Ultrafast Intramolecular Charge Transfer with <i>N</i>-(4-Cyanophenyl)carbazole. Evidence for a LE Precursor and Dual LE + ICT Fluorescence

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 → 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 ⇄ 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 → 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 → 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(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 ρ. 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.本研究采用吸收光谱、荧光光谱、皮秒级荧光衰减测试以及飞秒瞬态吸收光谱，对N-(4-氰基苯基)咔唑（N-(4-cyanophenyl)carbazole，NP4CN）的光物理行为展开了系统探究。在非极性溶剂正己烷（n-hexane）与极性溶剂乙腈（acetonitrile，MeCN）中，均检测到局域激发（locally excited，LE）态，其作为分子内电荷转移（intramolecular charge transfer，ICT）态的前驱体。通过分析620 nm附近激发态吸收（excited state absorption，ESA）峰的衰减动力学，测得22℃下丙腈（ethyl cyanide，EtCN）中LE→ICT的反应时间τ₂为0.95 ps，乙腈（MeCN）中则为0.32 ps。当泵浦-探测延迟时间为100 ps时，正己烷中NP4CN的ESA光谱仍存在LE态吸收带与ICT态吸收带的显著叠加，这与丙腈和乙腈中的观测结果相悖。该现象表明该溶剂中建立了LE⇌ICT平衡，且ICT反应时间0.5 ps等于正向与逆向ICT速率常数之和的倒数1/(k_a + k_d)。在正己烷与乙腈中NP4CN的光稳态S₀→Sₙ吸收光谱中，均出现了额外的电荷转移（CT）吸收带，而将其电子给体（donor，D）亚基咔唑与电子受体（acceptor，A）亚基苯甲腈的光谱直接叠加后，并无该吸收带。该CT吸收带的能量比N-苯基咔唑（N-phenylcarbazole，NPC）低约4000 cm⁻¹，这归因于NP4CN的苯甲腈部分相较于NPC的苯基亚基具有更高的电子亲和能。25℃下正己烷中NP4CN的荧光光谱主要以结构化的LE发射为主，仅伴随少量ICT组分，说明在此条件下LE→ICT反应刚刚启动。在二正戊基醚（di-n-pentyl ether，DPeE）与二正丁基醚（di-n-butyl ether，DBE）中，低温下可在ICT荧光带的高能边观测到LE发射，这源于溶剂介电弛豫的起始发生。而在极性更强的溶剂如二乙醚（diethyl ether，DEE）与乙腈中则无此现象，此时无结构的ICT发射带完全覆盖了被显著猝灭的LE荧光。针对苯基亚基电子亲和能依次递增的系列给体-受体分子：N-苯基咔唑（NPC）、N-(4-氟苯基)咔唑（N-(4-fluorophenyl)carbazole，NP4F）、N-[4-(三氟甲基)苯基]咔唑（N-[4-(trifluoromethyl)phenyl]carbazole，NP4CF）与NP4CN，实验发现25℃下正己烷中仅NP4CN表现出ICT发射；而在乙腈中，NP4F与NP4CN均可观测到ICT荧光。当ICT态的能量E(ICT)与最低激发单重态的能量E(S₁)满足E(ICT) ≤ E(S₁)时，即可观测到ICT发射。E(ICT)可通过该系列N-苯基咔唑的给体（D）与受体（A）亚基的氧化还原电势之差E(D/D⁺) − E(A⁻/A)计算得到。通过对NP4CN的溶剂致变色效应测试，测得其ICT态偶极矩μ_e(ICT)=19 D，该值略高于文献报道的10~16 D，这源于不同的昂萨格半径（Onsager radius）ρ。通过X射线晶体衍射分析确定，NP4CN基态S₀下咔唑与苯基的扭转角θ=45°，而其ICT态的扭转角更小，这与文献中相关N-苯基咔唑及其他给体-受体分子的研究结论一致。