Physics Department, University College Cork, Cork, Ireland.
Phys Chem Chem Phys. 2011 Apr 28;13(16):7485-99. doi: 10.1039/c0cp02601h. Epub 2011 Mar 21.
The S(0) → S(1) fluorescence excitation spectrum of jet-cooled 1H2N with origin at 25484 cm(-1) has been measured. Twelve totally symmetric modes and five non-totally symmetric modes have been assigned in the excitation spectrum. Theoretical calculations at DFT B3LYP/6-31G** and CIS/6-31G** levels indicate that the 1H2N molecular geometry is more planar in the S(1) state than in the ground state. The geometry of the naphthalene ring changes upon excitation and promotes a number of totally symmetric ring stretching modes, in the excitation spectrum. As a result of the geometry change upon excitation a number of non-totally symmetric modes gain intensity. Based on a rotational envelope fitting procedure the average excited rotational state lifetime was estimated to be between 7 and 16 ps for 0 ≤E≤ hc × 800 cm(-1) (E is excess energy above the S(1) origin). The decay rate coefficients, k, of the rotational S(1) states, are not constant over this range of excess energies. By applying a Golden Rule model, it was determined that internal conversion to S(0) is unlikely to be the sole non-radiative process contributing to the decay of the excited states. It was concluded that excited state intramolecular proton transfer (ESIPT) plays a role in the observed behaviour of the rate co-efficient with excess energy. The observation of (i) a sharp increase in rate coefficient, k, above an excess energy of ∼550 cm(-1), and (ii) a significant number of high intensity fluorescence excitation spectrum features above an excess energy of ∼700 cm(-1), may indicate the presence of an energy barrier of ∼550 cm(-1), between the enol and keto geometries in the S(1) state. This result supports the conclusions of S. De, S. Ash, S. Dalai and A. Misra, J. Mol. Struc. Theochem, 2007, 807, 33-41, who estimated a barrier to ESIPT of ∼750 cm(-1). It was concluded that ESIPT occurs in 1H2N, across an energy barrier with a rate constant, k(pt)≤ 10(11) s(-1). Hence, at low excess energies (≤ 550 cm(-1)), the observed emission band originates predominantly from the keto tautomer. Above an excess energy of ∼1600 cm(-1), 1H2N decays predominantly via a non-radiative mechanism.
已测量到喷射冷却的 1H2N 的 S(0)→S(1)荧光激发光谱，其起源位于 25484 cm(-1)。在激发光谱中，已分配了 12 个完全对称模式和 5 个非完全对称模式。在 DFT B3LYP/6-31G和 CIS/6-31G水平上的理论计算表明，1H2N 分子在 S(1)态下的几何形状比在基态下更平面。萘环的几何形状在激发时发生变化，并在激发光谱中促进了一些完全对称的环拉伸模式。由于激发时的几何形状变化，一些非完全对称模式获得了强度。基于旋转包络拟合程序，估计 0 ≤E≤ hc × 800 cm(-1)（E 是 S(1)起源上方的过剩能量）范围内的平均激发旋转态寿命在 7 到 16 ps 之间。在这个过剩能量范围内，旋转 S(1)态的衰减速率系数 k 不是常数。通过应用金规则模型，确定向 S(0)的内部转换不太可能是导致激发态衰减的唯一非辐射过程。得出结论，激发态分子内质子转移（ESIPT）在观察到的速率系数与过剩能量的行为中起作用。在过剩能量大于约 550 cm(-1)时，观察到速率系数 k 的急剧增加，以及在过剩能量大于约 700 cm(-1)时，观察到大量高强度荧光激发光谱特征，这可能表明在 S(1)态下，烯醇和酮几何形状之间存在约 550 cm(-1)的能量势垒。这一结果支持了 S. De、S. Ash、S. Dalai 和 A. Misra 在 J. Mol. Struc. Theochem，2007，807，33-41 中的结论，他们估计 ESIPT 的能垒约为 750 cm(-1)。得出的结论是，在 1H2N 中，ESIPT 发生在具有速率常数 k(pt)≤ 10(11) s(-1)的能量势垒中。因此，在低过剩能量（≤ 550 cm(-1)）下，观察到的发射带主要来自酮式互变异构体。在过剩能量大于约 1600 cm(-1)时，1H2N 主要通过非辐射机制衰减。
