Takashige Fujiwara

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

We present here the results of time-resolved absorption and emission experiments for 4-(dimethylamino)benzonitrile in solution, which suggest that the fluorescent intramolecular charge transfer (ICT) state may differ from the twisted ICT (TICT) state observed in transient absorption.

Role of the πσ* State in Molecular Photophysics

Photosynthesis, which depends on light-driven energy and electron transfer in assemblies of porphyrins, chlorophylls, and carotenoids, is just one example of the many complex natural systems of photobiology. A fuller understanding of the spectroscopy and photophysics of simple aromatic molecules is central to elucidating photochemical processes in the more sophisticated assemblies of photobiology. Moreover, developing a better grasp of the photophysics of simple aromatic molecules will also enhance our ability to create and improve practical applications in photochemical energy conversion, molecular nanophotonics, and molecular electronics. In this Account, we present a concerted experimental and theoretical study of aromatic ethynes, aromatic nitriles, and fluorinated benzenes, illustrating the important roles that the low-lying pisigma* state plays in the electronic relaxation of these aromatic compounds. Diphenylacetylene, 4-dialkylaminobenzonitriles, 4-dialkylaminobenzethynes, and fluorinated benzenes exhibit fluorescence that strongly quenches as the excitation energy is increased for gas-phase systems and at elevated temperatures in solution. Much of this interesting photophysical behavior can be attributed to the presence of a dark intermediate state that crosses the fluorescent pipi* state. Our quantum chemistry calculations, as well as time-resolved laser spectroscopies, indicate that this dark intermediate state is the pisigma* state that arises from the promotion of an electron from the pi orbital of the phenyl ring to the sigma* orbital localized in the C[triple bond]X group (where X is CH and N) or on the C-X group (where X is a halogen). These crossings not only lead to the strong excitation energy and temperature dependence of fluorescence but also induce highly interesting pisigma*-mediated intramolecular charge transfer in 4-dialkylaminobenzonitriles. Most previous studies on the excited-state dynamics of organic molecules have examined aromatic hydrocarbons, nitrogen heterocycles, aromatic carbonyl compounds, and polyenes, which have low-lying excited states of pipi* character (hydrocarbons and polyenes) or npi* and pipi* character (carbonyls and N-heterocycles). These studies have revealed important involvement of selection rules (promoting vibrational modes and spin-orbit coupling) and Franck-Condon factors for radiationless transitions, which have important effects on photophysical properties. The recent experimental and time-dependent density functional theory (TDDFT) calculations of aromatic ethynes, nitriles, and perfluorinated benzenes described in this Account demonstrate the importance of the bound excited state of a pisigma* configuration in these molecules.

The low-lying πσ* state and its role in the intramolecular charge transfer of aminobenzonitriles and aminobenzethyne

Electronic absorption spectra of the low-lying pipi(*) and pisigma(*) states of several aminobenzonitriles and 4-dimethylaminobenzethyne have been studied by time-resolved transient absorption and time-dependent density functional theory calculation. In acetonitrile, the lifetime of the pisigma(*)-state absorption is very short (picoseconds or subpicosecond) for molecules that exhibit intramolecular charge transfer (ICT), and very long (nanoseconds) for those that do not. Where direct comparison of the temporal characteristics of the pisigma(*)-state and the ICT-state transients could be made, the formation rate of the ICT state is identical to the decay rate of the pisigma(*) state within the experimental uncertainty. These results are consistent with the pisigma(*)-mediated ICT mechanism, L(a) (pipi(*))-->pisigma(*)-->ICT, in which the decay rate of the pisigma(*) state is determined by the rate of the solvent-controlled pisigma(*)-->ICT charge-shift reaction. The pipi(*)-->pisigma(*) state crossing does not occur in 3-dimethylaminobenzonitrile or 2-dimethylaminobenzonitrile, as predicted by the calculation, and 4-aminobenzonitrile and 4-dimethylaminobenzethyne does not exhibit the ICT reaction, consistent with the higher energy of the ICT state relative to the pisigma(*) state.

Photophysics of aromatic molecules with low-lying πσ* states: Fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S1) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S1 state of ππ* character for fluorinated benzenes with four or less F atoms, and S1 state of πσ* character for PFB and HFB. The πσ* character of the S1 state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the π1π* (Lb)←S0 transition, which can be identified with the predicted low-energy π1σ*←S0 absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the π1σ* state and efficient S1 (πσ*)→S0 internal conversi...

Highly effective quenching of the ultrafast radiationless decay of photoexcited pyrimidine bases by covalent modification: photophysics of 5,6-trimethylenecytosine and 5,6-trimethyleneuracil.

5,6-Trimethylenecytosine (TMC) and 5,6-trimethyleneuracil (TMU), in which the twist of the C5-C6 bond (or the pyrimidalization of C5) is strongly hindered, do not exhibit the subpicosecond excited-state lifetime characteristic of the naturally occurring pyrimidine bases. This result demonstrates the important role the out-of-plane deformation of the six-membered ring plays in the ultrafast (subpicosecond) internal conversion of photoexcited nucleobases. The dramatically shorter fluorescence lifetime of TMU ( approximately 30 ps) relative to TMC ( approximately 1.2 ns), in aqueous solution at room temperature, is attributed to the presence in TMU of an efficient, secondary nonradiative decay channel of S(1)(pipi*) involving a low-lying (1)npi* state.

Spectroscopy and Photophysics of 1,4-Bis(phenylethynyl)benzene: Effects of Ring Torsion and Dark πσ* State

A combination of supersonic-jet laser spectroscopy and quantum chemistry calculation was applied to 1,4-bis(phenylethynyl)benzene, BPEB, to study the role of the dark pisigma* state on electronic relaxation and the effect of ring torsion on electronic spectra. The result provides evidence for fluorescence break-off in supersonic jet at high S1(pi pi*) <-- S0 excitation energies, which can be attributed to the pi pi*-pi sigma* intersection. The threshold energy for the fluorescence break-off is much larger in BPEB (approximately 4000 cm(-1)) than in diphenylacetylene (approximately 500 cm(-1)). The high-energy barrier in BPEB accounts for the very large fluorescence quantum yield of the compound (in solution) relative to diphenylacetylene. The comparison between the experimentally derived torsional barrier and frequency with those from the computation shows overall good agreement and demonstrates that the low-energy torsional motion involves the twisting of the end ring in BPEB. The torsional barrier is almost an order of magnitude greater in the pi pi* excited state than in the ground state. The finding that the twisting of the end ring in BPEB is relatively free in the ground state, but strongly hindered in the excited state, provides rationale for the well-known temperature dependence of the spectral shape of absorption and the lack of mirror symmetry relationship between the absorption and fluorescence at elevated temperatures.

The role of the πσ* state in intramolecular charge transfer of 4-(dimethylamino)benzonitrile

The solvent-polarity dependence and temporal characteristics of the transient absorption of 4-(dimethylamino)benzonitrile, DMABN, and 4-(dimethylamino)benzethyne, DMABE, demonstrate the presence of the πσ*-state absorption at about 700 nm and the ππ* (LE)-state absorption at about 520 nm and 450 nm. The rise and decay times of the πσ*-state transient differ from those of the ππ*-state transients in both compounds. Moreover, the peak position of the πσ*-state absorption is blue-shifted and more intense in acetonitrile as compared to n-hexane, whereas the band positions of the ππ*-state absorptions are essentially the same in the two solvents. For DMABN in acetonitrile, the rise time (∼4.3 ps) of the twisted intramolecular charge transfer (TICT)-state transient at 330 nm is identical to the decay time of the πσ*-state transient. The 4.8 ns decay time of the TICT-state absorption of DMABN is longer than the 2.9 ns decay time of the intramolecular charge-transfer (ICT) fluorescence, indicating that the fluorescent ICT state differs from the TICT state observed in transient absorption. These results are consistent with the presence of a low-lying πσ* state in DMABN (and DMABE), and the role the πσ* state plays in the formation of the TICT state of DMABN.

Coupled electron and proton transfer processes in 4-dimethylamino-2-hydroxy-benzaldehyde.

TDDFT calculations, picosecond transient absorption, and time-resolved fluorescence studies of 4-dimethylamino-2-hydroxy-benzaldehyde (DMAHBA) have been carried out to study the electron and proton transfer processes in polar (acetonitrile) and nonpolar (n-hexane) solvents. In n-hexane, the transient absorption (TA) as well as the fluorescence originate from the ππ* state of the keto form (with the carbonyl group in the benzaldehyde ring), which is produced by an intramolecular proton transfer from the initially excited ππ* state of the enol form (OH group in the ring). The decay rate of TA and fluorescence are essentially identical in n-hexane. In acetonitrile, on the other hand, the TA exhibits features that can be assigned to the highly polar twisted intramolecular charge transfer (TICT) states of enol forms, as evidenced by the similarity of the absorption to the TICT-state absorption spectra of the closely related 4-dimethylaminobenzaldehyde (DMABA). As expected, the decay rate of the TICT-state of DMAHBA is different from the fluorescence lifetime of the ππ* state of the keto form. The occurrence of the proton and electron transfers in acetonitrile is in good agreement with the predictions of the TDDFT calculations. The very short-lived (∼1 ps) fluorescence from the ππ* state of the enol form has been observed at about 380 nm in n-hexane and at about 400 nm in acetonitrile.

A one-photon laser induced fluorescence and a sequential two-photon optical–optical double resonance excitation study of the vibrational structure of the B̃ 1A1(ππ*) state of thiophosgene, Cl2CS

The vibrational structure of the B 1A1(ππ*) electronic state of jet cooled thiophosgene, Cl2CS, has been studied by laser induced fluorescence (LIF) excitation spectroscopy through the one-photon process B 1A1←X 1A1 and by optical–optical double resonance (OODR), spectroscopy. Two OODR schemes were used to probe the B 1A1 state. One uses selected vibronic levels of the singlet A 1A2(nπ*) state while the second scheme uses the companion triplet, a 3A2(nπ*) as the intermediate level. The one-photon LIF and the two-photon OODR schemes use different optical selection rules and the two sets of spectra contain very different information. The analyses of the vibrational structure confirmed the existing assignments for the Q1 (C=S stretch), Q2 (C–Cl stretch), and Q4 (out-of-plane) modes, and reversed the assignments for the Q3 (sym. Cl–C–Cl bend) and Q6 (antisym. Cl–C–Cl bend) modes. The barrier height to molecular inversion was revised upwards to 1495 cm−1 from an analysis of the (−)−(+) inversion splittings...

Photophysical and spectroscopic manifestations of the low-lying πσ* state of 4-(dimethylamino)benzethyne: solvent-polarity dependence of fluorescence and excited-state absorptions

A concerted experimental and computational study of 4-(dimethylamino)benzethyne, DMABE, has been carried out to probe the low-lying pisigma* state and the role it plays in the photophysics of the molecule. The subpicosecond transient absorption spectra reveal the presence of a strong excited-state absorption at about 700 nm and a weaker absorption at about 520 nm. The observed absorption maxima are in excellent agreement with the TDDFT calculations that place a strongly allowed pisigma* <--pisigma* transition at 750 nm, and a weaker pipi* <--pipi* (LE) transition at 528 nm. This agreement combined with the differing decay times, and differing solvent shifts of the two transients, allow assignments of the 700 nm absorption to the pisigma* state and the 520 nm absorption to the LE (pipi*) state. The bifurcation of the initially excited L(a) (pipi*) state into the pisigma* state and the LE state, as probed by transient absorption, is strongly influenced by solvent polarity, with polar environments favoring the L(a)-->pisigma* decay channel over the competing L(a)--> LE decay channel. The nanosecond radiationless decay of the LE state to the dark pisigma* state is also strongly enhanced in polar environments, thus accounting for the dramatic quenching of fluorescence in solvents of high polarity.