.4 126.1 107.9 125.3 359.3 77.8 82.7 72.9 7.3 1.0110

142.7 139.4 139.7 137.9 139.2 138.0 138.0 139.6 137.8 139.5 139.6 138.6 139.1 140.4 144.6 141.0 139.5 137.5 137.1 137.1 138.2 139.9 125.5d 108.3 126.2 360.0 54.4 54.3 54.5 0.1 1.0062

a See atom numbering in Chart 1 and Figure 1. The bond lengths are in picometers (pm), the angles in degrees. b X-ray analysis at 200 K. Two symmetric independent molecules A and B in the asymmetric unit. d Data from ref 32. X-ray analysis at 293 K. The article only contains information on the twist angle θ (defined as C(13)N(9)C(14)C(19), different from our definition; see Figure 1, footnote e, and ref 33). e X-ray analysis at 173 K f Data from ref 33. X-ray analysis at room temperature. g Sum of the angles around the carbazole nitrogen (Figure 1). h Twist angle θ: (C(10)N(9)C(14)C(15) + C(13)N(9)C(14)C(19))/2 (Figure 1). i Pyramidal angle φ: angle between the vector N(9)C(14) and the plane C(10)N(9)C(13) (Figure 1). j Quinoidality: ((C(15)-C(16))/(C(16)-C(17)) + (C(18)-C(19))/(C(17)-C(18)))/2 (Figure 1). c

Photophysics of N-(4-Cyanophenyl)carbazole

Figure 2. Calculated (AM1) energies as a function of the twist angle θ around the N(9)-C(14) bond between the carbazole and phenyl subgroups for (a) NP4CN and (b) NPC. For NP4CN the energies are 106.0 kJ/mol (θ ) 0°) and 8.0 kJ/mol (θ ) 90°), relative to zero at θ ) 43.0°. For NPC, 110.3 kJ/mol (θ ) 0°) and 5.6 kJ/mol (θ ) 90°), relative to zero at θ ) 44.7°.

141.8 pm substantially longer than the N-phenyl bond of 136.5 pm in DMABN.41 Its length is similar to the 141.4 pm of the N-phenyl bond in MMD, but remains shorter than the 143.8 pm in 3-(di-tert-butylamino)benzonitrile (mDTABN).41,42 For the NPC crystal, a somewhat larger N(9)-C(14) bond length of 142.7 pm is found than for NP4CN (Table 1). It is concluded from these results that the electronic coupling of the carbazole and benzonitrile or phenyl moieties in NP4CN and NPC will be smaller than that between the dimethylamino and benzonitrile subgroups of DMABN, becoming similar to that present in the case of MMD.42

J. Phys. Chem. A, Vol. 114, No. 48, 2010 12625 Quinoidality: Reduced Carbazole/Phenyl Electronic Coupling. The quinoidality ((C(15)-C(16))/(C(16)-C(17)) + (C(18)-C(19))/(C(17)-C(18)))/2 (Figure 1) in the phenyl rings of NP4CN (0.9986) and NPC (1.0131) is close to unity, i.e., a quinoidal phenyl structure is practically absent (Table 1). The negligible bond length alternation is attributed to the relatively long carbazole-phenyl bond N(9)-C(14), see above, which reduces the electronic coupling between the carbazole and phenyl subgroups. With the 4-aminobenzonitriles, the quinoidality is more pronounced, because of the shorter amino-phenyl bond: 0.9793 (4-aminobenzonitrile (ABN), 137.0 pm) and 0.9870 (DMABN, 136.5 pm).39,41,42 For MMD, with a strongly twisted amino group (θ ) 59°) and a consequently lengthened amino-phenyl bond (141.4 pm), the phenyl bond length alternation has also become very small (quinoidality ) 0.9964).41,42 For comparison, in strongly quinoidal molecules, much larger values are obtained: 0.9032 (p-benzoquinone) and 0.9518 (p-benzoquinodimethane).43,44 Calculated Carbazole/Phenyl Twist Angles of NP4CN and NPC. The calculated (AM1)45,46 energy involved in twisting around the carbazole/phenyl bond of NP4CN and NPC is depicted in Figure 2. The twist angle θ at the lowest energy is 43.0° for NP4CN and 44.7° for NPC. The calculated twist angle of NP4CN is only slightly smaller than the angle θ of 45.2° determined from the crystal (Table 1). A reasonable agreement between computation (44.7°) and experiment (53.7°) is likewise found for one of the crystal twist angles of NPC (Table 1). From calculations (AM1) with NPC in the literature, a similar angle θ of ∼45° is obtained.33 For the structurally related compound NP4CNdtBu, θ ) 41° is calculated (AM1).19 Absorption Spectra of NPC and NP4CN in n-Hexane and MeCN at 25 °C. NPC Ws Subgroups. Before discussion of a possible electronic coupling between the carbazole and benzonitrile subunits of NP4CN in the ground state, the absorption spectra of NPC in n-hexane and MeCN at 25 °C are compared with those of carbazole, N-methylcarbazole (NMC), and toluene in these solvents, employing molar absorption coefficient data (Figure 3a,b). Between the two lowest-energy peaks (29 000-

Figure 3. Absorption spectra at 25 °C of N-phenylcarbazole (NPC), carbazole, N-methylcarbazole (NMC), and toluene in (a) n-hexane and (b) MeCN and of NP4CN, NMC, and 4-methylbenzonitrile (4MBN) in (c) n-hexane and (d) MeCN. In the determination of ε, the temperature dependence of the solvent density has been taken into account.

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TABLE 2: Data Obtained from the Fluorescence and Absorption Spectra of NP4CN and NPC in a Series of Solvents at 25 oC NP4CN solvent 25

a

ε ν˜ max(LE) (cm-1) ν˜ max(ICT) (cm-1) Φ(LE) Φ′(ICT) Φ(ISC)c ν˜ max(S1,abs) (cm-1) εmax (S1,abs) (M-1 cm-1) ν˜ max(S2,abs) (cm-1) εmax (S2,abs) (M-1 cm-1) ν˜ max(S3,abs) (cm-1) εmax (S3,abs) (M-1 cm-1) E(S1) (cm-1)d ∆E(S1,S2) (cm-1)e

n-hexane toluene DPeE 1.88 29210

DBE

DEE

NPC THF

EtCN MeCN n-hexane

2.37 2.86 3.05 4.24 7.39 29.2 36.7 26390 27310 27050 26220 24920 23270 22750

0.371b 0.322

0.42 29870 22430 34430 13250 43000 55710 29690 180

0.284

0.271 0.366 0.58 29650 29780 29800 29850 29730 29790 14070 34190 34320 34340 34390 34280 34380 14610 42870 42930 43030 42830 42990 53970 28720 29130 29140 28880 28330 27800 930 650 660 970 1400 1990

0.365 0.59 29810 13500 34410 14710 43040 54110 27630 2180

DBE

DEE

THF

EtCN MeCN

1.88 29220 0.365

3.05 4.24 7.39 29.2 36.7 29020 29010 28820 28760 28750 0.39

0.41 29500 5140 34250 25100 41700 47715 29380 120

0.41 0.43 29470 29510 29430 29480 29470 4150 34210 34270 34190 34290 34310 18030 41680 41860 41640 41830 41880 50425 29260 29280 29140 29140 29140 210 230 290 340 330

a Solvents: di-n-pentyl ether (DPeE), di-n-butyl ether (DBE), diethyl ether (DEE), tetrahydrofuran (THF), ethyl cyanide (EtCN), and acetonitrile (MeCN). b With a small amount of ICT fluorescence, see text. c Measurements as in ref 17a. T-T absorption spectra in Figure S1 in the Supporting Information. d Crossing point of the fluorescence and absorption spectra (Figure 4). 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 (refs 2 and 14).

34 000 cm-1), the absorption spectra of NPC, NMC, and carbazole have a similar structure. The spectra of NPC and NMC are both red-shifted with respect to carbazole, this red shift being larger for NMC (1060 cm-1) than for NPC (750 cm-1), taking the ν˜ max(S1,abs) values in n-hexane from Table 2. At energies higher than the second absorption peak ν˜ max(S2,abs) at 34 250 cm-1 in n-hexane (Table 2), an additional absorption appears in the spectrum of NPC as compared with NMC or carbazole, thereby indicating that the contribution of toluene to the sum spectrum (starting at about 36500 cm-1) is only minor. In MeCN a similar situation is observed (Figure 3b). The difference in the molar absorption coefficient εmax of the S1 lowest-energy peaks of NP4CN in n-hexane and MeCN will be due to the broadening of the vibronic structure (loss of sharpness) of the S1 absorption band when going from n-hexane to MeCN. That such an effect can occur is seen from a comparison of the S1 absorption band of NPC, NMC, and carbazole in the two solvents (Figure 3a,b). NP4CN Ws Subgroups. Especially between 30 000 and 34 000 cm-1, the absorption spectrum of NP4CN in n-hexane and in MeCN (Figure 3c,d) has an additional absorption band as compared with its subgroup NMC, the absorption of 4methylbenzonitrile (4MBN) (above 35 000 cm-1) having only a minor contribution to the sum spectrum of NMC and 4MBN. This difference is attributed to a CT state (D+A-) of the NMC (D) and 4MBN (A) subgroups. That the CT band of NP4CN appears at lower energies than that of NPC is attributed to the larger electron affinity of the benzonitrile as compared with the phenyl subgroups, as can be deduced from their reduction potentials E(A-/A): -2.36 V47 vs SCE for benzonitrile and -3.42 V48 vs SCE for benzene. Such an additional CT absorption was also identified in the absorption spectrum of the D/A molecules NP4CNdtBu, 4-(1-pyrenyl)-N,N-dimethylaniline (PyDMA), 4-(9-anthryl)julolidine, and 3,5-dimethyl-4-(9-anthryl)julolidine.19,49 It is somewhat surprising that the extra CT absorption of the last two molecules has a substantial absorbance, notwithstanding the D/A twist angle θ of close to 90°,49b,c which minimizes the electronic coupling between A and D. This may be due to the exceptionally large electron density at the meso C atoms of anthracene. It should be noted that the nomenclature CT (charge-transfer character in the valence-bond sense) employed here refers to a state reached directly from S0 by light absorption. Such CT states are different from the ICT states formed from the LE precursor (see Scheme 1).

Fluorescence and Absorption Spectra of N-Phenylcarbazoles in n-Hexane and MeCN. The fluorescence and absorption spectra of NP4CN and NPC in n-hexane and MeCN at 25 °C are depicted in Figure 4a-d. For the interpretation of these spectra, those of N-(4-fluorophenyl)carbazole (NP4F), N-[4(trifluoromethyl)phenyl]carbazole (NP4CF), and N-(4-cyanophenyl)-3,6-dicyanocarbazole (NPTCN) (Figure 4e-j) are also shown. The spectra of NPTCN in Figure 4i are presented in diethyl ether (DEE), because of insufficient solubility in n-hexane. NP4CN and NPC in n-Hexane and MeCN. Whereas the fluorescence spectrum of NPC in n-hexane shows a pronounced vibrational structure (Figure 4c), such a structure is much less developed with NP4CN in this solvent (Figure 4a). This is considered to be a first indication that the fluorescence spectrum of NP4CN is a superposition of a broad unstructured ICT emission band and a structured LE emission. Also with NP4F and NP4CF in n-hexane (Figure 4e,g) a structured fluorescence spectrum similar to the LE band of NPC is observed, which means that a LE f ICT reaction does not occur with these molecules. Further support for this assumption will come from experiments presented later in this article: the temperature dependence of the fluorescence spectrum, picosecond fluorescence decays, and femtosecond transient absorption spectra.50 The fluorescence spectrum of NP4CN in MeCN (Figure 4b) consists of a broad band, similar to the ICT emission spectrum of DMABN3 in this solvent. It is hence assigned to the ICT state. With NPC (Figure 4d) only a structured LE emission appears, showing that an ICT reaction does not occur in MeCN. For NP4CN, a LE emission cannot be observed with certainty. Before analysis of the spectra of NP4CN and NPC, together with those of NP4F, NP4CF and NPTCN (Figure 4), the energy of the possible ICT states of these molecules will be discussed. Note that the electron affinity of the phenyl moieties, related to the reduction potential E(A-/A), increases in the series NPC (phenyl), NP4F (fluorophenyl), NP4CF ((trifluoromethyl)phenyl), and NP4CN (cyanophenyl); see Table 3. A structured fluorescence spectrum in n-hexane and a broad ICT emission in MeCN is also found with 4-(9-anthryl)-N,N-dimethylaniline (AnDMA) and PyDMA (Figure 5). The fluorescence spectra of AnDMA and PyDMA in n-hexane (Figure 5a,c) show some vibrational structure and have a shape that is completely different from the broad ICT emission bands of these molecules in MeCN (Figure 5b,d). The shape of the spectra in MeCN closely

Photophysics of N-(4-Cyanophenyl)carbazole

J. Phys. Chem. A, Vol. 114, No. 48, 2010 12627 D/A molecules can then be calculated from the oxidation and reduction potentials E(D/D+) and E(A-/A) of their D and A subgroups in MeCN51-53 by using eq 1: the semiempirical Weller equation, valid for intermolecular exciplexes1 (A-D+) in the solvent n-hexane.23,39,54 The results are collected in Table 3. E(ICT) has been determined for two situations, taking the carbazole subunits to be either the donor or the acceptor. By comparing E(ICT) with the energy E(S1) of the S1 state, it follows that the carbazole group acts as the electron donor.

E(A-D+) in n-hexane ) E(D/D+) - E(A-/A) + 0.15 ( 0.10 eV (1)

Figure 4. Fluorescence and absorption (Abs) spectra in n-hexane and MeCN at 25 °C of NP4CN (a,b), NPC (c,d), NP4F (e,f), and NP4CF (g,h). For NPTCN the spectra in diethyl ether (DEE) and MeCN (i,j) are shown. The fluorescence spectra in n-hexane of NPC (c), NP4F (e), NP4CF (g), and of NPTCN in DEE (i) consist of a single LE emission, whereas in the case of NP4CN (a) the spectrum may contain an ICT contribution under the major LE fluorescence, see text. In MeCN, a single LE emission band is observed with NPC (d), NP4F (f), and NPTCN (j). With NP4CN in this solvent (b), an ICT emission is found with a possible contribution from LE (see text), whereas a dual ICT + LE fluorescence is present with NP4CF (h).

SCHEME 1

resembles that of NP4CN in this solvent (Figures 4b and 8), whereas the structured spectrum of NP4CN in n-hexane (Figure 4a) is similar to those of AnDMA and PyDMA in this solvent (Figure 5a,c). We therefore conclude that the fluorescence spectra of AnDMA and PyDMA in n-hexane cannot be due to a single ICT emission, as implied in ref 49a but consist of overlapping LE and ICT bands, as indicated in the spectra. ICT State Energetics. An estimate of the energy E(ICT) for the ICT state of NP4CN and the other N-phenylcarbazoles in Figure 4 can be obtained when this energy is taken to be similar to that of the intermolecular exciplex E(A-D+). E(ICT) of these

Fluorescence in n-Hexane. It is seen from Table 3 that, for NP4CN in n-hexane, the E(A-D+) of 3.67 eV is about the same as E(S1) ) 3.68 eV of its LE state. This would be in accordance with the observation that for NP4CN in this solvent a LE f ICT reaction just starts to occur. With the other N-phenylcarbazoles NPC, NP4F, NP4CF, and NPTCN (in DEE), only a LE emission is found (Figure 4c,e,g,i). Looking at Table 3, the difference between E(A-D+) and E(S1) is positive and equals 1.09 eV (NPC), 0.40 eV (NP4F), 0.22 eV (NP4CF), and 0.59 eV (NPTCN), which would make clear that an ICT reaction does not take place with these molecules in n-hexane. With NPTCN, the two CN substituents reduce the donor properties of its carbazole moiety (oxidation potential E(D/D+) ) 1.71 V vs SCE for 3,6-dicyanocarbazole and E(D/D+) ) 1.16 V vs SCE for carbazole)54 to such an extent that the energy of the ICT state (A-D+) is no longer below that E(S1) of the S1 state (Table 3). Fluorescence in MeCN. With NP4CN in MeCN, the fluorescence spectrum consists of a broad ICT emission (Figure 4b). Only a LE emission is found for NPC, NP4F, and NPTCN in MeCN (Figure 4d,f,j), but a dual (LE + ICT) fluorescence appears with NP4CF (Figure 4h). On the basis of the calculated E(A-D+) in MeCN, which is 0.4 eV lower than in n-hexane,21,54a,55,56 the following differences E(A-D+) - E(S1) are obtained: -0.41 eV (NP4CN), 0.69 eV (NPC), 0.0 eV (NP4F), -0.18 eV (NP4CF), and 0.19 eV (NPTCN). It so becomes understandable that only with NP4CN and NP4CF an ICT reaction takes place in this solvent. These results for E(A-D+) - E(S1) mean that, with NP4CN in MeCN, the reaction equilibrium has moved further toward ICT than with NP4CF. This then would lead to a larger ICT/ LE fluorescence quantum yield ratio Φ′(ICT)/Φ(LE) for NP4CN than for NP4CF (0.36, Figure 4h) in such a way that the LE emission of NP4CN is covered by the predominant ICT fluorescence (Figure 4b). Another example of such a situation has been found with FPP4C and PP4C.16 Our interpretation that there may be a LE contribution to the overall fluorescence spectrum of NP4CN in MeCN is supported by the observation of a dual ICT + LE emission with NP4CF in MeCN (Figure 4h). As a general conclusion, the presence or absence of an ICT fluorescence with the N-phenylcarbazoles in Figure 4 clearly is governed by the redox potentials of their carbazole and phenyl subgroups (eq 1). A linear correlation between the ICT emission maximum ν˜ max(ICT) and the difference between the redox potentials E(D/D+) - E(A-/A) has also been established for a series of NP4CNdtBu derivatives.19 CT Absorption. It is seen from Figure 4 that the additional CT absorption band between 30 000 and 34 000 cm-1 present in the spectrum of NP4CN (Figure 4a,b) is absent for NPC and NP4F (Figure 4c-f). With NP4CF, the extra band is located around 32 000 cm-1, at a higher energy than the onset of the

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TABLE 3: ICT Energetics for N-Phenylcarbazoles, PyBN, PyDMA, and AnDMA in n-Hexane AD

Aa

Da

NP4CN

benzonitrile 4MBN carbazole benzene carbazole fluorobenzene carbazole CFB carbazole benzonitrile DCCarb benzonitrile pyrene pyrene anthracene

carbazole NMC 4MBN carbazole benzene carbazole fluorobenzene carbazole CFB DCCarb 4MBN pyrene 4MBN DMA DMA

NPC NP4F NP4CF NPTCN PyBN PyDMA AnDMA

E(A-/A)b (V vs SCE) -2.36 -2.40 -2.71 -3.42 -2.71 -2.74 -2.71 -2.60 -2.71 -2.36 -2.36 -2.10 -2.10 -1.96

E(D/D+)c (V vs SCE) 1.16 1.10 2.62 1.16 2.38f 1.16 2.40 1.16 2.68 1.71 2.62 1.25 2.62 0.80 0.80

E(A-D+),d eq 1(eV) 3.67 3.65 5.48 4.73 5.24 4.05 5.26 3.91 5.54 4.22 3.76 4.87 3.05 2.91

E(S1)e (eV)

E(S1)e (cm-1)

3.68 3.68 3.68 3.64 3.64 3.65 3.65 3.69 3.69 3.63 3.63 3.32g 3.32g 3.16 3.04

29690 29690 29690 29380 29450 29730 29280 29280 26770 25500 24530

a 4MBN, 4-methylbenzonitrile; NMC, N-methylcarbazole; CFB, trifluoromethylbenzene; DCCarb, 3,6-dicyanocarbazole; DMA, N,N-dimethylaniline. b Data from benzonitrile (ref 47); 4MBN (ref 51a); carbazole (ref 51b); benzene (ref 48); fluorobenzene (ref 51c); CFB (refs 51c, d, 52); pyrene (ref 51e); anthracene (ref 51f). c Data from carbazole, NMC (ref 53a); 4MBN (ref 51a); benzene (ref 51a); fluorobenzene, CFB (ref 51c); DCCarb (ref 53b); pyrene (ref 51g); DMA (ref 51a). d (A-D+) in MeCN is 0.4 eV lower (ref 55). e Crossing point of the fluorescence and absorption spectra (Figure 4). f A different value, E(D/D+) benzene ) 2.48 V vs SCE, is reported in ref 51h. g From spectra in ref 57.

Figure 5. Fluorescence and absorption (Abs) spectra in n-hexane and MeCN at 25 °C of 4-(9-anthryl)-N,N-dimethylaniline (AnDMA) (a,b) and 4-(1-pyrenyl)-N,N-dimethylaniline (PyDMA) (c,d). The fluorescence spectra of AnDMA and PyDMA in n-hexane consist of a combination of LE and ICT emission, whereas the spectra in MeCN show an ICT fluorescence band with a minor, strongly quenched, LE emission.

CT band of NP4CN. These differences correlate with the electron affinities (E(A-/A)) of the phenyl subgroups, as to be expected for a S0 f CT absorption. ICT Structure of Other D/A Molecules. For other D/A compounds structurally similar to NP4CN, such as pyrene substituted with a 4-benzonitrile acceptor (4-(1-pyrenyl)benzonitrile (PyBN))57 or a 4-N,N-dimethylaniline donor (PyDMA),49a,57b,58 the ICT state was reported to have a nonperpendicular structure, with twist angles θ(ICT) of the D and A moieties between 55 and 70°. As the excited-state conformation is flattened relative to that in the ground state, the involvement of a TICT state can be excluded.57c With PyBN, as an example, θ decreases from 69° in S0 to 40° for the ICT state.57a This is seen as an example of the general trend that molecules tend to become more planar in the excited state, due to increased mesomeric interaction.57 Other examples are AnDMA and 9,9′-bianthryl.59 9,9′-Bianthryl changes its twist angle θ from about 90° in S0 to around 70° in S1.59b On the

basis of these conclusions, it is assumed that also for NP4CN the ICT twist angle θ(ICT) is smaller than θ(S0) ) 45° (Table 1). Fluorescence Spectra of NP4CN and NPC in n-Hexane as a Function of Temperature. The question whether there is indeed an ICT emission band under the structured LE fluorescence of NP4CN in n-hexane, as discussed in the previous section, leads us to compare the fluorescence spectra of NP4CN with those of NPC (only LE emission), in n-hexane as a function of temperature. The fluorescence spectrum of NPC in n-hexane keeps its vibrational structure over the entire temperature range between 60 and -90 °C (Figure 6a). As generally observed, the spectral structure becomes sharper upon cooling. With NP4CN from 60 to -90 °C (Figure 6b), the fluorescence spectra are substantially less structured than those of NPC. Moreover, the peaks in the spectra undergo a red shift when the temperature is lowered, 350 cm-1 for the highest-energy peak between 60 °C (29 390

Photophysics of N-(4-Cyanophenyl)carbazole

J. Phys. Chem. A, Vol. 114, No. 48, 2010 12629 the fluorescence lifetime of the ICT state. kf(LE) and k′f(ICT) are the radiative rate constants. When kd . 1/τ′0(ICT), the high-temperature limit (HTL), with a small ICT reaction enthalpy difference -∆H

Φ′(ICT)/Φ(LE) ) k'f(ICT)/kf(LE)ka/kd

(3)

Alternatively, when kd , 1/τ′0(ICT), the low-temperature limit (LTL) (large -∆H)

Φ′(ICT)/Φ(LE) ) (k'f(ICT)/kf(LE))kaτ′0(ICT)

Figure 6. Fluorescence (Flu) and absorption (Abs) spectra of (a) NPC and (b) NP4CN in n-hexane, at temperatures between -90 and 60 °C.

cm-1) and -90 °C (29 050 cm-1), clearly larger than that observed with NPC (40 cm-1). The temperature dependence of the absorption spectra of NP4CN over this temperature range is much smaller: a red shift of 160 cm-1 (from 29 930 to 29 770 cm-1) for the lowest-energy peak (Figure 6b), only slightly larger than that of NPC (135 cm-1, Figure 6a). The reason for the red shift of the fluorescence spectra of NP4CN in the nonpolar n-hexane is not clear (small temperature dependence of ε and n: ε25 ) 1.88, ε-95 ) 2.06; n25 ) 1.3724, n-95 ) 1.4393).56a,b The shift could in principle be due to a change in the twist angle θ at different temperatures. In view of the similarity of the potential energy curves of NP4CN and NPC (Figure 2), this does not seem to be likely, as the peaks in the fluorescence spectrum of NPC undergo a much smaller temperature shift. Another possible explanation is that the overall fluorescence spectrum of NP4CN in n-hexane consists of a dual emission from a structured LE emission (as with NPC) and a structureless broad ICT fluorescence band, the ratio Φ′(ICT)/ Φ(LE) of their quantum yields (eqs 2-4) increasing with temperature.60 The question whether or not an ICT emission is present under the predominant LE fluorescence of NP4CN in n-hexane therefore remains for the moment unresolved. This will be further discussed when the femtosecond transient absorption spectra of NP4CN in n-hexane are treated, which in fact indicate the occurrence of a subpicoseond LE f ICT reaction (see below). Dual ICT + LE Fluorescence. For systems with two kinetically interconverting states, such as LE and ICT in Scheme 1, the ratio Φ′(ICT)/Φ(LE) can be expressed by the following equations.3,13a,17a,61a

Φ´(ICT)/Φ(LE) ) k'f(ICT)/kf(LE){ka/(kd + 1/τ′0(ICT))} (2) In eqs 2-4 and Scheme 1, ka and kd are the rate constants of the forward and backward ICT reaction, τ0(LE) is the fluorescence lifetime of the model compound (no ICT), τ′0(ICT) is

(4)

Fluorescence Spectra of NP4CN in Di-n-pentyl Ether, Din-butyl Ether, and Diethyl Ether as a Function of Temperature. Appearance of LE Emission upon Cooling. To investigate whether a small increase in solvent polarity as compared with n-hexane (ε25 ) 1.88) will lead to a clear appearance of LE fluorescence of NP4CN besides the dominant ICT emission (larger Φ(LE)/Φ′(ICT)), its fluorescence was measured in di-n-pentyl ether (DPeE, ε25 ) 2.86), at temperatures down to its melting point (Figure 7a). Whereas the fluorescence spectrum at 25 °C consists of a single broad emission band which is attributed to the ICT state, a weak LE fluorescence band becomes visible upon lowering the temperature, starting at -45 °C. At this temperature, Φ(LE)/Φ′(ICT)

Figure 7. Absorption (Abs) spectra at 25 °C and normalized fluorescence (Flu) spectra at different temperatures of NP4CN in (a) di-n-pentyl ether (DPeE), (b) di-n-butyl ether (DBE), and (c) diethyl ether (DEE). In DPeE and DBE below -35 °C, a LE emission starts to appear at the high-energy side of the main ICT band. The Φ(LE)/ Φ′(ICT) ratios in DPeE and DBE mentioned in the text are determined by subtracting the spectra at -35 °C (only ICT) from the overall (ICT + LE) fluorescence spectra at lower temperatures.

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Figure 8. Absorption (Abs) and normalized fluorescence (Flu) spectra of NP4CN in a series of solvents of increasing solvent polarity (Table 4): n-hexane, di-n-pentyl ether (DPeE), diethyl ether (DEE), tetrahydrofuran (THF), ethyl cyanide (EtCN), and acetonitrile (MeCN) at 25 °C.

) 0.011, increasing to 0.074 at -69 °C. A similar temperature dependence of the fluorescence spectrum is observed for NP4CN in the slightly more polar di-n-butyl ether (DBE, ε25 ) 3.05; ε-85 ) 4.62);56a see Figure 7b, and Figure S2 in the Supporting Information. The ratio Φ(LE)/Φ′(ICT) likewise increases upon cooling, from 0.010 at -45 °C to 0.019 at -69 °C and 0.081 at -94 °C. The contribution of the LE emission to the total fluorescence spectrum is smaller than for DPeE. With NP4CN in the more polar solvent DEE (ε30 ) 4.14; ε-102 ) 9.87),56a only a single ICT fluorescence band is observed, without an indication of a LE emission (Figure 7c). This is the case over the entire temperature range investigated, from 25 to -115 °C. The maximum ν˜ max(ICT) undergoes a red shift upon lowering the temperature, from 26 290 cm-1 at 30 °C to 24 870 cm-1 at -115 °C (Figure 7c), caused by the increase of the solvent dielectric constant and refractive index over this temperature range. These results show that for NP4CN the LE f ICT reaction, which just starts to appear in n-hexane, has become very efficient already in the low-polarity solvents DPeE and DBE. Solvatochromic Measurements. Dipole Moments µe(ICT) of NP4CN and µe(LE) of NPC. The fluorescence spectra of NP4CN in solution at 25 °C, measured over a large polarity range from DPeE (ε25 ) 2.86) to MeCN (ε25 ) 36.7), consist of a single ICT emission band, as shown in Figure 8. The ICT emission maximum ν˜ max(ICT) undergoes a red shift with increasing solvent polarity from 27 310 cm-1 in DPeE to 22 750 cm-1 in MeCN (Figure 8 and Figure S3 (in the Supporting Information), Table 4). By employing these solvatochromic data, the ICT dipole moment µe(ICT) can be determined from the slope of the plot

Figure 9. Solvatchromic plots of the ICT fluorescence maxima ν˜ max(ICT) of NP4CN vs (a) the solvent polarity parameter f(ε) - 1/2f(n2), and (b) ν˜ max(ICT) of DIABN (ref 64), see eqs 5-7 and Table 4. For the ν˜ max(flu) and the numbering of the solvents, see Table 4. From the slopes of the plots, the ICT dipole moment µe(ICT) of NP4CN is calculated: 18.6 D for (a), and 19.3 D for (b), see eq 5 and text.

of ν˜ max(ICT) vs the solvent polarity parameter f(ε) - 1/2f(n2) (eqs 5-7),10,18,47,61b,63 resulting in µe(ICT) ) 18.6 D (Figure 9a). By plotting the ν˜ max(ICT) of NP4CN against those of DIABN (18 D)64 in the same solvents (Figure 9b), a comparable µe(ICT) ) 19.3 D is calculated; see Table 5. With this procedure, the scatter in the data points is generally reduced by mutually compensating the specific solute-solvent interactions.13a,47,62,64 For NPC, a much smaller LE dipole moment µe(LE) is determined: 5.1 D vs f(ε) - f(n2)18,47 (Figure S4a in the Supporting Information) and 5.4 D relative to DIABN (18 D)65,66 (Figure S4b in the Supporting Information). The µe values obtained for NP4CN in the literature, range between 10.1 and 16 D, see Table S2 in the Supporting Information.6,10,67-69 For a discussion of these data, see Supporting Information.65,70-74

ν˜ max(flu) ) -

1 µe(µe - µg)g(ε, n) + const 2hcF3

(5)

TABLE 4: ICT Fluorescence Maxima ν˜ max(ICT) of N-(4-Cyanophenyl)carbazole (NP4CN) and 4-(Diisopropylamino)benzonitrile (DIABN) and LE Fluorescence Maxima ν˜ max(LE) of N-Phenylcarbazole (NPC) at 25 °C in a Series of Solvents Spanning the Polarity Scale f(ε) - 1/2f(n2) (Eqs 6 and 7), and the Half-Width ∆(1/2) of the Fluorescence Spectra of NP4CN solvent

ε

n

f(ε)

f(n2)

f(ε) - 1/2f(n2)

NP4CN (1000 cm-1)

∆(1/2)NP4CNa (1000 cm-1)

NPC (1000 cm-1)

DIABNb (1000 cm-1)

n-hexane (1) toluene (2) di-n-pentyl ether (3) di-n-butyl ether (4) diethyl ether (5) tetrahydrofuran (6) ethyl cyanide (7) acetonitrile (8)

1.88 2.37 2.86 3.05 4.24 7.39 29.2 36.7

1.372 1.494 1.412 1.397 1.350 1.405 1.363 1.342

0.185 0.239 0.277 0.289 0.342 0.405 0.470 0.480

0.185 0.225 0.199 0.194 0.177 0.197 0.182 0.174

0.092 0.126 0.177 0.192 0.253 0.307 0.384 0.393

29.21/28.23c 26.39 27.31 27.05 26.22 24.92 23.27 22.75

3.00 3.79 3.66 3.74 3.87 4.18 4.52 4.65

29.22

25.72 23.84 24.37 24.30 23.52 22.38 20.87 20.49

a

Full width at half-maximum (fwhm). b ν˜ max(ICT) of DIABN (refs 13a, 39, 42, 47, and 64). spectrum with small ICT contribution (Figure 4a).

c

29.02 29.01 28.82 28.76 28.75

Fluorescence maxima of LE fluorescence

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TABLE 5: Data from the Solvatochromic and Thermochromic Analysis of the ICT Fluorescence Maxima ν˜ max(ICT) of NP4CN and NPC NP4CN (eq 5) NP4CN (vs DIABN)g NP4CN (vs DMABN)i NP4CN (thermochromism in DEE) NPC (eq 5) NPC (vs DIABN)g NPC (vs DMABN)i

F (Å)a

µg (D)b

slope

µe(ICT) (D)c

5.15 5.15 (4.68) 5.15 (4.20) 5.15 4.98 4.98 (4.68) 4.98 (4.20)

3.58e 3.58e (6.8) 3.58e (6.6) 3.58e 1.43e 1.43e (6.8) 1.43e (6.6)

-20680 ( 1200f 1.13 ( 0.03h 0.86 ( 0.04j 9.25 ( 0.2k -1520 ( 190l 0.09 ( 0.01m 0.06 ( 0.01n

18.6 ( 0.5 19.3 ( 0.3 (18) 18.6 ( 0.4 (17) 15.3 ( 0.2

µe(LE) (D)d

5.4 ( 0.5 5.1 ( 0.3 (18) 5.0 ( 0.4 (17)

a Onsager radius (eq 5), determined from a density equal to 0.78, based on DMABN (ref 18). b Ground-state dipole moment. c ICT dipole moment (eqs 5-7, with g(ε,n) ) f(ε) - 1/2f(n2)). d LE dipole moment (eqs 5-7, with g(ε,n) ) f(ε) - f(n2)). e Calculated by using AM1, scaled by µg(DIABN) ) 6.78 D (ref 47). f From a plot of ν˜ max(NP4CN) vs f(ε) - 1/2f(n2), eqs 5-7, see Figure 9a. Slope in cm-1. g For DIABN, F ) 4.68 Å, µg ) 6.8 D, µe(ICT) ) 18 D, refs 18 and 47. h From a plot of ν˜ max(ICT) of NP4CN vs ν˜ max(ICT) of DIABN (Figure 9b). i For DMABN, F ) 4.20 Å, µg ) 6.6 D, µe(ICT) ) 17 D, refs 18 and 47. j From a plot of ν˜ max(NP4CN) vs ν˜ max(ICT) of DMABN. k From a plot of ν˜ max(ICT) of NP4CN in DEE vs temperature (Figure S5), see eq 8. Slope in cm-1 K-1. l From a plot of ν˜ max(NPC) vs f(ε) - f(n2), eqs 5-7, see Figure S4a. Slope in cm-1. m From a plot of ν˜ max(NPC) vs ν˜ max(ICT) of DIABN, see Figure S4b. n From a plot of ν˜ max(NPC) vs ν˜ max(ICT) of DMABN.

f(ε) )

ε-1 2ε + 1

(6)

f(n2) )

n2 - 1 2n2 + 1

(7)

In eqs 5-7, µg and µe are the ground and excited-state dipole moments, ε is the dielectric constant, n is the refractive index, h is Planck’s constant, c is the speed of light, F is the Onsager radius of the solute, and g(ε,n) ) f(ε) - 1/2f(n2) for NP4CN, as the ICT state is not directly populated by absorption from S0, but is formed from the LE precursor (see below).13a,18,47 In the case of NPC, with a single LE emission, g(ε,n) ) f(ε) - f(n2); see Table 4.18,47 Thermochromic Analysis for NP4CN in DEE. From the temperature dependence of the maxima ν˜ max(ICT) of the singleband fluorescence spectra of NP4CN in DEE between 30 and -85 °C (Figure S5 in the Supporting Information), a dipole moment µe(ICT) ) 15.3 D is obtained (eq 8),18 employing the temperature dependence of the f(ε) - 1/2f(n2) and f(n2) functions with the data56a,b for ε and n at each temperature. This dipole moment is somewhat smaller than those (18.6-19.3 D) determined by solvatochromic analysis at 25 °C (Table 5).

2µe(µe - µg) d(f(ε) - f(n2)/2) dν˜max(ICT) )dT dT hcF3 µe2 - µg2 df(n2) dT hcF3

decay times: τ1 ) 9.88 ns in n-hexane and τ1 ) 10.89 ns in MeCN (Table 6 and Figure S6 in the Supporting Information). The results support our conclusion, made from an inspection of the fluorescence spectra of NPC in n-hexane and MeCN (Figures 4c,d, and 6a) that a LE f ICT reaction does not take place with NPC. In eqs 9 and 10, if(LE) and if(ICT) are the double-exponential LE and ICT fluorescence intensities with the decay times τi and the amplitudes A1i and A2i (i ) 1, 2)

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

Picosecond Fluorescence Decays of NP4CN and NPC in n-Hexane and MeCN at 25 °C. The picosecond fluorescence SPC decays of NP4CN in n-hexane and MeCN at 25 °C are single exponential (Figure 10, Table 6). The single-exponential character of these fluorescence decays makes clear that a LE f ICT reaction with NP4CN at 25 °C in these solvents, when it occurs (see below), will be faster than 3 ps, the time resolution of the picosecond decay measurements.3,13c,16 As NP4CN in n-hexane shows mainly LE fluorescence (Figures 4a and 6b), the decay time is labeled τ1(LE). With NP4CN in MeCN only ICT fluorescence is observed (Figures 4b and 8), hence the decay time τ1 is equal to the ICT lifetime τ′0(ICT).3 With NPC in n-hexane and MeCN at 25 °C, singleexponential fluorescence decays are obtained, with nanosecond

(10) (11)

The expressions for τ1, τ2, and A appearing in eqs 9-11 are3,66

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

(8)

(9)

X - 1/τ1 1/τ2 - X

(12)

(13)

X and Y in eqs 12 and 13 are given by eqs 14 and 15:

X ) ka + 1/τ0(LE)

(14)

Y ) kd + 1/τ′0(ICT)

(15)

RadiatiWe Rate Constants. From the decay times τ1(LE) of NP4CN in n-hexane and τ1(ICT) ) τ′0(ICT) in EtCN and MeCN, together with the fluorescence quantum yields Φ(LE) and Φ′(ICT), the radiative rate constants kf(LE) and k′f(ICT) can be determined, via kf ) Φ/τ1 (Scheme 1, Table 6). It is found that kf(LE) ) 8.2 × 107 s-1 (n-hexane) and k′f(ICT) ) 3.3 × 107 s-1 (EtCN) and 3.0 × 107 s-1 (MeCN), kf(LE) being larger than k′f(ICT), as is generally observed with D/A molecules. As an example, for DMABN in MeCN at 25 °C, kf(LE) ) 6.5 × 107 s-1 and k′f(ICT) ) 0.79 × 107 s-1.3 Hence,

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Figure 10. Single-exponential picosecond fluorescence decays of NP4CN at 25 °C in (a) n-hexane and (b) acetonitrile (MeCN), measured at the fluorescence maxima (Figure 8): 333 nm (a) and 430 nm (b). The decay time τ1 is given in the figure. The weighted deviations σ, the autocorrelation functions A-C, and the values for χ2 are also indicated. Excitation wavelength: 272 nm. Time resolution: 0.497 ps/ channel, with a time window of 900 effective channels.

TABLE 6: Fluorescence Decay Times τ1, Fluorescence Quantum Yields Φ(LE) and Φ′(ICT), and Radiative Rate Constants kf(LE) and k′f(ICT) at 25 °C

NP4CNb n-hexane EtCN MeCN NPCc n-hexane MeCN

τ1 (ns)

Φ(LE)

4.53 11.11 12.16

0.371

9.88 10.89

0.30 0.39

Φ′(ICT)

kf(LE)a (107 s-1)

k′f(ICT)a (107 s-1)

8.2 0.366 0.365

3.3 3.0 3.0 3.6

a kf )Φ/τ1. b In ref 1, Φ(LE) ) 0.34 in n-hexane, Φ′(ICT) ) 0.38 in MeCN. k′f(ICT) in n-butyl chloride (5 ( 1) × 107 s-1. c In ref 1, Φ(LE) ) 0.33 in n-hexane, Φ(LE) ) 0.40 in MeCN.

for NP4CN in MeCN (Table 6), k′f(ICT) is with 3.0 × 107 s-1 considerably larger than for DMABN. Such a relatively large value for k′f(ICT) has also been obtained with PyDMA49a (14.6 × 107 s-1, MeCN), PyBN57a (22.8 × 107 s-1, MeCN), and AnDMA75 (2.1 × 107 s-1, n-propyl cyanide (PrCN)). It was hence concluded that the ICT reaction of these molecules cannot be explained in terms of the TICT model.19,20,57,75,76 With NPC at 25 °C, the kf(LE) are somewhat smaller than for NP4CN: 3.0 × 107 s-1 in n-hexane and 3.6 × 107 s-1 in MeCN (Table 6). Double-Exponential Picosecond ICT and LE Fluorescence Decays of NP4CN in n-Hexane at -75 and -95 °C. The picosecond fluorescence decays of NP4CN in n-hexane at -75

Figure 11. Picosecond fluorescence decays of NP4CN in n-hexane at (a) -75 °C and (b) -95 °C. Emission wavelengths: 333 nm (LE), 350 nm (ICT). Excitation wavelength: 272 nm. Time resolution: 0.497 ps/ channel, with a time window of 900 effective channels. See the caption of Figure 10.

and -95 °C are shown in Figure 11. The decays were measured at two wavelengths: 333 nm (30 030 cm-1, at the blue edge of the fluorescence spectrum, see Figure 6b) and 350 nm (28 570 cm-1, at the center of the spectrum). From a global analysis of both decays at -75 °C, two decay times are obtained: τ2 ) 15.5 ps and τ1 ) 4190 ps (Figure 11a). At 350 nm, a growingin is observed: A22/A21 ) -0.17. Such a growing-in is absent at 333 nm, A12/A11 ) 0.38 (Table 7). Also at -95 °C, the decays are double exponential (Figure 11b), with τ2 ) 24.7 ps and τ1 ) 4040 ps. The growing-in at 350 nm has become more pronounced, having a larger amplitude ratio (A22/A21 ) -0.29) than that found at -75 °C. The LE amplitude ratio A12/A11 ) 0.77 (Table 7). Its increase as compared with -75 °C is similar to that observed upon cooling for other D/A systems, such as DMABN in MeCN.3 From the double-exponential decays of NP4CN in n-hexane (Figure 11), it follows that a LE f ICT reaction takes place, a support for our conclusion (see above) that its overall fluorescence spectrum consists of two overlapping LE and ICT emission bands. At the blue edge of the spectrum, there is mainly LE emission (positive A12/A11, eq 9), while the contribution of the ICT band leads to an overall negative amplitude ratio A22/ A21 (eq 10). Picosecond ICT and LE Fluorescence Decays of NP4CN in Di-n-Butyl Ether at Various Temperatures. Influence of Dielectric Solvent Relaxation. The global analysis of the picosecond fluorescence decays of NP4CN in DBE at 25, -85, and -95 °C is shown in Figure 12. The decays are measured at two wavelengths: at the blue edge of the spectrum (340 nm)

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TABLE 7: Picosecond Fluorescence Decay Times τi, and Amplitude Ratios A1i/A11 and A2i/A21 (Eqs 9, 10, 16, and 17) and Femtosecond ESA Decay Times τ2 for NP4CN, the ICT Reaction Rate Constants ka and kd, and the ICT Lifetime τ′0 (ICT) (Scheme 1) T τ4 τ3 (°C) (ps) (ps) NP4CN n-hexane (ε25 ) 1.88) DBE (ε25 ) 3.05) EtCN (ε25 ) 29.2) MeCN (ε25 ) 36.7)

25 -75 -95 25 -85 -95 22 22

48 83

τ2 (ps)

τ1 (ps)

0.5b 4530 15.5 4190 24.7 4040 10.7 4250 243 625 5560 426 1220 5720 0.95b 11110d 0.32b 12160d

A12/A11 ka kd ka/kd τ′0(ICT) τ0(LE) A14/A11 A13/A11 (eq 12) A24/A21 A23/A21 A22/A21 (1010 s-1) (1010 s-1) () A12/A11)a (ns) (ns)

126 218

91 196

0.38 0.77 3.69 21 69

-0.15 -0.17

-0.37 -0.37

-0.17 -0.29 -0.10 0.26 0.28