ARTICLE pubs.acs.org/JPCA
Revisiting Fluorenone Photophysics via Dipolar Fluorenone Derivatives Leandro A. Estrada, James E. Yarnell, and Douglas C. Neckers* Center for Photochemical Sciences at Bowling Green State University, Bowling Green, Ohio 43403, United States
bS Supporting Information ABSTRACT: The nonradiative decay of four dipolar fluorenone derivatives (FODs) was systematically investigated using steady state and time-resolved UVvis absorption and fluorescence measurements combined with cyclic voltammetry. Analysis of the frontier orbital localization of the global minimum geometry and the vertical transitions was carried out from DFT calculations. The first singlet excited state was found to be ππ* in all derivatives regardless of the polarity of the solvent. Charge separation/recombination dominates the singlet excited state deactivation for carbazole-containing FODs. Intersystem crossing (ISC) operates exclusively in the 3,6-disubstituted variants as evidenced by phosphorescence experiments. In the case of CPAFO36, ISC competes disadvantageously with CT deactivation.
1. INTRODUCTION Fluorenone (FO) shows a weak UVvis absorption band at λ > 320 nm, often assigned as nπ*.1 However, its solvatochromic behavior is dissimilar to that of nπ* transitions: in FO the low energy band shifts to the red slightly with the increasing of solvent polarity. This discrepancy led Kuboyama,2 based on molecular orbital (MO) theory, to suggest that in FO this low energy transition is ππ*. Although this divergence was irregular on its own, the later assignment of the triplet state character3 plus the interpretation of the solvent effects on steady state and time-resolved photoluminescence,4 and photochemistry,5 added more controversy to the understanding of the photophysics of this compound. Until this point, the only converging arguments were that FO photophysics was solvent-dependent and that the rapid intersystem crossing was due to an intervening nπ* triplet state. While derivatization of FO supported Kuboyama’s initial assignment,6 it was not until a decade later that Kobayashi clarified this by laser flash photolysis (LFP).7 He found that in nonpolar solvents the S1 state was nπ*, in polar solvents it was ππ*, and in all cases the T1 state was ππ* (Figure 1).8,9 Berces et al. later reported two types of intersystem crossing (ISC) involved in FO’s S1 relaxation: S1(ππ*) f T3(nπ*) and S1(ππ*) f T2(ππ*).10 The temperature dependence of the S1(ππ*) f T3(nπ*) transition experimentally supported Kobayashi’s electronic energy diagram. Internal conversion (IC) might play an important role in the depopulation of S1 in polar solvents,11 especially in the case of dipolar derivatives. Berces et al. also reported photophysical studies on 2-methoxyfluorenone (MFO) and 2-fluorofluorenone (FFO).12 FFO displayed similar photophysics to FO (ISC is the dominant S1 deactivation pathway), while MFO excited state deactivation was governed by an IC that gains in importance with increasing polarity of the medium. The formation of an intramolecular r 2011 American Chemical Society
charge transfer (ICT) state is a strong reason for the bigger rate constants of IC (kIC) in polar aprotic solvents.13 Further studies demonstrate similar trends in kIC for the cases of 2-amino-, 2-Nmethylamino-, and 2-(N,N-dimethylamino)fluorenone as the previously discussed case of MFO.14 Berces’ work initially implied that the covalent bonding of electron donating groups (EDGs) to FO stabilizes the S1 state energy thereby increasing the S1T3 energy gap (Figure 1). While this reasoning is counterintuitive since πEDGs function as σ-electron withdrawing groups (EWGs),15 such an energy increase also highlights the presence of an energetic barrier between S1(ππ*) and T3(nπ*) avoiding ISC while favoring IC in polar solvents. The temperature dependence of this state might be closely related to the increasing amplitude of CdO nonplanar vibrations. More recently, Biczok et al. reported studies of dipolar derivatives in which the EDG was anchored to C3 of FO.16 Phosphorescence from FODs was undetected at room temperature and in rigid matrices for C2-substituted dipolar FODs (2-FODs) exclusively. LFP revealed that the absence of phosphorescence in 2-FODs was a consequence of a low ISC quantum yield (ΦISC < 0.02 for 2-FMDs in toluene and null in acetonitrile). Dipolar 3-FODs also decay with IC being the dominant S1 deactivation mode (ΦIC ∼0.6 for 3-FODs in toluene, ∼0.9 in acetonitrile), although to a lesser extent than for the case of 2-FODs. After comparison with the fluorescence quantum yield (ΦF) in polar and nonpolar media, it was concluded that ISC was still an active S1 deactivation mode for 3-FODs. Its importance was more apparent in solvents of low polarity. The structuremechanism relationship under which the IC takes place is still unclear.17 Fermi’s golden rule establishes that kIC Received: January 17, 2011 Revised: May 4, 2011 Published: May 18, 2011 6366
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Figure 1. Energy level diagram for FO in polar (left) and nonpolar (right) solvents according to Kobayashi et. al. Adapted from ref 9.
Figure 2. Molecular structures of FODs. Since all structures have the FO residue in common, their acronyms are based on their functionalities and the position of such in the FM ring. PA stands for phenylacetylene, while CPA stands for 1-(3,6-di-tert-butylcarbazol-9-yl)-4-ethynylbenzene.
is directly proportional to the excited state electronic coupling matrix element (HAD).18 Previous observations imply that dipolar 2-FODs should have better electronic coupling than 3-FODs in the excited state, since their IC rates were always larger regardless of solvent polarity. This is in line with the observation that metaconjugation in phenylacetylene (PA) dendrimers increases the excited state electronic coupling.19 However, there is no clear correlation between the pattern of substitution and the nonradiative decay of the dipolar FOD as a whole. In order to clarify some of the aspects of FO photophysics, we now report a detailed study on dipolar FODs based on experiments and density functional theory. These compounds, previously reported by our group as synthetic intermediates, incorporate carbazole (Cz) as electron donor in a donor acceptordonor (DAD) configuration (Figure 2).20 We intend to establish clearer structuremechanism relationships between pattern of substitution and photophysics.
2. EXPERIMENTAL SECTION
1-yl)fluoren-9-one (PAFO36), and 3,6-bis(2-(4-(3,6-di-tert-butylcarbazol-9-yl)phenylethyn-1-yl)fluoren-9-one (CPAFO36) were synthesized and characterized previously.20 All spectroscopic grade solvents used for optical measurements were used as received from commercial suppliers with the exception of THF and toluene. The latter solvents were purified through a solvent purification system using activated alumina. Anhydrous DMF and electrochemical grade ferrocene and tetra-n-butylammonium hexafluorophosphate (NBu4PF6) were purchased from commercial sources and used as received for electrochemical measurements. 2.2. Electrochemistry. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed at 298 K with a potentiostat/galvanostat in a three-electrode cell at a scan rate of 100 mV/s. An Ag/AgCl, a platinum disk (2 mm diameter, polished to a mirror finish), and a Pt wire were used as reference, working, and auxiliary electrodes, respectively. Ferrocene was used as internal standard for all measurements. 2.3. Steady State Absorption and Photoluminescence Spectroscopy. Absorption spectra were recorded at a concentration of approximately 1.0 μM in spectroscopic grade solvents using a double-beam spectrophotometer accurate to (1 nm. Steady-state and time-resolved fluorescence measurements were performed on a single-photon-counting spectrofluorimeter equipped with pulsed NanoLEDs (tpulse ∼ 0.8 ns) for emission lifetime measurements. The solutions were prepared with optical densities below 0.1 at the wavelength of excitation. For photoluminescence studies at 77 K, the solutions had optical densities ranging from 0.1 to 0.3 at the wavelength of excitation. A transparent quartz Dewar containing liquid N2 was used to form glasses (in situ) and the Dewar plus sample were then inserted in the fluorimeter for data collection. For the measurements of phosphorescence spectra, the signal acquisition of the PMT was electronically gated to avoid residual fluorescence. Typically, delay times longer than 1 μs and gate widths of 102000 μs were used in these experiments. Fluorescence quantum yields of the luminophores were measured using fluorenone in acetonitrile as standard (Φstd = 3.2%).12 The quantum yield of an unknown sample was calculated using the comparative method21
2.1. Materials. 2,7-Bis(2-phenylethyn-1-yl)fluoren-9-one (PAFO27), 2,7-bis(2-(4-(3,6-di-tert-butylcarbazol-9-yl)phenylethyn-1-yl)fluoren-9-one (CPAFO27), 3,6-bis(2-phenylethyn-
Φ ¼ Φstd 6367
I Astd η2 Istd A η2std
ð1Þ
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Figure 3. Frontier orbital maps for dipolar FODs computed at the B3LYP/6-31G* level of theory. Contour value = 0.03.
where Φ is the quantum yield, I is the integrated intensity, A is the optical density, and η is the refractive index of the media. 2.4. Ultrafast Transient Absorption Spectroscopy. The instrumental setup for 100 fs laser pulses has been described previously.22 The sample was dissolved in 30 mL of spectroscopic grade toluene or DMF and transferred to a 50 mL flask, which was used as a reservoir for sample pumping through a flow quartz cell with an optical path of 2 mm. Steady state UVvis absorption measurements were carried out before and after laser data acquisition to monitor any sample decomposition. All measurements were carried out at room temperature (20 ( 2 C°). Signal decay/rise data analysis was carried out using (mathematical method) deconvolution of single exponential models. 2.5. Computational Methods. Gaussian03 was used to perform all calculations.23 The unconstrained geometries of the carbazole-containing compounds in the gas phase were optimized by density functional theory (DFT) using Becke’s threeparameter functional24 hybridized with the LeeYangParr correlation functional25 and the 6-31G* split-valence basis set.26 The solubilizing tert-butyl groups from carbazole were removed to speed up the computations. Time-dependent DFT (TDDFT) was used to calculate the vertical transitions of the first 10 states of each of the carbazole-containing compounds in the gas phase.27 RhoCI density analysis was performed to compute the first singlet excited state dipole moment.28 Tomasi’s polarized-continuum model (PCM) was used to compute the molecular energy in different solvent cavities (cyclohexane, THF, DMF).29
3. RESULTS 3.1. Energetics: DFT Evaluation. The gas-phase optimized geometries of PAFOs possess a fully planar system (Figures S1 and S5, Supporting Information), while CPAFOs possess two structural minima in which the Cz units are either parallel to or tilted (dihedral CzPh angle ∼44°) from the plane of the acceptor (Figures S3 and S7, Supporting Information). In both cases the phenyl group is coplanar to FO as in the PAFO variants. The tilted structures have the lowest energy in both CPAFO27 and CPAFO36 with difference (ΔE = 2428 kcal/mol). The frontier orbitals of the lowest energy geometries (Figure 3) show that the HOMOs are mainly localized in both Czs while LUMOs are localized in FO. Thus, any electronic transition involving this
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set of orbitals should comprise a charge transfer in accordance with the RehmWeller treatment (vide infra). Another implication is that derivatization of FO in 2,7 and 3,6 positions should not alter, significantly, the energy of the LUMO. Tuning of the lowest energy band gap could be controlled by introduction of either electron-donating (EDGs) or electron-withdrawing groups (EWGs) depending on the desired electronic properties. While the calculated ground state (GS) dipole moments for CPAFOs were low (CPAFO27, μS0 = 3.5 D; CPAFO36, μS0 = 2.5 D), the first excited state dipole moments (computed at the TDDFT level) were remarkably different (CPAFO27, μS1 = 1.8 D; CPAFO36, μS1 = 32 D). Such numbers anticipate clear changes in the electronic distribution for CPAFO36 after photoexcitation (Table 1). Furthermore, the computed values for CPAFO27 are closer to those reported experimentally for FO (μS0 = 3.4 D,30 μS1 = 5.9 D11) indicating that the photophysics of this variant should resemble that reported for the parent acceptor. Incorporation of Cz units to the PAFO system brings forth electronic destabilization of all π-orbitals as all HOMOs involved in the first 10 electronic transitions lack of n-orbitals (section S1, Supporting Information). In the case of PAFOs, the lowest energy electronic transitions are likely to include nπ* character which, in turn, could be responsible for an efficient ISC after photoexcitation. Additionally, the computed energy separation between the transitions S1 r S0 and S2 r S0 is higher for PAFO27 (ΔE = 13.0 kcal/mol) than for PAFO36 (ΔE = 6.6 kcal/mol) revealing that selective excitation of the lowest energy transition should exclude ISC as in the case of parent FO, whose phosphorescence is null if photoexcitation occurs at λ > 340 nm.6 Finally, single point energy (SEP) calculations using Tomasi’s PCM permitted the evaluation of the GS energy stabilization and the dipole change upon increases in solvent polarity. The results of such an exercise (Table 1) highlight the stabilization of the GS energy along with small increases in the resulting dipole moments. This suggests that changes in the relative GS electronic distribution of the frontier MOs should be insignificant upon increases in the external polarity. The ionization potential of Cz (IP = 5.3 eV)31 as well as the electron affinity of FO (EA = 3.1 eV)32 have been reported. Analysis of the driving force for electron transfer can be achieved through the Born dielectric continuum model,33 assuming solvent separated ion-pair (SSIP) formation 1 1 1 1 1 1 þ ΔGIP ¼ Eox Ered e2 rDA εS 2 rD rA εS εT ð2Þ where Eox and Ered are the reduction potentials of D and A, respectively, determined in the solvent S and which electron transfers are studied in the solvent T. The free energies of charge separation (CS) and charge recombination (CR) can be determined using the singlet energies, derived from the average energies of the lowest energy absorption and highest energy emission maxima, as follows34 ΔGCS ¼ ΔGIP ES1
ð3Þ
ΔGCR ¼ ΔGIP
ð4Þ
Assuming that the electrochemical experiments are carried out in a highly polar solvent (i.e., acetonitrile where εS = 37), and using the 00 singlet band gap of the donor (ES1 = 84 kcal/mol) 6368
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Table 1. Difference between S0 and S1 Dipole Moments along with Changes in S0 Dipole Moment and Energy solvent compound CPAFO27
vacuum
cyclohexane
tetrahydrofuran
dimethylformamide
rel. energy (kcal/mol)
0
15.7
24.7
28.5
μS0 (D)
3.5
4.1
4.8
5.2
b
μS1 (D)
1.8
c
|ΔμS| (D)
1.7 7.5
16.5
20.3
3.0
3.5
3.7
a
CPAFO36
rel. energy (kcal/mol)
0
μS0 (D)
2.5
a
a
b
μS1 (D)
32
c
|ΔμS| (D)
30
b
GS dipole moment. ES dipole moment. c Module of the difference between GS and ES dipole moments.
Table 2. Redox Potentials of FODs in DMF vs Fc/Fcþ compound
E1red0 (V)
E2red0 (V)
PAFO27
1.51
CPAFO27 PAFO36
1.51 1.47
2.14
CPAFO36
1.44
2.13
Epa (V)
IP (eV)
EA (eV)
þ0.76
5.7
3.4 3.4
þ0.75
5.7
3.5
3.4
and that of the acceptor (ES1 = 76 kcal/mol), the resulting driving forces for CS are 25.0 and 17.0 kcal/mol, respectively, in a slightly polar medium such as THF.35 The center-to-center distance is assumed to be 1.2 nm and the radius for Cz and FO are taken as 3.5 and 2.5 Å, respectively, upon evaluation of the optimized geometries.33e These numbers reflect a thermodynamically favored PET mechanism upon excitation of either donor or acceptor units within the assumed distance. Consequently, CS is expected to take place in the studied FODs if the kinetic barriers are not sufficiently high or the competing deactivation mechanisms (i.e., PL or ISC) are not significantly faster. Triplet energies bigger than 59 kcal/mol imply a thermodynamically favored mechanism for PET from the triplet manifold as well.
3.2. VOLTAMMETRY Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments against Fc/Fcþ allowed characterization of the charged species that could be produced after photoexcitation (Figure S9, Supporting Information). 2,7-FODs presented one reduction wave centered at 1.5 V and 3,6-FODs presented two reduction waves at ca. 1.5 and 2.1 V, all independent of the presence/absence of Cz. This implies that the LUMO should be localized exclusively in the FO unit. All Cz-derivatives demonstrate one reversible oxidation wave centered at þ0.75 V (corresponding to the anodic Cz/Czþ oxidation). Ionization potentials and electron affinities were calculated from the redox wave values and are summarized in Table 2. The electron affinity was lower in all cases than that reported for FO (∼3.1 V),36 indicating that extension of the π-system destabilizes the LUMO. 3.3. UVvis Absorption and Photoluminescence. All FODs show intense absorptions in the UVblue visible region of the spectrum with high extinction coefficients (Figure 4A). 2,7-FODs showed the most red-shifted absorption of all the derivatives. The similarity of the high energy absorption band of PAFO27 to the ππ* absorption of 2,7-bis(phenylethynyl)fluorene37 suggests the low energy transition has charge transfer (CT) character, with charge traveling from
the aromatic units to the carbonyl group, consistent with our DFT computations (vide supra). In addition, the CT absorption band is too strong for it to be an nπ* transition (log(εabs) ∼ 4.1).38 The CPAFO27 high energy band red shifts with respect to PAFO27 given the increased conjugation conferred by the carbazole units; however, their CT absorption bands are comparable in shape and intensity. These bands are red-shifted nearly 50 nm to those previously reported for FO,2 suggesting that extension of the π-system insignificantly affects this electronic transition. On the other hand, 3,6-FODs displayed higher energy absorption bands than 2,7-FODs, with similar absorption profiles. PAFO36 shows a blue-shifted low energy absorption (λabs g 380 nm) and red-shifted high energy absorption (λabs e 380 nm) with respect to those of CPAFO36. This implies a narrowing of the lowest energy transition band gap after incorporation of Cz units on FO for the 3,6-variants. What is more, the structural geometry of FODs determines the additive contribution of the DA dipoles through the carbonyl axis, and thus a higher transition dipole moment is expected for 3,6-FODs when this type of structure is compared to its 2,7-disubstituted analogues. This is in line with the large differences between ground and excited state dipoles computed at the TDDFT level. The minor solvent dependency of all FODs absorptions (Figure S10, Supporting Information) is consistent with that of parent FO,2 and its dimethylamino derivatives.13 While Berces et al. underline a more important solvatochromism for the case of dimethylamino-FODs, the lack of published spectra on his accounts makes corroboration impossible.14 PAFO27 shows negligible positive solvatochromism. The high energy band of CPAFO27 coalesces without much of an overall shift, while its low energy band exhibits only small positive solvatochromism in its red edge. This is indicative of a slightly higher transition dipole moment for CPAFO27 than PAFO27. 3,6-FODs show a consistent red edge positive solvatochromism of their low energy bands, yet their maxima behave unsystematically. Judging by these observations plus the magnitude of the extinction coefficients of the FODs, the assignment of the character of the S1 state to nπ* seems to be still inappropriate. Steady state photoluminescence (PL) spectra in THF showed some structure for PAFOs (ΔEvib ∼ 0.18 eV, corresponding to aryl CC stretch), corroborated in the PL spectra in MeTHF at 77 K (Figure 4B and Figure S11 in the Supporting Information). In THF at 25 °C, the spectrum of PAFO27 was blue-shifted compared to that of CPAFO27 and in contrast to the case of 3,6FODs where incorporation of Cz units affected significantly their 6369
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Figure 4. (A) Ground state UVvis absorption spectra of FODs in THF at 25 °C. (B) Normalized steady state photoluminescence spectra at room temperature of FODs after excitation at the lowest energy absorption band (λexc = 400 nm for 3,6-FODs. λexc = 450 nm for 2,7-FODs).
Table 3. Photophysical Properties of FMDs Recorded in THF at Room Temperature compound PAFO27
λabs (nm) [ε (M1cm1)]
λem max (nm)
ES1d (kcal/mol)
τF (ns)
350 [153000]
540
60.2
10.4a
560
59.0
krad (107 s1) 1.63
knrad (107 s1) 7.98
ΦPLe (%) 17
440 [12500] CPAFO27
350 [95800]
5.90a
4.07
12.9
24
440 [10800] PAFO36
338 [82200] 400[15800]
495
66.0
CPAFO36
315 [88800]
615
59.5
90 4.10
>99 21.5
0.9 16
395 [35600]
λexc = 460 nm. b λexc = 340 nm. c λexc = 370 nm. d Calculated from the 00 band gap. e ΦF was measured against fluorenone in CH3CN as fluorescence standard (ΦF = 3.2%). a
spectral position. At a glance, this suggests that the lower energy emission for CPAFO36 derives from electronic stabilization of the carbonyl unit conferred by donors in the para-positions. However, the dipoles generated are additive in the carbonyl axis and stabilized by the polar solvent. The fluorescence of the CPAFOs in MeCH and MeTHF at 77 K (Figure S11, Supporting Information) was similar to that of PAFOs, implying small differences in internal reorganization energies. Solvent reorganization possibly plays a major role in the excited state dynamics of FODs. All PL spectra show red-shifts with increasing solvent polarity (Figure S12, Supporting Information). These shifts become more important for CPAFO36, which displayed complete emission quenching in DMF. The solvatochromism of both 2,7FODs and PAFO36 is similar due, presumably, to the comparable magnitudes of their dipole moments for their S0 and S1 states. The PL maxima and Stoke’s shift depend on the Onsager’s solvent parameter f(ε,n) in a nonlinear fashion highlighting specific solvent effects in the excited state (Figure S30, Supporting Information). Other Cz-based DA compounds exhibit such behavior as a consequence of multiple excited states operating in solutions of high polarity such as planar and twisted intramolecular charge transfer (PICT and TICT) states.3941 There are geometry variations in the excited state with increasing solvent polarity that could determine its excited state deactivation (i.e., ISC or ICT). The fluorescence lifetimes of the FODs after excitation into their lowest energy band followed monoexponential decay and were insensitive to oxygen (Table 3). The low values of ΦF in THF highlight efficient NRD, which in the case of PAFO36 was the most effective. Introduction of Cz units to the
FO acceptor affected the emission lifetimes with dependence on the substitution pattern: 2,7-FODs decreased their lifetime while 3,6-FODs had the opposite effect. However, ΦF increases in both cases. Phosphorescence of the FODs at 77 K was measured using a gated-detection technique after exciting the samples at λ > 400 nm. Only 2,7- FODs lacked detectable phosphorescence (Figure S13, Supporting Information), despite attempting ISC enhancement via external heavy-atom effect (addition of 10% ethyl iodide).18,42 Lifetimes in the millisecond regime were recorded for phosphorescence decay in agreement with the long-lived nature of organic triplet states (τP = 7.0 ms for PAFO36 and τP = 9.2 ms for CPAFO36). The observed vibrational progression coincides with that of the fluorescence spectra at 77 K (Figure S11, Supporting Information). In addition, the triplet energies found for each compound were below the expected valued for the thermodynamically favored eT (ET < 59 kcal/mol, vide supra). These results suggest that 2,7disubstituted FODs either undergo IC from the triplet manifold (less likely) or that the access to their triplet manifold is forbidden (most likely) after excitation at λ g 400 nm. 3.4. Ultrafast Transient Absorption Experiments. THF solutions of 2,7-FODs displayed similar spectra, after excitation with 400 nm laser pulses consisting of ground state bleach at λ = 330360 nm and broad excited state absorption (ESA) bands with maxima centered at 375420 and 600620 nm, respectively, plus a shoulder centered at 500520 nm (Figures 5 and 6). The ground state absorption (GSA) of PAFO27 was strong enough to compete with the ESA around the 350 nm 6370
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Figure 5. (Left) UF-TAS evolution for PAFO27 in THF from 0 to 1450 ps after photoexcitation at 400 nm. Right: Kinetics traces for GS bleach and positive ESA bands.
Figure 6. (Left) UF-TAS evolution for CPAFO27 in THF from 0 to 1450 ps after photoexcitation at 400 nm. (Right) Kinetics traces for GS bleach and positive ESA bands.
region impeding its kinetic tracing. This case is contrary to that of CPAFO27, where a negative feature extending from 350 to 400 nm is observed. The positively absorbing bands resemble those reported for the UVvis absorption of the FO radical anion (FO•), with a small red shift and broadening due to the extended conjugation.43 The triplettriplet absorption (TTA) maximum for FO shows a broad band (350500 nm) centered at ∼420 nm in various degassed solvents,44 with concomitant increase of the ISC lifetime with the polarity of the medium (i.e., τISC ∼ 3 ns in THF).45 The lack of spectral resemblance to FO’s TTA, the faster ESA growth than the ISC in THF for FO, and the fact that these signals decay within 0.15 ps to 1.5 ns support their assignment to singletsinglet absorption. The kinetic traces of all ESA bands of PAFO27 are similar (Figure 5 and Figure S14 in the Supporting Information). Destructive interference between GSA and ESA in such a region involved similar optical densities (ODs) preventing a correlation between the bleach kinetics and those from the other ESA bands. ESA bands grow monoexponentially (τ(g) = 0.330.44 ps) while decaying as the sum of a monoexponential term plus a constant (τ1(d) = 123157 ps, τ2(d) > 1.5 ns). The growth is associated with charge separation (CS) out of the FranckCondon (FC) region. While the longer lived decay component might be associated to 1CT relaxation (S1 f S0) processes (see that τF ∼ 10 ns), the nature of the short-lived component is unclear. Apparently, more than one excited state might be present in the form of an equilibration between the
planar and twisted ICT (TICT) states. The existence of an overlapping TTA band, though less likely, is yet to be disregarded. To probe the presence of higher states involved in excited state deactivation, the same sample was pumped with 360 nm laser pulses (Figures S1517, Supporting Information). The Sn r S1 signal growth was comparable to those found for the ESAs upon excitation with 400 nm laser pulses (τ(g) = 0.32 ps). Nevertheless, the feature changes in the ESA band extending in the 370450 nm region after 100 ps highlight a possible transition between potential energy surfaces (PESs). The presence of an isosbestic point at 475 nm plus the resemblance of band shapes and location to those reported for the TTA of FO at λ < 450 nm suggest that ISC might be occurring from upper states (Figure S17, Supporting Information), possibly mediated by an upper nπ* state according to the FO case44 and our TDDFT calculations (vide supra). These observations, plus the lack of phosphorescence following irradiation at λ > 400 nm, imply that the access to the triplet manifold via S1 state is forbidden. Conversely, the kinetics of CPAFO27 is more complex than that of PAFO27 (Figure 6 and Figures S19 and S20 in the Supporting Information). The smaller excited state molar absorptivity of CPAFO27 with respect to that of the GS at 380 nm allowed kinetic tracing of the bleach signal which exhibits complex recovery (τ1(g) = 5.3 ps, τ2(g) = 0.90 ns, τ3(g) > 1.5 ns). Furthermore, the band centered at 420 nm grows biexponentially (τ1(g) = 0.54 ps, τ2(g) = 4.6 ps) and decays monoexponentially (τ(d) > 1.5 ns). The kinetic behavior of both bands 6371
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Figure 7. (Left) UF-TAS evolution for PAFO36 in THF from 0 to 1450 ps after photoexctitation at 400 nm. (Right) Kinetics traces for GS bleach and positive ESA bands.
Figure 8. (Left) UF-TAS evolution for CPAFO36 in THF from 0 to 1450 ps after photoexctitation at 400 nm. (Right) Kinetics traces for GS bleach and positive ESA bands.
is attributable to contributions of both GS recovery and ESA decay. Considering the features from the GSA spectrum and comparing with the UF-TAS at 20 ps time delay (Figure S20, Supporting Information), subtraction of the former signal to the latter partially reconstructs the positive ESA band of PAFO27 extending from 380 to 420 nm. From this spectral resemblance it can be concluded that the positive features at λ > 500 nm can be directly associated with the band centered at 420 nm as they are assumed to come from the same ESA as in the PAFO27 case. The ESA bands centered at 520 and 620 nm grow monoexponentially (τ(g) = 0.250.40 ps) while decaying as the sum of two components (τ1(d) = 0.831.09 ns, τ2(d) > 1.5 ns). In contrast, the longest growth contribution in the signal at 420 nm is correlated to the shortest recovery from the signal at 380 nm and assigned to solvent reorientation. The shortest growth lifetime component for 420 nm is parallel to those from λ > 500 nm, associated with CS out of the FC region. The longer of the other two lifetime components can be assigned to ICT relaxation (1CT f S0) and the shorter decay component attributed to the equilibration lifetime between CT and a relaxed TICT state as evidenced by the blue shift observed from the band centered at 520 to 520 nm. Interestingly, photoexcitation at 355 nm revealed ISC in the same region as PAFO27 (Figure S22, Supporting Information). The spectral evolution of the 3,6-FODs is dissimilar (Figures 7 and 8). At 0.5 ps, PAFO36 spectrum shows two bands with
maxima centered at 430 and 580 nm (the former with a shoulder centered at 400 nm) that grow consistently at t < 20 ps (Figure 7). Destructive interference of the GSA and ESA is apparent in the high energy region (Figure S25, Supporting Information). The low energy band (580 nm) shifts bathochromically, while becoming more intense from 0 to 20 ps. For t > 20 ps, this band shifts to the blue while incrementing its absorbance until the end times of the experiment (Figure S24, Supporting Information). Meanwhile, the high energy band decays without much of an overall shift after reaching its absorption maximum at t ∼ 10 ps. The shoulder centered at 390 nm transforms after 100 ps, decaying until its intensity equaled about one-third of the original maximum. The most likely mechanism taking place is ISC as supported by the presence of an isosbestic point ca. 480 nm. The EDGs in 3,6-positions change the energy of the upper states and thus different ESA energies should be expected if compared to FO. The TTA absorption band for FO is reported to have a maximum centered at 420 nm,7,9,44 which in this case appears to be red shifted ∼150 nm. Kinetic modeling of the ESA also supports an ISC mechanism (Figure S26, Supporting Information). The ESA at 600 nm presented a biexponential growth (τ1(g) = 0.39 ps, τ2(g) = 412 ps). The shortest component was assigned as population of the S1 state and the longest was assigned to ISC (Sn r S1 and Tn r T1 absorptions overlap in this region). The latter component correlated well with one of the decay parameters of the bands 6372
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Figure 9. Schematics of electron delocalization for 2,7- and 3,6-FODs.
centered at 385 and 430 nm which presents complex kinetics (τ1(d) = 43 ps, τ2(d)= 438 ps, τ3(d) > 1.5 ns). While the shortest of these three terms can be speculatively assigned to vibrational energy relaxation, the longest component is undoubtedly TTA. The rate for ISC, kISC = 2.3 109 s1, is in agreement with the calculated knrad (knrad > 9.91 108 s1, see Table 3), implying that ΦISC for this compound must be close to unity. The spectral evolution of CPAFO36 was different from that of PAFO36 (Figure 8 and Figure S26 in the Supporting Information). At early times, bands with maxima centered at 355 and 490 nm rapidly grow until reaching a plateau at 2 ps. This is also evident in the shoulder centered at 643 nm. While the high energy band (355 nm) is constant, the low energy band (490 nm) and the shoulder (643 nm) fuse into a single broad band after 3 ps with maximum centered at 500 nm (Figure S27, Supporting Information). The intensity of these bands persists from this time up to 50 ps followed by continual decay until the end times of the experiment when it reaches about half of its maximum intensity. The valleys centered at 330 and 380 nm can be attributed to the destructive overlap between GSA and ESA, with the former signal being dominant over the latter. Given the long-lived character of this excited state as judged by the kinetics of the band centered at 500 nm (no GSA), the bleach recovery is mostly due to, presumably, decay of the positive feature. The lack of persistence of the band along the experimental time frame prompted us to momentarily disregard any possibility of TTA and focus our attention in the ICT mechanism. The positive ESA could be assigned to the contributions of Cz•þ and FO•, respectively. This is even more apparent in the spectrum of CPAFO36 in a more polar solvent, such as DMF (Figure S28, Supporting Information) where the ESA bands clearly resemble those reported for uncoupled Cz•þ (750800 nm),46 and FO•. The latter signal is clearly blueshifted from the reported values due to the extended conjugation conferred by the PA groups.43 The lack of coupling between charged radicals implies a dominant TICT mechanism. Back to the spectral evolution of CPAFO36 in THF, the negative absorption band presented monoexponential decay (τ(d) = 1.62 ps) and complex recovery (τ1(g) = 58 ps, τ2(g) > 1.5 ns), respectively. The fast decay component correlates with the signal growth of the ESA at 550 nm (τ(g) = 2.8 ps), and the first decay component of the shoulder at 650 nm (τ1(d) = 2.1 ps, τ2(d) > 1.5 ns). Thus, this fast element is assigned to CS. The long decay component correlates with the longest of the biexponential components for the decay traced at 550 nm (τ1(d) = 1.01 ns, τ2(d) > 1.5 ns), and is thus assigned as CT f S0 relaxation. The trace at 500 nm showed complex kinetics (τ1(d) = 79 ps, τ2(d) = 1.31 ns, τ3(d) >1.5 ns) whose numbers signified an apparent equilibration between ICT and TICT states and the decay of one of these. The fastest component correlates well with that from the negative absorption recovery (58 vs 79 ps). The fact that the TTA band might be obscured by the observed ESA signals alerts of its possible presence; however, the
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upper limit established for ISC by the results from the fluorescence measurements (knrad = 2.15 108 s1 w τISC ∼ 5 ns) prompts its oblivion for the time being. All the same, access to the triplet manifold is evidenced by the presence of phosphorescence after excitation at 400 nm (vide supra) which suggests that a possible CT f Tn mechanism could be taking place outside the experimental time window.
4. DISCUSSION The first reduction potentials are identical for 2,7-FODs and very similar for their 3,6-variants. This reflects more stabilized LUMOs with respect to that from parent FO (Table 2) and poor energetic contribution of the Cz donors to the LUMO. The LUMOs of all FODs must be localized in the FO unit as predicted by DFT calculations. The identical oxidation potentials of both CPAFOs highlight HOMO localization on the Cz units (compare with DFT results from section S1, Supporting Information). This indicates poor electronic communication between D and A units in the ground state and could be, in principle, extended to PAFOs given the harmony between experiment and theory found thus far. This case is consistent with those reported for other types of dipolar Cz-based chromophores.47 The absorption profiles of 2,7-FODs are similar in which the low energy absorption bands extending from 400 to 500 nm are nearly identical. This suggests negligible contribution of Cz to lower this transition energy. TDFFT calculations reveal that the HOMO of PAFO27 is similar to the HOMO-4 of CPAFO27 (Figures S2 and S4, Supporting Information), explaining the overlap in the low energy region. While the spectral features of PAFO36 and CPAFO36 are comparable, the lower energy absorption for PAFO36 blue shifts from that of CPAFO36 and vice versa when compared to the high energy profile. The higher energy of the S1 r S0 transition of 3,6-FODs compared to those of the 2,7-variants might be due to the more extended conjugation found in the 2,7-FODs (Figure 9). Such π-extension destabilizes the energy of the HOMO orbitals while decreasing the optical band gap. The assignment of the low energy band of 2,7-FODs to ππ* bands is consistent with its high molar decadic coefficient (log(εabs) ∼ 4.1) and lack of negative solvatochromic effect with increments in solvent polarity. This is similar to the case of parent FO whose low energy band is ππ*.2,6,7 The molar decadic coefficient of the FODs grows after expansion of the πsystem via derivatizations in the 2,7 positions of FO, without much of a change in its absorption features aside from an obvious red shift. This helps to support the initial ππ* assignment for the S1 r S0 transition. The solvatochromic behavior of FODs is similar to that of parent FO and can be explained by small ground state dipole moments. However, the trend established by the fluorescence spectra is different. The important positive solvatochromism of CPAFO36 reflects the highly polar character of its emissive state, while confirming that the S1 state for this compound is of CT character. In polar enough medium such as DMF, it displays complete charge separation and emission quenching (viz. TICT). Low temperature fluorescence spectra reveal small internal reorganization energies for all FODs (λv ∼ 56 kcal/ mol) and the importance of solvent reorganization for the stabilization of the S1 excited state. Positive solvatochromism was observed in the fluorescence spectra of all other FODs to a minor extent, reflecting small differences between ground and 6373
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Figure 10. (Left) Proposed excited state deactivation diagram for FODs. The kinetic barriers for CS are variable depending on the case. (Right) Possible intermediate structures of PICT and ICT states described in the diagram.
excited state dipole moments. This is also evidenced by LippertMataga plots and TDDFT calculations (Figure S30, Supporting Information). Forward electron transfer is thermodynamically favored for CPAFO27 (i.e., E00 ∼ 60 kcal/mol w ΔGCS < 0 for THF and DMF). However, the long conjugation of 2,7-disubstituted geometry helps delocalize the generated hole which translates into a smaller excited state dipole (Figure 9). This could explain why the solvent reorganization in this molecule is not as important as in the case of CPAFO36 where the electron delocalization is done in one of the axis of the Cz group, thus generating a strong dipole. The access to the triplet manifold is restricted for the case of 2,7-FODs. After photoexcitation at λ g 400 nm, only 3,6-FODs exhibits phosphorescence. The fact that 2,7-FODs lacked of such signal is common to the case of FO: its phosphorescence is only apparent if excited at λ < 350 nm.6 Forward eT is unexpected in the case of PAFO36 as its high oxidation potential forbids such a mechanism under RehmWeller treatment (vide supra). Therefore, its low ΦF points toward a very efficient ISC. Different is the case of CPAFO36, wherein competition of ICT and ISC is predicted by steady state absorption, fluorescence, and phosphorescence results. Further experiments of triplet sensitization monitored by LFP are necessary to study the quantum yield of triplet formation of all FODs. UF-TAS of PAFO27 after excitation at 350 nm revealed a minor contribution of ISC to its nonradiative deactivation. Such contribution is negligible when the same compound is excited at 400 nm. This suggests an upper nπ* state as responsible for the ISC, in accordance to the DFT calculations and the case of parent FO. Technical limitations forbid us to pump at higher energies to verify this. The features present in the transient spectrum of PAFO27 are similar to those from CPAFO27, and both are similar to the reported signal for FO•.43 ICT after photoexcitation is likely to take place as expected from the DFT orbital mapping. The extension of the conjugation might play an important role in the excited state deactivation of FODs. The S1 state of PAFO36 deactivates via efficient ISC, contrary to the case of PAFO27. Finally, CPAFOs kinetics highlight a PICTTICT equilibration where in the case of the 3,6-variant it is more evident in polar solvents, such as DMF, as the equilibrium favors the CS species (Figure 10). This highlights effective electronic communication from positions 2,7 which helps delocalize the charge through the p-phenylene system.
5. CONCLUSION In summary, four novel FODs were studied in order to establish clear structureactivity relationships from the photophysics and electronic point of view. The photophysical properties of the materials were clearly dependent on the position of substitution. All FODs presented low ΦF values which prompted further experiments to unveil the dominant S1 state relaxation mechanism. After UVvis absorption, steady-state plus timeresolved fluorescence spectroscopy, CV, and UF-TAS were performed, it was concluded that 2,7-disubstituted FODs presented ICT as main deactivation mechanism.14 3,6-Variants presented different excited state deactivation modes as the Cz units became more active in the relaxation mechanism lowering the energy of a possible 3nπ* mediator state which promotes ISC. PAFO36 deactivates from the S1 state with ΦISC ∼ 1, while CPAFO36 deactivates under competing ISC and ICT mechanisms. Further experiments based on LFP are needed in order to unveil completely the excited state deactivation of all FODs, such as sensitization and excitation wavelength dependence experiments. ’ ASSOCIATED CONTENT
bS
Supporting Information. Cartesian coordinates of FODs optimized geometries, TDDFT data and orbital maps, cyclic and differential pulse voltammograms, UVvis absorption and photoluminescence spectra in various solvents, PL at 77 K in various matrices, UF-TAS data of all FODs with corresponding kinetic traces and mathematical fits, LippertMataga plots of all FODs, and complete Gaussian 03 reference. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Professor Massimo Olivucci for support with theoretical computations and fruitful discussions. We thank Dr. K. Glusac from the Ohio Laboratory for Kinetic Spectroscopy (OLKS) for support with the 100 fs laser system. 6374
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The Journal of Physical Chemistry A Professor F. N. Castellano, Dr. F. Sp€anig, and Ms. V. Prusakova are also acknowledged for assistance with voltammetry and photoluminescence measurements. DFT calculations were performed with time allotted from of the Ohio Supercomputer Center. L.E. thanks the McMaster Endowment for a fellowship and Ms. S. Ergun for support with phosphorescence measurements. This work was supported by the Endowment Fund of the Center for Photochemical Sciences.
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