Vibronic Coupling Drives the Ultrafast Internal Conversion in a Functionalized Free-Base Porphyrin.

Internal conversion (IC) is a common radiationless transition in polyatomic molecules. Theory predicts that molecular vibrations assist IC between excited states, and ultrafast experiments can provide insight into their structure-function relationship. Here we elucidate the dynamics of the vibrational modes driving the IC process within the Q band of a functionalized porphyrin molecule. Through a combination of ultrafast multidimensional spectroscopies and theoretical modeling, we observe a 60 fs Qy-Qx IC and demonstrate that it is driven by the interplay among multiple high-frequency modes. Notably, we identify 1510 cm-1 as the leading tuning mode that brings the porphyrin to an optimal geometry for energy surface crossing. By employing coherent wave packet analysis, we highlight a set of short-lived vibrations (1200-1400 cm-1), promoting the IC within ≈60 fs. Furthermore, we identify one coupling mode (1350 cm-1) that is responsible for vibronic mixing within the Q states. Our findings indicate that porphyrin-core functionalization modulates IC effectively, offering new opportunities in photocatalysis and optoelectronics.

Estimating the number of available states for normal and tumor tissues in gene expression space.

The topology of gene expression space for a set of 12 cancer types is studied by means of an entropy-like magnitude, which measures the volumes of the regions occupied by tumor and normal samples, i.e., the number of available states (genotypes) that can be classified as tumor-like or normal-like, respectively. Computations show that the number of available states is much greater for tumors than for normal tissues, suggesting the irreversibility of the progression to the tumor phase. The entropy is nearly constant for tumors, whereas it exhibits a higher variability in normal tissues, probably due to tissue differentiation. In addition, we show an interesting correlation between the fraction (tumor/normal) of available states and the overlap between the tumor and normal sample clouds, interpreted as a way of reducing the decay rate to the tumor phase in more ordered or structured tissues.